Feeling Like An Idiot

I've not run into an issue with using longer extensions like that, but, that makes me wonder if your head might not be getting too weak to hold & lock that long a build into a repeatable position every time.

I had a "drooping" issue years ago using a 5X20MM tip on a 50mm extension in the past when I was using a standard force module. I switched over to the stiffer module and that had solved my issue. I don't know if this is related to your issue, but I figured it might help.

My head is a HH-A-M5. I'm probing the part in A0 most of the time. I probe a large cone where I switch to various angles like A45B0, B45,B90, B135, B180, B-135, B-90, B-45. Those angles probe lines which I construct into a cone which is actually by datum. I probably need to rethink this.

Using a hexagon medium force module. I could change to an extended force module to see if that helps.

That is a head scratcher. 

I had a problem like that once.  I went nuts trying to figure out why things were measuring wrong on just one part after adding an extension to our probe build.  I wasted so much time trying so many things before I just stopped and watched the CMM run through the whole routine.  It turned out the probe was bumping the part during the last angle change.  And, since the bump happened during the angle change there were no movement errors.  It wasn't bumping hard enough to throw anything out of calibration, but it did move the part enough to mess up the measurements on the last feature being measured.  D'oh!

acgarcia said:

I don't understand why it would fail those angle measurements on the large part with the extension but pass them on the smaller still with the extension.

I would have suggested that the extension might have a crack or structural anomaly - but as you said... seems that the extension is not now the perp. How about shelling the program? Don't know what release you're using, but I get some extreme 'hiccups'  (more like the Demon is blowing chunks ) in 2023.2 current where one feature just is plain FUBAR... even cutting it out and recreating it. But shelling the .prg actually fixed it, at least for me.

(if you're lucky when deleting the suspect code... you might get the infamous pop-up "you are deleting stuff that is not shown") 

Ooof, worst feeling ever... having to go tell the shop floor or management that the problem was the CMM and not the part.  

I had a customer that had to use long extensions to access tight bearing bore IDs in a deep bore from both ends of the part.

They had variation in their results and had placed a ring gage on their fixture to measure the this in each index position (A90B0/A90B180) and they would print this result on the report for verification or a measure of how good the part maybe. It was confusing for their machinists.

I set up a test for them to measure the ring gage, index measure the exact same hit points from the other side, then loop and repeat. It showed that the head was not repeatable at these long extensions.

I then showed them that they could perform an autotip calibration after they indexed within the program.

We repeated the above test but included the autocalibration. The repeatability became excellent and a fraction of their tight tolerance, where as without the autocal, it was similar to the tolerance they were trying to measure.

If you have room on the table to place the calibration sphere so that you can autocalibrate after tip changes with long extensions, this will get around your problem.

Quick question, David. When doing the auto calibration on the tips with long extensions in your example, the error in calibration that is removed isn't from the probe/tip build itself, but from the head of the machine? How long of tips should we start trying this with?

Our machine is a Hexagon Global S. We use the HH-AS8-T2.5 Head + HA-TM-31 + TP 20, and there is very few programs with long tips. But I have seen programs with 100mm and 130mm long probe builds here that don't include a hard extension or something like an EM1 or EM2 due to needing as skinny and long of as a probe as possible. Should we be trying your method in these cases? I haven't noticed any parts being out when these programs are ran, but I also do not always know when they are ran due to us having 3 shifts of regular inspectors.

it will depend on extension length and what tolerances you are trying to measure too.

You can do this repeatability test to understand the effect of the head indexing repeatability for a given extension length.

At typical probe build lengths, indexing heads are generally sub micron repeatable (otherwise your would not be able to use them). This repeatability is magnified by the extension length as when you perform a single calibration the head is in a certain position but next time it indexes, it will be in an ever so slightly different position contributing to an error. Indexing heads are convenient but not as accurate as a fixed head (all the leitz high end machines are fixed head for example) 

my customer had a renishaw PH10 head and 200mm/300mm extensions on it for this one part. The auto calibrate increased cycle time but greatly improved accuracy and also removed the need for a verification measurement of a ring gage in both index positions to try and judge if the measurement was good. We also restructured their program so that we only indexed a minimum number of times.