Abrading – Precision device or process - or with condition responsive... – Computer controlled
Reexamination Certificate
2002-04-26
2004-10-26
Rachuba, M. (Department: 3723)
Abrading
Precision device or process - or with condition responsive...
Computer controlled
C451S011000, C451S057000, C451S062000
Reexamination Certificate
active
06808438
ABSTRACT:
FIELD OF INVENTION
This invention concerns the grinding of workpieces and improvements which enable grind times to be reduced, relatively uniform wheel wear and improved surface finish on components such as cams. The invention is of particular application to the grinding of non cylindrical workpieces such as cams that have concave depressions in the flanks, which are typically referred to as re-entrant cams.
BACKGROUND TO THE INVENTION
Traditionally a cam lobe grind has been split into several separate increments typically five increments. Thus if it was necessary to remove a total of 2 mm depth of stock on the radius, the depth of material removed during each of the increments typically would be 0.75 mm in the first two increments, 0.4 m in the third increments, 0.08 mm in the fourth, and 0.02 mm in the last increment.
Usually the process would culminate in a spark-out turn with no feed applied so that during the spark-out process, any load stored in the wheel and component was removed and an acceptable finish and form is achieved on the component.
Sometimes additional rough and finish increments were employed, thereby increasing the number of increments.
During grinding, the component is rotated about an axis and if the component is to be cylindrical, the grinding wheel is advanced and held at a constant position relative to that axis for each of the increments so that a cylindrical component results. The workpiece is rotated via the headstock and the rotational speed of the workpiece (often referred to as the headstock velocity), can be of the order of 100 rpm where the component which is being ground is cylindrical. Where a non-cylindrical component is involved and the wheel has to advance and retract during each rotation of the workpiece, so as to grind the non-circular profile, the headstock velocity has been rather less than that used when grinding cylindrical components. Thus 20 to 60 rpm has been typical of the headstock velocity when grinding non-cylindrical portions of cams.
Generally it has been perceived that any reduction in headstock velocity increases the grinding time, and because of commercial considerations, any such increase is unattractive.
The problem is particularly noticeable when re-entrant cams are to be ground in this way. In the re-entrant region, the contact length between the wheel and the workpiece increases possibly tenfold (especially in the case of a wheel having a radius the same, or just less than, the desired concavity), relative to the contact length between the wheel and the workpiece around the cam nose and base circle. A typical velocity profile when grinding a re-entrant cam with a shallow re-entrancy will have been 60 rpm around the nose of the cam, 40 rpm along the flanks of the cam containing the re-entrant regions, and 100 rpm around the base circle of the cam. The headstock would be accelerated or decelerated between these constant speeds within the dynamic capabilities of the machine (c & x axes), and usually constant acceleration/deceleration has been employed.
The power demand on the spindle motor driving the grinding wheel is dictated in part by the material removal rates i.e. the amount of material the wheel has to remove per unit time. The increased contact length in the re-entrant regions has tended to increase this and very high peak power requirements have been noted during the grinding of the concave regions of the flanks of re-entrant cams.
For any given motor, the peak power is determined by the manufacturer, and this has limited the cycle time for grinding particularly re-entrant cams, since it is important not to make demands on the motor greater than the peak power demand capability designed into the motor by the manufacturer.
Hitherto a reduction in cycle time has been achieved by increasing the workspeed used for each component revolution. This has resulted in chatter and burn marks, bumps and hollows in the finished surface of the cam which are unacceptable for camshafts to be used in modern high performance engines, where precision and accuracy is essential to achieve predicted combustion performance and engine efficiency.
The innovations described herein have a number of different objectives.
The first objective is to reduce the time to precision grind components such as cams especially re-entrant cams.
Another objective is to improve the surface finish of such ground components.
Another objective is to produce an acceptable surface finish with larger intervals between dressings.
Another objective is to equalise the wheel wear around the circumference of the grinding wheel.
Another objective is to improve the accessibility of coolant to the work region particularly when grinding re-entrant cams.
Another objective is to provide a design of grinding machine, which is capable of rough grinding and finish grinding a precision component such as a camshaft, in which the cam flanks have concave regions.
These and other objectives will be evident from the following description.
SUMMARY OF THE INVENTION
According to the present invention, there is provided a method of grinding a component which is rotated by a headstock during grinding, comprising the steps of removing metal in a conventional way until shortly before finish size is achieved, thereafter rotating the component through only one revolution during a finish grinding step, and controlling the depth of cut and the headstock velocity during that single rotation, so as to maintain a substantially constant load on the grinding wheel spindle drive motor.
The depth of cut and/or speed of rotation of the component during the one revolution may be adjusted to ensure that the demand on the spindle drive does not exceed the maximum rated power capability of the motor.
In order to maintain a constant power requirement for the spindle without exceeding the maximum power capability of the spindle motor, the component speed of rotation may be altered during the finish grind rotation.
When used to grind a component the profile of which will increase and decrease the loading on the spindle motor during a single revolution of the component, the speed of rotation of the component may be altered as between one point and another during the single revolution so as to maintain a substantially constant load on the spindle motor.
Preferably the instantaneous rotational speed of the component is varied so as to accommodate load variations due to component profile, such as non-cylindrical features of a component.
The headstock speed of rotation may be varied to take account of any variation in contact length between the wheel and the workpiece such as where the component is non-circular or where parts of the surface being ground are to be finished with a concave profile as opposed to a flat or convex profile.
When using a CBN wheel in the range 80-120 mm diameter for grinding a steel component, and with 17.5 kw of power available for driving the grinding wheel, wheelfeed has been adjusted to achieve a depth of cut during the single finish grinding step in the range 0.25 to 0.5 mm, and the headstock drive has been adjusted to rotate the component at speeds in the range 2-20 rpm.
The invention also provides a method of grinding a component which is rotated by a headstock during grinding to finish size, wherein the headstock velocity is linked to the power capabilities of the grinding wheel spindle drive, and a significant grinding force is maintained between the wheel and the component up to the end of the grinding process including during finish grinding, thereby to achieve a significant depth of cut even during the finish grinding step, for the purpose of reducing chatter and grind marks on the final finished surface and to achieve a short grind time.
The invention also lies in method of grinding a component which is rotated by a headstock during grinding wherein a substantially constant power demand on the spindle drive is achieved by controlling the headstock velocity during grinding, especially during final finish grinding, so as to accelerate and decelerate the rotational speed of
Rachuba M.
Reising Ethington Barnes Kisselle P.C.
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