M.Tech Machine Design Project
M.Tech Mechanical Engineering Machine Design Main Project
OPTIMAL DESIGN OF A DAMPED ARBOR FOR HEAVY DUTY MILLING :
ABSTRACT
The design method for a tuned mass damper embedded tool arbor was investigated which suppress chatter vibration and improve cutting performance during machining of large mechanical parts with a long slender cutting tool. Damped and undamped arbors are used for machining large mechanical parts. it is observed that vibrational forces and chatter noises are subjected to the undamped arbors during the machining process. Damped arbors are taken into account to reduce the vibration effect. According to the Toshiyuki Obikawa, Hayato Takahashi[2]., the cylindrical hollow damped arbor is used to reduce the chattering noise and improve the depth of cut. The objective is to redevelop the design of damped arbor by keeping the volume constant.
The design of the damped arbor is depended upon the dynamic stiffness and also the spring constant. Rayleigh's method coupled with displacement ratio is taken into account to calculate the dynamic stiffness of the damped arbor body. The design of the damped arbor is changed by making the cylindrical hollow to tapered. The optimal design is to be found by considering the different dimensions of the tapered hollow damped arbor by keeping the volume constant. Firstly the optimal stiffness constant is to be found from the minimum compliance value obtained from the Rayleigh's method and using this as the threshold constant, optimum design of the tapered hollow damper is achieved. The results of the tapered hollow arbor is taken into graph from which optimum dimensions are taken using the threshold spring constant.
The natural frequency of the damped and undamped arbors calculations done theoretically computational model also made by using catia and model analysis is carried out for the analytical result of natural frequencies. The material body used for the damped arbor is Chromium Molybdenum and fabrication of the arbor is will be according to the dimensions calculated and experimental tests conducted for the dynamic stiffness.
1. INTRODUCTION
In recent years, the efficiency of metal cutting process has been considerably improved in accordance with the increased feed speed of the machine tools and the improved performance of cutting tool materials. However, it is necessary to use long slender cutting tools in the case of machining deep cavities of large mechanical parts, molds and dies. When the cutting tool is long, chatter vibration due to the reduced stiffness of the tool is likely to occur. The chatter vibration causes the deterioration of the surface finish and the damage of cutting edges. Machining conditions such as spindle revolution speed and depth of cut, dynamic stiffness of the cutting tool and properties of work material significantly affect the chatter vibration.
As a means of suppressing chatter vibration, decreasing spindle revolution speed and depth of cut, and increasing dynamic stiffness of the machine tool, cutting tool, and work piece are effective. There are two ways to increase the dynamic stiffness of the structure: one is the increase in the static stiffness, the other is to improve damping behavior. Although it is difficult to increase the static stiffness because of the limitation in the size, the damping ratio of the structure can be improved by attaching a damper to the structure. Imbedding a tuned mass damper or viscous damper into a boring bar or ram of the machine tool was proposed and put into practical use. Furthermore, the optimal design and tuning of a dynamic damper for boring bars and end mills was also reported. As a result of these researches, chatter suppression and highly efficient cutting became possible in the machining of deep holes or cavities. On the other hand, it is desirable to change the length of cutting tools and cutting conditions from place to place in order to reduce total machining time because the depth of cutting position changes dramatically in the machining of giant parts. However, since preparing several tool arbors with different length leads to a cost increase, it is better to assemble a very long arbor by connecting modularized damped arbors. In this paper, a design method of a long slender arbor with high vibration absorption performance for machining deep cavity is reported. For this objective, an accurate analytical design method is investigated by introducing displacement ratio of the end of the arbor and the center of gravity of mass. Furthermore, the relationship between the geometry of a damper and damping performance are also studied as well as, when the arbor is used in various lengths. Methods are available for increasing dynamic stiffness by attaching a mass damper into the boring bar or ram of a machine tool (Seto and Yamanaka, 1980, Seto, 2010, Madoliat, et al., 2011, Rashid and Nicolescu, 2008). Moreover, optimal designing and tuning methods have been reported for the mass damper built in an end mill or boring bar (Sims, 2007, Saffury and Altus, 2009, Rivin and Kang, 1992, Pratt and Nayfeh, 2001, Semercigil and Chen, 2002), which, according to their research results, make it possible to avoid chatter vibration and increase cutting efficiency in the machining of deep hole or cavity shape.
1.1 Machine Tool Vibrations And Their Adverse Effects
Machining and measuring operations are invariably accompanied by relative vibrations between work piece and tool. These vibrations are due to one or more of the following causes:
(1) Inhomogeneities in the work piece material.
(2) Variation of chip cross section.
(3) Disturbances in the work piece or tool drives.
(4)Dynamic loads generated by acceleration/deceleration of massive moving components.
(5) Vibration transmitted from the environment.
(6) Self-excited vibration generated by the cutting process or by friction (machine-tool chatter).
The adverse and undesirable effects of these vibrations include reduction in tool life, improper surface finish, unwanted noise and excessive load on the machine tool. A machine tool is expected to have high stiffness in order to avoid such effects. Hence the machines are to be made of robust structured materials through passive damping technology to suppress the chatter vibrations and thereby increasing the production rates.
2. LITERATURE REVIEW
2.1 Overview
Merritt implies that by increasing damping capacity of the machining system, it can be used more effectively. Increase of damping leads to increase the dynamic stiffness of the system which is a product of modal stiffness and damping ratio. Dynamic stiffness proportionally affects stable chatter-free depth of cut. In order to increase the damping capacity of system in passive structural control, various kinds of dampers are used. The simplest kind of passive damper is a dynamic absorber mass connected to the main system through passive spring and damper elements (tuned mass damper) used in. Rashida and Nicolescu present the development and testing of tuned viscoelastic dampers for vibration control through their application on a work piece in milling operations. It is well known that for vibration absorbers to function effectively their stiffness and damping must be accurately tuned based upon the natural frequency of the vibrating structure. Impact dampers are also used to suppress chatter in boring and drilling in. These dampers are composed of a free mass equipped in a hole with a certain clearance and act like tuned mass dampers. Also, Semercigil and Chen attempted suppression of chatter vibration in end milling process by using an impact damper. Mass and clearance of impact dampers like stiffness and damping of tuned mass dampers need to be accurately tuned according to the natural frequency of the structure. Another kind of passive dampers is frictional damper that is successfully used for chatter suppression with no need to tuning. Damping in friction dampers is generated due to friction between parts of damper e.g. in order to increase damping capacity of the system Marui et al. made a rectangular hole in a boring bar and inserted a plate a bit larger than the hole in it. Friction between the hole and plate during bending vibration causes energy dissipation that improves damping and stability of the tool.
Edhi and Hoshi introduce a typical design of a frictional damper with experimental proof of its effectiveness in eliminating high frequency chatter of fine boring. This damper is characterized by a simple structure that consists of an additional mass attached to the main vibrating structure with a small piece of permanent magnet.
2.2 Introduction to Machine Tools
The function of machine tool is to produce a work piece of the required geometric form with an acceptable surface finish at high rate of production in the most economic way. In fact, general purpose machine tools, CNC lathes and machining centers are designed to cope with low cutting speeds with high cutting forces as well as high cutting speeds with low cutting forces. Machine Tool Structure must possess high damping, high static and dynamic stiffness. High cutting speeds and feeds are essential requirements of a machine tool structure to accomplish this basic function. Therefore, the material for the machine tool structure should have high static stiffness and damping in its property to improve both the static and dynamic performance. The static stiffness of a machine tool can be increased by using either higher modulus material or more material in the structure of a machine tool.
However, it is difficult to increase the dynamic stiffness of a machine tool with these methods because the damping of the machine tool structure cannot be increased by increasing the static stiffness. Sometimes high specific stiffness is more important than stiffness to increase the natural frequency of the vibration of the machine tool structure in high speed machining. Often the most economical way of improving a machine tool with high resonance peaks is to increase the damping rather than the static stiffness even though it is not easy to increase the damping of the machine tool structure. The chatter is a nuisance to the metal cutting process and can occur on any chip producing tool. Chatter or Self-excited vibrations occurs when the width of cut or cutting speed exceeds the stability limit of the machine tool. The effects of chatter are all adverse, affecting surface finish, dimensional accuracy, tool life and machine life. When the machine tool is operated without any vibration or chatter, the damping of the machine tool plays no important role in machining. However, the machine tool structure has several resonant frequencies because of its continuous structural elements.
If the damping is too small to dissipate the vibrational energy of the machine tool, the resonant vibration occurs when the frequency of the machining operation approaches one of the natural frequencies of the machine tool structure. Therefore the material for the machine tool structure should have high static stiffness and damping in its property to improve both the static and dynamic performance.
2.3 Damping overview
The three essential parameters that determine the dynamic responses of a structure and its sound transmission characteristics are mass, stiffness and damping. Mass and stiffness are associated with storage of energy. Damping results in the dissipation of energy by a vibration system. For a linear system, if the forcing frequency is the same as the natural frequency of the system, the response is very large and can easily cause dangerous consequences. In the frequency domain, the response near the natural frequency is "damping controlled". Higher damping can help to reduce the amplitude at resonance of structures. Increased damping also results in faster decay of free vibration, reduced dynamic stresses, lower structural response to sound, and increased sound transmission loss above the critical frequency. A lot of literatures have been published on vibration damping.
ASME published a collection of papers on structural damping in 1959. Lazan's book published in 1968 gave a very good review on damping research work, discussed different mechanisms and forms of damping, and studied damping at both the microscopic and macroscopic levels. Lazan conducted comprehensive studies into the general nature of material damping and presented damping results data for almost 2000 materials and test conditions. Lazan's results show that the logarithmic decrement values increase with dynamic stress, i.e., with vibration amplitude, where material damping is the dominant mechanism. This book is also valuable as a handbook because it contains more than 50 pages of data on damping properties of various materials, including metals, alloys, polymers, composites, glass, stone, natural crystals, particle-type materials, and fluids. About 20 years later, Nashif, Jones and Henderson published another comprehensive book on vibration damping. Jones himself wrote a handbook especially on viscoelastic damping 15 years later.
Sun and Lu's book published in 1995 presents recent research accomplishments on vibration damping in beams, plates, rings, and shells. Finite element models on damping treatment are also summarized in this book. There is also other good literature available on vibration damping. Damping in vibrating mechanical systems has been subdivided into two classes: Material damping and system damping, depending on the main routes of energy dissipation. Coulomb (1784) postulated that material damping arises due to interfacial friction between the grain boundaries of the material under dynamic condition. Further studies on material damping have been made by Robertson and Yorgiadis (1946), Demer (1956), Lazan (1968) and Birchak (1977). System damping arises from slip and other boundary shear effects at mating surfaces, interfaces or joints between distinguishable parts. Murty (1971) established that the energy dissipated at the support is very small compared to material damping.
2.4 Reviews On Research Done In Damped Arbor
Passive methods are other approaches for suppressing chatter which are less effective, but ease of usage causes them to be vastly used during recent years. Passive vibration attenuation has been employed successfully and efficiently in machining systems such as turning and milling. Merritt implies that by increasing damping capacity of the machining system, it can be used more effectively. Increase of damping leads to increase the dynamic stiffness of the system which is a product of modal stiffness and damping ratio.
Dynamic stiffness proportionally affects stable chatter-free depth of cut. In order to increase the damping capacity of system in passive structural control, various kinds of dampers are used. The simplest kind of passive damper is a dynamic absorber mass connected to the main system through passive spring and damper elements (tuned mass damper) used in. Rashida and Nicolescu present the development and testing of tuned viscoelastic dampers for vibration control through their application on a workpiece in milling operations. It is well known that for vibration absorbers to function effectively their stiffness and damping must be accurately tuned based upon the natural frequency of the vibrating structure. Impact dampers are also used to suppress chatter in boring and drilling.
These dampers are composed of a free mass equipped in a hole with a certain clearance and act like tuned mass dampers. Also, Semercigil and Chen attempted suppression of chatter vibration in end milling process by using an impact damper.
Mass and clearance of impact dampers like stiffness and damping of tuned mass dampers need to be accurately tuned according to the natural frequency of the structure. Another kind of passive dampers is frictional damper that is successfully used for chatter suppression with no need to tuning. Damping in friction dampers is generated due to friction between parts of damper e.g. in order to increase damping capacity of the system Marui et al. made a rectangular hole in a boring bar and inserted a plate a bit larger than the hole in it. Friction between the hole and plate during bending vibration causes energy dissipation that improves damping and stability of the tool. Edhi and Hoshi introduce a typical design of a frictional damper with experimental proof of its effectiveness in eliminating high frequency chatter of fine boring.
This damper is characterized by a simple structure that consists of an additional mass attached to the main vibrating structure with a small piece of permanent magnet. Ziegert et al. and Kim et al. propose another friction damper inside a milling tool. This damper is a hollow multi fingered cylinder inserted inside the tool. During the milling process when the tool is rotated Journal of Advanced Mechanical Design, Systems, and Manufacturing Vol. 5, No. 2, 2011 117 centrifugal force creates a pressure between fingers and tool body that causes energy dissipation via friction work during bending vibrations. This damper improves the efficiency of tool up to 53% in the best case. In this paper, the proposed damper in is modified and represented with a core among the fingers. The aim is to reach a higher level of tool efficiency. In order to investigate the effect of damper on stability of the milling process, a new analytical model is developed, and experimental cutting tests are performed.
3. ARBORS
3.1 Overview
An arbor is considered as an extension of the machine spindle on which cutters are securely mounted and rotated. This type of machine allows the tool to be placed in numerous positions in relation to the work piece. The arbors are made with taper shanks for proper alignments with machine spindles having taper hole on their nose. The taper shank of the arbor conforms to the Morse taper or self release taper whose value is 7:24. The arbor may be supported at the farthest end from the overhanging arm or may be of cantilever type which is called stub arbor. Miller's Tooling can boast an enormous amount of arbor and adapters to suit most milling machine or difficult requirements
Generally arbors are used in milling machines of all types to hold milling facemills of any brand or size. Then extending to milling arbor to hold boring heads, extensions and reducers to adapt arbors and cutters from one mill machine to suit another milling machine. Also required to secure the huge range of Morse Taper tools such as morse taper drills, reamers, etc.
Usually when a mill face cutter is purchased you will also require a suitable arbor to set the cutter into the mill machine's spindle. Some of the most common spindle arbor sizes are NT30 (ISO30), NT40 (ISO40), NT50 (ISO50), 2 & 3MT arbors, R8 arbors also can have metric and imperial drawbar threads. Suited for the CNC machines are BT30, BT40, BT50, SK30, SK40, SK50, HSK 50, HSK63 etc.
Various length facemill arbors can be provided to cater for longer reach into pockets and cavities as required. Arbors range to reducers whereby an ISO50 machine can accept an arbor using ISO40 tooling in the arbor, this can save huge expenses involved in tooling up for several types of machine spindles in the one shop, utilizing the same tools between machines for general and jobbing machineshops. Morse Taper tooling such as drills and reamers can easily be run in mills by the aid of an arbor to accept 1, 2, 3, 4, 5 MT tools.
Likewise, boring and facing heads will also require a suitable arbor to fit into any machine spindle. Arbors can accept all types of key and keyless chuck as well.
Rotary coolant inducer arbors are adapters to direct coolant thru the actual cutting tool mainly used for drilling and carbide end mills.
Figure: 3.1Tapers used for milling machine arbors
3.2 Standard Milling Machine Arbor
The standard milling machine arbor has a tapered, cylindrical shaft with a standard milling taper on the driving end and a threaded portion on the opposite end to receive the arbor nut. One or more milling cutters may be placed on the straight cylindrical portion of the arbor and held in position by sleeves and the arbor nut. The standard milling machine arbor is usually splined and keys are used to lock each cutter to the arbor shaft. These arbors are supplied in three styles, various lengths and, standard diameters. The most common way to fasten the arbor in the milling machine spindle is to use a draw bar. The bar threads into the taper shank of the arbor to draw the taper into the spindle and hold it in place.
Arbors secured in this manner are removed by backing out the draw bar and tapping the end of the bar to loosen the taper. The end of the arbor opposite the taper is supported by the arbor supports of the milling machine. One or more supports reused depending on the length of the arbor and the degree of rigidity required. The end may be supported by a lathe center bearing against the arbor nut or by a bearing surface 0f the arbor fitting inside a bushing of the arbor support.
Typical milling arbors are illustrated in Figure 3.2.2. Listed several types of Style C arbors. Style A has a cylindrical pilot on the end that runs in a bronze bearing in the arbor support. This style is mostly used on small milling machines or when maximum arbor support clearance is required.
Style B is characterized by one or more bearing collars that can be positioned to any part of the arbor. This allows the bearing support to be positioned close to the cutter, to-obtain rigid setups in heavy duty milling operations).Style C arbors are used to mount the smaller size milling cutters, such as end mills that cannot be bolted directly on the spindle nose. Use the shortest arbor possible for the work.
Figure: 3.2.2 Styles of Arbors
3.3 Screw Arbor
Screw Arbor: Screw arbors are used to hold small cutters have threaded holes. These arbors have a taper next to the threaded portion to provide alignment and support for tools that require a nut to hold them against a tapered surface. A right-hand threaded arbor must be used for right-hand cutters; a left-hand threaded arbor is used to mount left-hand cutters.
Slitting Saw Milling Cutter Arbor: The slitting saw milling cutter arbor is a short arbor having two flanges between which the milling cutter is secured by tightening a clamping nut. This arbor is used to hold the metal slitting saw milling cutters that are used for slotting, slitting, and sawing operations.
End Milling Cutter Arbor: The end milling cutter arbor has a bore in the end in which the straight shank end milling cutters fit. The end milling cutters are locked in place by means of a setscrew.
Shell End Milling Cutter Arbor: Shell end milling arbors are used to hold and drive
shell end milling cutters. The shell end milling cutter is fitted over the short boss on the arbor shaft and is held against the face of the arbor by a bolt, or a retaining screw. The two lugs on the arbor fit slots in the cutter to prevent the cutter from rotating on the arbor during the machining operation. A special wrench is used to tighten and loosen a retaining screw/bolt in the end of the arbor.
Fly Cutter Arbor : The fly cutter arbor is used to support a single-edge lathe, shaper, planer cutter bit, for boring and gear cutting operations on the milling machine. These cutters, which can be ground to any desired shape, are held in the arbor by a locknut. Fly cutter arbor shanks may have a Standard milling machine spindle taper, a Brown and Sharpe taper, or a Morse taper.
3.4 Materials Used in Arbors
Arbors are made of materials having the usual tool holder material properties, i.e., high strength, hardness, impact load resistance and good heat and wear resistance. For ease of manufacture and tapered. Arbors are mostly made of AISI 4140 Alloy Steel and MS (Mild steel). To enhance cutting speed, productivity and product quality, now-a-days cemented carbide segments (assembled) or replaceable inserts are also used specially for stronger and harder work materials like cast irons and steels. High Tensile Steel provide much longer tool life in Arbors.
These are available in different lengths and materials steel, titanium, aluminium, ceramic and carbon fiber. With long extensions consideration must be given to the thermal characteristics of the material.
In this project Chromium molybdenum is used for manufacturing for Arbor. This material characteristics are strength (creep strength and room temperature), rigidity, hardenability, wear resistance, corrosion resistance, fairly good impact resistance (toughness).
3.4.1 Chromium Molybdenum Properties
Element
|
Content
|
Carbon, C
|
0.380 - 0.430 %
|
Chromium, Cr
|
0.80 - 1.10 %
|
Iron, Fe
|
96.785 - 97.77 % (As remainder)
|
Manganese, Mn
|
0.75 - 1.0 %
|
Molybdenum, Mo
|
0.15 - 0.25 %
|
Phosphorous, P
|
≤ 0.035 %
|
Silicon, Si
|
0.15 - 0.30 %
|
Sulfur, S
|
≤ 0.040 %
|
Table: 3.4.1Chromium Molybdenum Properties
3.4.2 Physical Properties
The density of AISI 4140 is 7.7 to 8.03 kg/m3
Young's modulus is 2.1X102 GPa
poison's ratio is 0.29
4. MILLING MACHINES
4.1 Milling Machine
A milling machine is a machine tool used to machine various materials. Milling machines are often classed in two basic forms, horizontal and vertical, which refers to the orientation of the main spindle. Both types range in size from small, bench-mounted devices to room-sized machines. Unlike a drill press, which holds the workpiece stationary as the drill moves axially to penetrate the material, milling machines also move the workpiece radially against the rotating milling cutter, which cuts on its sides as well as its tip. Workpiece and cutter movement are precisely controlled to less than 0.001 in (0.025 mm), usually by means of precision ground slides and lead screws or analogous technology. Milling machines may be manually operated, mechanically automated, or digitally automated via computer numerical control (CNC). They can perform a vast number of operations, from simple to complex (slot and keyway cutting, planing, drilling to contouring, diesinking). Cutting fluid is often pumped to the cutting site to cool and lubricate the cut and to wash away the resulting swarf. The different types of milling machines are
4.2 Types of milling machine
The milling machines are classified according to the general design of the machine.
1. Column and knee type
• Plain milling machine
• Universal milling machine
• Vertical milling machine
2. Table type milling machine
3. Planer type milling machine
4. Special type milling machine
4.2.1 Vertical milling machine
It is very similar to a horizontal milling machine in construction as it has the same parts of base, column, knee, saddle, and table. The spindle of the machine is positioned vertically. The cutters are mounted on the spindle. The spindle is rotated by the power obtained from the mechanism placed inside the column. Angular surfaces are machined by swiveling the spindle head.
Figure: 4.2.1 Vertical Milling Machine
4.3 Milling Operation
Milling is the process of cutting away material by feeding a workpiece past a rotating multiple tooth cutter. The cutting action of the many teeth around the milling cutter provides a fast method of machining. The machined surface may be flat, angular, or curved. The surface may also be milled to any combination of shapes. The work piece is mounted on the table with the help of suitable fixtures. The desired contour, feed and depth of cut for the job are noted down.
A suitable milling cutter for the specified job is selected and mounted on the arbor. The knee is raised till the cutter just touches the work piece. The machine is started. By moving the table, saddle and the knee, for the specified feed and depth of cut, the desired job may be finished. The machine may then be switched off. The different methods of milling are
1) Up milling
2) Down milling
4.3.1 Up Milling
Up milling is also referred to as conventional milling. The direction of the cutter rotation opposes the feed motion. For example, if the cutter rotates clockwise , the work piece is fed to the right in up milling.
Figure: 4.3.1 Up milling or Conventional Milling
4.3.2 Down Milling
Down milling is also referred to as climb milling. The direction of cutter rotation is same as the feed motion. For example, if the cutter rotates counter clockwise , the workpiece is fed to the right in down milling.
Figure: 4.3.2 Down milling or climb milling
The chip formation in down milling is opposite to the chip formation in up milling. The figure for down milling shows that the cutter tooth is almost parallel to the top surface of the workpiece. The cutter tooth begins to mill the full chip thickness. Then the chip thickness gradually decreases
4.4 Milling Cutters
Milling cutters are cutting tools typically used in milling machines or machining centers to perform milling operations. They remove material by their movement within the machine or directly from the cutter's shape.
4.5 Types Of Milling Machine Cutters
End mills (middle row in image) are those tools which have cutting teeth at one end, as well as on the sides. The words end mill are generally used to refer to flat-bottomed cutters, but also include rounded cutters (referred to as ball nosed) and radiused cutters (referred to as bullnose, or torus). They are usually made from high speed steel or cemented carbide, and have one or more flutes. They are the most common tool used in a vertical mill.
Figure: 4.5.1 End Mill
4.5.2 Face Mill
A face mill is a cutter designed for facing as opposed to e.g., creating a pocket (end mills). The cutting edges of face mills are always located along its sides. As such it must always cut in a horizontal direction at a given depth coming from outside the stock. Multiple teeth distribute the chip load, and since the teeth are normally disposable carbide inserts, this combination allows for very large and efficient face milling.
Fig: 4.5.2 Face Mill
4.6 Vibration in Machine Tools
The Machine, cutting tool, and workpiece from a structural system have complicated dynamic characteristics. Under certain condition vibrations of the structural system may occur, and as with all types of machinery, these vibrations may be divided into three basic types:
1. Free or Transient vibrations: resulting from impulses transferred to the structure through its foundation, from rapid reversals of reciprocating masses, such as machine 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.
2. Forced vibration: 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 frequency of the structure, the machine will resonate in the corresponding natural mode of vibration.
3. Self-excited vibrations: usually resulting from a dynamic instability of the cutting process. This phenomenon is commonly referred to as machine tool chatter and,typically, if large tool-work engagements are attempted, oscillations suddenly build up in the structure, effectively limiting metal removal rates. The structure again oscillates in one of its natural modes of vibration.
4. The sources of vibration excitation in a machine tool structure are vibration due to inhomogeneities in the work piece , cross sectional variation of removed material, disturbances in the vibration of tool drives , rotation unbalanced members guide ways gears, drive mechanisms and others.
4.7 Chatter in the Milling machine
The milling operation is a cutting process using a rotating cutter with one or more teeth. An important feature is that the action of each cutting edge is intermittent and cuts less than half of the cutter revolution, producing varying but periodic chip thickness and an impact when the edge touches the work piece. The tooth is heated and stressed during the cutting part of the cycle, followed by a period when it is unstressed and allowed to cool.
The consequences are thermal and mechanical fatigue of the material and vibrations, which are of two kinds: forced vibrations, caused by the periodic cutting forces acting in the machine structure and chatter vibrations, which may be explained by two distinct mechanisms, called “mode coupling” and “regeneration waviness”, explained in Tobias (1965), Koenigsberger & Tlusty (1967) and Budak & Altintas (1995). The mode coupling chatter occurs when forced vibrations are present in two directions in the plane of cut.
The regenerative chatter is a self-excitation mechanism associated with the phase shift between vibrations waves left on both sides of the chip and happens earlier than the mode coupling chatter in most machining cases, as explained by Altintas (2000). In milling, one of the machine tool workpiece system structural modes is initially excited by cutting forces. The waved surface left by a previous tooth is removed during the succeeding revolution, which also leaves a wavy surface due to structural vibrations.
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