Process for avoiding overstressing a workpiece during grinding

Details Process for avoiding overstressing a workpiece during grinding Process for avoiding overstressing a workpiece during grinding

1996-02-08

1998-06-02

Rose, Robert A.

Abrading

Precision device or process - or with condition responsive...

With indicating

451 5, 451 9, 451 28, 451403, 451 47, B24B 4900

Patent

active

057590850

DESCRIPTION:

BRIEF SUMMARY
The invention relates to a process for avoiding thermal overstressing in a workpiece during grinding.


BACKGROUND OF THE INVENTION

In the manufacture of qualitatively high-grade parts, such as gears or other workpieces having special profiles, it becomes increasingly important for obtaining a great precision and accuracy of shape to add a fine machining after the previous machining and hardening. Grinding is often used as fine machining process. Here is removed an overmeasure resulting, on one hand, from the machining prior to hardening and, on the other, from hardening distortions. In the hard machining of gears, the overmeasure mostly amounts to between one and three tenths of a millimeter.
By virtue of the removal of said overmeasure, the grinding operation must produce the final shape for which a partial difference of only a few micrometers are admissible. On the other hand, economic considerations require that the hard or fine machining be effected in the shortest possible time in order to obtain a maximum workpiece output per machine and per time unit.
These two contradictory requirements have led in the past to the development of more efficient grinding processes wherein diamonds and cubically crystalline boron nitride are used together with the conventional grinding materials (such as silicon carbide, electrocorundum).
In the practice, the fine machining operation is carried out with high specific rates of metal removal, that is, with a maximum number of cubic millimeters of overmeasure removed per second of grinding time and per millimeter of grinding wheel width. But the limit of optimation capacity is reached where the work is effected with such large feeds that the grinding wheel locally produces on the surface of contact with the workpiece temperatures so elevated that thermal structural changes appear in the workpieces. The expert here speaks of overheating and thereby means a thermal damage of the workpiece such that it becomes waste. Different causes can produce the overheating. The most frequent causes are:
Therefore, monitoring the overheating is an important criterion of quality for industrial production. For this purpose there are often used etching methods in which workpieces used as samples are etched in different baths in order to make visible therefrom any change occurred in the rim zones of the texture. But the process is costly and does not permit an examination of all workpieces. It can be used only after the machining and allows no inspection of the workpieces for overheating during the grinding operation.
German patent 40 25 552 has now disclosed a process and an apparatus for recognizing thermal overstressing of a workpiece. The increase in temperature on the surface subjected to grinding is here determined and said actual value is compared with a reference value. The reference value has been determined under the same parameters of the grinding operation with a grinding wheel which has led to generation of overheating on a reference workpiece. The grinding operation with the grinding wheel concerned is terminated as soon as determining an increase of temperature corresponding to the critical temperature producer of the overheating taking into consideration a presettable safety margin. Since determining the temperature on the workpiece to be processed has been admitted to be difficult and inaccurate, it has been proposed to make use, instead of the temperature increase produced by the grinding operation, a geometric change in shape of the workpiece conditioned thereby. But for this reference measurement, costly measuring devices must be applied to the workpieces to be machined which prevent a simple handling of the overheating detection and do not allow reliable measurement results. This process also offers only the possibility of recognizing overheating that has already been generated, that is, of detecting damage. Over-heating cannot be avoided thereby.


SUMMARY OF THE INVENTION

The invention is based on the problem of improving a process for avoiding thermal overstresses

REFERENCES:
patent: 3984213 (1976-10-01), Kelso
patent: 4505075 (1985-03-01), Salmon et al.
patent: 4744179 (1988-05-01), Mockli
patent: 5111562 (1992-05-01), Burka
patent: 5148637 (1992-09-01), Byron
patent: 5174068 (1992-12-01), Pickert
patent: 5174070 (1992-12-01), Losch et al.

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