Around this time, CTMC installed several Blohm high-precision surface grinders to increase our production capacity and meet the demand for our products. Two of these machines were programmable CNC versions with the capability to grind parallel curved surfaces to sub-micron accuracy. This got me thinking about the possibility of compensating for the angular deflection by grinding a compensating curve in the linear guide rail mounting surfaces.
With the rails mounted on the bottom of the Y-axis casting, a 13 micron high arc in the center of the rail mounting surface would compensate for the total angular deflection, and provide a correction to the angular error in proportion to the axis travel. One of the CNC Blohm grinders was programmed to produce parallel surfaces for the linear rail mounting with the 13 micron high arc in the middle of the ground surfaces. We produced a test casting to this specification and built it into one of the machines in our normal production run. We tested on the machine and it showed a reduction from the expected 8 arc-seconds of pitch error to a deflection of less than 2 arc-seconds.
We implemented an engineering change to modify all 40 series Y-axis rail-mounting surfaces to this specification. All subsequent machines showed an average pitch error of less than 2 arc-seconds. This eliminated the machine rework, and yielded an accuracy improvement for this important component of overall geometric accuracy of 400 percent.
This entry was submitted by Frank Samples and edited by Rob Spiegel.
Frank Samples led development on a variety of manufacturing equipment, including the FI 230 wire EDM, FO41 drop tank EDM, and a laser welding system with more than 90 percent weld productivity. He currently works as an engineering consultant at Steelcase Inc.
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