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- Jun 12, 2014
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I think I have a number of small issues; all adding up to give me the errors reported above.
I agree. You also have a lot of cumulative errors and also at the limit of of your caliper/equipment resolution. You can often measure a part with a caliper 3X and each measurement will be different. Just switch from a drill to a boring head and changing the height of the head or knee cen change position, just lots of sources of errors.
You are really at the limit of what you can get out of the mill as setup, and are doing pretty decent at that. You can get a DRO for as little as $250 or less with 5 micron scales (which have their own deviations), this takes some guesswork out of the dials and table movement locking, which can easily account for the error you are seeing. It would also give you more precise center to center hole dimensions. As others mentioned, just locking down the gibs can introduce significant error, although there are a few tricks to minimize it. Mechanical edge finders and also the basic electronic touch ones, have not been very accurate in my experience, certainly not at the 0.001" level. I use a mechanical Haimer which is good for both edge and center to ~0.0004" and also have an electronic probe center finder which is about the same accuracy or a bit worse. You also have eccentricity of the collet, angular changes in the head, etc. Part of the process is reproducibility of procedure, but you still have movement of the machine, and also the process of drilling or boring holes. Aluminum can deform or twist on clamping. Drills tend to walk very easily, the longer drills can deflect quite a bit. When I need hole location precision I center drill the hole with a Keo countersink first and than follow with a stub drill. On precise boring dimensions I use a reamer. My goal is to hit 0.001" or better tolerance when it is required. Without a DRO, decent edge finder, etc. I was lucky if I was at 0.003" on a good day.
If you are making multiple parts, then I agree with others in setting up a jig with alignment pins that reference off of known holes.