Modern Machine Shop

MAR 2017

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Page 62 of 156

QUALITY GAGING TIPS Columnist chuck (producing a three-lobed workpiece), or a centerless grinder (which may create a five-lobed condition). Asymmetrical lobing cannot be mea- sured by the means described here. It is evidenced by irregular travel of an indicator and is usually indicative of a problem in the tool. When an even number of lobes is arranged geometrically on the part, each lobe is opposed by one diametrically opposite. The piece, therefore, will have major and minor diameters. Knowing this, we can gage the part using simple two-point, or diametrical, measurement methods. The difference in the measurements will generally be twice the out-of-round value due to the diametric-versus- radial method of assessment. For example, if our specs call for a part that is round within 0.0001-inch variation in radius, we can measure using a simple comparator and reject any part where the total indicator reading (TIR) is larger than 0.0002 inch. Parts with an odd number of lobes pose a slightly more complicated problem and really should be measured with a gage that has three measuring points 120 degrees apart, such as a tri-bore gage or three-jet air plug/ring. With the proper gage, the same two-point-gage method used above would work, but in this case any total variation of three times the diametric variation would be acceptable. It's clear from these two examples that measur- ing the part to the roundness spec in effect cuts down the tolerance and ensures that that roundness spec is apt to be met. When tolerances are at the 0.0001-inch level, this roundness spec is probably acceptable. But cut this by a factor of 10, and things can change fast. Other form checks, such as cylindricity, might be able to be estimated by exploring the part more, or by creating special fixtures to inspect for square- ness or straightness to approximate these form inspections. But these are a topic for another day. Again, while these methods are only approxi- mate, the techniques described here provide a good, practical means to determine an indication of out-of-roundness on the shop floor when time is of the essence and parts need to be moved. Renishaw Inc West Dundee, IL A full range of probes available in optical or radio transmission confi gurations • Lower contact forces for less stylus bend and pre-travel • Excellent 3D performance • CMM-level accuracy • Excels in the harshest machine conditions Renishaw brings proven strain gauge technology and ultra compact electronics to a wide variety of small, medium and large machining centers. RENGAGE ™ addresses the performance limitations of conventional probes with true 3D measurement and unequalled accuracy in any plane. Reap the benefi ts of automated job set-up, reduced scrap and lower fi xture costs. RENGAGE ™ strain gauge technology—for the most accurate probes in the world. I N N O V A T I O N O V E R 4 0 Y E A R S O F

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