[Th...] Posted January 30, 2019 Share Posted January 30, 2019 I'm using a Contura 7/7/6, and I'm having an issue with the CMM under-reporting the actual diameter of a pin gauge. I've measured the pin w/ a high-accuracy micrometer at 0.1246" in diameter, but the CMM diameter is between 0.1241" and 0.1243" depending on how far down I'm measuring. I'm struggling to figure out why this is happening. I've repeated the same checks on a pin that measures at 0.1871" and gotten results practically dead-on identical, using the same stylus system (1mm) at almost the same location within the machine's measuring volume, similar measurement strategies (500 points, 400 degree range, speed around half the diameter e.g. 0.065" per second for the small pin) and identical evaluation settings (OTE, cookbook standard filters and outlier elimination). Each measurement repeats exactly like it should through several checks, and yet the small pin is consistently reporting smaller than reality. I've re-qualified the 1mm stylus, replaced it with a new one and qualified that, changed eval settings, slowed the scan speed way down, etc. With the majority of features on our parts being this size or smaller, and with tolerance ranges that don't really allow for this sort of deviation, I'd really like to figure out how to fix this. Is there something I've missed? Link to comment Share on other sites More sharing options...
[Ro...] Posted January 31, 2019 Share Posted January 31, 2019 Well everything in your strategy seems to be on point so far..... In my experience with small or partial features i like to switch to points instead of scanning, i know that's sacrilege in here but i've found it to be accurate over time. Try a 6 or 8 point circles and see what that does for you. Link to comment Share on other sites More sharing options...
[Da...] Posted January 31, 2019 Share Posted January 31, 2019 Thomas, how is your spatial align like? Cylinder? Link to comment Share on other sites More sharing options...
[Cl...] Posted January 31, 2019 Share Posted January 31, 2019 Is the gage pin bent? Is the gage pin standing perfectly vertical? If it's not, you will be measuring an oval, which could account for the size discrepancy. Link to comment Share on other sites More sharing options...
[Ow...] Posted January 31, 2019 Share Posted January 31, 2019 What is the roundness/form of the pin? If it's an actual Gage pin, form/roundness should be <0.0001(?) and if it's not, it's a scanning issue, go with single points like Robert suggested and see if it changes size, especially with a 1mm probe Scanning filter should be 15 UPR . The other possible cause is temperature, when measuring with the high-accuracy gauge, are you holding the pin with your hands for an extended length of time? Link to comment Share on other sites More sharing options...
[Th...] Posted January 31, 2019 Author Share Posted January 31, 2019 I'll try to answer these in sequence. 8 discrete points made no apparent difference in measurement. Spatial and Z origin are being done with a plane that represents the end of the pin. X and Y are a circle some 0.020" down. Rotation left empty. As far as I know the pin is not bent, but I haven't checked that yet. I'm putting a cylinder feature together with circle paths and form lines to look at straightness, but the stylus shaft is stepped and I can only get about 0.160" down. As for fixturing, I'm using an R&R mini vise clamp with V-notches in one of the jaws, so it should be vertical. Roundness for 7 circles, ranging from -0.020" to -0.050" from the end, are all at 50 millionths. Filter is 15 upr Gauss, as specified in the cookbook. The pin is not being held for very long when measuring with the micrometers, which themselves have a hand shroud to keep heat from transferring to the instrument. The CMM was just calibrated earlier this month by a Zeiss tech, and the service report shows the machine tracking well within the allowable spec for length and form measurement errors. The room is temperature controlled to 68F, with minor fluctuation over time totaling around 1 degree. The pin resides in that room normally and is the correct temperature as well. Link to comment Share on other sites More sharing options...
[Cl...] Posted January 31, 2019 Share Posted January 31, 2019 On a pin that small, I would use a cylinder that is at least 3x the length of it's own diameter for the spatial. That plane is too small. Link to comment Share on other sites More sharing options...
[Th...] Posted January 31, 2019 Author Share Posted January 31, 2019 In that case I'll step up to our 1.5mm stylus system. The way the 1mm I'm using is stepped, I can't get far enough down the pin. Update: Ran using a 0.400" long cylinder w/ 2 circle paths, both 0.050" away from the end of the feature, with the same settings as before, along with 4 vertical lines starting and ending 0.030" away. Used this cylinder as spatial, with a 2D circle 0.100" down the pin as my X/Y origin, and a point on the center of the pin end as my Z origin. With OTE evaluation, my cylinder diameter is about 0.1243", which is closer (the origin circle still reports 0.1241") but is still undersized. Straightness doesn't seem to work properly, but I'm getting a cylindricity value of 0.0002". Link to comment Share on other sites More sharing options...
[Ri...] Posted January 31, 2019 Share Posted January 31, 2019 I'd be interested to dive deeper into the "high-accuracy micrometer". Is this a laser mic? A spec or some more clarification on this is required in my own opinion. If this is a laser mic, I'd be interested in the setup as these tend to be the Achilles Heel of these instruments. If none of the advice above seems to help, you could always send it to a 3rd party to get a different opinion. Link to comment Share on other sites More sharing options...
[Th...] Posted January 31, 2019 Author Share Posted January 31, 2019 It's a Mitutoyo High-Accuracy Digimatic micrometer, model no. 293-130. Link to comment Share on other sites More sharing options...
[Ty...] Posted January 31, 2019 Share Posted January 31, 2019 When was the last time the pin or mic was calibrated? Is the CMM measuring at the same depth as the mic check? Have you remeasured the other pin that was correlating? Link to comment Share on other sites More sharing options...
[Th...] Posted January 31, 2019 Author Share Posted January 31, 2019 I could've saved you all a lot of time if I'd been a little more attentive the first time I did this. It's the measuring force throwing me off here. Standard probing is done at 200mN and 100% probing dynamic, which isn't an issue for any of our parts, but the vise clamp only holds probably the lower quarter to third of the pin. By measuring at the upper end, the CMM is exerting enough force to flex the pin in the opposite direction, and since it's flexing away from the stylus the whole time, it's shrinking my measured diameter. If I'd taken the time yesterday to throw a 0.0700" pin on the machine (which I just now did), the 0.007" deviation would have clued me in. I took a diameter on the 0.1247" pin at a much closer proximity to the vise clamp, at 100mN of force and 25% dynamic, and got 0.1246". Don't know why I never thought to try changing those values before. I thought I was going insane. That totally explains why the larger pins weren't under-reporting, too - they're stout enough that the CMM wasn't bending them. Link to comment Share on other sites More sharing options...
[Ow...] Posted January 31, 2019 Share Posted January 31, 2019 How sturdy is the mechanism your holding the pin with "was" going to be my next question... 🤣 Glad you figured it out. Anybody that's run a CMM for while has been in the same situation you just found yourself, it drives you crazy. 🫣 Link to comment Share on other sites More sharing options...
[Ri...] Posted February 4, 2019 Share Posted February 4, 2019 Please sign in to view this quote. Something I have found, many of the adapter plates (VAST) DO NOT hold a probe even remotely close to the Z axis movement, which will also affect X and Y axis. I've seen these things out out of parallel from the locating pins to the cube surface over .005 (inches), guess what that does on a 180mm long stylus setup. So, check if your shanking out. Link to comment Share on other sites More sharing options...
[Th...] Posted February 4, 2019 Author Share Posted February 4, 2019 I observed the measurements as they occurred and saw light between the shank and the pin the whole time. Roundness values and graphs were basically spot-on, too. As far as I know, our adapter plates and extensions are good. Link to comment Share on other sites More sharing options...
[Da...] Posted February 5, 2019 Share Posted February 5, 2019 Probe stem deflection? How does the radius on the probe qualification report compare to the actual probe size? Link to comment Share on other sites More sharing options...
[Th...] Posted February 5, 2019 Author Share Posted February 5, 2019 This was already resolved, the pin was deflecting during measurement. Link to comment Share on other sites More sharing options...
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