Originally Posted By cd johnson:
I've made some test cuts and had them measured via a Faro arm. I then use the scale compensation feature in my Acu-Rite DRO 300M,
and make additional test cuts and have them measured.
I do the measurement from same side cut surfaces. Left side of cut to left side of another cut, spaced exactly X inches away. This way
I don't have to worry about diametric cutter tolerances throwing the results off.
When I first got this setup, the scale compensation was set to a ridiculous number and the work was way off, several thou per inch.
I guess that's understandable as I got the DRO from one source, and the scales from another.
I'm now zeroing in on as close to zero error as can be reliably measured.
I made a compensation adjustment and then found out I'd gone in the wrong direction. The manual for the DRO really wasn't clear
if adding points to the compensation adjustment would shorten or increase the actual length of travel. I was wrong the first time.
I'm not saying I can hold tolerances of .0001 or better. I'm saying I want to iron out the scale compensation factor so that there's as
little error as I can get. If I make an error, I want to be able to only blame myself, not the scales or the machine.
The nice thing about an error of .0001 per inch is that it's really easy to calculate the error in PPM. It's 100 PPM. The compensation
factor is entered in PPM.
i don't even bother to read the dials anymore. It wouldn't do me any good anyway, as they're graduated for inch measurements and
the ball screws in the mill (Yes, it has ball screws instead of lead screws on the X and Y axes) are metric. Relying on the dials would
guarantee that you'd always be totally off by insane amounts.
I got the mill with those ball screws already installed. Given that they really make the dials useless, I'm not so sure that I wouldn't
rather just install fresh lead screws instead if I had to do it all over again.
But now, I'm so used to using the DRO that doesn't matter.
I'm not crazy about Faro arm measurements but in my case it was measuring things that were 6 feet or more apart. Even the CMM has it's flaws but usually a good operator can work them out. With 25 years behind me you can bet I never want to have to use the dials on a mill ever again. Mostly because I have been spoiled with digital read outs. I can't even imagine trying to make something with a lead screw machine and dials.
I have a friend who also has a tool grinding shop. That is one area I haven't worked in yet. He invited me to come out and see if I like it or not. So hopefully by next year I will have some tool grinding experience too. May be a good part time job after I retire.
Good luck with your project and thanks for not going ballistic on me.