It is not always possible to correct a vibration problem as soon as that problem begins to appear. Rather than stopping production, here is a means of continuing to realize smooth, efficient grinding until the underlying cause of vibration can be addressed.
Some of the ground surfaces on this workpiece show waviness resulting from process vibration. The wavelength of this wavinessthe distance from peak to peakmight offer a clue to the cause of the vibration, and can be used to calculate parameters that might result in a smooth surface despite the vibration.
Fig. 2. How to calculate the chatter wavelength given frequency. In testing, researchers used a sensor mounted near the grinding machines spindle to measure frequency precisely.
In a production-grinding operation, waviness on the part surface is a potential clue that the machine or process has developed a vibration problem. The effect might be seen in inspection, or if there is a lapping or polishing step, more time might be spent during that step removing the waves. According to grinding wheel manufacturer Norton Saint-Gobain Abrasives, this is the point at which shops almost always attempt to solve the vibration problem by making some simple change to the process, which is a pretty good approach.
Indeed, those waves on the surface, often called chatter, could indicate the appropriate fix. (Others use chatter to refer to regenerative waviness. The use here is not that specific.) On a part machined on a surface grinder, for example, vibration frequency (cycles per minute) is equal to the work speed (inches per minute) divided by the distance between two consecutive chatter marks (inch). Find the vibration frequency using this relationship, and if it matches the rotation speed of the grinding spindle, then this indicates that the grinding wheel, wheel flanges or the grinding spindle itself is a likely culprit. Change the wheel, tighten the flange bolts, or perhaps just change speed, and that much might be enough to cure or control the vibration problem.
But in other casessome involving other parts of the machine, some involving the natural frequency of the systema simple fix is not enough to sufficiently address the problem. In these cases, the very best response is to have the machine serviced, repairing whatever failing machine element is allowing vibration to affect the workpiece. Service, however, takes time, and it means taking the machine out of production. For shops that need to keep going, if only for the short term, researchers have proven out a process for overcoming vibrations effects without compromising productivity and without stopping the machine for service.
What follows is derived from a paper about a technique called contact-length filtering written by Saint-Gobain corporate applications engineers John Hagan and Mark Martin. By reducing the work feed rate while increasing the depth of cut, the effects of severe vibration can be eliminated without any net effect on overall productivity.
The aim of contact-length filtering is to get the wheel-to-work contact length very large relative to the wavelength of the surface affected by vibration. When the former is high enough relative to the latter, the wheel effectively removes vibration-related peaks from the workpiece, smoothing out the surface even though vibration is still occurring. When the depth of cut is increased by the same multiple that feed rate is decreased, material removal rate (and therefore productivity) can remain the same.
The depth of cut controls contact length. Obviously, the contact lengths increase needs to avoid unfavorable effects such as material burn, workpiece deflection and so on. Usually this is accomplished by increasing the wheels depth of cut to a level that is heavy compared to standard cutting conditions, but still avoids these ill effects.
Contact-length filtering begins to achieve a smooth surface when double the wheel-to-work contact length surpasses the wavelength of the chatter, or surface waviness. In other words, the condition required for chatter amplitude reduction is.
The technique wont always work, the researchers say. It wont be possible in every process to get the vibration wavelength low enough or the contact length high enough. In these cases, the only remaining solution is the one that needs to be performed anyway, namely, take the time to identify and correct the vibrations underlying cause. What follows is a case in which contact-length filtering was effective for machining a smooth surface in spite of extreme vibration.
This study was designed to show the effect contact-length filtering can have on reducing chatter due to vibration. The grinding test was performed at the Saint-Gobain Higgins Grinding Technology Center near Boston, Massachusetts. The test machine was an Elb creep-feed/surface grinder. The operation was slot grinding using an 8-inch-diameter, -inch-wide conventional abrasive wheel. The material ground was 4340 hardened steel. The wheel was intentionally thrown out of balance by adding weights to one side of the wheel hub. The vibration due to wheel unbalance was measured at 0.00019-inch displacement. The first test involved grinding three slots at Conditions 1, as shown in Figure 3. The feed rate was 120 inches per minute, and depth of cut was 0.001 inch. Six passes were made for each slot to achieve a total depth of 0.006 inch. The material removal rate was 0.12 cubic inch per inch of wheel width. The effects observed on the workpiece at these conditions was significant, as seen in Figure 4a.
The second part of the test involved grinding three slots at Conditions 2 shown in Figure 3. Here, the feed rate was reduced to 20 inches per minute, and the total depth was achieved in a single pass at a depth of cut of 0.006 inch. The wheel imbalance remained the same at 0.00019 inch. The material removal rate also remained the same. The vibration amplitude observed on the workpiece at these conditions was greatly reduced, as can be seen in Figure 4b. The vibration amplitude at the second set of conditions was measured at 8 microinches versus 79 microinches when grinding at first set of conditions.
The technique has its limitations, the researchers stress. The case study represents an ideal scenario. It wont be possible in every process to achieve the condition of twice the contact length being larger than the vibration wavelength, let alone without any loss to material removal rate. But in the right applications, this technique is potentially a powerful option. It is a way to keep going, and to continue realizing an acceptable surface through production grinding, until the right moment comes when the valuable machine can be taken offline for repair.
In vertical grinding,the workpiece is heldupright in a rotary chuck with the grinding spindle overhead. Thisconfigurationcan improve roundness, facilitate single-setup processing and prolong the life of the machine. Loading and unloading may gets easier, too. Workpieces with relatively large diameters and short lengths benefit the most from vertical grinding.
When precision dies grinding, it is very difficult to solve the problem of vibration of the grinding machine. Because the vibration of the machine tool directly affects the effect of the dies processing. For example, the surface finish is poor.
If it is precision molding grinding such as plug-in connector molds or cutting grooves, it cannot be dressed because of the vibration of the machine. Because sometimes the grinding wheel needs to be dressed to a thickness of 0.1-0.13mm, if there are unstable factors of the vibration of the grinding machine, the grinding wheel exploded. So how do we deal with the vibration of the surface grinder?
First, check the flat of the ground, if the level of the anchor screw is not adjusted, it will cause the machine to resonate. Because sometimes the ground level of the processing site is not very good, so adjust the horizontal screw to make the machine reach the level of height. First, we need to check whether every screw of the foot is put into place on the floor mat.
Secondly, if the above actions are carried out, the vibration is still not eliminated. We should check whether the ground floor is empty, if the ground is terrazzo or floorboard, it will be relatively strong. If it is cement ground, the ground will be empty and cause resonance. How to deal with this situation? Dont worry, go to the rubber hardware store to buy several black rubber pads with a thickness of 6-10mm and a size of 10cm. And the size of the floor mat is slightly larger. Loosen the anchor screw and put the rubber pad under the horizontal foot mat, which can reduce the vibration.
Thirdly, if the above actions are taken and the vibration is still not eliminated, we should check the problems of the grinding wheel and flanges fixed by the grinding wheel. First is the grinding wheel. The quality of the grinding wheel on the market is not the same. Two factors affect the vibration of the grinding wheel.
One is that the grain size of the grinding wheel is not uniform, and the center of gravity is unstable when it is rotated. The other is that the inner hole is not standard and too large. For example, the grinding machine flange hole diameter is 31.75mm, but the grinding wheel hole diameter is 32mm, it will cause a vibration when it is fixed on the grinding machine. So we should ask the specifications when purchasing the grinding wheel. It is suggested that we should buy a better grinding wheel.
Thirdly, the poor balance of the grinding wheel will seriously affect the life of the grinding spindle. For the flange, everyone will think of the proofreading balance, in fact, the flange of the small precision hand grinder is not to be balanced. If there is vibration, it may be the above reasons. Try another wheel, or if the flange is not balanced, then load the balance block, then remove the balance block and install it. If not, change one new flange.
Fourthly is the environment. It is impossible to place the machine tool with large vibration such as a punching machine or lathe near the same working site of the precision surface grinder. It is easy to cause resonance.
How to judge? It is very simple. You should need to stop the grinding machine and start the punching machine or lathe. And touch the grinding machine, you feel the vibration, that is affected. The treatment method is to separate the workshop.
Fifthly, if the above series of actions still fail to eliminate the vibration, it may not be the scope that you can handle. It may be that the spindle of the grinding machine has a long time to vibrate itself, the inner bearing or the motor is broken, or it may be the problem of the structure of the machine tool and the assembly, should notify the manufacturer to deal with it.
With an overhang of more than 4 times the tool diameter, vibration tendencies can become more apparent and damped tools come into the picture as a good solution.With a damped pre-tuned boring bar machining of holes with a depth of up to 14 times the diameter of the bar can be performed with good results.
Precision internal machining operations are now carried out almost exclusively using hard-metal or diamond tipped cutting tools. Tool holders are available in a variety of forms to suit the specific machining requirements. The material properties of the toolholder have a large influence on both the surface quality and dimensional accuracy of the machined component (workpiece), and on the life of the cutting tool. This becomes critical when machining deep holes, because it is necessary to use a tool with large length to diameter (L/D) ratio or overhang. A high degree of overhang, together with the material properties of the workpiece and various other factors, can lead to excessive vibrations in the tool shaft, which in turn causes undesirable chattering. By the use of passive damping elements, integrated in the tool shaft, the dynamic behavior of the tool can be optimized.
The development within production engineering is accompanied by increasing quality requirements of the produced workpieces. In addition to the product-related high-quality features such as the shape, dimensional tolerances and surface qualities, the effectiveness and controllability of the manufacturing process are relevant factors. As a result of intense development work of the cutting edge, the capability of the cutting tools has been increased considerably.
Seen as a whole, the machine, cutting tool, and workpiece form a structural system having complicated dynamic characteristics. Under certain conditions vibrations of the structural system may occur, and as with all types of machinery, these vibrations may be divided into three basic types:
Free or transient vibrations: resulting from impulses transferred to the structure through its foundation, from rapid reversals of reciprocating masses, such as machining tables, or from the initial engagement of cutting tools. The structure is deflected and oscillates in its natural modes of vibration until the damping present in the structure causes the motion to die away.
Forced vibrations: resulting from periodic forces within the system, such as unbalanced rotating masses or the intermittent engagement of multitooth cutters (milling), or transmitted through the foundations from nearby machinery. The machine tool will oscillate at the forcing frequency, and if this frequency corresponds to one of the natural frequencies of the structure, the machine will resonate in the corresponding natural mode of vibration.
Self-excited vibrations: usually resulting from a dynamic instability of the cutting process. This phenomenon is commonly referred to as machine tool chatter (chatter vibrations) and, typically, if large tool-work engagements are attempted, oscillations suddenly buildup in the structure, effectively limiting metal removal rates.The structure again oscillates in one of its natural modes of vibration.
It is important to limit vibrations of the machine tool structure as their presence results in poor surface finish, cutting edge damage, and irritating noise.The causes and control of free and forced vibrations are generally well understood and the sources of vibration can be removed or avoided during operation of the machine. Chatter vibrations are less easily controlled and metal removal rates are frequently limited because the operator must stop the machine to improve the machining conditions, which often means reducing the depth of cut or feed rate.This article deals with chatter vibrations and how these disturbances can be minimized by the use of damped tools.
The basic cause of chatter is the dynamic interaction of the cutting process and the machine tool structure. During cutting, a force is generated between the tool and workpiece, which acts at an angle to the surface.The magnitude of this cutting force depends largely on the tool-work engagement and depth of cut. The cutting force strains the structure elastically and can cause a relative displacement of the tool and workpiece, which alters the tool-work engagement (undeformed chip thickness). A disturbance in the cutting process (e.g., because of a hard spot in the work material) will cause a deflection of the structure, which may alter the undeformed chip thickness, in turn altering the cutting force. There is a possibility for the initial vibration to be self-sustaining (unstable) and build up, with the machine oscillating in one of its natural modes of vibration.
To make the increased capability of modern cutting edges, the cutting-tool materials available, powerful and stable machine tools, holding tools and cutting tools are required. The shape accuracy of the produced parts is determined by the kinematic machine tool behavior and the static, dynamic and thermal stiffness of the machine tool system. The surface quality that can be achieved depends on the geometry of the cutting edge, the machining parameters and the dynamic behavior of the system: machine tool - cutting tool - workpiece. The metal removing capacity that can be achieved without chatter vibrations is clearly defined by the dynamic machine tool behavior.
For machining complex shapes of dies and moulds, usually tools with a long overhang are used. Equally the machining of the integral components of aircrafts and cars requires the use of tools with a large length to diameter (L/D) ratio. Also for the machining of boreholes and for the inside machining of cylindrical workpieces long boring bars are required.
With increased overhang the tool can become the deciding weak link in the system of machine tool - cutting tool - workpiece. Furthermore, the low static stiffness and material damping characteristics of metallic materials also causes a high dynamic compliance. This can lead to instability of the chip removal process and chatter vibrations.
... are usually derived from chatter vibrations in machining, with cutting tool damage and unsatisfactory workpiece quality. In order to achieve sufficient process stability, the metal removing rate should be reduced or the cutting tool geometry changed. Material substitution as well as geometric shape optimization leads to increased dynamic stability of boring bars. By the use of passive damping elements, integrated in the boring bars, the dynamic behavior of the tools can be optimized.
Generally, machining up to four times the diameter of boring bars does not cause any problems from the vibration point of view, provided that correct conditions apply as regards cutting data and inserts.
There are three different types of damped bars, depending on the length of overhang. Standard bars with a short damping system for machining up to 7 times the bar diameter, standard long bars for machining up to 10 times the bar diameter and cemented carbide reinforced bars (CR bars) for machining up to 14 times the bar diameter.
... made of sintered tungsten carbide or machinable sintered tungsten alloys are an excessively costly solution. In case of a tool collision, a solid carbide bar will snap off in one or more pieces, often with fragments going like projectiles out of the machine tool. Carbide reinforced (CR) bars have some advantages compared to solid carbide bars. A CR-bar is an assembly of carbide rings or sleeves held together by compressive stress from a steel tension bar going through the center of the sleeves. In case of a collision, this will stress the steel parts beyond material yield limit and it will bend away. In a worst case scenario, one or two of the carbide sleeves are also damaged, but these can be replaced at a relatively low price, and the bar can be repaired for a total price which is far less than that of a replacement.
Stability is critical for any machining operation.Vibrations can often be avoided by choosing the right insert, the best standard tool holders, and the right cutting data. An essential part of the service provided by Sandvik Coromant and Teeness, is assisting customers in the application of tools, providing information and training of personnel.
Generally, a solid boring bar will perform adequately in general turning out to 4 times its diameter without vibrations. But in more demanding applications, such as internal threading and grooving, vibrations may start at an overhang between 2 and 3 times the bar diameter. In comparison, the most highly developed bars stretch 15 times beyond their own diameter. Some information on how limits may be stretched using existing tools follows.
A vibration is a variable deflection, thus, no defection means no vibration. Vibrations in a cutting tool are triggered and maintained by a dynamic cutting force. Even in a continuous cut, the cutting force will have small rapid changes in both size and direction around a certain average. The keys to eliminate vibrations are the following: Increase static stiffness, reduce cutting forces, and increase dynamic stiffness.
To increase static stiffness, choose the largest possible diameter for the boring bar and the minimum length. Special bars can also be shape-optimized, for instance be made tapered or elliptic, to utilize all available space in the workpiece. Bars can also be reinforced with materials that are stiffer than steel.
An increased length from 4 to 10 times the bar diameter will give a 16 times larger deflection for a bar taking a constant cutting force.A further extension from 10 to 12 times the bar diameter, gives another 70% increase in deflection from the same cutting force. Holding the bar length constant while changing the bar diameter from 25 to 32 mm, reduces deflection with 62% for equal cutting forces. It is important to cut with sharp edges to minimize the cutting force required to perform the machining operation - a positive cutting geometry reduces cutting forces.
An entering angle close to 90 will direct a maximized feed force in the axial direction of the bar. However, this is not effective unless the nose radius is smaller than the radial depth of the cut. If there is no vibration in the radial direction the boring bar will make a good surface even with small vibrations in the tangential direction.
The smallest acceptable insert point angle will give good clearance of a trailing surface, and small chip area variations if the tool starts vibrating in a radial direction. Reduced weight of cutting units will minimize the kinetic energy in a possible vibration. This will make it easier for the tool to damp possible vibrations, and thus stretch the maximum overhang for both solid and damped tools.
For some machining applications, the above given guidelines are not enough to sufficiently reduce vibration tendencies. These are in many cases regarded as impossible operations, with damped tools being the only option. The choice can sometimes lie between using damped tools or turning away the work, the latter course usually being unthinkable. In addition to better productivity, better surface finish, increased tool-life and better tolerances, the ever increasing higher environmental demands being set in machine shops also creates work for damped tools. Vibration from machining creates noise and sometimes it is necessary to use damped tools to keep within the maximum noise level permitted in a workshop.
The use of damped tools was in the past considered to be exotic and complicated, but this is not the case today. The most critical issue in manufacturing is to perform machining operations in the most efficient way. Today`s damped tooling includes tools that are pre-tuned to the correct frequency in relation to the tool length that is ordered, requiring basically for the damped boring bar and machine to be set up as you would with a conventional, solid boring bar.
Several techniques are known for enhancing dynamic stiffness and stability (chatter-resistance) of long cutting tools and, thus, increasing allowable overhang. The four most widely used and most universal approaches are:
The use of anisotropic bars is based on a theory explaining the development of chatter vibrations during cutting by an intermodal coupling in the two-degrees-of-freedom system referred to the plane orthogonal to the bar axis and passing through the cutting zone. According to this theory there is a specific orientation of stiffness axes relative to the cutting forces, resulting in a significant increase in dynamic stability.
The most frequently used high Youngs modulus materials are sintered tungsten carbide, and machinable sintered tungsten alloy with an added 2-4% of copper and nickel. Both materials are expensive. Solid bars made of both these materials allow stable cutting with ratios L/D < 7.
At present, the commonest approach to enhancing the dynamic stability of long bars is the application of passive dynamic vibration absorbers (DVA) with an inertia mass (pre-tuned bars). The effectiveness of a DVA for a given mass depends on the vibration amplitude at its attachment point. Accordingly, absorbers are usually installed at the furthest available position along the bar. Another factor determining their effectiveness is the mass value of the inertia weight of the DVA. In boring bars, the DVA has to be placed inside the structure. This limits both the position of the DVA along the bar axis and the size of the inertia weight, which has to be positioned in an internal cavity whose diameter must be much smaller than the bar diameter. To achieve a reasonable degree of DVA effectiveness, materials with very high specific density must be used for inertia weights.
If harmonic oscillation (vibration) tendencies should arise during the machining process, the tuning system will immediately come into force, and the kinetic energy of the bar will be absorbed in the tuning system. As a result, vibration is minimized and cutting conditions optimized. It is quite feasible to machine components that require a tool overhang of more than 10 times the tool diameter. Furthermore, with cemented carbide reinforced special bars, overhangs of 14 times the tool diameter can be dealt with successfully.
... are effective, but require vibration sensors and actuators generating forces opposing the deflections of the tool during the vibratory process. The most frequently used are active systems with cavities in the mandrel (bar) body, which are filled with pressurized oil. Pressure values in the cavities vary according to the output of the control system, thus generating dynamic deformations to cancel the chatter vibrations. Present design of active dampers is not very reliable in the shop environment, and may require frequent adjustment.The ultimate L/D ratio for a boring bar with an active damper is 10 12.
The experimental work is a comparative study of the metal removal rate for different types of boring bars at varying L/D ratios performed at SINTEF. Long and slender boring bars for single-point turning are particularly susceptible to self induced vibration. The productivity is traditionally low when using these types of tools.This study includes machining tests with conventional and damped boring bars of steel and tungsten alloy.
With internal tooling the problem of instability is a result of the machining. The only cutting force component which does not need to be counteracted with support is the axial force (Ff) which is directed along the axis of rotation, along the shank of the working bar. The radial cutting force (Fp) bends the tool out and away from the cutting zone in such a way that the diameter of the hole is affected.The tangential cutting force (Fc) bends the tool downwards and away from the center line along which the tool is designed to cut. Poor surface finish is the first sign that the cutting force is not being sufficiently damped.
All the damped boring bars are Sandvik Coromant 570 style produced by Teeness, strong and compact with coolant through center. These bars features the best anti-vibration performance due to minimum weight in front, given by the short distance from the insert tip to damper. The damping system is based on use of passive dynamic absorbers.
In order to minimize the effect of the tool wear on surface roughness and process stability, the inserts were slightly worn by machining 3 min before testing. Each insert was used for maximum 6 min. The depth of cut was corrected for deflection of the boring bars.The surface roughness was measured according to ISO 4288:1998. Two tests were carried out.
A comparative study of workpiece surface roughness for different types of boring bars at varying L/D ratio. The test was performed as a finishing operation with one kind of inserts and appurtenant cutting data.
A comparative study of metal removal rate (productivity) for different types of boring bars at varying L/D ratio. The tests were performed at a constant depth-of-cut to feed ratio. The surface roughness constraint was Ra = 3.6 m.
Damped boring bars can be used up to L/D 13. However, each damped bar has a restricted working area regarding the L/D ratio. It is also shown that the metal removal rate can be sustained up to L/D 12.
The least rigid components of machining systems are long and slender (cantilever) tools and cantilever structural units of machine tools (rams, spindle, sleeves, etc.). These components limit machining regimes due to the development of chatter vibrations, limit tool life due to extensive wear of cutting inserts, and limit geometric accuracy due to large deflections under cutting forces. Use of materials having a high Youngs modulus (such as cemented carbides) to enhance the dynamic quality of cantilever components has only a limited effect and may be costly.
The use of passive vibration absorbers combine exceptionally high dynamic stability and performance characteristics with cost effectiveness.The performance and interaction of these tools were validated by extensive cutting tests with damped boring bars from Teeness. Stable performance with length-to diameter ratios up to L/D = 13 were demonstrated. Comparative tests with similar, commercially available bars from Sandvik Coromant demonstrated the advantages with boring bars featuring passive vibration absorbers, both in surface conditions and productivity.
Teeness is a modern tool manufacturer with 70 employees. Their knowledge about basic metal cutting and mechanical vibrations have made them Sandvik Coromants specialist partner on machining with minimized vibration tendencies. The Teeness office and factory are situated two kilometers west of downtown Trondheim, the third largest city in Norway. All anti-vibration tool adapters from Sandvik Coromant are manufactured at Teeness in Trondheim,Norway. Teeness also manufactures supporting products such as solid adapters and cutting heads for internal turning. Teeness manufactures hundreds of standard Sandvik Coromant products. As partners, the companies team up to support metalworking industries to provide the best solutions.
Demanding applications may require special solutions. Each year Teeness manufactures more than 250 engineered special tools according to specifications from customers around the world.As each one of these makes an impossible operation possible,Together, Sandvik Coromant and Teeness offers customers unique manufacturing advantages.
SINTEF, the Foundation for Scientific and Industrial Research at the Norwegian Institute of Technology, is the largest independent research organization in Scandinavia. SINTEF has 1700 employees and the turnover in 2001 was 232 million Euro. Contracts for industry and the public sector generated more than 90% of this income, while 7% came in the form of basic grants from the Research Council of Norway. SINTEF sells research-based knowledge and related services to Norwegian and international clients. SINTEF collaborates closely with the Norwegian University of Science and Technology (NTNU) and the University of Oslo. Personnel from NTNU work on SINTEF projects, while SINTEF staff teaches at NTNU.The SINTEF-NTNU community involves the widespread joint use of laboratories and equipment. SINTEF is committed to produce high quality deliverables.This means that the products are to be relevant and useful for the clients, maintain high scientific and quality standards, and be presented professionally. If required by clients, SINTEFs quality assurance ensures that the projects meet the requirements of NS-EN ISO 9001 quality standards. Most of the laboratories are accredited according to EN 45001 or the GLP scheme.
Vibrating hand-held grinding, sanding and polishing tools (GSP) are used in production processes to control surface finish and quality. Their use has been associated with vibration disease for half a century, and may result in damage to the vascular, sensory or musculoskeletal systems. Few GSP manufacturers have addressed the problem of vibration in their products, with most of the grinders on the market only suitable for daily exposure of up to one hour. Observations and discussions in Swedish industry suggest that more controlled and consistent production processes are likely to remove, or reduce, the need for post-production quality control using GSP tools. As the production requirement for this operation is reduced, the duration of the operator's exposure to vibration will also be lessened. Where an elimination of the problem cannot yet be achieved through production quality improvements, better tool design may help to reduce some of the vibration transmitted to the operator. The relatively recent availability on the market of a grinder with an automatic balancing device, as well as the development of antivibration grinders, less vibration prone grinding wheels, and more effective antivibration handles and gloves, may lead to a reduced incidence of vibration disease.