Disclosure of Invention
The invention aims to provide a three-dimensional grinding force calculation method for a planetary roller screw internal thread grinding process, which can realize accurate calculation and prediction of grinding force in a grinding area, can be applied to the prediction of the grinding force of different grinding parameters and feed times, and has important significance for grinding mechanism research, stable control of the grinding force and improvement of grinding quality.
The practical problems solved by the invention are as follows: according to the grinding force prediction formula, grinding force numerical values under different grinding parameters (grinding wheel rotating speed, workpiece feeding speed and grinding depth) and feeding times can be obtained through calculation, the influence of factors such as vibration and grinding heat on the internal thread machining quality caused by the change of the grinding force in the internal thread grinding process of the planetary roller screw is reduced by controlling the change of the grinding force, the internal thread grinding precision of the planetary roller screw is improved, and the grinding force prediction formula has guiding significance for the control of the grinding force in the grinding process and the analysis of the internal thread grinding mechanism.
Starting from the research and control of the thread grinding process, a grinding process analysis model which accords with the high-speed grinding characteristics of the internal thread is established, the internal thread grinding process is theoretically analyzed based on a generalized finite element model analysis method, the internal thread grinding process is simplified into a plane grinding process, and a plane grinding process grinding force formula of primary feed is obtained. Based on a single abrasive grain grinding force model, according to the characteristics of the internal thread grinding process, considering the removal volume change of the internal thread grinding process converted from the two-dimensional plane grinding process and the difference between the ith removal volume and the first removal volume, and converting the grinding force of the two-dimensional plane grinding process into the grinding force of the internal thread grinding process by introducing a correction coefficient to obtain the grinding force prediction formula of the ith feed.
The technical scheme of the invention is as follows: a grinding force prediction method in a planetary roller screw internal thread grinding process comprises the following steps:
step 1: the grinding process is simplified: the grinding process of the internal thread of the planetary roller screw is simplified into a two-dimensional plane grinding process;
step 2: correcting the grinding force: the conversion from two-dimensional plane grinding force to three-dimensional internal thread grinding force is realized by introducing an internal thread grinding correction coefficient, and a three-dimensional grinding force formula in the internal thread grinding process is obtained;
and step 3: calculating the grinding correction coefficient of the internal thread, obtaining grinding force data through orthogonal finite element simulation calculation, calculating the correction coefficient of the internal thread grinding by using a control variable method, obtaining a grinding force prediction formula in the internal thread grinding process, and finally predicting the grinding force in the internal thread grinding process.
The further technical scheme of the invention is as follows: the specific content in the step 2 is as follows: the three-dimensional grinding force formula is as follows:
Fa,i=Fn,itanλ (3)
in the formula, vsIs the rotational speed v of the grinding wheelwIs the feed speed, apIs the grinding depth; fiTotal grinding force for ith feed, Fn,i、Ft,iAnd Fa,iNormal grinding force, tangential grinding force and axial grinding force of the ith feed are respectively; k is a radical ofnAnd ktNormal and radial grinding force coefficients, respectively, of positive value, b1、b2、b3、c1、c2、c3Is an index related to the rotation speed, feed speed and grinding depth of the grinding wheel, the numerical values are positive numbers, lambda is a lead angle, K1,iThe correlation coefficient of the ratio of the ith feed removal volume to the 1 st feed removal volume, K2,iRemoving a correlation coefficient of a volume ratio of the ith feed internal thread grinding to the plane grinding;
the further technical scheme of the invention is as follows: the grinding force prediction formula in the step 3 is as follows:
wherein v issIs the rotational speed of the grinding wheel, vwFor the feed rate, apTo grind depth, knAnd ktIs a coefficient of grinding force, which is constant, K1,iThe correlation coefficient of the ratio of the ith feed removal volume to the 1 st feed removal volume, K2,iAnd lambda is a thread lead angle, and is a correlation coefficient of the volume ratio of the ith feed internal thread grinding to the planar grinding removal.
The further technical scheme of the invention is as follows: the solving process of the grinding force prediction formula is as follows:
according to the geometrical relationship of internal thread grinding processing, the following results are obtained:
wherein R is the radius of the grinding wheel is a known quantity, R is the inner diameter of the workpiece to be processed is a known quantity, and thetaRFor the angle theta corresponding to the arc length of the grinding wheel cutting into the workpiecerFor the arc length of work to be cut intoD is the length of the line segment corresponding to the arc length, apA known amount for the total grinding depth; according to the formula, the angle theta corresponding to the arc length of the grinding wheel cut into the workpiece is obtainedRAngle theta corresponding to arc length of cut into workpiecerThe length d of the line segment corresponding to the arc length;
from the geometric relationship, the area S can be obtainedRAnd SrIn which S isRAnd SrThe areas respectively comprise the arc length of the grinding wheel cut-in workpiece and the cut-in arc length of the workpiece and a line segment d, and the difference value of the two areas is the i-th axial section removal area delta Si(i=1, 2,3……)。
Then the ith removal volume:
the 1 st removal volume was:
obtaining:
flat grinding for the ith removal of volume:
Vi=ap×d×iap (13)
obtaining:
and finally obtaining a planetary roller screw internal thread grinding force prediction formula.
Effects of the invention
The invention has the technical effects that: 1) because the internal thread machining precision requirement of the planetary roller screw nut is high, the grinding process can be completed only by feeding for many times, and the defect that the conventional internal thread grinding force formula cannot calculate the grinding force of feeding for many times is overcome, the grinding force prediction formula obtained by the invention can accurately calculate the grinding force of the internal thread grinding process of feeding for many times, and a specific grinding force value can be calculated only by setting specific grinding wheel and workpiece size parameters, the grinding wheel rotating speed, the feeding speed, the grinding depth and the feeding times; 2) grinding parameters such as the influence rule of the grinding wheel rotating speed, the feeding speed and the grinding depth on the grinding force can be explored according to the grinding force prediction formula obtained by the invention, and an internal thread grinding mechanism is further explored.
Detailed Description
In the description of the present invention, it is to be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", and the like, indicate orientations and positional relationships based on those shown in the drawings, and are used only for convenience of description and simplicity of description, and do not indicate or imply that the device or element being referred to must have a particular orientation, be constructed and operated in a particular orientation, and thus, should not be considered as limiting the present invention.
Referring to fig. 1 to 6, the grinding force calculating method for the internal thread grinding process of the present invention comprises the steps of:
s1, simplifying the internal thread grinding process into a two-dimensional plane grinding process. FIG. 2 is a schematic diagram showing the decomposition of the grinding force during the grinding of internal threads, which can be decomposed into three components perpendicular to each other, i.e., a normal grinding force F along the radial direction of the grinding wheelnTangential grinding force F in the tangential direction of the grinding wheeltAnd axial grinding force F in the direction of the axis of rotation of the grinding wheela. The grinding process of the internal thread is complex, and the grinding process of the internal thread of the planetary roller screw can be simplified into a two-dimensional plane grinding process to obtain a two-dimensional plane grinding force formula in consideration of the fact that in the actual grinding process, the lead angle of the planetary roller screw is small and the axial force in the grinding process is small. The grinding force of two-dimensional plane grinding is composed of two parts, normal grinding force FnAnd tangential grinding force Ft,
Wherein v issIs the rotational speed of the grinding wheel, vwFor the feed rate, apTo grind depth, knAnd ktTwo grinding force coefficients, which are constants.
FIG. 3 is a schematic diagram of the relationship between axial force and normal force during the internal thread grinding process, which can be obtained from the geometric relationship between axial force and normal force:
tanλ=L/S=L/πdw (2)
wherein, lambda is a lead angle of the screw thread, L is a lead of the nut, S is an inner circumference of the nut, and dwIs the inner diameter of the nut.
The axial force is:
Fa=Fntanλ (3)
the actual grinding force in the grinding process is
And S2, converting the grinding force in the two-dimensional plane grinding process into the grinding force in the internal thread grinding process by introducing a correction coefficient. Because only the removal area is calculated in the two-dimensional plane grinding process, the removal volume in the real grinding process cannot be calculated, the influence of the removal volume on the grinding force and the change of the removal volume caused by different feed times are ignored, and only the rotating speed v of the grinding wheel is consideredsA feeding speed vwAnd grinding depth apInfluence on grinding force. According to a plane grinding force formula, comparing the characteristics of the internal thread grinding process and the two-dimensional plane grinding process, considering the removal volume change converted from the two-dimensional plane grinding process to the internal thread grinding process, and introducing a grinding depth coefficient K by removing the volume change at different feed times of the internal thread grinding1,iAnd K2,iThe grinding force formula is corrected, under the condition of the same grinding wheel rotating speed, feeding speed and grinding depth, the grinding force is different due to different removal volumes of different cutting times, the grinding force formula of two-dimensional plane grinding is converted into the grinding force formula of three-dimensional internal thread grinding, and the three-dimensional grinding force formula of the internal thread grinding process is obtained,
Fa,i=Fn,itanλ (7)
in the formula, vsIs the rotational speed v of the grinding wheelwThe feed rate is,apIs the grinding depth; fiTotal grinding force for ith feed, Fn,i、Ft,iAnd Fa,iNormal grinding force, tangential grinding force and axial grinding force of the ith feed are respectively; k is a radical ofnAnd ktNormal and radial grinding force coefficients, respectively, of positive value, b1、b2、b3、c1、c2、c3The numerical values of the indexes related to the rotation speed, the feeding speed and the grinding depth of the grinding wheel are positive numbers, and lambda is a lead angle.
In order to ensure the grinding precision of the internal thread of the planetary roller screw, the internal thread grinding needs to be finished by feeding for many times, and the same grinding depth is ensured by feeding every time. Finite element simulation studies the grinding process of the first feed. K1,iThe correlation coefficient of the ratio of the ith feed removal volume to the 1 st feed removal volume, K2,iAnd removing the correlation coefficient of the volume ratio of the internal thread grinding and the plane grinding for the ith feed.
S3, establishing a finite element model of the plane grinding process, obtaining data of normal grinding force and tangential grinding force of the grinding process through orthogonal finite element simulation calculation, and solving a coefficient k by using a control variable methodn、kt、K1,i、K2,i、b1、b2、 b3、c1、c2、c3To obtain the normal grinding force Fn,iAnd tangential grinding force Ft,iPrediction formula given corresponding grinding wheel speed vsA feeding speed vwGrinding depth apThe normal grinding force F can be obtained by the feed times in,iAnd tangential grinding force Ft,iIs calculated to calculate the axial grinding force Fa,iAnd total grinding force Fi。
(1) Further, the coefficient k in the plane grinding force formula is solved in the step S3n、ktAnd index b1、b2、 b3、c1、c2、c3The specific method comprises the following steps:
①b1、c1and kn、ktIs calculated byThe method comprises the following steps:
due to b1And c1Is related to the rotational speed v of the grinding wheelsRelated index, thus maintaining the feed speed vwAnd grinding depth apFor constant value, by taking different values of the rotational speed v of the grinding wheelsTo obtain the values of the normal grinding force and the tangential grinding force of the 1 st feed, in this case, it is considered that
Wherein d issIs the diameter of the grinding wheel, vsIs the rotational speed of a grinding wheel, k'1And k ″)1Substituting the data into a formula (8) for curve fitting to obtain a fitted curve of the radial grinding force and the tangential grinding force along with the change of the rotating speed of the grinding wheel, and obtaining b1And c1The numerical value of (c). During this calculation, multiple sets of k 'are available'1And k ″)1The calculated values of (1) are respectively taken as a plurality of groups of k'1And k ″)1The average of the values is kn、ktThe value of (c).
②b2And c2The calculating method of (2):
due to b2And c2Is related to the feed speed vwRelative index, thus maintaining the grinding wheel speed vsAnd grinding depth apFor constant value, by taking different values of the feed speed vwTo obtain the values of the normal grinding force and the tangential grinding force of the 1 st feed, in this case, it is considered that
Wherein v iswIs feed speed, k'2And k ″)2Substituting the data into formula (9) for fitting curve to obtain fitted curve of radial grinding force and tangential grinding force varying with the rotation speed of the grinding wheel, and calculating b2And c2The numerical value of (c).
③b3And c3The calculating method of (2):
due to b3And c3Is related to the grinding depth apRelative index, thus maintaining the grinding wheel speed vsAnd a feed speed vwFor constant value, by taking different values of grinding depth apTo obtain the values of the normal grinding force and the tangential grinding force of the 1 st feed, in this case, it is considered that
Wherein a ispIs grinding depth, k'3And k ″)3Substituting the data into a formula (10) for curve fitting to obtain a fitted curve of the radial grinding force and the tangential grinding force along with the change of the rotating speed of the grinding wheel, and solving b3And c3The value of (c).
(2) Solving the grinding depth coefficient in the plane grinding force formula in the step S3
And
the solution process of (2) is as follows:
FIG. 4 is a schematic tangential view of the grinding process of the internal thread, where R is the radius of the grinding wheel, R is the inner diameter of the workpiece to be machined, and θ isRFor the angle theta corresponding to the arc length of the grinding wheel cutting into the workpiecerThe angle corresponding to the arc length of the cut workpiece, d is the length of the line segment corresponding to the arc length, apIs the grinding depth.
According to the geometrical relationship, the following are obtained:
the radius R of the grinding wheel, the radius R of the workpiece to be processed and the grinding depth apAll the known quantities are known, the simultaneous equations (11) - (13) can solve the angle theta corresponding to the arc length of the grinding wheel cutting into the workpieceRAngle theta corresponding to arc length of cut into workpiecerAnd the length d of the line segment corresponding to the arc length.
Further, the area S can be obtained from the geometric relationshipRAnd SrIn which S isRAnd SrThe areas respectively comprise the arc length of the grinding wheel cut-in workpiece and the cut-in arc length of the workpiece and a line segment d, and the difference value of the two areas is the i-th axial section removal area delta Si(i=1,2,3……)。
Then the ith removal volume:
the 1 st removal volume was:
obtaining:
fig. 5 is an axial schematic diagram of internal thread grinding, and the volume is removed in the ith time of flat grinding:
Vi=ap×d×iap (20)
obtaining:
therefore, the final internal thread grinding force prediction formula is as follows:
wherein v issIs the rotational speed of the grinding wheel, vwFor the feed rate, apTo grind depth, knAnd ktIs a coefficient of grinding force, which is constant, K1,iThe correlation coefficient of the ratio of the ith feed removal volume to the 1 st feed removal volume, K2,iAnd lambda is a thread lead angle, and is a correlation coefficient of the volume ratio of the ith feed internal thread grinding to the planar grinding removal.
The invention is illustrated below with reference to specific examples:
the invention comprises three steps, the grinding process is simplified, the grinding force is decomposed, the high-speed grinding of the internal thread is simplified into two-dimensional plane grinding, and the two-dimensional plane is obtainedAn empirical formula of grinding force for face grinding; according to the characteristics of internal thread grinding and two-dimensional plane grinding, the conversion from two-dimensional plane grinding force to three-dimensional internal thread grinding force is realized by introducing a correction coefficient, and a three-dimensional grinding force formula of the internal thread in the high-speed grinding process is obtained; the coefficient k is obtained by the grinding force obtained by orthogonal finite element simulation calculation and the control variable methodn、kt、K1,i、K2,i、b1、b2、b3、c1、c2、c3And further obtaining a grinding force prediction formula.
A two-dimensional plane grinding model of a plurality of abrasive grains was created in Abaqus, and as shown in fig. 5, the grinding force at the 1 st feed was studied, the maximum rotation speed of the grinding wheel spindle was 30000rpm, the grinding wheel was a diamond grinding wheel, and the radius R was 15 mm. The workpiece material is GCr15, the radius of the inner circle is r-20 mm, and the diameter of the outer circle is dw50mm, length of workpiece lw80 mm. The internal thread model is M42 x 3, and the lead angle is lambda equal to 2 degrees 48'.
Four groups of different feeding speeds are set for the grinding wheel in the Abaqus, wherein the four different feeding speeds are 10000r/min, 12000r/min, 13000r/min and 20000r/min respectively, the grinding depth is set to be 0.03mm, the feeding speed is 30mm/s, the negative rake angle of the abrasive particles is-45 degrees, and the number of the abrasive particles is 9. The average grinding force values at the respective rotational speeds were obtained as shown in table 1.
TABLE 1 mean value of grinding force corresponding to each rotation speed
| Rotating speed (r/min)
|
10000
|
12000
|
13000
|
20000
|
| Tangential force (N)
|
3.12
|
2.56
|
2.35
|
1.513
|
| Radial force (N)
|
4.57
|
3.75
|
3.44
|
2.42 |
Importing the data into MATLAB to fit a curve according to a formula (8) to obtain a nonlinear parameter b1=-1.082, c1=-0.9833。
Five groups of different feeding speeds are set in the Abaqus, the feeding speeds are respectively 40mm/s, 60mm/s, 80mm/s, 90mm/s and 100mm/s, the grinding depth is set to be 0.03mm, the rotating speed of the grinding wheel is 10000r/min, the negative rake angle of the abrasive particles is-45 degrees, and the number of the abrasive particles is 9. The average values of the grinding forces at different feed rates are shown in table 2.
TABLE 2 grinding force corresponding to different feed rates
Importing the data into MATLAB to fit a curve according to a formula (9) to obtain a nonlinear parameter b2=1.493,c2=2.268。
Three groups of different grinding depths are set in the Abaqus, the grinding depths are respectively 0.01mm, 0.02mm and 0.03mm, the feeding speed is set to be 100mm/s, the rotating speed of the grinding wheel is 10000r/min, the negative rake angle of the abrasive grains is-45 degrees, the number of the abrasive grains is 9, and the average values of the grinding forces under the different grinding depths are obtained and are shown in the table 3.
TABLE 3 mean value of grinding force peak value applied to each cutting depth
| Grinding depth/(mm)
|
Radial force/(N)
|
Tangential force/(N)
|
| 0.01
|
2.39
|
2.11
|
| 0.02
|
4.57
|
3.12
|
| 0.03
|
7.06
|
4.92 |
Importing the data into MATLAB to fit a curve according to a formula (10) to obtain a nonlinear parameter b3=0.8243, c3=0.5107。
The above calculations are all grinding forces for the 1 st feed, so the formula for obtaining the two-dimensional plane grinding force is
The feed rate is 30mm/s, the cutting depth is 0.03mm, and the rotating speed is 1000, 12000, 13000,The grinding force value of 20000r/min is introduced into the formula (26) to obtain knHas an average value of 6.918 × 10-2,ktHas an average value of 9.782 × 10-4And the grinding force formula of the finally obtained plane grinding process is as follows:
according to a plane grinding force formula, the characteristics of the internal thread grinding process and the plane grinding process are compared, the change of the removal volume caused by different cutting times is considered, and the grinding depth coefficient K is introduced by the difference between the internal thread grinding process and the plane grinding process1,iAnd K2,iCorrecting the grinding force formula, converting the grinding force formula of the plane grinding into the grinding force formula of the internal thread grinding to obtain a three-dimensional grinding force formula of the internal thread in the high-speed grinding process, wherein the grinding force formula is
In the research, the grinding of the internal thread of the planetary roller screw nut needs to be finished by five times of feed, each time of feed is 0.02mm, and the total cutting depth of the five times of feed is 0.1 mm.
The i-th cut removal volume of the internal thread grinding calculated from equations (11) to (18) is shown in table 4.
TABLE 4 ith feed removal volume
| Number of feeds
|
1 st time
|
2 nd time
|
3 rd time
|
4 th time
|
5 th time
|
| Volume removed (mm)3)
|
0.000532
|
0.0025
|
0.0053
|
0.0081
|
0.0126 |
The ith removal volume V can be obtained by flat grinding from the formulae (11) to (13) and (20)1=0.0238mm3,V2=0.0476mm3, V3=0.0714mm3,V4=0.0952mm3,V5=0.119mm3。
From the equations (19) and (21), the grinding depth correction coefficients are obtained as:
therefore, the formula of the corrected 1 st feed grinding force is as follows:
the formula of the grinding force of the 2 nd feed is as follows:
the formula of the 3 rd feed grinding force is as follows:
the formula of the grinding force of the 4 th feed is as follows:
the formula of the 5 th feed grinding force is as follows:
wherein v issIs the grinding wheel rotation speed (r/min), vwFor the feed rate (mm/min), apIs the grinding depth (mm).