On 25/05/2020 18:19, et...@whidbey.com
> Greetings Peter,
> I was way off in my dimensions, as David pointed out. I don't know
> what I was thinking. Anyway, I can measure accurately to 1/2 micron
> with my 20 millionths of an inch per division indicators. I have had
> to make parts that were round within 30 millionths of an inch so
> that's why I have the inspection equipment to check this kind of
> stuff. And temperature REALLY changes dimensions at that resolution.
> Even touching a part with your fingers can make the part measure out
> of round because of uneven heating. Is your machine really accurate
> enough to want sub micron resoultion? In any case a digital encoder
> would still probably be the cheapest way for you to get sub micron
> positioning. The way CNC machines determine position using digital
> encoders is to use a switch for coarse position feedback and then the
> machine moves slowly until it sees the index on the encoder. You will
> need to do the same if you don't want to constantly overshoot your
> position marker. Just how small are the parts you are making? And how
> accurate do they need to be made?
Us metric folks use "hundredth's" almost exclusively as the measurement
division while hand machining - that is 1/100th of a millimeter or 10
microns. This division is approximately equivalent to the imperial "few
We are taught to estimate the dial reading to a tenth of a division, ie
one micron, though we would only do that on the most accurate parts.
My analogue micrometers read to a hundredth and can be estimated to a
micron or two, my digital mikes read to a micron. My (small, A4 size)
granite surface table is accurate to 1/4 micron. Wanting a part accuracy
of a few microns, requiring a basic mill reference accurate to +/- 1
micron or better is not really a big deal.
(I don't do submicron work - as yet anyway)
For this mill a large part would be 80mm long and 50mm diameter. 10mm to
30mm part dimensions would be more typical.
Mill is a converted BCA with hella-expensive 2mm pitch ball screws and
chunky overpowered 200 step motors using 16 microsteps/step to give
0.625 micron resolution, and a 30,000 rpm 600W ER8 fluid damped spindle
to reduce cutting forces.
XY repeatability (without zeroing) is within 2 microns using LinuxCNC
motion control software. With present zeroing (standard microswitches)
repeatability gets considerably worse.