An old toolmaker in Erie told me to stop trusting my DRO, and he was right

I was setting up a run of 200 small bushings last month and the readout said I was dead on, but the first part came out .003 oversize on the bore. An old toolmaker I know from the shop in Erie swung by and watched me for a bit, then asked when I last checked my machine against a real standard instead of just trusting the numbers on the screen. I told him I zero it every morning, and he laughed and said zeroing a screen doesn't mean the screw is where the screen thinks it is. He had me touch off on a 2 inch gauge block and the DRO was reading .0045 off over that span, which is a mile when you're holding a plus or minus .001 tolerance. I ended up sweeping the table with an indicator, found the scale bracket had come loose just enough to shift things, and snugged it down. After that the run went clean and I didn't scrap a single part the rest of the shift. What do you guys use to check your machines against a known standard, and how often do you actually do it?

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river_scott98h ago

Man, I had almost the exact same thing happen with a lathe a couple years back, DRO was lying to me by about .003 over 6 inches and I was chasing my tail on a shaft job for two days thinking I forgot how to run a machine. Now I keep a 4 inch gauge block and a test bar in the top drawer and I check both axes at least once a week, more often if the machine has been bumped or moved. A loose scale bracket will get you every time because the numbers look so clean you never question them. Good catch on sweeping the table, that indicator will tell you the truth even when the screen won't.

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