Abrading – Precision device or process - or with condition responsive... – With feeding of tool or work holder
Reexamination Certificate
2000-01-04
2001-10-16
Morgan, Eileen P. (Department: 3723)
Abrading
Precision device or process - or with condition responsive...
With feeding of tool or work holder
C451S056000, C451S443000, C451S253000
Reexamination Certificate
active
06302764
ABSTRACT:
BACKGROUND OF THE INVENTION
The continuous generating grinding method of gear teeth has been shown to be a good finishing process also in mass production because of its high efficiency and outstanding constant precision of ground workpieces. In most cases, grinding tools were used in the past that were at the outer circumference gear-worm shaped corundum wheels—the so-called grinding worms—which rarely turned faster than at a speed of approximately 40 m/s at their circumference.
The already very high efficiency of the process may be increased even more if the circumferencial speed of the grinding tool is increased further. The problem thereby is the fact that the grinding worm is deformed by the effect of the centrifugal forces at high speed. Thereby the deformation is not only caused by the complicated stress condition, as it exists in case of a rotating disk, but also by the worm profile, which has at each angle position around the rotational axis a different axial position, whereby an uneven distribution of force is applied to the worm body circumference. Furthermore, the non-homogeneity of the specific gravity and of the modulus of elasticity of the grinding wheel body are also responsible that the grinding worm shape is deformed with increasing speed. A grinding worm rotating at high speed is therefore not only larger in its diameter than the one that is not moving, but it is generally also not round, and the once established worm profile takes on a shape that cannot be predicted in advance. This is however basically true for tools of all grinding machines, only this phenomenon is not a hindrance in cases where the active form of the grinding disk is shaped at a working speed, which means, where the deformations effected by the centrifugal force are eliminated by the dressing process to a certain degree.
Unfortunately, grinding worms are, for obvious reasons, much more difficult to be shaped than grinding disks. In the rule it is therefore necessary to conduct the dressing process at very low speed. Therefor there are a number of processes known wherein the most efficient and currently most widely known process the one with two profiling disks: each profiling disk layered with diamond grains dresses thereby one worm flank in a process, which is similar to the thread cutting process on a lathe. In another more universal method, grinding worm flanks are dressed by making contact at specific points along a line by means of a rotating dressing tool that has a layer of diamond grains at its active outer circumference. This process is performed in such a manner that line after line are placed very close to one another until the entire active flank surface is dressed. This method is however slower than the one mentioned above but it allows—within certain limits—the creation of an arbitrary topology on the worm flanks. For grinding worms shaped in this manner, there is determined in advance a specific assignment of each point of the tooth flanks to be ground to a specific point on the worm flank whereby, during subsequent grinding it must be ensured by relative motion between the tool and workpiece that the respective points are actually touched or are a common meshing point or machining point. Through this method it is possible to manufacture topologically corrected gear teeth by a continuous generating grinding process.
DE-PS 196 19 401 C1 discloses a process by which grinding worms may also be topologically dressed at top grinding speeds. However, this process places high demands on the mechanical device and on the quality of the necessary servo-drives and control systems, which leads in any case to high investment costs. In addition, dressing tools used in this process can only be used for one specific modulus pitch on the grinding worm.
SUMMARY OF THE INVENTION
It is the object of the present invention to disclose a process and a device wherein grinding worms that are operated at high to very high speeds may be dressed (trued) in a known and tested dressing process at low speeds and which have nevertheless the required precise profile geometry at operating speed, which means, at a stressed condition under centrifugal force.
According to the invention, the process comprises the following steps performed sequentially:
1. Dressing of a grinding worm according to a known method in respect to the shape of the tooth flanks of the gear teeth that are to be ground.
2. Measuring the entire grinding worm profile with the grinding worm turning at operating speeds. This measuring may be performed, for example, directly by means of a non-contact measuring system, as by laser optical distance scanning or the like, or it may be performed indirectly by grinding and measuring of a sample (specimen) workpiece. The results of this measuring are in any case a table or a set of data, which contain precise coordinates of surface points that are distributed across the worm flanks matrix-like with sufficient small distances between one another.
3. Conversion of measured data into control data for the dressing device for a correcting, redressing process of the grinding worm profile. This conversion must determine the specified geometry of the grinding worm flank in the first phase on the basis of the specified geometry of the workpiece teeth; whereby in a second phase, the difference must be determined between the specified data of the worm flanks and the measured actual values; and in a third phase, corrected control data must be determined by using the differential values for the necessary movement of the dressing device.
4. The redressing of the grinding worm profile with the newly computed data whereby the previously determined form error is in a way used as a correction factor in dressing the grinding worm so that the grinding worm obtains the desired shape at operating speed.
The measuring of the worm grinding profile at operating speed is of great importance during this process. Should it be measured directly as mentioned above, as it may be performed by laser optical means, for example, then the measuring process may be completed relatively quickly and the data is readily available for further machining. There is a certain difficulty in the relative rough grinding worm flank surface, which requires careful filtering of the measured values when using sensitively reacting measuring devices.
The more costly measuring method is the indirect measuring with a sample workpiece. Thereby a suitable, sufficiently wide sample toothed wheel must be ground in the continuous shift-grinding process, which corresponds to the workpiece relative to the modulus, number of teeth, meshing and pitch angle, and precisely so that the entire grinding worm profile is reproduced on the complete gear teeth width of the sample wheel. This is accomplished if during grinding the entire possible shifting path of the grinding worm is simultaneously run off on the gear teeth width of the sample wheel. Naturally, the specified operating speed of the grinding tool must thereby be maintained.
Tooth flanks of the sample wheel, which are ground in such a manner, contain now in the transformed shape the actual geometry of the grinding worm profile, which means, all form deviations of the tool caused by the centrifugal force, which as mentioned above cannot be predicted, are reproduced on these sample gear teeth. From there, the actual geometry may be taken by any tooth-flank measuring machine.
Even though the second method is more costly than the direct measuring method of the worm profile, it has the great advantage that taken into consideration are not only the geometric distortions of the grinding worm caused by the centrifugal forces, or out-of-round conditions, profile distortions, changes in pitch etc, but also the deviations on the ground tooth flank surface, which are based on the technological influences such as meshing shocks, co-grinding of the tooth root, influence of the cooling lubricant, or even machine errors. In other words, the second method causes the total of all errors during the grinding process and makes po
Morgan Eileen P.
Reishauer AG
Sughrue Mion Zinn Macpeak & Seas, PLLC
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