Whether you have a vintage grinder with hydraulic feed or a newer CNC machine, grinding accuracy is limited because part size during grinding is really only an “estimate”, inferred from the expected position of the wheel surface combined with the expected amount of material removed during a timed sparkout. The estimate does not include variables such as wheel wear between dressing, part/tooling deflection, thermal changes or pressures accumulated before sparkout starts. HOW CAN I GET BETTER OUTPUT FROM MY GRINDER? Add a high-accuracy, closed-loop gaging system and you have a system that steps down infeed rates, starts sparkout, and causes retraction, all based on “realtime” part size. Produce parts consistently within single-digit-micron or low-tenths tolerances, faster than ever before. HOW DOES THE “UN-GAGED” GRINDING CYCLE WORK? ON MOST HYDRAULIC MACHINES: 1. When the cycle starts, the wheelslide makes a rapid advance using a hydraulic “jump cylinder”. 2. After rapid advance, the hydraulic handwheel begins turning, advancing the wheelslide at a fixed rate until the wheelslide reaches a mechanical stop. 3. A sparkout timer starts, retracting the wheelslide when it expires. ON A CNC MACHINE: 1. When the cycle starts the wheelslide feeds at rapid rate until just before the wheel touches the part. 2. The wheelslide feeds at a coarse rate to a programmed wheelslide position. 3. The feed may make one or more additional rate reductions, all based on wheelslide position. 4. At a final programmed position a timer starts, retracting the wheel when it expires. WHAT ARE THE DOWNFALLS OF AN UN-GAGED CYCLE? The first problem with un-gaged cycles is that, although the position of the wheelslide is known, the position of the grinding wheel face is only inferred, since the exact diameter of the wheel is continually shrinking. The second problem is that the amount of material removed during sparkout can vary greatly from part to part, depending on pressures built up during the cycle. Factors that contribute to variations are: . hardness of individual parts . incoming stock amount on individual parts . sharpnes of the wheel . stiffness of the parts and/ or tooling . amount of stock removed at each feed rate WHAT IS BETTER ABOUT A GAGED GRINDING CYCLE? .. The feed rate transitions are controlled by how much stock remains on the part, not by the inferred position of the cutting surface. This provides greater consistency of grinding forces .. The sparkout also starts based on stock remaining, not inferred position. More importantly, sparkout ends and the wheel retracts at the instant the actual part size is on-target, not at the expiration of a timer. .. With greater consistency between cycles, feed rates can often be increased for greater productivity. WHAT IS ADDED TO THE GRINDING SYSTEM WITH A GAGE? . Gage head on hydraulic or pneumatic actuator . Control valve to advance/ retract actuator . Gage controller to interpret size and control the wheelslide feed rates, sparkout, and retract. . Feed control modifications: Hydraulic machines: two control valves installed in the feed cylinder exhaust line - one initiates a "fine" feed and the other starts sparkout, both based on real - time part size. CNC machines: determine from the machine manufacturer if the machine already has a "gage ready" program, or if one can be installed. WHAT IMPROVEMENTS CAN BE EXPECTED BY ADDING AN IN-PROCESS GAGE TO MY GRINDER? Each process is different, but is not uncommaon to achieve a doubling of your Cpk after adding an in-process gage. In addition to less scrap and re-works, you can expect a faster average cycle time, less size variation after dressing and less size variation after operator breaks. For more information please contact David Hayes, President & Sales Manager - Control Gaging, Inc.
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