Abrading – Precision device or process - or with condition responsive... – Computer controlled
Reexamination Certificate
2002-04-26
2004-11-02
Rachuba, M. (Department: 3723)
Abrading
Precision device or process - or with condition responsive...
Computer controlled
C451S011000, C451S062000, C451S197000, C451S209000, C451S210000
Reexamination Certificate
active
06811465
ABSTRACT:
FIELD OF THE 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.
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 a nd 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, in a method of grinding a component, such as a cam, a reduction in the finish grinding time is achieved by rotating the component through only a single revolution during a final grinding step and controlling the depth of cut and the component speed of rotation during that single revolution, so as to maintain a substantially constant specific metal removal rate during the final grinding step.
The advance of the wheelhead during the final grinding step may be adjusted to produce the desired depth of cut.
Preferably the depth of cut is kept constant but the workpiece speed of rotation is altered during the final grinding step to accommodate any non-cylindrical features of a workpiece so as to maintain a constant specific metal removal rate.
When grinding a cam the headstock velocity may be varied between 2 and 20 rpm during the single revolution of the cam during the final grinding step, with the lower speed used for grinding the flanks and the higher speed used during the grinding of the nose and base of the cam.
During the final grinding step using a grinding machine having 17.5 kw of available power for rotating the wheel, and using a grinding wheel in the range 80-120 mm diameter typically the depth of cut will be in the range of 0.25 to 0.5 mm.
The headstock drive may be programmed to generate a slight overrun so that the wheel remains in contact with the workpiece during slightly more than 360° of rotation of the latter, so as not to leave an unwanted step, hump or hollow at the point where the grinding wheel first engages the component at the beginning of the single revolution of the final grinding step.
During the single revolution of the workpiece the headstock velocity may be further controlled so as to maintain a substantially constant power demand on the wheel spindle drive during the final grinding step so as to reduce chatter and grind marks on the component surface.
When grinding non-cylindrical workpieces, the headstock velocity may be varied to take into account any variation in contact length between the wheel and workpiece during the rotation of the later, which ensures that the material removal rate is maintained truly constant so that all parts of the circumference of the grinding wheel perform the same amount of work, with the result that substantially constant wheel wear results.
Headstock acceleration and deceleration, as well as headstock velocity, may be controlled during the single rotation of the final grinding step, so as to achieve the substantially constant wheel wear.
Where the grinding is to leave at least one concave region around the component profile the grinding is preferably performed using a small diameter wheel, for both rough and finish grinding the component, so that coolant fluid has good access to the region in which the grinding is occurring during all stages of the grinding process, so as to minimise the surface damage which can otherwise occur if coolant fluid is obscured, as when using a larger wheel.
A grinding machine may be used which has two small wheels mounted thereon, either of which can be engaged with the component for grinding. One of the wheels may be used for rough grinding and the other for finish grinding.
A preferred grinding material for the or each grinding wheel is CBN.
A grinding machine adapted to perform a method according to the invention, preferably includes a programmable computer-based control syst
Rachuba M.
UNOVA U.K. Limited
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