r/Metrology • u/Hydragirl68 • 6d ago
General Things Engineers say:
After giving results of a flatness test (yes it failed.) I had an engineer come to me today to ask if I could put my probe on an angle. His colleague has had some positive results when the probe was on an angle and measuring flatness. I clarified you mean instead of laying the part down flat, put the part on its side? No, put the probe on a slight angle and measure it.
🤦♀️
15
u/CthulhuLies 6d ago edited 6d ago
What probe head?
A TP20 has about 10x the trigger force normal to the shank ijk vs perpendicular.
Ie straight into the mechanism vs using the shank as a lever.
On plastic this absolutely matters. If you don't have metal clamped well this absolutely matters.
For a scanning head like LSP-X1h from Leitz it calibrates the trigger force on the ball itself so it should be the same force regardless of Surface IJK relative to the Shank
9
u/FrickinLazerBeams 6d ago
Guys you can use an angled probe to measure a plane without cosine error. The stalk doesn't have to be aligned to the normal.
8
u/causewaymanatee 6d ago
You made the same mistake our machinist make.
" I run the same part on 2 machines and one passes, one fails...fix it, bye."
They leave out one passed by 0.002mm and one failed by 0.001mm. You left out the magnitude of the failure.
7
u/Thethubbedone 6d ago
Putting the stylus orthogonal to the surface can, with some probing systems, sometimes give a small amount of error (or if you're using Revo with RSP2 totally invalidate the result). Tilting the stylus can give better results and not because you're adding cosine error. For two reasons-any modern cmm handles tilted styli just fine, AND the result isn't different if its taken from ball-center anyway.
3
u/kalistyi 6d ago
So just joined this today. I can handle my engineers. The F.-- k business people at my company told me I have to be doing 3 people's jobs and just except it because AI can do it. Almost made walk out right then and there.
7
u/referenceonly77 6d ago
Didn't a vehicle company just hire back all engineers this year because AI could not in fact do it all correctly?
4
1
u/drdunningat 6d ago edited 6d ago
"measure the flatness with a micrometer on multiple spots of the part instead, if the deviation of thickness doesnt exceed the flatness tolerence then the part is fine and the cmm machine is wrong" /s
3
1
1
1
u/Taborlin_the_great 5d ago
You should be running your test per your SOP. If your SOP says probe is square to the part that’s the way you run it. The lab isn’t supposed to care if the results are good or bad. The lab should only be concerned with accurate results
1
1
u/milehighxr 6d ago
I like propping my part on 3 points and running my indicator under it to check flatness. Its what they taught me at the mold shop 25+yrs ago. All done on a granite plate of course. This way you are checking the flatness of the face in question, reactive to the granite. If you're doing it on a cmm, the part should not be laying on the surface imho, cuz yer just checking parallelism then.
0
u/Quality-Panda 6d ago
Is he suggesting intentionally adding cosine error will give him the result he needs? Lol
Assuming you started with a normal vector.
6
u/Hydragirl68 6d ago
I offered to create a point map so he could see where it is failing. He didn’t want that. 🤷♀️
5
u/ShaneCoryPlays 6d ago
This is my go to! Point map so you can isolate the issue. The reply i get? "Can't we just remove those points?" Lol
4
u/Pathfinder_Dan 6d ago
I've gotten that one a few times. My response is "Would you be comfortable asking me that if our customers were all standing next to me?"
4
u/Hydragirl68 6d ago
Omg laughed so hard this. Worked at a place that would insist I do that. Not for long though
4
u/CthulhuLies 6d ago
This wouldn't add cosine error, and even if you tried to mimic like a 30 degree cosine error unless the surface is irregular the plane would just be offset by the cosine error with the flatness being unaffected.
0
0
u/Delicious-Engine7363 6d ago
Yeah. It might look a little better with the cosine error he's suggesting, but that isn't the proper way to measure as 99.9% of us know.
4
u/f119guy 6d ago
What do you mean? There is nothing in any standard that says flatness must be measured along a vector normal to a surface. The probe tip compensation in each of your probe angles should account for it. It’s a spherical probe, shouldn’t matter which surface of the probe contacts the part
46
u/gaslightredditor 6d ago
Depending on your setup, there could be some truth to that, in theory. Let's assume the plane in question is parallel to the granite. If the actuation force at tool angle 0, 0 is greater than the actuation force at 30, 0, for example, then the tool at an angle could be more accurate and repeatable. Calibration should be accounting for that when calculating the offsets though. Now you aren't going to see any huge changes, but if you're only failing by microns, then it could make a difference.
Have you ran a Gage R&R?