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Nothing throws a wrench into a production machining operation like variation in the incoming parts. This is particularly true for an abrasive machining process like honing, which is typically used to improve the quality of workpieces produced with upstream processes that may include boring, plating, drilling, reaming, and heat treating. Once dialled in for one set of part conditions, an established production honing process can be thrown off balance when the mix of incoming part conditions or material parameters changes. In some cases too, workpieces possess complicating variations of their own, such as the wall-thickness differences inherent to cartridge valves, connecting rods and injector bodies.
Typically, a production honing process is setup to use an abrasive tool with a specific grit size and bond optimised for incoming part conditions. Tool expansion to achieve the desired results and final size is programmed based on rate of time. The tool feed system then performs the same way on each cycle, starting with touch-off, sensed through spindle load or the force sensed on the tool feed system. Cycle time is always the same. However, when a batch of parts comes in with a different heat treatment than the prior batch, the operator must intervene. In this case, the tool may expand too quickly for conditions and be damaged. In the opposite case with a softer-than-normal workpiece, the tool will still expand at its programmed rate, when it might have been able to expand faster to reduce cycle time. Expansion at too slow a rate may also result in glazing of the honing stones, which won't self dress if the cutting force is too low. Typically, the operator tweaks the feedrate periodically to compensate for these variables.
New tool feed technology that servo-controls the force in the honing tool feed system, changes this dynamic entirely. The new Controlled-Force feature, which works in concert with the machine's standard rate-feed system, functions like cruise control to maintain the optimum cutting load on the honing abrasive throughout a cycle, irrespective of the incoming part's hardness, geometry or size variation. In effect, the machine can detect what is happening in the bore when the abrasive contacts it, whereas with rate-feed alone, the machine feeds the tool per the program, not according to the real conditions the part is experiencing.
With Controlled-Force honing, the machine still uses a programmed feedrate, which is then increased or decreased to maintain a setpoint for the force load on the tool feed system. If the feed force drops off the system increases the tool feed rate to compensate – resulting in faster cycle time, as just one example. Feed force is monitored every few milliseconds during the cycle to ensure the tool always feeds at the highest rate possible for part conditions.