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Stifness Measurements as a Check of Machine-Tool Spindles
Stifness Measurements as a Check of Machine-Tool Spindles C.A. van Luttervelt (2) and H. R. Willemse - Submitted by A. J. Pekelharing (1)
In many cases poor performance of machine-tools is related with ill-functioning of the spindle. With the aid of a simple set-up w e made many diagrams of spindle deflection as a function of load on a variety of old and new machine-tools. In fact it is already an old method, which is however seldom used. T h e difference is that w e use inexoensive electronic equipment which produces continuous diagrams instead o f the point to point diagrams obtained by t h e classical mechanical means. This makes the method usefull for routine measurements and gives m o r e information. In the paper w e will show how those diagrams enabled us t o trace defects in spindle assembly such as insufficient preload of bearings, misalignment of bearings, defects in spindle drives etc. It seems that the method can serve as a valuable tool for the inspection of machine-tool spindles.
T h e quality of mechanical parts is to a great extend detemined by the accuracy of shape, dimensions, location and roughness. This quality is to a certain extend determined by pecularities of the metal removal processes and to a certain extend by the accuracy of the machine-tools on which the process is carried out. A certain interaction between process and machine-tool cannot be neglected. Especially in metal cutting operations like fine turning and fine milling we found this interaction to be a significant factor and in fact progress of our research work on the relation between variables of the cutting proces and product accuracy w a s serverly hampered by the unknown reaction of the machine tools on which the investigations were carried out. This forced us to do some work on the effects of machine tools on product quality. Several reconnaissance tasks were given to students of the technological university and technical colleges. Investigations were carried out in laboratories and in industry. In all cases we found machine-tool spindles t o be a major source of problems. We then measured error-motions according to the CIRP-Me unification document "Axes of rotation" [11. With that method we were able to differentiate machinetool spindles according to the magnitude of error motion in each of the three components: axial motion, radial motion and angular motion while running idle. In this form the method does not give any information about the magnitude of error motions while cutting takes place and it is not easy to find the origin of t h e error motions found and thus it is not well possible to decide on what has to be done to rectify a machine-tool showing a high level of error motions. This means that the method is not a good means of diagnosis to assist the maintenance crew. We tried to improve the method in two ways:
1) By trying to measure the error motions while a force is excerted on the spindle running at normal service speeds. 2 ) By using signal analysis techniques for the interpretation of the error motion signals. [2] T h i s has not yet lead to a practical solution. There are mainly two reasons: 1) We constructed several types of equipment to exert a force (if wanted consisting of a static and a dynamic component) on the running spindle with simultaneous measurement of the error motion components. Each of the constructions had several advantages and disavantages. The main problem is to find a construction that can easily be mounted on a variety of machine tools and can exert a force and measure the error motions in a location where cutting usually takes place. 2) The results of the application of digital signal analysis techniques did not yet convince the maintenance crew. Due to the instrumentation needed it is an expensive method. There are transportation problems and qualified people are able to use it. The results obtained were not yet in balance with those disadvantages. During this period we made various kinds o f measurements on quite a number of spindles in the laboratory but also in workshops in the university and in various companies. In general the condition of the machine-tool spindles proved to be poorer than expected, even on machine-tools which are inspected periodically by an experienced maintenance people. The classical Schlesinger type checks on run out of spindles do not guaranty that spindles with a run-out within acceptable limits while rotated slowly by hand perform well at service speeds and under service load.
Annals of the CIRP Vol. 31/1/1982
There are several causes: 1 ) According to accepted theories bearings of machinetool spindles should have a certain preload. This preload, determined during assembly of the machines, disappears after a certain period of service. Normally maintenance personnel do not have the means and the knowledge to restore the proper amount of preload. Instructions on adjustment of preload are in most cases not provided by the machine-tool builder or are far from being usefull. Even more disapointing is the fact that on quite a significant number of completely new or rebuilt machine-tools there was no preload and in some cases even considerable play. 2) Error motions are to a certain extend caused by basic errors of the shape of the bearings. The magnitude of those error motions is not importantly influenced by the rotational speed of the spindle. 3) Another group of error motions is caused by dynamic errors of the bearings and is thus influenced by the rotational speed. 4) A third group of error motions is caused by the spindle drive (electric motor, gears, belts, etc.1. T h i s group of error motions depends not only on the rotational speed of the spindle but also on the speed of other elements in the drive. The classical Schlesinger type of check only considers cause 2 and neglects all the others. Cause no. 1 is more important at relatively light loads than at very heavy loads, especially when the light load is variable. Causes 3 and 4 usually become more important when speeds increase. (our work on a diagnoses system by use of application of signal analysis techniques is mainly in relation with causes 3 and 4). After some time we decided to concentrate on cause 1 "insufficient preload" for the following reasons: a) Insufficient preload occurs rather frequently. b) There is not a quick and reliable method available t o check preload. c) Only on sufficiently preloaded spindles one can expect that measuring error motions while turning idle provides information of interrest to the behaviour of the spindle under load. Consequently, we think that for routine assesment of spindle accuracy the following steps should be taken: 1) Assesment of spindle run out according to I S 0 3070 [ 6 ] or similar standards. 2) Assesment of the static stifness of the spindle assembly according to the method described underneath. 3 ) Assesment of error motions of the spindle at service speeds according to 111. It goes without saying that when the results of a step gives unsatisfactory results the next step needs not t e be taken. T h e greatest problem lays in the determination of the limiting values. Where is the good/no good boundary ? T o a certain extend this depends on the kind of work that will be done on the machine-tool considered. Good starting values could be obtained by considering a number of similar new machine-tools which show satisfying performance. In this way an amount of experience can be collected which can form the base for a standard. A problem is that the results of the measurements depend to a certain extend on the measuring conditions. This means that in order to obtain comparable results the measuring conditions should be standardized as much as possible, otherwise we will never be able to agree on limiting values which could for instance be quoted in buying contracts [ 3 , 4 1 .
25 1
Description of the method.
This lead to the conclusion:
Stifness of a construction is defined as the force needed to reach a certain amount of deflection. In this case we consider the deflection of the spindle mounted in its bearings with respect to the spindle housing. Stifness of this spindle assemby can be measured in any direction. Up to now we concentrated on the radial direction. When the spindle is horizontal like on a lathe, we measured the stifness in the vertical direction. When the spindle is vertical, measurements are made in de X- or Y-direction with usually little differences in results. When the spindle is mounted in a quill we usually measured with respect to the housing, with amazingly dissatisfying results. It was shown that much more information can be obtained from a graph showing deflection as function of applied force than by just quoting the total deflection at a specified force. This latter method is extremely sensitive for the position where the force is applied and where the deflection i; measured and the value obtained provides no information about what the causes are. In fact the shape of the graph proved to be more helpful1 than the numerical values. In Order to obtain that graph quickly the following method is used: A bar is provided with straingauges s, see fig. 1. One end of the bar is placed inside the spindel nose. The other end is fixed on the tool post of the lathe OK on the table of the milling machine. The signal of the straingauges depends only on the force exerted at A and the fixed length ll. Length l2 is of no importance. An inductive displacement transducer D (e.g. a Tesatast) is fixed on the housing (OK the quill) of the spindle with the feeler on the spindle in A.
Deviation from linearity is a better measure for the quality of a spindle assembly than the total deflection at a specified force.
li=%-"l
*I
spindle a n d bearing w i l h play._,'' . . _
Fig. 3 . Examples of the effect of spindle and bearings on the shape of the deflection curves. The vertical part is seldom purely vertical since there is nearly always some friction and elastic deformation of e.g. oil sealings. With vertical spindles there is no effect of weight of the spindle on the radial stifness graph, but also in that case no OK insufficient preload causes a deviation from linearity in the graph deflection as a function of force. Example I. Small lathe.
toolpost
U
Fig. 1. Principle of the set-up on a lathe. Results. Let us first consider what can be expected. Therefore we start with a horizontal spindle with play in the front bearing, on which a load is applied in the vertical direction, see fig. 2 .
In this section we will consider several lathes of exactly the same construction. Fig. 4 shows a good one. There is no play in the bearings, the four curves taken with the spindle rotated Over 90' are nearly straight lines and identical. FOK reasons of clarity the zero points of the lines in the figure are shifted in the vertical direction. Also there is hardly any difference between the cold machine and after one hour running idle at maximum spindle speed. (This lathe is the only one in which we found that the deflection under load after running idle for one hour is more than the deflection measured with a cold machine. We are not able to explain this phenomenon.)
-Cdd
Fig. 2. Principle of the stifness measurement of a horizontal spindle and ChaKaCteKiStiC curve when play is present. In interval 1 for forces smaller than the component F of the weight of the spindle at the position of loadilg A, the displacement is caused by the decrease of the deflection of the bearing which was due to the weight of the spindle. A certain force P compensates the weight of the spindle and at that'force the spindle is lifted until it contacts the upper part of the bearing, causing a vertical part 2 of the curve. When the force increases further, the displacement increases also. In principle part 1 and part 3 of the curve are symmetrical. The shape of these parts of the curve depends largely on the deflection characteristics of the bearing (causing a non linear component) and of the spindle (causing a lineair component), as shown in fig. 3.
Depending on the construction and on the location where the force is exerted and the deflection is measured the lineair characteristic is more or less distorted by the non-lineair component. The main point is that the vertical part 2 of the curve should not be present, since that shows absence of preload. The more preload there is, the more lineair the curve should be.
252
0
--- warm
FCN]
-
333
Fig. 4 . Example of a small lathe with good preload of the bearings, measured cold and after one hour running idle. Fig.5 shows another machine. Here the bearings have play as is demonstrated by the significant increase of the deflection at a certain load. This becomes less after the machine has warmed-up by running idle for one hour. An example of a terribly poor machine (of which we found several) is shown in fig. 6. Even after warmingup play in the bearings remains present. The four lines are significantly different, which was found to be caused by misalignment of the rearbearing. It was possible to rectify the situation by adjustment of the bearings as fig. 7 shows. This figure shows also intermediate stages of rotation of the nut which determines the preload of the bearing. The maximum amount of preload was determined by the best stifness (least inclination of the curve) found on similar lathes which gave satisfactory results in practice.
c
b
F
"1-
333
Fig. 7. Variation of the stifness of the small lathe of fig. 6 as a result of a stepwise increase of preload.
Fig. 5. Example of a small lathe with insufficient preload of the bearings, measured cold and warm. Practical example 11. Machining center. T h e method was applied t o a brand new vertical machining center with the result shown in fig. 8. Again four lines of deflections versus load were made with the spindle turned over 90' in between. In the upper part of fig. 8 the machine tool was cold. The lower part shows results of the same machine but now after one hour running idle with maximum spindle speed. The temperature in the spindle housing increased from 22,5O c t o 380 c. At the extremes of the lines tangents can be drawn. The inclination of those lines is mainly determined by the stiffness of the spindle and t o a little extend by the stifness of the bearings. Also at zero load tangents can be drawn. Here the inclination is mainly determined by the bearings and only little by the spindle itself. I n this way the stifness values of table I were obtained :
1
---warm
0
F [NJ
-
333
Fig. 6. Example of a small lathe with play, measured cold and after one hour running idle. Fig. 9. A good machining centre of similar design a s in fig. 8 measured cold.
253
Another machining centre of the same manufacturer and of similar design showed the curve in fig. 9. Here the stifness is about 330 N/pm or about double the value for high forces in table I and about six times the value for small forces. The total deflection of the good machine for F from -500 to 500 N is 3 pm(see fig. 9). For the bad machine the values of table I1 can be derived from fig. 8: Deflections in bm Temp. = 22.5O C
F F
= -500/0 = 0/500
1"
5,3 6.5
-ii;B
F = -500/500 Temp. = 38'
'0
C
F -5OO/ [N] F = 0/500 F = -500/500
489
90° 5,0 581 11,l
.
5,O
180° 5,o 7,4
12.4 5,4 5,o
270' 4,5 6,O
1015
av 580 6,2
11.4
4,0 4,4
The strange shape of the curve with the force on the nose was found to be caused by play in the axial spindle bearing. The average stifness on the tool nose is about 113N/pm and at 120 mm from the tool nose 6 2 N/pm. The deflections were measured relative to the quill. In addition to this came the displacement of the quill relative to the housing. The values depend on the location of measurement and did not reproduce very well, but were something between 14 and 18 rrm for 1000 N force in the direction of measurement. On the basis of this investigation adjustments were made with the result that the measured deflections became roughly half the original values.
4.8
Tabel 111: Deflection under load of the spindle of a bad new machining centre at two different temperatures and four positions of the spindle. Due to the great deflection under load of the spindle the machine was not able to machine the test piece according to NAS 979 171 with acceptable accuracy even although the positioning accuracy was reasonably good.
Conclusion
W e think that the method described here may become a helpful1 tool for acceptance testing of new or rebuilt machine-tools and for periodical inspection during their service live. The method is East and needs no too expensive or sensitive equipment. After a short period of instruction people are able to use the method and to distinguish those spindles which need attention. More qualified personal can use the results to adjust unsatisfactory spindles. A n important question remains to be solved: How many times can the preload be adjusted before bearings should be changed 7 If the main reason for the disappearance of preload is wear, one can not expect that the wear is completely uniformely distributed over the runways. After some wear the magnitude of the error motions is expected to increase and this may lead to the need to change bearings. No information about this problem is available yet. Our findings confirm those of Pruvot, who showed that designing a bearing assembly for machine-tool spindles is not so easy as many people think [51. He designed elegant new solutions. for the machine-tools of the future. However, the majority of the production of next ten to fifteen years will be carried out on machinetools which are already installed today. Every method which can contribute to the good performance of these machine-tools should be considered seriously.
1000
Literature references [11
CIRP-STC "Me" unification document "Axes of rotation", Annals of the CIRP Vol 25/2/1976 pp. 545-564. [2] C.A. van Luttervelt, Op weg naar een diagnose de bepaling systeem voor gereedschapswerktuigen van de nauwkeurigheid van hoofdspillen. Metaalbewerking 46 (1980) no. 2 pp 35-42: no. 3 pp 65-70. Technology of Machine Tools t31 Machine Tool Task Force report, October 1980. Summarv in American Machinist October 1980.- DO. 105-12-7. M. van Krimpen and J.L. Remmerswaal, Acceptance test for machine-tools a must? Discussion paper CIRP-STC "0" August 1981. F.C. Pruvot, High speed bearings for machine tool spindles. Annals of the CIRP, Vol. 29/1/1980 pp. 293. IS0 3070 Acceptance conditions for boring and milling machines. National Aerospace Standard NAS series metal 979. Uniform cutting tests cutting equipment specifications.
-
__
Fig. 11. Vertical spindle of a milling machine. Practical example 111: Vertical milling machine It is wellknown that milling machines with a horizontal and a vertical spindle usually show a big difference in construction and performance of the two spindles. The difference becomes even more important if the vertical spindle is mounted in a quill and if the state of maintance is poor. This is illustrated with the following graphs obtained from measurements on a five years old milling machine. Fig. 10 shows the deflection force diagram of the horizontal spindle in the horizontal and vertical direction. In this case the force is applied and the deflection is measured at a position which is closer to where normally the cutting forces are located. The average stifness is ca. 120 N/ym. Fig. 11 shows a similar diagram for the vertical spindle. Two cases are shown: one with the force on the spindle nose and one with the force at a more realistic position.
254
-
In many cases poor performance of machine-tools is related with ill-functioning of the spindle. With the aid of a simple set-up w e made many diagrams of spindle deflection as a function of load on a variety of old and new machine-tools. In fact it is already an old method, which is however seldom used. T h e difference is that w e use inexoensive electronic equipment which produces continuous diagrams instead o f the point to point diagrams obtained by t h e classical mechanical means. This makes the method usefull for routine measurements and gives m o r e information. In the paper w e will show how those diagrams enabled us t o trace defects in spindle assembly such as insufficient preload of bearings, misalignment of bearings, defects in spindle drives etc. It seems that the method can serve as a valuable tool for the inspection of machine-tool spindles.
T h e quality of mechanical parts is to a great extend detemined by the accuracy of shape, dimensions, location and roughness. This quality is to a certain extend determined by pecularities of the metal removal processes and to a certain extend by the accuracy of the machine-tools on which the process is carried out. A certain interaction between process and machine-tool cannot be neglected. Especially in metal cutting operations like fine turning and fine milling we found this interaction to be a significant factor and in fact progress of our research work on the relation between variables of the cutting proces and product accuracy w a s serverly hampered by the unknown reaction of the machine tools on which the investigations were carried out. This forced us to do some work on the effects of machine tools on product quality. Several reconnaissance tasks were given to students of the technological university and technical colleges. Investigations were carried out in laboratories and in industry. In all cases we found machine-tool spindles t o be a major source of problems. We then measured error-motions according to the CIRP-Me unification document "Axes of rotation" [11. With that method we were able to differentiate machinetool spindles according to the magnitude of error motion in each of the three components: axial motion, radial motion and angular motion while running idle. In this form the method does not give any information about the magnitude of error motions while cutting takes place and it is not easy to find the origin of t h e error motions found and thus it is not well possible to decide on what has to be done to rectify a machine-tool showing a high level of error motions. This means that the method is not a good means of diagnosis to assist the maintenance crew. We tried to improve the method in two ways:
1) By trying to measure the error motions while a force is excerted on the spindle running at normal service speeds. 2 ) By using signal analysis techniques for the interpretation of the error motion signals. [2] T h i s has not yet lead to a practical solution. There are mainly two reasons: 1) We constructed several types of equipment to exert a force (if wanted consisting of a static and a dynamic component) on the running spindle with simultaneous measurement of the error motion components. Each of the constructions had several advantages and disavantages. The main problem is to find a construction that can easily be mounted on a variety of machine tools and can exert a force and measure the error motions in a location where cutting usually takes place. 2) The results of the application of digital signal analysis techniques did not yet convince the maintenance crew. Due to the instrumentation needed it is an expensive method. There are transportation problems and qualified people are able to use it. The results obtained were not yet in balance with those disadvantages. During this period we made various kinds o f measurements on quite a number of spindles in the laboratory but also in workshops in the university and in various companies. In general the condition of the machine-tool spindles proved to be poorer than expected, even on machine-tools which are inspected periodically by an experienced maintenance people. The classical Schlesinger type checks on run out of spindles do not guaranty that spindles with a run-out within acceptable limits while rotated slowly by hand perform well at service speeds and under service load.
Annals of the CIRP Vol. 31/1/1982
There are several causes: 1 ) According to accepted theories bearings of machinetool spindles should have a certain preload. This preload, determined during assembly of the machines, disappears after a certain period of service. Normally maintenance personnel do not have the means and the knowledge to restore the proper amount of preload. Instructions on adjustment of preload are in most cases not provided by the machine-tool builder or are far from being usefull. Even more disapointing is the fact that on quite a significant number of completely new or rebuilt machine-tools there was no preload and in some cases even considerable play. 2) Error motions are to a certain extend caused by basic errors of the shape of the bearings. The magnitude of those error motions is not importantly influenced by the rotational speed of the spindle. 3) Another group of error motions is caused by dynamic errors of the bearings and is thus influenced by the rotational speed. 4) A third group of error motions is caused by the spindle drive (electric motor, gears, belts, etc.1. T h i s group of error motions depends not only on the rotational speed of the spindle but also on the speed of other elements in the drive. The classical Schlesinger type of check only considers cause 2 and neglects all the others. Cause no. 1 is more important at relatively light loads than at very heavy loads, especially when the light load is variable. Causes 3 and 4 usually become more important when speeds increase. (our work on a diagnoses system by use of application of signal analysis techniques is mainly in relation with causes 3 and 4). After some time we decided to concentrate on cause 1 "insufficient preload" for the following reasons: a) Insufficient preload occurs rather frequently. b) There is not a quick and reliable method available t o check preload. c) Only on sufficiently preloaded spindles one can expect that measuring error motions while turning idle provides information of interrest to the behaviour of the spindle under load. Consequently, we think that for routine assesment of spindle accuracy the following steps should be taken: 1) Assesment of spindle run out according to I S 0 3070 [ 6 ] or similar standards. 2) Assesment of the static stifness of the spindle assembly according to the method described underneath. 3 ) Assesment of error motions of the spindle at service speeds according to 111. It goes without saying that when the results of a step gives unsatisfactory results the next step needs not t e be taken. T h e greatest problem lays in the determination of the limiting values. Where is the good/no good boundary ? T o a certain extend this depends on the kind of work that will be done on the machine-tool considered. Good starting values could be obtained by considering a number of similar new machine-tools which show satisfying performance. In this way an amount of experience can be collected which can form the base for a standard. A problem is that the results of the measurements depend to a certain extend on the measuring conditions. This means that in order to obtain comparable results the measuring conditions should be standardized as much as possible, otherwise we will never be able to agree on limiting values which could for instance be quoted in buying contracts [ 3 , 4 1 .
25 1
Description of the method.
This lead to the conclusion:
Stifness of a construction is defined as the force needed to reach a certain amount of deflection. In this case we consider the deflection of the spindle mounted in its bearings with respect to the spindle housing. Stifness of this spindle assemby can be measured in any direction. Up to now we concentrated on the radial direction. When the spindle is horizontal like on a lathe, we measured the stifness in the vertical direction. When the spindle is vertical, measurements are made in de X- or Y-direction with usually little differences in results. When the spindle is mounted in a quill we usually measured with respect to the housing, with amazingly dissatisfying results. It was shown that much more information can be obtained from a graph showing deflection as function of applied force than by just quoting the total deflection at a specified force. This latter method is extremely sensitive for the position where the force is applied and where the deflection i; measured and the value obtained provides no information about what the causes are. In fact the shape of the graph proved to be more helpful1 than the numerical values. In Order to obtain that graph quickly the following method is used: A bar is provided with straingauges s, see fig. 1. One end of the bar is placed inside the spindel nose. The other end is fixed on the tool post of the lathe OK on the table of the milling machine. The signal of the straingauges depends only on the force exerted at A and the fixed length ll. Length l2 is of no importance. An inductive displacement transducer D (e.g. a Tesatast) is fixed on the housing (OK the quill) of the spindle with the feeler on the spindle in A.
Deviation from linearity is a better measure for the quality of a spindle assembly than the total deflection at a specified force.
li=%-"l
*I
spindle a n d bearing w i l h play._,'' . . _
Fig. 3 . Examples of the effect of spindle and bearings on the shape of the deflection curves. The vertical part is seldom purely vertical since there is nearly always some friction and elastic deformation of e.g. oil sealings. With vertical spindles there is no effect of weight of the spindle on the radial stifness graph, but also in that case no OK insufficient preload causes a deviation from linearity in the graph deflection as a function of force. Example I. Small lathe.
toolpost
U
Fig. 1. Principle of the set-up on a lathe. Results. Let us first consider what can be expected. Therefore we start with a horizontal spindle with play in the front bearing, on which a load is applied in the vertical direction, see fig. 2 .
In this section we will consider several lathes of exactly the same construction. Fig. 4 shows a good one. There is no play in the bearings, the four curves taken with the spindle rotated Over 90' are nearly straight lines and identical. FOK reasons of clarity the zero points of the lines in the figure are shifted in the vertical direction. Also there is hardly any difference between the cold machine and after one hour running idle at maximum spindle speed. (This lathe is the only one in which we found that the deflection under load after running idle for one hour is more than the deflection measured with a cold machine. We are not able to explain this phenomenon.)
-Cdd
Fig. 2. Principle of the stifness measurement of a horizontal spindle and ChaKaCteKiStiC curve when play is present. In interval 1 for forces smaller than the component F of the weight of the spindle at the position of loadilg A, the displacement is caused by the decrease of the deflection of the bearing which was due to the weight of the spindle. A certain force P compensates the weight of the spindle and at that'force the spindle is lifted until it contacts the upper part of the bearing, causing a vertical part 2 of the curve. When the force increases further, the displacement increases also. In principle part 1 and part 3 of the curve are symmetrical. The shape of these parts of the curve depends largely on the deflection characteristics of the bearing (causing a non linear component) and of the spindle (causing a lineair component), as shown in fig. 3.
Depending on the construction and on the location where the force is exerted and the deflection is measured the lineair characteristic is more or less distorted by the non-lineair component. The main point is that the vertical part 2 of the curve should not be present, since that shows absence of preload. The more preload there is, the more lineair the curve should be.
252
0
--- warm
FCN]
-
333
Fig. 4 . Example of a small lathe with good preload of the bearings, measured cold and after one hour running idle. Fig.5 shows another machine. Here the bearings have play as is demonstrated by the significant increase of the deflection at a certain load. This becomes less after the machine has warmed-up by running idle for one hour. An example of a terribly poor machine (of which we found several) is shown in fig. 6. Even after warmingup play in the bearings remains present. The four lines are significantly different, which was found to be caused by misalignment of the rearbearing. It was possible to rectify the situation by adjustment of the bearings as fig. 7 shows. This figure shows also intermediate stages of rotation of the nut which determines the preload of the bearing. The maximum amount of preload was determined by the best stifness (least inclination of the curve) found on similar lathes which gave satisfactory results in practice.
c
b
F
"1-
333
Fig. 7. Variation of the stifness of the small lathe of fig. 6 as a result of a stepwise increase of preload.
Fig. 5. Example of a small lathe with insufficient preload of the bearings, measured cold and warm. Practical example 11. Machining center. T h e method was applied t o a brand new vertical machining center with the result shown in fig. 8. Again four lines of deflections versus load were made with the spindle turned over 90' in between. In the upper part of fig. 8 the machine tool was cold. The lower part shows results of the same machine but now after one hour running idle with maximum spindle speed. The temperature in the spindle housing increased from 22,5O c t o 380 c. At the extremes of the lines tangents can be drawn. The inclination of those lines is mainly determined by the stiffness of the spindle and t o a little extend by the stifness of the bearings. Also at zero load tangents can be drawn. Here the inclination is mainly determined by the bearings and only little by the spindle itself. I n this way the stifness values of table I were obtained :
1
---warm
0
F [NJ
-
333
Fig. 6. Example of a small lathe with play, measured cold and after one hour running idle. Fig. 9. A good machining centre of similar design a s in fig. 8 measured cold.
253
Another machining centre of the same manufacturer and of similar design showed the curve in fig. 9. Here the stifness is about 330 N/pm or about double the value for high forces in table I and about six times the value for small forces. The total deflection of the good machine for F from -500 to 500 N is 3 pm(see fig. 9). For the bad machine the values of table I1 can be derived from fig. 8: Deflections in bm Temp. = 22.5O C
F F
= -500/0 = 0/500
1"
5,3 6.5
-ii;B
F = -500/500 Temp. = 38'
'0
C
F -5OO/ [N] F = 0/500 F = -500/500
489
90° 5,0 581 11,l
.
5,O
180° 5,o 7,4
12.4 5,4 5,o
270' 4,5 6,O
1015
av 580 6,2
11.4
4,0 4,4
The strange shape of the curve with the force on the nose was found to be caused by play in the axial spindle bearing. The average stifness on the tool nose is about 113N/pm and at 120 mm from the tool nose 6 2 N/pm. The deflections were measured relative to the quill. In addition to this came the displacement of the quill relative to the housing. The values depend on the location of measurement and did not reproduce very well, but were something between 14 and 18 rrm for 1000 N force in the direction of measurement. On the basis of this investigation adjustments were made with the result that the measured deflections became roughly half the original values.
4.8
Tabel 111: Deflection under load of the spindle of a bad new machining centre at two different temperatures and four positions of the spindle. Due to the great deflection under load of the spindle the machine was not able to machine the test piece according to NAS 979 171 with acceptable accuracy even although the positioning accuracy was reasonably good.
Conclusion
W e think that the method described here may become a helpful1 tool for acceptance testing of new or rebuilt machine-tools and for periodical inspection during their service live. The method is East and needs no too expensive or sensitive equipment. After a short period of instruction people are able to use the method and to distinguish those spindles which need attention. More qualified personal can use the results to adjust unsatisfactory spindles. A n important question remains to be solved: How many times can the preload be adjusted before bearings should be changed 7 If the main reason for the disappearance of preload is wear, one can not expect that the wear is completely uniformely distributed over the runways. After some wear the magnitude of the error motions is expected to increase and this may lead to the need to change bearings. No information about this problem is available yet. Our findings confirm those of Pruvot, who showed that designing a bearing assembly for machine-tool spindles is not so easy as many people think [51. He designed elegant new solutions. for the machine-tools of the future. However, the majority of the production of next ten to fifteen years will be carried out on machinetools which are already installed today. Every method which can contribute to the good performance of these machine-tools should be considered seriously.
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Literature references [11
CIRP-STC "Me" unification document "Axes of rotation", Annals of the CIRP Vol 25/2/1976 pp. 545-564. [2] C.A. van Luttervelt, Op weg naar een diagnose de bepaling systeem voor gereedschapswerktuigen van de nauwkeurigheid van hoofdspillen. Metaalbewerking 46 (1980) no. 2 pp 35-42: no. 3 pp 65-70. Technology of Machine Tools t31 Machine Tool Task Force report, October 1980. Summarv in American Machinist October 1980.- DO. 105-12-7. M. van Krimpen and J.L. Remmerswaal, Acceptance test for machine-tools a must? Discussion paper CIRP-STC "0" August 1981. F.C. Pruvot, High speed bearings for machine tool spindles. Annals of the CIRP, Vol. 29/1/1980 pp. 293. IS0 3070 Acceptance conditions for boring and milling machines. National Aerospace Standard NAS series metal 979. Uniform cutting tests cutting equipment specifications.
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Fig. 11. Vertical spindle of a milling machine. Practical example 111: Vertical milling machine It is wellknown that milling machines with a horizontal and a vertical spindle usually show a big difference in construction and performance of the two spindles. The difference becomes even more important if the vertical spindle is mounted in a quill and if the state of maintance is poor. This is illustrated with the following graphs obtained from measurements on a five years old milling machine. Fig. 10 shows the deflection force diagram of the horizontal spindle in the horizontal and vertical direction. In this case the force is applied and the deflection is measured at a position which is closer to where normally the cutting forces are located. The average stifness is ca. 120 N/ym. Fig. 11 shows a similar diagram for the vertical spindle. Two cases are shown: one with the force on the spindle nose and one with the force at a more realistic position.
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