Please help me understand this... calibration results

[Mike Johnston]

Good morning everybody...

I am hoping that you can help me understand a couple of things about the calibration results window. First of all, I don't understand where the CMM is getting the X,Y, and Z numbers from. My actuals are sometimes not even close to what is reporting as the theoretical values. I'll have an X theoretical of 0.000 and an actual sometimes of 10mm or more. I don't understand why this is. I've even seen a difference of 20mm at times.

The next thing is the actual diameter of my tip. I've attached an image of my last calibration which is on a 1.5mm probe tip and the results are 1.4746mm. This calibration was done at 80% movespeed and 2% touchspeed. When I decrease it to 20% movespeed and 1% touchspeed I see almost identical results. I measure this tip with a micrometer and it comes out perfect. 1.5mm. I don't understand why the CMM is reporting the diamter at almost .025mm off. Any insight and wisdom from you guys and gals would be tremendously appreciated since I am being asked to explain this difference to my bosses. Thanks in advance!

[Mike N.]

well several things to fix here....

1. make sure you calibrate at 100% on the hand control pendant and run at the same values ALWAYS.

2. it appears that you most likely have the wrong probe head defined or something wrong with your probe selections to account for such a large Z offset. Your probe config must match exactly to what is installed in your machine.

3. the diameters reported will always have a slight difference in value. This is because of the physics of the calibration and the touchspeed used. I would watch for great deviation in your standard deviation value. A probe that is 2mm dia and approx 2-3" long shouldn't have much more than .0003 max std dev.

[Wes Cisco]

You do not say what type of probe you are using. I have both a tp20 and tp200 rack and I have found that when I switch between the two the Calibration routine does not "snap" to the difference in the length of the probe body. Programs do, but for some reason the calibration routine does not! If I tell it the sphere has moved and recalibrate A0B0 1st, then all tips and modules for that probe are fine until I switch to the other probe, then I have to tell it the sphere moved again. Even though I have the quick change adapter, I have been bitten, so I always recalibrate *EVERY* time I switch between probes anyway.
I do not agree that you should always run 100% on the jogbox. I never do. *BUT* whatever you calibrate at you must run at. So if you calibrate at 2 bars on the job box you must run at 2 bars. Also your prehit and retract must be large enough to allow the the tip to "settle" out of move speed and into touch speed before it takes the hit. I have heard .1" as an approximate recommended distance. This is what I use and it seems to be quite adequate.
Your measured diameter will always be .0001-.0004" smaller than the ruby mics. This is due to flex in the shank and the algorythems used to calculate the measured diameter. My B&S instructor told us we could use calibrations with up to .0004" max deviation, but not over. HTH

[Mike N.]

I do not agree that you should always run 100% on the jogbox. I never do. *BUT* whatever you calibrate at you must run at. So if you calibrate at 2 bars on the job box you must run at 2 bars.

Slight missunderstanding here.....

like you said and I tried to say.....cal and run at the same speed no matter what it is.... ALWAYS

I said that regarding the jog box because most often the box speed is changed and sometimes forgotten when calibrating.....so its best to cal at a constant value and create a routine for yourself to stick to.

[Mike Johnston]

First of all, thanks for the feedback...

A little more info... Mistral CMM, PH10M head, TP200, and a standard force module. I do run at the same speed that I calibrate and I usually calibrate and run at 80% move speed, 2% touch speed, and 7-8 bars on the controller. Something just isn't making sense. I agree about the flex but with the same exact hardware on our microexcel I don't see the .025mm difference. In fact, the results look very good.

I'm sure that I have the probe file built correctly so I still don't understand why it's so far off in the Z-axis. I don't seem to see any "bad" results in the data that I produce I just like to see things a little closer to where they "should" be.

Thanks again you guys for your comments. If I find anything I'll be sure to post it here. Thanks!

Mike

[Wes Cisco]

First of all, thanks for the feedback...

A little more info... Mistral CMM, PH10M head, TP200, and a standard force module. I do run at the same speed that I calibrate and I usually calibrate and run at 80% move speed, 2% touch speed, and 7-8 bars on the controller. Something just isn't making sense. I agree about the flex but with the same exact hardware on our microexcel I don't see the .025mm difference. In fact, the results look very good.

I'm sure that I have the probe file built correctly so I still don't understand why it's so far off in the Z-axis. I don't seem to see any "bad" results in the data that I produce I just like to see things a little closer to where they "should" be.

Thanks again you guys for your comments. If I find anything I'll be sure to post it here. Thanks!

Mike

Not trying to just nit pick, but which is it? 7 bars or 8. You should set a standard and stick to it! If you calibrate at 7 and run at 8 you will loose some accuracy. And the same vice versa.
Have you tried telling it "yes" the sphere has moved and retouching with this problem tip? I would bet that if you do that and then recalibrate the tip again you will not see the large Z error. If you do, then you have some kind of a problem and I would not trust the machine until I had either solved the problem or proven to myself that I was getting good data by measuring some known features/parts. HTH

[Jeffrey Lovely]

delete that probe and rebuild a new one
worth a try

[NikoM]

Good morning everybody...

I am hoping that you can help me understand a couple of things about the calibration results window. First of all, I don't understand where the CMM is getting the X,Y, and Z numbers from. My actuals are sometimes not even close to what is reporting as the theoretical values. I'll have an X theoretical of 0.000 and an actual sometimes of 10mm or more. I don't understand why this is. I've even seen a difference of 20mm at times.

The next thing is the actual diameter of my tip. I've attached an image of my last calibration which is on a 1.5mm probe tip and the results are 1.4746mm. This calibration was done at 80% movespeed and 2% touchspeed. When I decrease it to 20% movespeed and 1% touchspeed I see almost identical results. I measure this tip with a micrometer and it comes out perfect. 1.5mm. I don't understand why the CMM is reporting the diamter at almost .025mm off. Any insight and wisdom from you guys and gals would be tremendously appreciated since I am being asked to explain this difference to my bosses. Thanks in advance!

Your results looks pretty good for me. I can see only slight deviation on Z and it can be due to your actual A angle ~90.5 deg (for example) instead of 90 deg. And your probe diameter can be smaller because of long (35mm) shank. As longer your extension as smaller you measured probe diameter. And .025mm is only ~.001" - it is not that much.

[#2#]

The X, Y and Z values are the location of your tip relative to the "perfect" position. Each time you unscrew and re-screw the probe parts this relation will change. Why will it change? Because your probe parts are not perfectly straight and the angle at which you stop tightening will be different each time.

You don't need to worry about the X, Y and Z's. These are for PC-DMIS to calc. where the tip actually is rather than its "perfect" position.

You don't need to worry about the diameter being different from "perfect" either. PC-DMIS will use this diameter to compensate for probe flex, etc.

Why are you so far off in Z? My guess is because you are indexing your probe to A90 and your probe isn't really at 90º to the z axis. It looks like the numbers are similar (4.2 and 4.4) at 180 degrees apart in angle B (B-90 and B90). I would guess that your head is square but may not be rotating to the exact 90º angle. If the head weren't square the numbers would be high at one B angle and low at the opposing B angle. The longer the probe the more error in Z. Of course, this shouldn't matter as long as you don't shank since PC-DMIS uses this to compensate for any error.

The standard deviation (as stated before) is what you DO need to worry about. That is the standard deviation of all the hits taken on the sphere for that angle. So, it is telling you how spherical your tip and cal sphere are. Hopefully your cal sphere is very spherical so basically it reports how sperical your tip is.

[BOB PARSONS]

CALIBRATION

Remember also that the dia. reported is the effective dia. and thus should
not be the actual dia. Your dia. looks very reasonable to me. The effective
dia. takes into account flex, triggerring, and other physical factors. As stated
above the software uses the effective dia. and is not an indication of error.
Your deviation gives you a better idea of your accuracy.

[cmmguy]

Unless your testing a program, why would you get in the habit of ever running less than 100% on the jogbox? Wouldn't it be better to control your speed in the program instead? I am just curious.

[Wes Cisco]

Well I am in the downhole oilfield tool business, not sheet metal or dies. Most of my parts are essentially hydraulic manifolds. I often need to measure small diameters and cylinders deep inside the part. Therefore I have many long tips. I also have many airtriggers. I usually run at 2 bars on the jog box for everything; although I have a couple of jobs with long programs and short tips, that I run 3 or 4 bars on. Those special programs have lots of big notes about which speed to calibrate and run at. So to answer your question, I am pushing my hardware to it's limits and beyond in regards to tip length. The only way I can do this is to slow things way down to minimize air triggers.

[cmmguy]

I still dont understand why you dont program the speeds you need in the part program. Then it doesnt effect your probing speed.

[John Kingston]

Unless your testing a program, why would you get in the habit of ever running less than 100% on the jogbox? Wouldn't it be better to control your speed in the program instead? I am just curious.

I agree, I may "test run" programs at lower speeds on the jogbox, but all programs for part verification are run at full jogbox speed. I don't want the operators messing with that and they are told that. BUT, we (ok I am) are very picky about who runs the CMM.

[Wes Cisco]

I still dont understand why you dont program the speeds you need in the part program. Then it doesnt effect your probing speed.

I am not sure I understand what you mean when you say, "then it doesn't effect your probing speed". Do you mean touch speed? I do not see what advantage would be gained by touching slightly faster, and with my long tips, that might cause more bad hit data. As a concrete example I have two tips I use a great deal. One is a 5x100 carbon fiber (one piece), and the other is a 1.5x90 (1.5x20 tip & 70 carbon fiber ext). With both of these tips the main application is A90B0 & A90B180. I have a B&S Xcel 9-15-9 with Sharpe 32 controller. If I try to run at more than 2 bars on jog box and 40-50% move speed in the program I have too many airtriggers. Even at these speeds it is not uncommon to have an airtrigger every minute or two. With my shorter tips I run 70-100% movespeed in the program, but still 2 bars on jogbox, except for a very few programs as noted previously. If I tried to run everything at say 5 bars on the jogbox I would be running maybe 10 or 20 % movespeed in the program. I could do it that way, but 2 bars was the standard when I came here and there were already hundreds of programs in existence using that method, so I was sort of stuck with it. I really need a scanning head, mostly for it's ability to handle longer tips. They are looking to relocate this company, (locally, just need bigger building), and when they do I am pushing for a second large dcc cmm with scanning head. We will see...