micromilling machine

understanding micro-milling machine technology |
 

 production machining

understanding micro-milling machine technology | production machining

Micro-milling can be a companion process to turning-based production machining. This article looks at some of the technologies that go into a micro-milling machine and why they are important to successful operation. #micromachining

Technology transitions, along with moving outside your comfort zone, can be rather painful, particularly in the manufacturing sector. Management, engineering and the movers and doers out on the shop floor dont always see eye to eye regarding any new technology that gets introduced into the company.

But in todays highly competitive production market, change is inevitable in order to survive. What you are doing today and how you are doing it will not be the same in 5 to 10 years. However, its not about creating an immediate paradigm shift for tomorrows work, but rather subtle changes into new technology and new markets over time.

One such technology that compliments Swiss-type production machining is micro-milling. Micro-milling has traditionally held its roots in the European market, but throughout the last few years it has been rapidly expanding into the U.S. market. For those already embracing small part production on Swiss-type machines, micro-milling is a developing market that can provide competitive leadership compared to those with little or no experience working with small parts.

Unfortunately, machine technology for micro-milling is different. Further, it goes beyond the boundaries of traditional milling equipment and, like any technology, it can be a jungle out there. In order to take the first steps into micro-milling, its important to unravel some of the machine technology behind the equipment as well as the supporting technology that goes with it.

The success of any company is highly dependent upon each individual department. If one department is not running at 100 percent, it has the potential of affecting the whole company. The affecting potential is dependent on the business model of the company and what it produces.

Likewise, a machine is made up of individual components. If one component is not at 100 percent, the machine may fail at producing a quality part. How the machine is used and what its producing will naturally determine the affecting potential on the part.

However, unlike traditional milling, which is less forgiving, micro-milling requires a tighter quality relationship within every machine component. In other words, if there is a slight quality issue with the spindle, chances are it will have a negative effect on the part quality and may result in total failure. For micro-milling, each component is critical in the success of producing a good micro-machined part.

Machine geometry plays an important role on the overall performance of the machine. It will determine the stiffness, accuracy, thermal stability, damping properties, work volume and ease of operator use. The two most popular vertical machine geometry types are bridge and C-frame construction, each offering various pros and cons.

However, a C-frame construction typically offers the best stiffness for micro-machining since stiffness directly affects accuracy. In a C-frame design, the only moving axis is the spindle or the Z axis, thus there is less weight offering better dynamic stiffness.

Stiffness decreases in C-frames as the length of Z travel increases. The ideal C-frame construction is one that has a suitable balance between the design intent and the length of Z travel. With a bridge construction, both the X axis and the Z axis are suspended above the X-axis table. The bridge therefore carries a greater amount of weight offering less dynamic stiffness for micro-milling. Most bridge constructions are better suited for high-speed machining of medium to larger parts where maximum Z travel is commonly required.

One of the more devastating machining attributes of milling small, delicate and accurate parts is vibration. Like stiffness, damping is a critical element that needs to be under control during micro-milling. Machine tools with increased damping will absorb more of the vibrations induced by cutting. Many machines frames are constructed using cast iron or steel weldments.

Unfortunately, these types of structural materials are not suitable for micro-milling. The most suitable machine frame material for micro-milling is polymer concrete. Polymer concrete often provides as much as 10 times higher absorption of vibrations than cast iron. Polymer concrete also provides superior dynamic and static rigidity than cast iron and has substantially better thermal stability properties, which are crucial for achieving small-part accuracy.

The machine tool way system includes the load-bearing components that support the spindle and table, as well as guiding their movement. There are two primary guideway systems: box ways (sometimes called hydrodynamic ways) and linear guides. Each system has its positive and negative characteristics.

Unfortunately, one type of way system is not appropriate for all applications. Box ways are used on a large percentage of machines and are most commonly found on large metal removal machining centers. Because of their design, box ways are problematic where frequent axis reversals are required and low friction motion is needed for extreme accuracy. A linear guideway system is the choice for a micro-milling machine. They offer low static and dynamic friction and are well suited for a high degree of multi-axis and complex motion.

How small of a part a shop can machine and how successful a shop is depends on the drive and motion technology built into that shops micro-machine. There are several types of drive and motion technology on the market today. However, ballscrew technology is still the driving force behind the axis drive mechanism on most machine tools.

Ballscrews are driven by servomotors. This combined technology of ballscrew and servomotor still remains suitable for micro-milling machines. Technology such as linear motors do not provide significant advances compared with traditional ballscrew technology for micro-milling. What does remain important is how the drive and servomotors work together to provide precise and accurate motion in order to produce miniature-size 3D features. Feedback devices, such as glass scales and motor encoders, are placed on machine tools to determine position.

Many machine tool manufacturers only use rotary encodes to determine actual position of an axis. However, rotary encoders only determine distance travel or the speed of travel and do not account for backlash, wear or thermal changes with the ballscrew. Any of these geometrical changes with the ballscrew will cause errors in the actual position. To counteract these geometrical changes and to ensure the most precise axis position, glass scales are placed close to the guideways to provide additional feedback to the control.

Glass scales come in a range of precision, but most precision high-end machine tools commonly use a 0.5-micron increment in conjunction with the rotary encoder. Since many applications in micro-milling are small and detailedrequiring the smallest incremental motion, 0.5 micron glass scales may not be significant for producing small, ultra-precision parts. In this case, 0.1 micron glass scales are available to produce even the most detailed micro-machined parts.

Spindle technology has come a long way through recent years. There are many types of spindles on the market: gear driven, belt driven, motorized, air driven and hydrostatic. The more common high-rpm spindles are motorized spindles. A motorized 160K-rpm spindle was unheard of only a few years ago. Although a 160K spindle has its applications, the more common high-speed spindles are more applicable to have an rpm of as high as 50K.

With micro-milling, tool size is relative to the application. Commonly, a 6-mm-diameter tool would be considered large and a 0.3-mm-diameter tool would be considered quite small. In this range, a spindle of 50K rpm would provide an adequate solution.

Motorized spindles come in two basic forms: open loop or closed loop. A closed loop spindle is commonly called a vector spindle. Open loop spindles are generally used when cutting forces are relatively small such as in micro-milling. They are also less expensive, but have a number of drawbacks. Open loop spindles have no encoder feedback. Therefore, operations such as rigid tapping and spindle orientation are not supported. Further, the ratio between minimum and maximum spindle speed is limited. For example, an open loop spindle of as high as 40K rpm may have a low-end rpm of only 2K. A spindle less than 2K rpm does not develop enough torque for cutting.

The ideal spindle for micro-milling is a closed loop or vector controlled spindle. They offer the range of rpm, full torque at low speeds, rigid tapping capabilities and spindle orientation. A well-designed vector control spindle on a micro-milling machine will offer a large amount of flexibility along with the abilities of cutting even the most difficult-to-cut materials.

The toolholder and spindle interface is the design configuration between the spindle and the toolholder. There are a number of different toolholder interfaces for milling. Some of the more common ones are called steep tapered toolholders such as CAT, BT and ISO. These are used on the majority of milling machines and come in various sizes. Another type of interface is called HSK. HSK tooling has rapidly been adopted for high-speed spindles and for use on high precision machining centers.

Tapered toolholders establish their axial position in the spindle through the mating of two tapers. One of the disadvantages of this interface is that as spindle speeds increase, the spindle shaft tends to expand because of centrifugal force and thermal effects. When this occurs, the taper of the toolholder can be drawn further into the spindle thus causing a number of problems including inaccuracies in Z-axis motion.

Because micro-milling uses high rpm, tapered toolholders are not the ideal toolholder type. HSK toolholders offer a number of advantages for high rpm spindles and thus are the preferred choice for micro-milling machines. HSK toolholders are retained in the spindle by a set of internal grippers located inside the spindle.

As rotational speeds are increased, metal-to-metal contact between the toolholder and the spindle is maintained because centrifugal forces cause the internal grippers to expand within the toolholder, pressing it firmly against the inside of the spindle shaft. HSK tooling is also a double-contact interface. It locates on both a shallow taper and a flange, creating a rigid precision fit for both axial and radial cutting forces.

This precision fit allows the interface to have superior runout conditions compared to steep tapered tooling. When working with very small cutters, runout inaccuracies can cause premature cutter failure. Further, excessive runout can also reduce the life expectancy of the spindle. For micro-milling machines, the ideal runout inaccuracy should be 1 micron or less.

Control technology is another area on the machine tool that has seen advances. Thanks to advanced hardware and software technology, todays CNC controls are fast and powerful. Unfortunately, the topic of CNC control technology is complex. Books have been written on the topic alone.

However, there are a number of important aspects regarding control technology that can be pointed out herecontrol interface, motion control and feedback, processing speed and support. A control interface doesnt seem like a logical problem, but high-tech machine tools require high-tech controls and most high-tech controls are packed with numerous features.

These features can be quite overwhelming to an end user, thus creating an intimidating work environment. The interface should be logically laid out and simple to use, yet flexible enough to handle even the most complex toolpaths output from any CAD/CAM system.

Because micro-milling tool paths can be complex, containing thousands of blocks of information, its important that the control is able to accept several types of storage media along with an Ethernet connection. Motion control and feedback are crucial for precision micro-milling applications. The control must be able to process high density complex data fast and be able to command the motion to the axis in a precise manner.

How fast the control is able to do this will depend on a number of internal factors that are beyond the scope of this text. The best solution is to select a micro-milling machine with proven control technology from a respected and well-known company. In this case, support will become a natural spin-off providing years of excellent support.

Working with small parts and tools can be frustrating at times. During milling, cutting tools need to be measured for both length and diameter size. This information is fed back to the control to make offset compensations for the tool path.

Measuring tools that can hardly be seen by the human eye are almost impossible to measure mechanically. Many machine tools are using laser measuring devices to automatically measure both the tools length and diameter using a small laser beam of light. With an adequate laser measuring system, tools as small as 40 microns are able to be measured in a reliable and easy manner.

Small parts and fixtures can also be challenging to set up. The use of a touch probe can make setting up a micro-milling machine much easier. Automatic centering, part zeroing and part alignment can be used to quickly establish part orientation. Additionally, part measuring can also be done using many of the touch probe routines that are commonly found on controls that feature probing.

A good micro-milling machine alone will not guarantee success at micro-part manufacturing. Work environment including a temperature control work area, cleanliness and organization are all important areas that need attention during a transition toward micro-milling. Further, working with tools as small as 50 microns takes a bit of knowledge and a great deal of experience. Be sure the machine tool builder has knowledge and experience to fully support you and guide you towards successful micro-milling.

The Pursuit of Perfection is how Acero Precision defines its mission and while such lofty words make nice copy, all one needs to do is visit this contract manufacturer to experience how serious the company is about the technology, personnel and processes necessary to make it more than an abstract goal.

new best mini milling machine reviews - updated in july 2021

new best mini milling machine reviews - updated in july 2021

Once upon a time, milling machines were only relevant to large garages and factories. And mini mill remained so for a long time, because who would want to bring home a machine weighing hundreds or thousands of pounds anyway?

In this post, were going to help you find the best mini mill by exploring the top models in the market. With the models in our list, you can mill a wide variety of materials at home and achieve great, professional results.

If youre looking for the best mini milling machine for small to medium projects, this would definitely be a great selection. Its loaded with many other features that make it easy to use, such as the push-button speed control.

Sturdiness is one of the biggest joys of owning this model. Its one-piece cast iron column is super tough and hard to break. Its the kind of construction you can rely on when dealing with hard materials.

When lateral forces are exerted against the bit, its possible to for the cutting tool to get accidentally disconnected from the spindle. When that happens, a bad cut on the material might be executed or worse, you could get injured.

A table size of 10 5/8 by 3 5/32 inches allows you to work on your small scale projects without a problem. The two hand wheels allow you to adjust the worktable quickly and conveniently so you can finish your task without stress.

Once you get this mini mill, youre provided with three collets of varying sizes. These are very useful, as they prevent your cutting tools from falling off the spindle. That means you get to work faster and more safely.

One of the main issues that machinists face when using a mini mill is vibration. On this model, the axes are locked in position to alleviate this issue. Minimal vibration means you get to work peacefully.

When looking for the mini mills to include in our review, we went through a significant number of models. Many of the did not make it to our list, as we were using a fixed criterion to determine the finest models.

A heavier milling machine normally performs better and lasts longer than a lightweight mill. However, you also have to account for the ease of transportation and the place where you will keep the mill.

Electric mini mills have different power needs. Some need 110V, some 230V, and so on. Fortunately, you can alter them to work with your shops or garages power output, though you might need an electrician for that.

Check the motor too. How much power can it deliver? There are several ratings you can look at. The first on is the HP rating, which can be 0.something, 1, 2, and so on. The higher the rating, the more/better the output.

And lastly, theres the spindle speed. Youll see a rating like 0 to 2000 RPM (rotations per minute) or 0 to 3000 RPM. The higher the spindle speeds a milling machine can attain, the wider the range of tasks it can handle. Again, higher speeds create a smoother finish.

Therefore, ensure you keep your manual nearby for reference or details. Find a picture in there that labels the parts of your machine and be sure to understand the parts and functions. That will help you use the appliance more efficiently.

A mill is a machine that uses a rotating cutter attached to a spindle to adjust the shape of solid pieces such as metal or wood or to cut shapes on them. It works by removing material from the work piece.

A mini mill is a small version of an industrial mill. Its a versatile machine used to drill, produce slots, bore holes, cut gears, and achieve a range of other functions by removing material from solid items.

Over to you now. What model are you going to secure? Will you go for the no-fuss littleJET JMD-18 Mini Milling Machine or will you get the more robust Klutch mini milling machine that is first on our list?

Remember, getting the best mini mill machine requires you to speculate the features carefully. Follow our links to see more features on amazon while checking what other machinists are saying about your preferred model.

A few years back, it all started with my first blog website. It was about to deal and heal with automotive hand tools. Well, it brought me a good audience base for sure, which then dragged me out of my major and got me to sit and write, and be a blogger. Read more

benchtop micro milling machine : 7 steps (with pictures) - instructables

benchtop micro milling machine : 7 steps (with pictures) - instructables

This is a micro sized benchtop mill that is suitable for milling small parts in soft metals such as Aluminum and Brass. While it is a manual mill it wouldn't be too difficult to convert it to CNC by adding suitable stepper motors and a CNC controller.

This design uses off the shelf assembled linear slides and 80/20 Aluminum extrusion for simple assembly and accurate alignment. The beauty of this design is its modular nature- you can make it any size you want and it's super easy to build.

I literally had almost all of these materials on hand- some of it I had been hanging on to for years. Having said that I have checked to make sure all of the materials used are widely available. I think the only new parts I bought were the ER11 collet chuck and the 12V power supply.

Linear slides- I used Igus SLW prebuilt linear slides. The size I used is the 1040, which has a 10x2 trapezoidal lead screw. One full rotation of the lead screw = 2mm table travel. The beauty of these is that they are configured as bolt on units so the rails and the lead screws are in perfect alignment and you don't have to worry about them binding- getting this alignment right is one of the most difficult aspects of building a milling machine. Igus has a huge variety of linear slides available in all sorts of configurations.

A less expensive alternative would be to use a Proxxon KT70 XY table for the base and an Igus linear slide for the Z axis. I've seen some really nice machines built using the Proxxon table but you're a bit more limited with small table size.

Note that with these Igus linear slides the supported load must be less than 2X the bearing distance- If your Z axis (vertical axis) has bearings placed 2" apart your spindle must be placed less than 4" out from the Z axis base or you run the risk of premature bearing failure or binding. The further apart the linear slide bearings are placed the more rigid the machine will be.

Spindle- I used an old Foredom #44 handpiece for the body (check eBay) and a ER11 8mm shaft collet chuck. A Foredom #30 handpiece should also work as it has the same 8mm x 7mm x 22mm bearing size. It is also possible to retrofit machine tool high speed angular contact bearings in this size.

Speed control- I used an old Ace R/C 30A speed control I had sitting in a box for 25 years (I knew my hoarding would pay off!) but just choose a speed control suitable for your motor that can take a PWM control input- almost all R/C speed controls are set up like this.

Hobbyking is a great source for inexpensive electronic speed controls (also called an ESC.) Note that the majority of modern R/C speed controls are designed to be used with brushless motors but you can still get inexpensive brushed controllers if you happen to find a suitable DC brushed motor.

Begin with the 80/20 extruded Aluminum base. This is the backbone of the machine so it's important to get it setup nice and square- use a machinist's square to get the alignment correct. An 11" long section forms the back upright and a 10.5" long section is used for the lower base. These are bolted together using braces made from 3/4" thick Aluminum measuring 5" tall by 3" wide at the base (the exact shape of these is unimportant.)

Mounting the plates like this allows them to slide up/down and forward/back for positioning adjustment. This makes adjusting the mounting height of the Z axis a breeze so you can raise it later on if you need more room under the spindle to hold a tall part in place.

The reason for using 3/4" thick Aluminum plate is twofold: First, It gives a nice solid surface to mount the linear slides and second, it drastically increases the mass of the machine making it much more rigid and less prone to vibration.

The spindle is made from an old Foredom #44 handpiece that had been disassembled so only the body and bearings remained. The ER11 collet chuck simply slides right in place and a MXL timing belt pulley is fastened to the collet chuck shaft using a set screw (a spacer is needed so the pulley clears the top of the spindle housing.) It's not the fanciest spindle out there but it works pretty well for small sized end mills and it's very inexpensive and easy to make. There is much debate about using deep groove radial ball bearings in a spindle but for lower RPM applications (under 20,000 rpm) on small mills it seems to work just fine as both Taig and Sherline milling machines use them. For high speed/high loading spindles that are running all day long angular contact bearings are the way to go and proper balancing of the assembly becomes critical. For more info about home made high speed spindles have a look here, here and here.

The spindle mount was made from an old bicycle 1" round tubing clamp block. The tube clamp block is bolted to an Aluminum plate that attaches to the Z axis slide. To the front of this block is mounted a 3/4" thick by 1 1/2" wide Aluminum bar that serves as the motor mount support. The upright plate that supports the motor mount is made from 1/4" thick Aluminum plate and has slotted holes at the top for adjusting belt tension. The slots were made drilling holes and then filing with a small round file.

The motor came from an old 14V Makita drill I found and the mounting plate for it was made from 1/2" thick Aluminum. I had to turn a recess in the mounting plate for the motor using my lathe so the motor shaft has enough clearance to mount a MXL timing belt pulley on the other side. This is by no means the only way to mount a motor- there are probably fifty different mounting methods so choose whatever works with the motor of your choice.

I only had one suitable handwheel in my junk box so I made the other two by turning them from round Aluminum bar stock using my lathe. I made the knobs on the handwheels from some 1/2" diameter bronze rod I had laying around. There is a recessed hole in the knob and they are attached to the handwheels using 4mm screws.

The handwheels are attached to the linear slide lead screws using 4mm set screws ( I used socket head cap screws instead of traditional set screws as they're easier to remove later on if the handwheels need to be replaced.)

There is a voltage regulator connected to the 12V power supply to provide the necessary 5V to the Arduino. While the Arduino Pro Mini I used can handle the 12V output from the power supply I wanted to use a separate 5V regulator in case I ever swapped out the power supply for something with a higher output if I ever go to a more powerful brushless motor.

The Arduino just acts like a servo tester in this circuit- it provides the necessary PWM signal to the motor controller that tells it to change speed based on the position of the potentiometer. Instead of the wires going to the servo they go to the R/C input of speed controller. The wiring and code for this is extremely simple. Of course you can also use an inexpensive pre made servo tester to output the signal to the speed control.

void loop() { val = analogRead(potpin); // reads the value of the potentiometer (value between 0 and 1023) val = map(val, 0, 1023, 0, 180); // scale it to use it with the servo (value between 0 and 180)

Since the top of the X axis linear slide is bare I needed to make a tooling plate. A tooling plate is what allows you to bolt things like spoil boards and vises/hold downs to the X axis so you can hold parts to machine.

I made the plate from 3/8 thick Aluminum and drilled holes on a 1" square pattern and tapped them with a 10-32 tap. Since there were a lot of holes to tap (and the material wasn't too thick) I mounted the tap in my drill and used that to drive the tap (with lot of cutting fluid.) Worked great and a total time saver!

As well as just using the tooling plate I wanted to be able to mount a T-slot plate for greater versatility. The profile I used is 80/20 1030 (1" x 3") that has slots on 1" centers. This is done by drilling holes through the T-slots at the ends just large enough for a allen head wrench to pass through to tighten the 10-32 socket head screws that secure it to the tooling plate. Super simple and it only takes a couple of minutes to bolt it in place. Now I can mount things like sliding low profile vices and the small rotary table I'm building.

Yes - absolutely sacrilegious but there's a good reason for this. My Taig lathe uses this 1" bolt spacing and 10-32 thread mounts for all of its accessories and I wanted to be able to swap parts easily between my lathe and my mill. So totally justified. :)

And there you have it! I powered it up and did some test cuts and it works wonderfully. I have been wanting a small mill like this for a very long time as I have so many projects that can make use of it. :)

Yes you can buy milling machines relatively inexpensively these days but the beauty of building something like this is you can get exactly what you want, you know what goes into it and you can modify it later on to suit your needs.

If I didn't already have a motor and speed control I would have definitely gone with an inexpensive brushless setup as they are very quiet and quite powerful (this drill motor is not quiet.) Ideally I would make the entire spindle from scratch using matched precision angular contact bearings with proper preload adjustment, etc. for higher spindle speeds but for right now this setup works quite well.

Hello, the project looks interesting.I think I will build up something similar.One recommendation for you.When you make connections with Al-profiles,(see pic)try to use more contact area (bolt head/notch)!The marked connections are unnessisarily instable.Better use flat bolt head with washer in the notch.You win much more stability for no extra price!regards

Hello, I am looking to mill small silver metal objects and I haven't seen anyone doing this. Does anyone know what power spindle and what sort of bits I should/can use to do this?Is silver considered a hard metal?Thank you

Hi there!Silver is pretty soft- much softer than Aluminum. It's probably similar hardness to Delrin. It will want to gum up a bit so I'd take small cuts using a two flute end mill. It won't take much power if you're using small end mills and taking light cuts so maybe a 100W to 200W spindle would work. If you're using end mills larger than 2mm you will probably want more power. A lot of it just depends on what it is exactly you are trying to machine.

Great Project Honus!! Very practically done. I've been thinking about doing something with a similar but bit larger build. I think you might find some of my work interesting, including some idea's on how to integrate 3D printed parts, with pretensioned wire and polymer reinforced hard plaster. If you are looking for an interesting next project to work on, we should talk. https://www.instructables.com/member/Drewrt/instructables/

On the note of filling beams with stuff, i've had really great success just using a mix of sand and epoxy. Ratio was about 10% epoxy 90% sand by weight. Should be a bit on the wet side of things. Helps a ton on my little mill!

Very nice project. If you took the next step and added steppers, you could at least double the value of your efforts sofar. An arduino + drivers + powersupply + steppers would cost perhaps 200-250 US$. Parts to be milled can be drawn in Fusion 360 (free for hobbyists), which also can generate g-codes to the steppers.

micromilling machines | springerlink

micromilling machines | springerlink

In this chapter micromilling machines are presented, starting with a clear differentiation between ultraprecision milling machines and micromachines. The main characteristics of the micromilling process are also summarised, including size effects affecting milling at the micro scale and more usual applications. Furthermore, a more detailed description of the different subsystems and components specific for micromilling machines is shown: guideways, structural materials, drives, measuring systems, spindles and additional equipment. Finally some commercial micromilling machines from well-known machine tool builders are presented.

micromilling process - an overview | sciencedirect topics

micromilling process - an overview | sciencedirect topics

Micromilling processes bear different material removal mechanisms compared to the conventional milling, as the milling tool edge radii are at a close level to the uncut chip thickness, which brings the well-known size effects.

The analysis of the high-speed mechanical micromilling process is described in this chapter. The present work not only compares computational approaches to the solution of shear plane and tool face temperatures, but also explains why there is a difference when calculating temperatures generated during the micromachining process. The analysis shows that the computed temperature of the shear plane never exceeds 35C when micromilling at spindle speeds approximately 310,000rpm. Machining AISI 1040 steel at significantly high speeds presents significant challenges to prevent the accelerated wear of the cutting tool that is caused by the frictional interactions between chip and tool and the nature of the intermittent contact. The analysis also shows the effect of coating at reducing the interface temperatures between chip and tool, and concludes that each coating has very little effect at reducing temperature at the tool face and at the primary shear zone. The results shown here are reported in Ref. [11]. Reprinted with kind permission from Springer Science + Business Media B.V.

Figs. 3.1A and B show the schematic of the micromilling process and the tips orbital motion based the micromachining process, respectively. The pyramidal tip is used as the example. The symbol V and the arrow represent the moving direction of the workpiece during machining. The micromilling process in Fig. 3.1A, shows the microcutter rotating around its center, usually driven by a high speed spindle. The section of the microchannel machined by this process is a rectangle. The width of the channel is determined by the diameter of the microcutter used. As shown in Fig. 3.1B, the pyramidal diamond tip revolves around an orbital center. This motion is realized by a 3D nanopositioning stage in this study, which will be explained in the next section. The section of the microchannel machined by this method is trapezoidal and influenced by the diamond tip shape. The width of the channel is basically determined by the radius of the orbital motion. The radius is changed according to the moving range of the nanopositioning stage. But for the micromilling process, it is impossible to machine the channel with a width less than the diameter of the microcutter. The relative motion between the tool and the workpiece in three dimensions can be realized in the same way with the conventional micromilling process. The coarse moving stages can be used. The main difference between the micromilling process and the orbital motion based micromachining process is that no high speed and high accuracy air bearing spindle is employed in the latter condition.

This case comes from HSG-IMAT and combines material-removal by mechanical processes (micromilling), with ECF. In principle the roughing and bigger shapes (up to 100m) of the tool insert are machined by micromilling, usually in steel; and then the ECF process is used where higher resolution and sharper edges are required. Its main advantages are the small tools available (up to 5-10m), no tool wear and no generation of burrs. However the inclusions in the steel should be small.

Within 4M activities a test part was done consisting in five ribs each 100m depth and 1mm long, and width from 20 to 100m (Figure2). The outer contour was micromilled and the grooves between ribs were ECF-processed into stainless steel 1.4301.

Posalux combi hybrid machine [7] integrates the LASER Ultrashort pulses FEMTO technology with micromilling process for machining automotive GDI nozzles. A femtosecond laser source with a short pulse down to 200fs can avoid heat-affected zones, recasts, deposits, and is able to machine a wide range of materials with high dimensional accuracy and good surface finish. However, this machine is designed only for machining small products.

Hamuel GmbH developed the first hybrid machine, HSTM1500 [8], as shown in Fig. 8.3, which accommodates high speed milling, 3D scanning, 3D laser cladding, 3D inspection, deburring/polishing, and laser marking processes. 3D laser cladding is essentially a welding-based 3D additive manufacturing technique which melts metals with a laser and deposits it onto a part. It is applicable to all conventional welding metals, together with the focused heat input and low dilution allowing cladding of difficult to weld materials. The machine provides a number of combinations of sequential machining processes to serve different purposes. For example, a combination of 3D inspection, 3D scanning, laser cladding, and high speed milling can be used to remanufacture a used part. A combination of high speed milling, 3D inspection, and laser deburring/polishing can be employed to machine complex components. The machine, therefore, has the ability to extend end-of-life of high value, complex components made from specialized materials in a cost-effective and environmentally friendly manner. The traveling distance of the linear axes (the X, Y, and Z axes) are 1930, 400, and 570mm, respectively.

The present state of microturning process is very similar to conventional turning process on a lathe that has been extended to provide better precision and accuracy in machining process. Similar to micromilling process, microturning has the capability to produce 3D structures on microscale (19). There are two major types of microturning cylindrical shaft turning for machining of micropins and face turning for machining of microgrooves, that has been reported for micromachining applications (19,46). Microturning for machining of micropins is possible but it is more difficult to realize due to the deformation of the fine workpiece which is very similar to the deflection of microend-milling cutter as could be seen in Figure 7(a). However, the situation is even more critical for microturning as often the microturning workpiece is much weaker than the tool in micromilling (5) and thus the major drawback of microturning process is that the machining force influences machining accuracy and the limit of machinable size. Significant work has been done to develop different cutting paths and schemes to reduce the effect of cutting force on the fine shaft. A micropin of around 350 m diameter (Figure 7(b)) with intricate shape and kinks has been fabricated (7,19). But, it is very difficult to achieve a straight shaft below 100 m diameter and in many cases, the workpiece is either broken, or starts to wobble due to excessive radial cutting force on the microshaft. Figure 8 shows one such microshaft machined using the conventional microturning process. The shaft was deformed plastically with very rough surface finish from plastic side flow caused by the strain gradient-induced strengthening due to the constant radial force during turning at a slower feed rate (47). When a faster feed rate is applied, the shaft breaks easily as the radial force increases to an excessive level.

In face turning, research has been conducted for many years on diamond turning and this has found wide applications in machining of various components such as microlenses, lens arrays, and parts for measurement references, for example, surface encoder for multiaxis position accuracy measurement (46,48,49). Diamond turning has been generally coupled with the term ultraprecision machining as single point diamond turning is probably one of the few processes achieving mirror surface finish. Finish of less than 10 nm and form error of less than 1 m can be obtained when machined using an ultraprecision machine tool capable of moving in high accuracy at nanometeric precision (14). It is also possible to fabricate microparts using conventional ultraprecision turning. After cutting microsteps on the surface of a plate, microparts can be cut out by other methods such as wire electrical discharge machining (WEDM) as shown in Figure 8 (5,48). Another important area of microturning is to support microgrooving and microthreading needs required for the fabrication of microfluidic sensors, microinductors, and microactuators. However, the major difficulty is in availability of such tools. Literature suggests that FIB can be a potential technique for machining of such tools (43,50).

The part feature accuracy rate and MRR in micromilling of difficult-to-machine materials are limited by the machine-tool system stiffness (especially for small-footprint machines), and the low flexural stiffness and strength of the microtools normally used. Rapid tool wear is another issue during the machining of hard and difficult-to-cut materials, since it negatively impacts part feature accuracy and finish (44). Those shortcomings can be overcome by introducing laser assistance during the machining process. The laser heating will induce localized thermal softening of the materials to be machined, which helps to reduce the cutting forces and tool wear. Tool deflection due to the extreme hardness of the materials can be reduced, which will enhance the dimensional accuracy. The hybrid laser-assisted micromilling process is able to machine freeform 3D microscale features in hard materials (45). Figure 35 shows the setup for the laser-assisted micromilling process (45).

Figure 35. The hybrid laser-assisted micromilling setup: 1 the rotary stage for orienting the laser; 2 the stacked linear stages X, Y, and Z; 3 spindle assembly; 4 a fiber-optic cable; and 5 collimator and micrometer assembly.

In this hybrid process, the laser nozzle is set at an angle, so that the laser radiation hits the surface to be machined just before it is machined by the micromilling process. The objective is the thermal softening of the materials by laser irradiation before machining with the milling tool. A relatively low-power, Ytterbium-doped, continuous-wave, near-infrared (1.06 m) fiber laser is used to achieve highly localized thermal softening of the material immediately in front of the cutting tool during the micromilling process. No assist gas is used. The laser spot size can be adjusted depending on the dimensions of the machined feature. The laser, spindle, and four axes are controlled simultaneously via a common computer interface. The dry micromilling is performed by TiAlN-coated WC four-flute ball end mills of 250 m diameter. The laser-assisted micromilling process can increase the MRR by increasing the depth of groove or depth of cut during the micromilling process. In addition, it can provide a comparatively better surface finish with less discontinuity and burr around the edge of the slots, as can be seen from Figure 36 (45). This is due to the fact that the workpiece materials become softer after laser irradiation before the final machining by the milling cutter. Moreover, the MRR and depth of groove also increase in the hybrid process due to the softening action of laser processing (Figure 37).

Figure 36. Comparison of the surface quality of the microgrooves produced by the (a) micromilling process and (b) laser-assisted micromilling process. The discontinuity along the grooves can be noticed when using the micromilling process alone.

Burrs are an undesired and unavoidable by-product of most conventional manufacturing processes, such as cutting, forming, blanking, and shearing operations, because of the plastic flow of material. A burr is defined as a projection of undesired material beyond the desired machined features (125). A novel application of micro-EDM for removing the smaller burrs generated during the micromilling process has been reported (125). Micro-EDM using low discharge energy and a small-diameter cylindrical tool is introduced for deburring microfeatures. The proposed method selectively removes only the burrs near the tool with minimum collateral damage of the machined features because the machining range can be controlled by the spark gap. As micro-EDM uses little electrical power and a small-diameter cylindrical tool, the tool can easily access microscale features and remove small amounts of material from the burrs as shown in Figure 45. Burrs with a height from 200 to 1000m and width from 80 to 200m were successfully removed by the three consecutive steps of rough deburring, finish deburring, and edge finishing, with an error in height of 1m or less and a deburred edge width from 50 to 60m.

Figure 45. (a) Concept of micro-EDM deburring: deburring on the top plane and side plane of the edge, (b) burrs generated after a microslot machining process by micromilling, (c) finishing of edges (one of two edges) by micro-EDM deburring.

Microfluidic devices have received much attention in recent years (Au et al., 2011) and offer important practical advantages for laboratory analyses. Enhanced portability, improved sensitivity, high throughput, and significantly reduced power consumption are some of the many advantages of these microfluidic devices when controlling fluids down to a picoliter volumetric level. This intricate microfluidic control has supported general biotechnological processing in pharmaceutical (Welch et al., 2006) and clinical (Li et al., 2010) settings. The demand for such microfluidic systems has been even greater, however, in biomedical research settings, with numerous technologies now benefiting from the superior analytical capabilities of microfluidics including, for example, enzymatic analyses (Miyazaki et al., 2008), DNA analyses (Humphreys et al., 2009), proteomics (Lee et al., 2009), and even bio-analytical mass spectrometry (Figeys et al., 1998). At the core of all of these applications lie two fundamental microfluidic operations, microdrop motion and sensing, and the further development of microfluidic systems must carefully consider the scalability of these important microdrop motion and sensing processes.

The contemporary approach for microfluidic microdrop motion makes use of continuous-flow architectures with in-line, one-dimensional (1-D) flow channels. The simplicity of these 1-D fluid flow systems supports standard device fabrication through wet-etching and micromilling processes (Becker and Locascio, 2002). The capabilities of these 1-D microfluidic devices have been tremendous, and perhaps the greatest results have been witnessed for fluid control applications demanding filtering (Zhu et al., 2004) and/or mixing (Wiggins and Ottino, 2004). A fundamental challenge does become apparent, however, from the practical use of 1-D flow channels. Fluid flow through the constrained 1-D flow channels is prone to clogs and blockages, and such failures can be catastrophic to operation. Permanent 1-D flow architectures offer few opportunities to reroute or adapt by way of reconfigurability or fault control.

If one wishes to introduce increased fluid control, a natural extension would involve the use of a two-dimensional (2-D) microfluidic architecture. Microdrop motion in a 2-D plane offers greater possibilities for adaptations via a real-time fault control, as well as increased opportunities for parallel on-chip microdrop mixing/splitting algorithms in bio/chemical reactors. It is with this 2-D vision in mind that the field of digital microfluidics has emerged (Fair, 2007). A digital microfluidic architecture employs a generalized microdrop motion platform with the potential for microdrop motion in a 2-D plane. (This digital microfluidics technology should not be confused with continuous-flow microfluidic systems with submerged microdrops being carried in a 1-D flow channel (Huebner et al., 2008; Song et al., 2006).) In digital microfluidics, microdrop motion is not pressure-based, thus it is not necessary to restrict motion to 1-D flow channels. Instead, microdrop motion is achieved by localized voltage signals throughout the 2-D plane. User-controlled voltage distributions are applied to carry out fluid processing tasks with adaptations provided by way of voltage signals. Real-time adaptations can therefore come about for path planning and fault control (in stark contrast to the 1-D constraints of continuous-flow microfluidic systems). Such reconfigurable operation can be carried out with especially low fluid volumes and high sensitivities. Moreover, the 2-D format of this generalized digital architecture can be adapted via voltage-control software for highly-parallel operation, with reaction-based microdrop mixing, splitting, routing, etc. being carried out simultaneously at multiple on-chip locations. Such digital microfluidic implementations are now being applied to immunoassays (Alphonsus et al., 2010), DNA ligations (which are very sensitive to reagent economy) (Liu et al., 2008), and DNA pyrosequencing (Fair et al., 2007). The low fluid volumes, high sensitivities, and parallel operation of 2-D digital microfluidic systems can together meet growing demands for high throughput on-chip analyses. At the same time, the need to work with larger fluid volumes can be met with digital microfluidic microdrop generation/extraction from adjacent on-chip reservoirs. Details for such splitting algorithms can be found in Cho et al. (2003), Elvira et al. (2012), and Ren et al. (2004).

A primary issue in any 2-D digital microfluidics design is microdrop motion scalability. Microdrop motion is dynamic and demands appropriate time synchronization and voltage localization from many system inputs to induce interfacial surface tension changes and microdrop motion (Dolatabadi et al., 2006). This is especially challenging in devices that are being scaled for use with increasing numbers of system inputs and finer and finer spatial resolutions. The contemporary approach for digital microfluidic microdrop motion has used a voltage-activated 2-D square electrode grid with M rows and N columns, but such a structure becomes grossly impractical for highly-parallel operation (requiring a tremendous number, MN, of independent input electrical address lines to control all MN square electrode grid locations). For this reason, such structures have been restricted to grid sizes on the order of 55 grid (Davids et al., 2006). With these electrical addressability issues in mind, our work on digital microfluidic multiplexing has eased these electrical addressability and control constraints (Collier et al., 2011) and is introduced in this work as a bi-layered electrode structure with upper row and lower column electrodes. Differential voltages, with values biased about the microdrop motion threshold voltage, are shown to establish microdrop motion at all MN gridpoints with only M+N electrical inputs. This technique overcomes the ubiquitous microdrop interference effect (Xu and Chakrabarty, 2008) and operates at an input voltage, 0.64Vrms (root-mean-squared volts) (Nichols et al., 2012), that is well within the 5V maximum for future CMOS/TTL lab-on-a-chip systems (Li et al., 2008).

A second issue in 2-D digital microfluidics relates to fluid sensing scalability. Fluid sampling sensitivity is diminished when devices trend toward smaller scales with lower reagent volumes. Signal levels associated with standard optical imaging (Madou and Cubicciotti, 2003) and capacitance sensing (Ahmadi et al., 2010) scale down proportionally with the sampling area, and the associated on-chip analyses can have unacceptably low signal levels. With this in mind, it is desired to improve the sensing abilities of the localized sampling elements being integrated on the smaller and smaller dimensions of emerging digital microfluidics devices. A new concept for onchip sensing is demonstrated in this work by way of a folded-cavity optical refractometry. An overhead microlens is integrated into the aforementioned digital microfluidic multiplexer to form a folded cavity with an especially sensitive relationship to internal fluid refractive indices. It is shown that the microoptical architecture can be tailored for on-chip optical sensing over wide refractive index ranges with user-controlled sensitivities. The on-chip sensing technique is ultimately shown to facilitate effective fluid sensing for integration with future generations of digital microfluidic devices.

micromilling: a top option for making microfluidic devices | cytofluidix

micromilling: a top option for making microfluidic devices | cytofluidix

When making the tiny channels, V-grooves, holes and other features that control fluids in microfluidic devices, there are a number of options to choose from, including chipmaking techniques and that headline-grabbing phenomenon known as 3-D printing.

Why? Milling microfluidic devices can be relatively fast, easy and inexpensive. Whats more, the results often exceed the capabilities of competitive manufacturing techniques. On the other hand, milling cant meet the requirements of every application, so makers of microfluidic devices must make sure in advance that milling can cut it in the role it will be asked to play.

Milling can be used to either directly machine microfluidic devices or to machine the molds used to make these devices. One way to decide which of these options is best for a particular situation is to consider manufacturing volume. Direct machining is probably more economical for small numbers of microfluidic devices, while milling a mold would make sense for higher-volume device production, according to Bob Brown, project engineer at microfluidics molder PEP microPEP, East Providence, R.I.

If the material is not suitable for machining, like some type of soft polymer, you would need to machine a mold, said Onik Bhattacharyya, vice president of sales and marketing for Chicago-based Microlution Inc., a builder of CNC micromachining centers. Microlutions machines can mill molds made from brass, aluminum and stainless steel.

A number of milling advantagesincluding easy access to the technology, fast turnaround, low cost and the ability to cut many materialsmake it a great method for rapid prototyping of microfluidic devices, according to Karel Domansky, a staff engineer at Harvard Universitys WyssInstitute for Biologically Inspired Engineering, which mills both microfluidic devices and the molds used to make them.

In addition, fabrication of multi-height features is much easier with milling than with photo-lithography, which usually produces features of the same height, noted Domansky, who has 15 years experience making microfluidic devices at Harvard and the Massachusetts Institute of Technology.

Another disadvantage of photo-lithography is that its more what they call a 2-D process, Bhattacharyya said. So, if you want to make a nice rolling hill, you cant do that with lithography. Milling, on the other hand, is a good option when you need to create a real 3-D geometry.

As for laser cutting, it may be faster and easier than milling, but it wont yield consistent results, noted Jack Heald, president of Minitech Machinery Corp., a Norcross, Ga.-based builder of desktop CNC machines used to mill microfluidic devices. For example, Heald said, using a laser to cut a 100m-thick micro-fluidic channel may result in a good bit of deviation along the channel because of inconsistencies in the burning process.

Then theres the option of using a 3-D printer to make microfluidic devices. Though it may be all the rage these days, 3-D printing is slower than milling and wont produce as smooth a surface due to the nature of the material-deposition process, according to Heald. In addition, he said, a 3-D printer may not give you the option of using metals, so you end up with a plastic or composite part that may or may not be as good as metal for a microfluidic application.

According to Heald, work materials commonly milled in microfluidic applications include aluminum, brass and PMMA (polymethylmethacrylate), an acrylic. All are easy to machine, he said, and PMMA isnt affected by fluids flowing in microfluidic channels.

Because different materials have different propertiesand interact with fluids in different waysmaterial choice for a microfluidic device depends in part on the type of fluid that will be flowing through it, Bhattacharyya said. Microlution machines have milled the devices from harder plastics, such as PEEK and polycarbonate, as well as from metals, such as steel, titanium, brass and aluminum.

Acrylic and polycarbonate are also common choices for microfluidic devices, according to Wyss Institutes Domansky. One reason is because they respond well to thermal bonding processes that are used to produce closed microfluidic channels. But care must be taken to avoid excessive stress-cracking when milling.

Both work materials and feature geometries impact accuracy and tolerances when milling microfluidic devices. For example, Domansky said, milling a more challenging material, such as polypropylene, will probably result in looser tolerances, while milling a more brittle and easier-to-machine material, such as acrylic, will allow tighter tolerances. In addition, he explained that tight tolerances will probably be more difficult to achieve when machining deep, narrow channels due to the tendency of the micromill to bend.

Another key factor is the construction of the milling machine. Heald noted that Minitechs latest machine has a solid-black-granite base and a Z-axis column that increases stability and significantly reduces vibration. This, he said, translates into higher milling accuracy and tighter tolerances, as well as smoother finishes.

As for feature sizes that can be achieved when milling microfluidic devices, that depends on the cutting tools. According to Heald, some of todays smallest tools are 10m-dia., side-cutting endmills. He also believes 5m-dia. tools may now be in use. But diminutive tools like these are difficult to work with and break very easily, he said.

A limitation of machining microfluidic devices is that endmills and drills may not be good options for creating small features with high aspect ratios. With a small tool like a 25m-dia. endmill, the highest aspect ratio that can be achieved is probably about 3:1, according to Microlutions Bhattacharyya. As endmill diameters increase, he added, aspect ratios can go higher because the tools become more rigid. So, a shop using a 100m-dia. endmill, for example, could potentially cut a microfluidic feature having a depth of 900m or more, he said.

At the Wyss Institute, researchers perform in-house micromilling to directly manufacture an extracorporeal blood cleansing device for sepsis therapy. The polysulfone or aluminum devices contain networks of surface channels milled to opposing surfaces of a thin sheet that are connected by 340m-wide and 340m-deep slits. (Image Credit:Wyss Institute)

In addition, he noted, the surface roughness of milled microfluidic devices is usually much greater than that produced by photolithography. This can be acceptable, however, when producing larger microfluidic features. For this reason, as well as the possible breakage and deflection of milling tools with dimensions in the tens of microns, Domansky thinks milling is best suited for making microfluidic structures with feature sizes measuring hundreds of microns or larger.

However, the smoothest possible surface isnt always desirable in microfluidic applications, according to Oliver Rapp, engineering and business development manager for PEP microPEP. Rapp has seen cases where less-than-smooth surfaces were actually advantageous for customers microfluidic devices.

Even when a smooth surface is important, the story doesnt necessarily end with the milling process. After milling, microfluidic device manufacturers that want finer finishes for their devices and molds can turn to secondary operations. When the microfluidic features are big enough, for example, mold polishing can be employed, Rapp noted. But if you have a channel thats so small that you cant get a stick or ultrasonic probe in there for polishing, all bets are off, he said. In a case like that, you might have to look at something like very fine EDMing.

A secondary operation, however, may not even be needed to improve upon a milled surface, Rapp maintained. Weve seen some pretty good surface finishes, especially from high-speed cutting tools, he said.

Small-diameter cutting tools require higher spindle speeds to achieve a good finish, noted Sebastian Garst, manufacturing manager for Melbourne, Australia-based MiniFAB, which designs and manufactures microfluidic devices and molds. According to Garst, milling can produce metal surfaces with a roughness of less than 100nm. But high-quality surface finishes are expensive to produce, he added, requiring cutters made of materials such as PCD (polycrystalline diamond), which is used to mill nonferrous materials.

In addition, Garst pointed out that fine surface finishes require longer machining processes, which conflict with the goal of rapid turnaround. So, in order to get a microfluidic device with a topnotch surface, theres often a tradeoff, he said.

William Leventon is a New Jersey-based freelance writer. He has a M.S. in Engineering from the University of Pennsylvania and a B.S. in Engineering from Temple University. Telephone: (609) 926-6447. E-mail: [email protected] Telephone:(609) 926-6447. E-mail: [email protected]

Microfluidics Congress: USA, 24th-26th Jul 2017, Philadelphia. http://www.global-engage.com/event NIST Workshop on Standards for Microfluidics 2017, Jun 1 and 2nd, Gaithersburg, MD

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