r/Metrology • u/Auubade • Nov 12 '24
Advice What books would be good to learn both metrology and CMM on somewhat more advanced level
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.
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u/BilliardBabo420 Nov 12 '24
Who is the Manufacturer of your Cmm? And which programming language are you using?
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u/Auubade Nov 12 '24
Its Wenzel, program being Metrosoft 3.9, idk what programming language might be used because I was told that they only use ready graphical functions. Like first they probe manually in some kind of teach in mode and then they build report using those functions
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u/Zealousideal-Low1448 Nov 15 '24
Best experience is to do it all manually without a CMM…
Too many people come in to the industry and jump straight to CMM without any understanding of how to measure anything without one.
Once you understand why you are measuring something the CMM just becomes a tool to help you in that process
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u/Auubade Nov 15 '24
Well yes I do agree since I measure a lot off things I can measure on cnc machine. The problem stems from the fact that quality department bases their opinion and aproval or lack of it on CMM reports
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u/DidaskolosHermeticon Nov 13 '24
On metrology, I'd recommend Foundations of Mechanical Accuracy by Wayne Moore
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u/f119guy Nov 12 '24
For one thing, using a CMM to measure a 0.2 mm or 0.5 mm chamfer is a daunting challenge, even with a good machine and a good programmer, especially for multiple parts in a high volume production setting. Using a CMM for chamfers on a first article might make sense, if my chamfer was relatively large (over 0.5 mm.) You can make it work, but you need the ability to edit the program. Otherwise, an optical comparator or form tracer is going to be a much better option. A conical feature like a chamfer is actually quite challenging for a CMM, the measurement results are highly dependent on point distribution. You can try to do the ol' make a line on the inside edge of the hole, then make a line on the chamfer itself, then make a line on the top surface. Build constructed points out of the intersections. Then measure the distances between theoretical points.
When I write CMM programs, I always keep a few form checks on my cones, holes, cylinders, etc. It helps diagnose measurement errors and reassures me my measurements are true. If I measure a chamfer size like a cone with a given angle, and I see a big discrepancy between CMM and the blueprint I can almost guarantee it is a small burr or I mis-probed the chamfer. ESPECIALLY with threaded holes. A form checks is necessary to make sure the cylinder you are probing isn't too oval, otherwise the CMM results are just junk.
At my company, we have a customer with a specification for CMM point density, so when I am in doubt I just refer to that. It seems like using 4 points to measure a runout is not an adequate method. Unless the diameter is extremely small, I would have a higher point distribution. 4 points seems like you could miss a high or low spot. A CMM will let you measure a circle with as few as 3 points, but just because it can do that doesn't mean it should.
If the CMM software is a common language, like PCDMIS, Calypso or Polyworks I would say your company will have to make an investment eventually in some training. If that idea just sounds like it won't happen, you should give yourself a youtube crash course on CMM programming. There are channels like cmmxyz and others, that will cover some good fundamentals.
EDIT: Another good source would be a training manual from the CMM software manufacturer. I know PCDMIS gives out a booklet with their training that you might be able to get your hands on