IMPROVED COLLET AND METHOD FOR OPTIMIZING GRIPPING ACTION THEREOF

Abstract: A method (70) for manufacturing a machine tool collet (10) and a machine tool collet (10) having design parameters optimized to maximize its gripping strength. The collet (10) is of the type having a plurality of gripping jaws (16) spaced around a longitudinal axis (18) with resilient material between the jaws (16). The method (70) includes calculating the total torque generated on a bar passing through the collet (10) for a given collet configuration, including the torque generated just prior to a toggle condition occurring between the jaws (16) and the bar and the maximum moment force generated at a toggle condition. At least one of the design parameters of the collet (10), such as jaw thickness or number of jaws, is varied and the total torque generated is recalculated until the total torque reaches a maximum value for the given collet configuration.

Description: TITLE OF THE INVENTION IMPROVED COLLET AND METHOD FOR OPTIMIZING GRIPPING ACTION THEREOF BACKGROUND OF THE INVENTION The present invention relates to a collet, and more particularly to an improved collet having optimized parameters to maximize gripping strength, and a method for making same. U.S. Patent No. 2,346,706 to Stoner and assigned to the Jacobs Manufacturing Company describes a collet having a plurality of flat-sided gripping members. The gripping members are held in relative position equally spaced about a common axis. The spaces between the gripping members are filled with an adherent resilient material, such as rubber or rubber composition. The gripping members are also transversely perforated. In this case, the resilient rubber material not only fills the space between adjacent gripping members, but it also passes through the perforations in the gripping members thereby forming a plurality of longitudinally spaced continuous annular rings which hold the gripping members in desired relative positions. This collet was a significant improvement over the conventional split steel collets and is widely recognized today by those skilled in the art and commonly known as the Rubber-Flex® collet. However, prior to applicant's present invention, it was not known to optimize the gripping strength of the Rubber-Flex® collet by determining optimum design parameters for the gripping blades or "jaws." Although a number of jaws having varying thicknesses were utilized in the collets, it was not commonly believed that gripping strength could be maximized by deducing the optimum number of gripping jaws for a given configuration which would fit around the inner diameter of the collet while maintaining at least a minimum amount of the resilient material between the gripping jaws. Additionally, the thickness of the gripping jaws was generally not considered as effecting gripping strength. Typically, the jaw thickness was dictated by the size or inside diameter of the collet. In other words, smaller diameter collets had thinner jaw blades. U.S. Patent No. 5,123,663 to Mizocmchi discloses a method for determining the minimum number of segments for a metal collet by determining the range of a central angle (theta) where the geometrical moment of inertia for each segment is constant with respect to a line passing through the centroid of the section perpendicular to a radial direction of the collet. The patent discloses that the range of angle theta is less than 30° regardless of the thickness of the segment. Once the central angle of about 30° is attained, any further increase in the number of segments will only increase the number of manufacturing steps and lower the tensile strength a cylindrical portion of the collet. OBJECTS AND SUMMARY OF THE INVENTION It is a principle object of the present invention to provide an improved collet of the type having independent gripping members molded in a rubber composition whereby the gripping strength of the collet is maximized. A further object of the present invention is to provide a method for determining the design parameters for gripping jaws in a collet of the type having independent gripping jaws molded in a rubber composition. It is also an object of the present invention to provide a collet having an optimized gripping strength which is compatible to conventional collet sizes and requirements and interchangeable with conventional split steel collets. Yet a further object of the present invention is to provide a process for determining the ideal gripping jaw thickness and number of jaws to optimize the gripping strength of a collet design. Still a further object of the present invention is to provide a design optimization tool for optimizing the design of a collet and which may be utilized on a personal computer. And yet another object of the present invention is to provide a method for predicting collet torque slippage useful for maximizing a collet design. It is also an object of the present invention to provide a collet torque slippage analysis method for determining the optimum characteristics or design parameters for a collet of specific dimensions and requirements. Yet a further object of the present invention is to provide a collet torque slippage analysis method for determining the optimum parameters for the gripping jaws within the collet for a collet of conventional dimensions and requirements. And still a further object of the present invention is to provide an analysis method for improving the gripping strength of the wide variety and sizes of conventional collets. And yet another object of the present invention is to provide an analysis method for determining maximum collet torque for a collet configuration having predetermined design parameters, the method being useful in optimizing design parameters of the collet. Still a further object of the present invention is to provide a system for optimizing collet design parameters, the system being useful in predicting optimum collet parameters to maximize gripping strength of the collet. It is also an object of the present invention to provide a machine tool collet wherein at least one of the structural dimensions or characteristics of the gripping jaws of the collet is optimized for maximizing the gripping strength of the collet. Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. To achieve the objects and in accordance with the purpose of the invention, as embodied and broadly described herein, a method is provided for optimizing the gripping strength of a collet of the type having a plurality of gripping jaws spaced around a common axis with resilient material between the gripping jaws. The method according to the invention comprises the steps of determining the radial force of the gripping jaws upon a bar disposed through the collet, the radial force being dependent upon an axial force applied to the collet, and determining the torque developed on the bar from the radial forces of the gripping jaws just prior to a toggle condition occurring between the gripping jaws and the bar. The method further calls for determining the total moment force generated between the gripping jaws and the bar at toggle condition for a given toggle angle. According to the method of the invention, the toggle angle is varied until the total moment force between the gripping jaws and bar at toggle condition reaches a maximum value. The method further calls for determining the total torque on the bar from the torque prior to toggle and the maximum total moment force at toggle condition. By employing the method of the present invention, an optimum design for a given collet configuration can be determined by varying parameters of the collet design and determining the total torque on the bar until a maximum torque value is determined for a given collet configuration. The present method may be utilized in determining the optimum design for all known collet configurations, including the ER and TG collet configurations. In a preferred embodiment of the method according to the present invention, the method includes the step of varying the thickness of the gripping jaws for a given collet configuration to determine the thickness of gripping jaw generating maximum total torque for a given collet configuration. Likewise, the method may include the step of varying the number of collet jaws for a given collet configuration to determine the number of gripping jaws generating maximum total torque for a given collet configuration. The method may further include the step of varying the jaw material, for example from steel to plastic, to determine the optimum material for maximizing gripping strength. In further accordance with the purpose of the present invention, a method is provided for determining the optimum design for a given collet configuration and comprises the step of calculating the total torque generated on a bar passing through the collet of a given collet configuration having predetermined design parameters, the total torque including torque generated just prior to toggle occurring between the gripping jaws and the bar and the maximum moment force generated between the gripping jaws and the bar due to toggle. The method also calls for varying at least one design parameter of the collet and recalculating the total torque generated until the total torque generated reaches a maximum value for the given collet configuration. In further accordance with the purpose of the invention, a system is provided for optimizing collet design parameters for a type collet having a plurality of gripping jaws spaced around a common axis with resilient material between the gripping jaws, whereby a bar passing through the collet is securely held by the gripping jaws. The system comprises means for variably inputting design parameters of the collet of a given configuration. Means are also provided for calculating the radial force of the collet gripping jaws upon the bar, the radial force being dependent upon an axial force applied to the collet through a collet holder and collet nut. Means are further provided for calculating the torque developed on the bar from the radial forces of the gripping jaws just prior to toggle between the gripping jaws and the bar. The system includes means for calculating the moment generated due to toggle between the gripping jaws and bar at a predetermined toggle angle. Means are provided for varying the toggle angle in the moment calculating means until the moment generated reaches a maximum value. The system also calls for means for summing the maximum moment at toggle and torque developed just prior to toggle to give a total torque developed on the bar by the gripping jaws for the inputted design parameters of the collet. In this way, the total torque developed on the bar can be maximized by varying at least one design parameter through the design parameter inputting means so that an optimum set of design parameters can be determined. In a preferred embodiment of the system according to the invention, a computer is provided with the radial force calculating means, torque calculating means, moment calculating means, toggle angle varying means, and summing means comprising respective software executed by the computer. Preferably, the computer includes a library of known design parameters for various known collet configurations for use by the software. The means for variably inputting design parameters is interfaceable with the computer so that at least one of the design parameters of the collet can be varied. Preferably, the computer further includes a library of known parameters for various collet nut geometries, with the radial force calculating means using the collet nut parameters to compute axial force imparted to the collet. Still in further accordance with the objects of the invention, a collet torque slippage analysis program is provided for calculating torque developed by a collet held by a collet nut and collet holder. In further accordance with the invention, a machine tool collet which is actuated by engagement with a conical surface of a collet holder in a collet nut is provided. The machine tool collet comprises a resilient material for holding a plurality of gripping jaws in a desired spaced relation. The collet further includes gripping means for optimally holding a machine tool within the collet. The gripping means includes a plurality of gripping jaws held by the resilient material in a desired longitudinally and angularly spaced relation about the longitudinal centerline axis through the collet. The gripping jaws have an exposed inner face parallel to the centerline axis whereby the plurality of gripping jaws define an inner radius of the collet. The gripping jaws also have an angled or slanted exposed outer face whereby the plurality of gripping jaws defines a conical outer surface to engage the collet holder conical surface. The gripping jaws further comprise at least one structural dimension or characteristic, such as thickness, number, or material, optimized for maximizing the gripping strength of the collet. In a preferred embodiment of the machine tool collet, the gripping jaws have a thickness optimized to maximize the gripping strength of the collet. Likewise, in another preferred embodiment, the gripping jaws are of a precise number optimized to maximize the gripping strength of the collet. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view of a collet according to the present invention, particularly a collet formed according to the process and method of the invention; Fig. 2 is a partial component view of the collet depicted in Fig. 1, particularly illustrating the gripping jaws of the collet; Fig. 3 is a front end view of the collet shown in Fig. 2; Fig. 4 is a flow chart diagram depicting the sequence of steps and calculations according to the method and process of the invention; Fig. 5 is a simplified schematic representation of the system according to the invention incorporating the process and method of Fig. 4; Fig. 6 is a flow chart diagram depicting the steps and calculations according to the method for calculating torque at toggle condition; Figs. 7a and 7b are diagrammatic depictions of collet thread geometries, particularly depicting the analysis parameters used in calculation of axial force; Fig. 8a is a simplified component view of a gripping jaw cooperating with a collet holder to grip a bar, particularly illustrating the condition of toggle; Fig. 8b is a diagram illustrating the contact width between a jaw and the bar; Fig. 9 is a diagrammatic sketch particularly pointing out the analysis parameters used to calculate torque just prior to toggle; Fig. 10 is another simplified diagrammatic sketch illustrating the concept of roll angle and maximum roll angle used in calculation of torque at toggle condition; Fig. 11a is a detailed diagrammatic sketch illustrating the analysis variables used in the calculation of torque at toggle condition; and Fig. lib is a similar to Fig. 11a and shows the parameters used in elastic deformation toggle analysis. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Reference will now be made in detail to the presently preferred embodiments of the invention, one or more examples of which are illustrated in the accompanying drawings and figures. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features or steps illustrated or described as part of one embodiment of the method of the invention, can be used on another embodiment of the method to yield a still further embodiment of the method. Thus, it is intended that the present invention cover such modifications and variations as come within the scope of the appended claims and their equivalents. The numbering of components in the drawings is consistent throughout the application, with the same components having the same number in each of the drawings. The method and apparatus according to the present invention relate to a collet of the type illustrated in Figs. 1 through 3. Collet 10 is actuated by engagement with the conical surface of a collet holder and collet nut (not shown) . Those skilled in the art understand the operation of collet 10 with a collet holder and collet nut. Collet 10 comprises resilient material 12 for holding a plurality of gripping jaws 16 in a desired spaced relation relative to each other. Preferably, resilient material 12 extends through perforations 28 within gripping jaws 16 so as to form essentially concentric rings of resilient material 12 through gripping jaws 16. Collet 10 also comprises gripping means A for optimally holding a machine tool within collet 10. In a preferred embodiment, gripping means A comprises a plurality of gripping jaws 16 held by the resilient material 12 in a desired longitudinally and angularly spaced relation about longitudinal centerline axis 18 r * « * s _ through collet 10. The gripping jaws have an exposed inner face 20 parallel to centerline axis 18, whereby the plurality of jaws 16 define an inner radius 22 of collet 10. The shaft or bar of a machine tool to be held by collet 10 is inserted through inner radius 22 of collet 10. Gripping jaws 16 further include an angled exposed outer face 24 whereby the plurality of jaws 16 define a conical outer surface 26. Conical outer surface 26 matches the conical surface of the collet holder (not shown) . According to the method and process of the invention described in this section, applicant has determined that the gripping strength of the present collet is significantly increased compared to conventional collets by analyzing the torque generated by the collet on the machine tool or a test bar and optimizing the design parameters of gripping jaws 16 so as to generate maximum torque for a given collet configuration. Thus, gripping jaws 16 comprise at least one structural characteristic or dimension optimized to maximize the gripping strength of collet 10. The structural characteristics or dimension of jaws 16 may include, for example, the thickness of jaw 16, the number of jaws 16 within collet 10, or the type of material from which jaw 16 is formed. For example, a standard ER25 collet configuration has collet geometries based on DIN standards, such as inner radius, outer radius, angle of outer conical surface, etc. Once a particular collet configuration is selected, the industry standards and requirements are basically dictated by the appropriate engineering standards. However, applicants have determined that by optimizing certain design parameters of the collet gripping jaws, the gripping strength of a collet of that particular configuration can be maximized. Applicant has also particularly determined that a degree of toggle between the gripping jaws and machine • ULE 26 tool or bar is desirable in that it provides a maximum torque generated between the collet jaws and bar. Applicant has incorporated this desirable condition in his analysis in determining the optimum design parameters for the gripping jaws for maximizing total torque between the collet and tool held within. The method and system for implementing the same according to the invention is represented generally in flow chart form in Figs. 4 to 6. The analysis steps indicated in the figures will be discussed in detail below. Table 1 provided below is a list of the variables and their meaning and default value used in the analysis steps. Table 1: Variables ' " Table 1 (continued) Table 1 (continued) , . «_. »-... *- ■ r*, r s__*^ iw T &T_»r-ι 14 It should be understood that the following description of the analysis steps according to the method and process of the present invention is but a preferred embodiment of the present method, and not meant as a limitation thereof. The values calculated in the steps may be calcuilated or predicted in any manner of process, all of which fall within the scope and spirit of this invention. Additionally, the order of analysis steps presented is not limited to the order discussed but, can include any effective order. Referring to Fig. 4, the various collet parameters and default values are entered at step 69, including collet holder and nut parameters, jaw design variables, material allowable stresses, coefficient of friction for collet nut, holder, and b'ar, test bar parameters, etc. At step 71 the maximum number of gripping jaws is determined for a given collet configuration and given jaw thickness. The maximum number of jaws is estimated using the inner radius of the collet, the .jaw thickness, and the rubber coating thickness on each side of the jaw as follows: Although not used particularly in the analysis, the angle of resilient material 12 between the jaws may be calculated assuming a uniform equal spacing between the jaws and a uniform resilient material coating thickness on each jaw face according to the following equation: Θ r 2π t • ! • Itcoat 'rubber Λ7 . n inner ' "jaws " As discussed above, the collet 10 is engaged with a collet holder and collet nut (not shown in the figures) . Engagement between the collet nut and collet holder imparts an axial force to the collet. As a result of this axial force, a radial force is developed between each jaw 16 and a test bar or machine tool sliaft through the collet. The axial force or thrust developed due to tightening of the 15 collet nut onto the collet holder is dependent upon the geometry of the threaded surfaces of the collet nut. The notation for this geometry is indicated in Figs. 7a and 7b. The thread geometry parameters may be obtained from engineering standards, such as the June, 1959 DIN 6341 standard (TR designation) or ISO 68 metric screw thread standard (M designation) . The values for the collet nut thread geometry parameters obtained or calculated from the engineering standards are preferably stored in a library 68 for later retrieval and analysis. At step 73, the parameters for a particular collet configuration are defined, preferably from stored in formation in» a library 66 (Fig.3). The collet geometry parameters are based on DIN standards and are basically defined once a particularly collet configuration is selected. The analyst is prompted to select a particular collet design or configuration and the system or program then defines the various geometric data needed for the analysis. For example, table 2 is provided below as an example of collet geometry standards obtained from DIN 6499 for an 8° cone for an ER11, ER25, ER32, and ER40 collet desi nation. The axial force calculation is executed at step 74. The total torque applied to the collet nut on the collet holder equals the sum of the torque necessary to develop an axial thrust on the collet and the torque necessary to overcome the collet thrust bearing friction. The applied torque to the collet nut is given by the equation: _*» i ' 16 , 2ττ r m f n -I- cos ,, nut w T In, r„, cos α„ — - /r„ϊ L- ) + f_ι, w R < If the term inside the parenthesis is defined as : then using the relationship for tan A_, the inverse relation is obtained: 1 - /. COStl, •tanA Ω = tan . + cos . As such, the expression for the applied torque to the nut may be written as: T nu , = W £ r + W f cb R c = W r.n fe + &j then solved for the axial force on the entire collet W to obtain: W - ■ τ,ιut (___ + / • 5s Now assuming that only the jaws carry any load and that each jaw is identical and located ideally so that each jaw is loaded in the same manner and in the same amount. That is, the axial force per jaw is: jaw _ W jaws It should be understood that the above calculations are but one means of predicting the axial force applied to the collet. The axial force may actually be predetermined and stored in an appropriate library for retrieval and analysis. The radial force calculation is performed at step 75. As a result of the axial force which develops due to the tightening of the collet nut, a normal force develops 17 between the collet holder surface and each gripping jaw. The total frictional force between the collet jaws and collet holder is the product of this normal force and the coefficient of friction between the jaws and collet holder. That is, irlniv _ r p)oιv 1 /e - J c ' y Summing forces in the axial and radial directions gives the following two equations in two unknowns : t ∑ F^l = h FZ ' ial - WJaW + F 7 " + fc r j y7 ∞* "c - 0 → Σ Predial = FZi l ~ F 7 + fc F^ SΪnα, = 0 From these two equations, expressions for the normal force between the collet holder and each jaw, for the radial force transmitted between each jaw and the bar, and for the axial force are obtained and given by: cjaw _ " • ' y (f b + f c ) cos c + (1 - fj c ) sin c F __ t β ^ (co β c - Λ inαc) In step 76, the torque due to Coulomb friction force per each gripping jaw is calculated for the condition of no slippage between the bar and gripping jaws. Slippage is essentially the condition where the gripping jaws have "rocked" to such a degree that they .are basically no longer applying force to the test bar which therefore is essentially free to rotate within the collet. The radial force transmitted between the jaws and the test bar has associated frictional orces that develop at the inner face between each jaw and the bar. The torque due to the Coulomb friction force per jaw is calculated as follows: 18 The total torque developed without slippage is this value times the number of jaws in the collet, according to the following: -riotal _ lyj fjaw _ ΛΓ pjaw D = y. f. F jaw R * . 1 CF ~ ' jaw l CF ' jaws r f Λ i> l jaws J b raaia! b The torque results from the frictional forces acting between the bar and each jaw edge. The frictional force is calculated based on Coulomb's Law of Friction which is independent of contact area. As such, in this part of the analysis, the thickness of the jaw and the engagement length along the inner face between the bar and jaw edges do not effect the torque resulting from Coulomb friction forces. At step 77, it is predicted whether slip will occur between the test bar and the jaws based on the geometry of the jaws. Slip will occur when: tanαJ > f b where α, = *-π-( ) - M- _ ^ _ . It is desired to inform the analyst whether the parameters for the jaw blade he has selected will result in slipping between the jaw blades and test bar. As illustrated in Fig. 8a, at some degree of slipping between the jaws and bar, the jaw will "toggle" or "rock" and lose contact with the bar and collet holder at certain points along the length of the jaw. Λt toggle condition, the contacting surface area will be reduced. Λ degree of toggling between the jaws and bar may be desired in that torque developed between the jaws and bar is actually increased. To account for differences due to geometric parameters of the jaws, such as jaw thickness, in the torque 19 calculation, the conditions "just prior" to toggle are examined according to an elastic contact analysis (Hertzian Contact Analysis) . Basically, the test bar is treated as a long cylinder in contact with a flat semi-infinite plane subjected to a line of force P as shown iniFig. 8b. The contact region between the cylinder and plane is equal to twice the semi-contact width (2a) for the entire cylinder length. The semi-contact width (a) is calculated according to the following: a - l VR > πE" This approach is valid as long as the jaw thickness is much larger than twice the semi-contact width. In step 78 according to the present method, torque is calculated at a condition just prior to toggle between the jaws and bar. As illustrated in Fig. 9, just prior to toggle occurring, the line of action of the radial force shifts to the outer corner of each jaw. Since the depth of the jaw varies along the engagement length thereof, the distance or height of the segment must be calculated. The engagement length between the jaw and the collet .holder is projected onto the bar and then this length is divided into a number of segments (N ) which may be set at default value such as 20. Then, by summing forces in the radial and tangential directions and summing moments about the center of the bar, the torque just prior to toggle for each segment is obtained. Summing these values for each segment gives 20 the total torque just prior to toggle for eacli jaw. This approach gives the following equations: The total torque developed just prior to toggle is this value times the number of jaws in the collet, according to the following: Method 70 further includes step 79 for calculating torque at tog # gle condition between the jaws and bar. The torque calculation at toggle condition analysis is charted in detail in Fig. 6. Since the ' toggle may not extend along the full length of each jaw, the jaw is divided at step 100 into a number of segments N s along the length thereof. The calculations proceed as a double loop. For each increment in- bar rotation, or "toggle angle," the elastic deformation for each segment of a jaw is computed as well as its radial frictional force. At step 101, the toggle angle for each jaw is determined. As shown in Fig. 10, the toggle angle is calculated based on the semir-contaσt width obtained from the Hertz contact analysis. The toggle angle is the angle through which.a jaw can rotate within the distance defined by the semi-contact width. The toggle angle is calculated as follows: 21 generated due to elastic deformation of the jaws and torque due to elastic radial (Coulomb friction) force once the jaw edge has "yielded." Thus, the analysis considers plastic deformation of the jaws as well. It should be understood that the type of material from which the jaws are formed will thus effect the torque at toggle analysis since yielding or plastic deformation is a factor of the allowable stress for the type material. Material composition of the jaws is thus a design parameter of the jaws which can be varied to optimize the gripping strength of the collet. The following analysis of the torque calculation at toggle condition is represented sequentially in flow¬ chart form in Fig. 6. Once the jaw has been segmented at step 100 and the toggle angle calculated at step 101 as above, the analysis is executed for each segment. Although the calculations will be explained in detail below, the basic steps are as follows. At step 103, the segment N A is checked for previous yielding and slip. If such occurred, this segment is ignored at 104 and the next segment analyzed, and so forth. If yielding and slip did not occur, the torque for the segment due to elastic deformation is calculated at 105. This torque is summed for all segments at 106. At step 107, the segment is checked for initial yielding. If such has not occurred, then no more torque is being generated and, at 108, the analysis skips to the next segment. If yielding has occurred, the jaw shortening is recalculated at step 109 to account for plastic deformation. Then, at step 110, slip is checked at the plastic deformation condition. If slip occurs, no torque is generated and the analysis skips to the next segment at step ill. If slip does not occur, the torque generated due to elastic radial force (Coulomb force) at the yielded condition is calculated at step 112. This torque is summed for all the segments at step 113. The next segment is analyzed at step 117. 22 At step 114, the total torque at toggle condition is calculated for all jaws from the sums of step 113 and 106. If this total torque is not a maximum valve at 115, then at 116 the toggle angle is incremented at the analysis repeated for that segment. The total torque at toggle condition is not a maximum if it is less than the previous value, the previous value therefore being the maximum value. The calculations and instructions for executing the steps just described are as follows: 23 Initialize variables IPASS=0, for all segments Increment the bar rotation angle Θ=Θ+ΔΘ where initial value of θ = αj (.. and Δθ = α , and set /RΛ55=/RΛ5 l S+; Calculate the length of each segment along the jaw length assuming uniform spacing b~llN seg where / is the engagement length of the collet and collet holder. Calculate At = ±t - a - R b θ For each segment i - 1, N seg , first check to see if IYLDi is less than zero. If it is, then this jaw segment is not in contact with the bar and so go to the next segment. If it is greater than or equal to zero then proceed. Set the iteration counter for segment yielding to zero (m=0) and do the following calculations: Average outer radius of collet for the i th segment (Re)/ = Rci 2N 'seg ( Λcl ~ Ra ^ Width of jaw at the i th segment h t = (R e ), - R b - δ clear where clear = R' nner - R, Calculate the roll angle for the i th segment θnll * a £ » = — tan - 1 i Af; 1 Calculate the maximum rotation angle for the i' h segment If θ ≥ θ' mΛX then go to the next segment. Angle between line AC and right edge of jaw (before toggle) for i th segment («*) / « tan " 1 X Length of the line AC for the i th segment before toggle Angle between line A' C and right edge of jaw (after toggle) for i ,h segment Length of the line A' C for the i th segment after toggle - h - {Zn)i ~ cos( 2n ),* 24 Shoπening of this diagonal line for the i' h segment averaged over each jaw is If (ό n )ι < 0 then go to the next segment. From strength of materials, the deformation of an axially loaded member is (PL)/(EA). Thus the force related to this shortening and acting along the line A'C is ( n)i = where A? = lab Total radial force at the jaw corner for the i th segment Bearing stress for the i th segment ( πbearsO _ V radial) j Ψn }i A 0 Check for yielding of the i th segment. If ( σ bean.O < -bear υ n >i ^ at ι Then calculate the moment arm from the center of the bar to the force x ι = ( R b + ( Λ , ta n(« 2n ) 1 - ~ (!' + α) tan( 2n ) t .j sin (α 2n ),. Calculate the moment for the i th segment (M x )i = x t (F zn )i ' Accumulate the moments for all segments along the jaw M τ = M τ + (M x ) t Go to next segment ' 25 If it is greater than or equal to the allowable bearing stress then decrease the deformations and re-calculate the force and bearing stress. Set yield flag IYLD t -lPASS where IPASS is the pass number for incrementing the toggle angle (δ n )T = (<5π), ~ m Δ w^ a = (c5 n ),* - ό? If the new deformation is less than zero, then set it Calculate the new force related to the new shortening ( F )m = (6 n) E jaw A T h A m = J_ Δ t_\ Calculate new total radial force at the jaw comer for the i th segment (Pradial = [[?% ) ( + ( F ,n)ή ∞s(a 2n ) i Calculate new bearing stress . ( nbeai- m _ J radial) j ' ~ A m Check for yielding of the i th segment. If n ^f > σ all then adjust the iteration counter to m=m+l and loop back to re-calculate the deformations, the radial force, and bearing stress. Maximum of 100 iterations is set. If this new bearing stress is less than the allowable value for the bearing stress, then the amount of yielding is determined. 26 If this is the initial yielding then calculate the maximum length of line A'C. At this condition the points O, A', and C are colinear and the line A'C has a maximum length of (Zπ)i = (Zπ)i - (< * + (<5„)f Now assign Othersvise use the previously calculated value. Calculate the elastic shortening of line A'C " ' - <-»< - [_±β ~ Λ ») - ^' ~ l < ~ where IQD i s the length required for contact. If d' is greater than zero then calculate the corresponding elastic force due to this elastic shoπening. d ' E jaw A ' (F a ) t = , Ja " where A = lab z ep)i Now calculate the moment due to the Coulomb friction force for the i :h segment W-) i -* R b f b {F m ) i Accumulate the moments M τ = M τ + (M x ) t . Go to next segment. If d' is less than or equal to zero then this segment has lost contact with the bar. ' Set the yield flag to a negative value IYLDi = - YLDi Go to next segment. • Calculate the total moment due to "toggle" for all jaws in the collet Mψ " N Jaws M T • Calculate the total torque •rtotal — -rtoial ■ λΛtoial l T ~ J PT τ m T • Increment θ and repeat "toggle" calculations until Mψ al is a maximum. 27 For each increment of toggle angle, the total moment due to toggle is calculated for all jaws in the collet. If this total moment is not a maximum value, then the toggle angle is incremented and the toggle calculations re-executed until the maximum torque at toggle is determined. The total torque on the bar is the sum of the torque just prior to toggle and the total moment at toggle. The gripping strength of the collet is directly related to the total torque generated on the bar or tool extending through the collet. Thus, it should be understood that by varying the design parameters of the collet jaws and calculating total torque for each parameter change, an optimum set of parameters can be determined for maximizing collet gripping strength. For example, the graph provided below illustrates the effect on total torque, and thus gripping strength, of varying the jaw thickness for an ER25-8 collet configuration in increments of 0.01 inches. It was assumed that the default values of table 1 are acceptable and that the applied torque to nut on the collet holder was 350 in.- lbs. It was also assumed that, at most, only 10 jaws should be used in the design and that both extremes in bar or tool shaft diameter should be considered. As the graph illustrates, as jaw thickness increases, there is an increase in total torque (including toggle) up to a thickness of approximately 0.10 inches. Above this thickness, the total torque decreases. The effect of bar diameter on the total torque for a given jaw thickness is also indicated. Thus it should be readily understood how the analysis method and system according to the present invention can optimize jaw thickness to maximize gripping strength of the collet. 28 Jaw thickness, inches The graph provided below illustrates the effect of varying the number of jaws for an ER25-8 collet configuration assuming a jaw thickness of 0.07 inches. As the graph shows, generally as the number of jaws increases, there is an increase in total torque. However, between 10 and 14 jaws the total torque increased only 9%. Also, the graph illustrates to 15 jaws, and depending on the degree of resilient material on the jaw faces, the maximum number of jaws may decrease. Number of jaws 29 The above graphs illustrate the effect of varying only one parameter while assuming default or constant values for the other jaw parameters. The analysis method may also be executed to determine the optimum combination of parameters, such as thickness, number, and type material, for the jaws of a given configuration. The analysis method of the present invention is preferably configured in a system 50, generally depicted in Figs 4 and 5. Means 52 are provided for varying the jaw parameters so as to optimize particular parameters. Means 54 are provided for calculating the radial force of the collet jaws upon the bar, as explained above. Means 56 are for calculating torque developed on the bar just prior to toggle between the jaw and bar, also as explained above. Means 58 are provided for calculating the maximum moment at toggle condition and for calculating the total torque developed on the bar 60. In a preferred embodiment, the various means comprise applicable software 64 for executing the calculations with system 50 being a computer, generally 62. System 50 preferably comprises a library 66 of parameters known for the wide variety of conventional collet configurations, and library 68 for collet nut geometry parameters. Interface means, generally 52, are provided for allowing an analyst to generate DIN ASME standard data, define default values, input collet parameters, etc. As described above, the present invention includes a machine tool collet having gripping means for optimally holding a machine tool. The gripping means include jaws having at least one parameter optimized for maximizing the gripping strength of the collet. For example, collet 10 may have the number of jaws 16, or thickness of jaws 16, or material composition of jaws 16 optimized. Table 3 below is a list of machine tool collets having gripping means for optimally holding a tool according to the appended claims. For a given collet configuration, the collets listed comprise an optimized jaw thickness and 30 optimized number of jaws for maximizing the gripping strength of the collet. For example, for an ER25-8 collet configuration, a machine tool collet of the present invention includes ten jaws and a jaw thickness of 0.0745 inches. 31 32 Table 3 (continued) 33 34 35 36

Claims: 37 WHAT IS CLAIMED IS: 1. A method for optimizing the gripping strength of a collet of the type having a plurality of gripping jaws spaced around a common axis with resilient material between the gripping jaws, said method comprising the steps of: determining the radial force of the gripping jaws upon a bar disposed through the collet, the radial force being dependent upon an axial force applied to the collet; determining the torque developed on the bar from the radial forces of the gripping jaws just prior to a toggle condition between the gripping jaws and the bar; determining the total moment force generated between the gripping jaws and the bar at toggle condition for a given toggle angle; varying the toggle angle until the total moment force between the gripping jaws and bar at toggle condition reaches a maximum; and determining the total torque on the bar by summing the torque prior to toggle and the maximum total moment force at toggle condition; whereby an optimum design for a given collet configuration can be determined by varying parameters of the collet design and determining the total torque on the bar until a maximum torque is determined for a given collet configuration. 2. The method as in claim 1, further including the step of varying the thickness of the gripping jaws for a given collet to determine the thickness of gripping jaw generating maximum total torque for a given collet configuration. 3. The method as in claim 1, further including the step of varying the number of collet jaws to determine the number of gripping jaws generating maximum total torque for a given collet configuration. 38 4. The method as in claim 1, further including the step of varying the collet jaw material to determine the type material generating maximum total torque for a given collet configuration. 5. The method as in claim 1, further including the step of varying the thickness and number of gripping jaws, to determine the combination of thickness and number of gripping jaws generating maximum torque for a given collet configuration. 6. The method as in claim 1, further including the step of varying the collet jaw thickness, number of collet jaws, and collet jaw material to determine the combination of thickness, number, and material of gripping jaws generating maximum torque for a given collet configuration. 7. The method as in claim 1, wherein said determining the radial force of each gripping jaw includes determining the axial force imparted to the gripping jaws from engagement of the collet with a collet holder and collet nut and calculating the radial force from the applied axial force. 8. The method as in claim 7, wherein the axial force imparted by the collet nut and collet holder is calculated using known values for the parameters of the collet nut threaded surface. 9. The method as in claim 1, wherein said determining the torque just prior to toggle includes segmenting a gripping jaw into a predetermined number of segments along the length thereof, calculating the torque prior to toggle for each segment, summing the torques for all segments of the gripping jaw, and multiplying the summed torque for the segment by the number of gripping jaws in the collet. 10. The method as in claim 1, wherein said determining the total moment at toggle condition includes segmenting a gripping jaw into a predetermined number of segments along the length thereof, calculating the toggle 39 moment for each segment due to elastic deformation of the segment and the moment due to frictional force between the segment and bar, summing the toggle moments and frictional force moments for all segments along the gripping jaw, and multiplying the summed moment for all segments by the number of gripping jaws in the collet. 11. The method as in claim 1, further including predicting if slip will occur between the collet gripping jaws and bar for the particular geometry parameters of the gripping jaws. 12. A method for determining the optimum design for a given collet configuration for a collet having a plurality of gripping jaws spaced around a common axis with resilient material between the gripping jaws, said method comprising the steps of: calculating the total torque generated on a bar passing through the collet for a given collet configuration having predetermined design parameters, the total torque including torque generated just prior to toggle condition between the gripping jaws and the bar and the maximum moment force generated between the gripping jaws and the bar at toggle condition; and varying at least one design parameter of the collet and recalculating the total torque generated until the total torque generated reaches a maximum value for the given collet configuration. 13. The method as in claim 12, including varying the gripping jaw thickness to determine the optimum gripping jaw thickness for a given collet configuration. 14. The method as in claim 10, including varying the number of gripping jaws to determine the optimum number of gripping jaws for a given collet configuration. 15. The method as in claim 12, including varying the collet jaw material to determine the optimum jaw material for a given collet configuration. 40 16. A method for determining maximum collet torque for a collet configuration having predetermined design parameters, said method comprising: calculating radial force of the collet gripping jaws upon a bar disposed through the collet, the radial force being dependent upon an axial force imparted to the collet from nut torque between a collet nut and collet holder; calculating torque developed on the bar from the radial forces of the gripping jaws just prior to toggle between the gripping jaws and the bar; calculating moment due to toggle between the gripping jaws and the bar at a presumed toggle angle; incrementing the toggle angle and recalculating the total moment due to toggle in a repetitive loop until the total moment due to toggle reaches a maximum; and summing torque developed from the radial forces of the gripping jaws just prior to toggle and the maximum moment due to toggle. 17. A system for optimizing collet design parameters for a type collet having a plurality of gripping jaws spaced around a common axis with resilient material between the gripping jaws, whereby a bar passing through the collet is securely held by the gripping jaws, said system comprising: means for variably inputting design parameters of the collet of a given configuration; means for calculating the radial force of the collet gripping jaws upon the bar, said radial force being dependent upon an axial force applied to the collet through a collet holder and collet nut; means for calculating the torque developed on the bar from said radial forces of the gripping jaws just prior to toggle between the gripping jaws and the bar; means for calculating the maximum moment at toggle condition between the gripping jaws and the bar; and 41 means for summing the maximum moment at toggle condition and torque developed just prior to toggle to give a total torque developed on the bar by the gripping jaws for the inputted design parameters of the collet, whereby the total torque developed on the bar can be maximized by varying at least one design parameter through said design parameter inputting means so that an optimum set of design parameters can be determined. 18. The system as in claim 17, further comprising a computer, and said radial force calculating means, said torque calculating means, said moment calculating means, said toggle angle varying means, and said summing means comprising respective software executed by said computer. 19. The system as in claim 18, further comprising a library of known design parameters for various collet configurations for use by said software, said means for variably inputting design parameters being interfaced with said computer so that at least one of the design parameters can be varied. 20. The system as in claim 18, further comprising a library of known parameters for various collet nut geometries, said radial force calculating means using the collet nut geometry parameters to compute axial force imparted to the collet. 21. A collet torque slippage analysis method for calculating torque developed by a collet held by a collet nut and collet holder, comprising the steps of: calculating the maximum number of gripping jaws which can be disposed around the inner radius of the collet; defining the thread geometry of the collet mis¬ calculating the axial force per gripping jaw imparted from the collet nut being tightened onto the collet holder using the collet nut thread geometry; deriving the radial force per gripping jaw from the axial force per gripping jaw; 42 calculating the total torque due to frictional force between the gripping jaws and a bar disposed through the collet without slippage between the jaws and bar, the total torque due to frictional force being dependent upon the radial force per gripping jaw, the coefficient of friction, and the number of gripping jaws; determining if slip occurs between the gripping jaws and the bar based upon the geometry of the gripping jaws and the coefficient of friction between the gripping jaws and the bar; calculating the total torque developed between the gripping jaws and the bar just prior to a toggle condition between the gripping jaws and the bar, comprising the following steps: determining the engagement length between a gripping jaw and the bar; dividing the gripping jaw into a number of segments along the engagement length thereof; determining the torque per segment by summing the forces generated in the radial and tangential direction and summing moments about the center of the bar for each segment; determining the torque per gripping jaw by summing the torques per segment; and multiplying the torque per gripping jaw by the number of gripping jaws in the collet; calculating the total torque developed between the gripping jaws and bar at toggle condition between the gripping jaws and the bar at a first incremental toggle angle, comprising following steps; dividing a gripping jaw into a number of segments along the length thereof; calculating the moment generated for each segment due to the degree of elastic deformation of each section of the gripping jaw, and summing the moments for the segments along the gripping jaw; 43 calculating the moment generated for each segment due to the Coulomb friction force between the segment of gripping jaw and bar, and summing the moments for the segments along the gripping jaw; summing the total moment due to elastic deformation and total moment due to Coulomb friction force for the gripping jaw to give total torque at toggle for the gripping jaw; and calculating the total torque at toggle for all gripping jaws by multiplying the total torque at toggle for the one gripping jaw by the number of gripping jaws in the collet; repetitively incrementing the toggle angle and recalculating the total torque at toggle for all gripping jaws until the total torque at toggle reaches a maximum value; and summing the maximum total torque at toggle with the total torque developed just prior to toggle to give a value of total torque with toggle. 22. The method as in claim 21, further comprising the step of varying at least one of the design parameters of the collet and re-executing said program so as to determine the optimum value of the particular design parameter which maximizes the gripping strength of the collet, the gripping strength being directly proportional to the total torque toggle calculated by said program. 23. The method as in claim 22, comprising varying the number of collet gripping jaws in the collet. 24. The method as in claim 22, comprising varying the thickness of the collet gripping jaws. 25. The method as in claim 22, comprising varying the collet jaw material. 26. A machine tool collet which is actuated by engagement with a conical surface of a collet holder and collet nut, said collet comprising: 44 a resilient material for holding a plurality of gripping jaws in a desired spaced relation; gripping means for optimally holding a machine tool within said collet, said gripping means comprising a plurality of gripping jaws held by said resilient material in a desired longitudinally and angularly spaced relation about the longitudinal centerline axis through said collet, said gripping jaws having an exposed inner face parallel to the centerline axis whereby said plurality of gripping jaws define an inner radius of said collet, said gripping jaws having an angled exposed outer face whereby said plurality of gripping jaws defines a conical outer surface to engage the collet holder conical surface, and said gripping jaws comprising at least one structural characteristic optimized for maximizing the gripping strength of said collet. 27. The machine tool collet as in claim 26, wherein said gripping jaws have a thickness optimized to maximize the gripping strength of said collet. 28. The machine tool collet as in claim 26, wherein said gripping jaws are of a precise number optimized to maximize the gripping strength of said collet. 29. The machine tool collet as in claim 26, wherein said gripping jaws are formed of a material optimized to maximize the gripping strength of said collet. 30. The machine tool collet as in claim 26, wherein said gripping jaws have a thickness and are of a number optimized to maximize the gripping strength of said collet. 31. The machine tool collet as in claim 26, wherein said gripping jaws are formed of a plastic resilient material. 32. A machine tool collet comprising a plurality of independent longitudinally disposed gripping jaws molded in resilient material around the longitudinal centerline axis through said collet, said collet comprising an 45 angular space between each said gripping jaw optimized for maximizing the gripping strength of said collet, said resilient material occupying at least a portion of said angular space.

Inventors: KANAAN ROGER J, KANAAN, ROGER, J.
Applicants: JACOBS CHUCK MANUFACTURING CO, JACOBS CHUCK MANUFACTURING CO.
Application year: 1993
Main classification: B23B31
IPC codes: B23B31, G01N3