Hello,
I come here as a cnc machinist that struggles with meeting QS demands and I most of the time don’t understand how some things are measured. Our company is being relocated from another country, what we got are the programs for CMM to measure our parts and since we don’t have enough know how and also a person that would be qualified enough to make decisions on their own, we are often stuck with what we got, and what we will get is a printed report from CMM. There were multiple ocassions on which I was told to fix things that simply didn’t make sense, like a chamfer starting point that is somehow 4mm smaller than all the other dimensions made by very same tool (the toolpath on my cnc was ok), and it seemed like chamfer was simply not concetric to raw part (it is serial production and the quality of raw parts isn’t the best since it has to be cheap and I have no other option than to machine it the way it is machined now, meaning I can’t add more steps to make it more concentric, like planning some surface for hard jaws to clamp on) and the probe would miss the chamfer. The chamfer was there to simply deburr the edge, but it would take us a couple of days of sending emails to start this thing up. Worse problem comes from the fact that I don’t think I can relay on CMM in some of it’s measurments. The program is often build in a way that a 0.2 or 0.5 mm chamfer with starting diamater of 200+mm is measured by a single probe touch which would often give stupid results like 50-55 degrees on 45 angle that would then be determined to be 45.3 degrees and measuring with CMM wouldn’t give same results (varied from 40 to 60). Also I don’t think I can understand some of GD&T. How is it possible to measure runout or TOTAL runout on CMM using what i think are 2 cylinders created out of measuring 2 diamaters (4 probe touches per diamater, same Z height)? We got some parts that measure runout or concectrity on threads or surfaces that are simply not oval (machine clamps on them and CMM doesn’t measure how oval thing is).
Is there a way to determine how many probe touching points and what kind of programming will yield me somewhat true results? Let’s say that I have such part (made a horrible paint drawing I’m sorry), How would I measure this tolerance and where can I find any info on this? Preferably some books, I don’t think that my employer will pay for any course and me paying for them won’t give me any pay rise.
submitted by /u/Auubade
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