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Need help stylii & speed selection, prehit/ retract & probe modules selection.

Hello everyone again!
This community is so awesome, I just wanna say thank you everyone. :-) I've already had so many questions answered by y'all just from reading here and learning. So, over the weekend, after taking y'alls advice and retweaking even more of my program, I ran into a little bit of a snafu. Basically, what happened is that some parts that were consistently checking good all of a sudden started checking bad. I traced the problem down to a portion of the program where I had re-tooled it to use a different size probe to take points on a smaller hole. (I went from probing the hole with a 1mm to a 4mm ruby) . The hole sizes are .290 inches and .286. I am also using the 1mm ruby to measure a hole that is .100 inches in diameter. The .100 inch hole has always measured really well on this part, even though the 1mm ruby barely fits in the hole. I only have 2 probe modules right now (we are currently in the process of buying more modules and looking at a tool changer). I have an SF probe module and an MF probe module. The 1mm ruby is in the green/MF module, and the 4 mm ruby is in the yellow/SF module. I am fairly confused here too. I thought that since the 4mm ruby was in the Standard force module, that it would actually be more sensitive in this case than the 1mm ruby because the 1mm ruby was in the MF module...??? Also, the data when using the 4mm ruby is so bad, it scared everyone in production. Some of the values were 6 or 7 thousandths off, but whenever we checked it with the 1mm ruby that error went down to the tenths, like .0002 off and such. I just don't understand, because I thought that CMM operators generally tried to use the biggest ruby within reason to cut down on time wasted (the ruby having to travel a big distance). I was having trouble with the 1mm ruby when I had the touch speed jacked up to 3mm/sec in the small .100 holes. When I changed that touch speed to 1mm/sec, the ghost and double hits in the hole went away. Which really confused me because you think I would have many, many more problems with the 1mm probe exploring a .100 hole. Nope! For some reason the 4mm ruby just does not like the .286 holes.

In general, I just need more experience with the ins and outs of CMM operation. I just really don't have enough experience here and am humbly asking anybody for more information for all these nuances/ if any of y'all have a really down and dirty guide for the behind-the-scenes data that doesn't seem to be talked about very much. i don't really understand the difference between the probe modules either (LF/SF/MF/, and EF). I don't really understand the relationships between accuracy and prehit distance and retraction either. I get that the slower the touch speed is the more inherent accuracy the operator will get. I'm just really stumped by the 1mm ruby outperforming the 4mm one. Especially since I was told from the beginning that the smaller probes can be finicky and unreliable, yet in this instance it is quite the opposite, and the smaller probes have been great. Selecting the right stylii and all the little accessories was not something that was explained to me very well. We used generic stuff at the Hexagon training labs, and it seems that every QC lab I've toured just has their own preferences, and when I've queried about different stuff in their operations, they just respond "it's what works/what we do and we just haven't changed it). I've found that a lot of the settings/preferences/data seems many times to be set by someone before the current operator/they just use what they always do and don't have an explanation.
Parents
  • Have both tips been calibrated? Every time you take something apart beyond popping the modules off the magnetic receiver on the head they need to be re-calibrated. If they're not calibrated at the same time, you can't use them both on the same part or you'll get whacko (and wrong!) results.
Reply
  • Have both tips been calibrated? Every time you take something apart beyond popping the modules off the magnetic receiver on the head they need to be re-calibrated. If they're not calibrated at the same time, you can't use them both on the same part or you'll get whacko (and wrong!) results.
Children
  • Yep!!! In my program, any time a tip is moved/swapped/anything, I use either the auto-cal or the calibrate single tip feature in PC-Dmis. (I prefer to use the single tip cal so far). I haven't had any issues with tips failing or bad calibration... That's why I thought maybe it was the fine-tune settings or perhaps the problems was that my 2 different probes were using 2 different module types (one is Standard force, and the other is medium force)
  • Are you using the 20mm long tips?
  • Both my probes I'm currently using are: A 1mm rubyx40 mm in a Medium Force module, and a 4mmx40mm ruby in a Standard force module. I've used longer for some aerospace parts that we have. In that example where I used a 70mm long styli, it was slightly due to time savings, but there is one hole that could only be accessed by a much longer stylii without having to write another separate program with the part flipped upside down or something.
  • I'm not sure what you have going on then; while lower force is better if you can get it to work consistently, I would expect either a standard or a medium force module to work with either of those tips.
  • @mbatten....what is even stranger is that today, the first thing I did when I came into work was re-tool the program so that the 4mmx40mm long styli I was using lost some length to a 4mmx20mm styli. In this configuration, it initially calibrated better (I was getting .0001 deviation before, I'm getting 0.0000 deviation the first time I cal'd it). It didn't help at all!!! And the shorter probe is even in my more sensitive module, the Standard force one. I only have a standard and a medium force module (I'm waiting on the Low force module). I don't know what to do at this point except hope that the Low force module takes care of my problems and gives me peace of mind? I guess it doesn't matter, because the holes that were coming up with error, I switched the program back to the 1mm ruby tip to measure the features, and all the data was accurate, repeatable and believable. It just slightly bothers me because I always heard that I should use as big of a ruby within reason to cut down on time and all that, and this ruby isn't ridiculously big for the hole either, I feel the 4mm is actually a better fit than the 1mm. Strange...
  • Make dead sure you've wrapped your head around the master probe discussion below. It sounds to me like you may have an issue with how the calibrations of the different tips relate to each other. The single-tip calibrations you're doing *may* be doing more harm than good.

    If you haven't gotten the chance, I highly recommend getting some level of CMM/PC-DMIS training from Hexagon. The 101 class will get you up to speed on the master probe calibration method among other things.