Opinion on Calibrating after Tip Change

[Tinyfxds]

I normally calibrate all my probes at the beginning of the day. However, with doing multiple tip changes daily and trying to hold tolerances in the .0001"-.0003" range, I'm wondering if I should calibrate after every tip change. I spoke with a local Hexagon engineer and he said it's not a bad idea but he didn't really seem definitive in his answer. What are your thoughts, opinion or experience on this subject? Thanks in advance.

P.S. I am asking this because we have seen slight differences in the cmm values compared to a hard check inspection. Usually it is a Z depth that is varying.

[Matthew D. Hoedeman]

It isn't a bad idea. Repeatable to what value for auto-changes? 0.00005" might be fine for 99.9999999999% of the people, but that would be half your tolerance.

Of course, this is assuming that you actually have hardware capable of really measuring to tolerances that small.

And you will never get a "definitive" answer for a question like this from TECH, if they say "NO" then they might be liable for telling you so, that kind of thing.

[JEFMAN]

I always do it, when I'm looking for fine accuracy... I believe auto calibration is done for it.
It's a little more time consuming, because of the displacements between part and sphere, but the difference of accuracy needs this...

[YGOLORTEM]

Along with what Matthew and Jefman said I'd be curious to know what type of environment you are measuring those types of tolerances repeatably at. Is the temperature and humidity controlled within a very tight range? Is the air control unit blowing directly at or onto the CMM or CMM bridge? Are the parts being inspected thoroughly cleaned to remove any manufacturing debris? Are the tips thoroughly cleaned and inspected under microscope for cleanliness before and after each use?

Tolerances that tight, there's many many variables.

For those that never look at probe tips under scope try it sometime you'd be amazed what those rubies pick up. They're like little debris magnets and that little bit of debris will greatly affect measurement.

[NinjaBadger]

0.0001-0.0003" - what's that? Diameters, locations, positions?

What's the hard check inspection equipment calibrated to? We used to get plug gauges calibrated but it didn't mean they were right (or suitable for the required tolerance), just that the sizes had been checked and recorded.

[Tinyfxds]

Let's just say everything is perfect. I'm trying to find if this is common practice or not. I will start putting an auto-calibrate into the program and see if we get different results. I know there are a million reasons why I could be seeing differences but I'm interested to see if this would help lessen the differences. I guess we will have to try it and see.

[DAN_M]

Regarding calibration...I work with tight tolerances on machines that are in a shop environment. We have AC but the temp stll fluctuates about 5°F/day and we're pretty humid. So, for us, calibration BEFORE running a program is important EVERY TIME.

To that end, I have a "group" that contains PC DMIS autocalibration commands at the beginning of EVERY program I make. The operator has to answer a YES/No question to get past it. This puts the decision making process on them, removes me completely from the equation, and even tells on them if they didn't calibrate lol. This is from my "sample" program so things would have to be moved around, parameter sets would have to be made, & etc to make it work... but you get the basic idea I'm sure.

QUAL_QUESTION =COMMENT/YESNO,NO,FULL SCREEN=NO,AUTO-CONTINUE=NO,
Has the CMM been calibrated??
IF_GOTO/QUAL_QUESTION.INPUT=="YES",GOTO = LABEL_END_CALGROUP
QUAL_ACTION =COMMENT/INPUT,NO,FULL SCREEN=NO,
1--> Let's calibrate the CMM now.

2--> Just run the program. I
don't need to calibrate.

3--> I need help.

IF_GOTO/LEN(QUAL_ACTION.INPUT)<>1,GOTO = QUAL_ACTION
ASSIGN/VAR_1=ARRAY(1,2,3)
ASSIGN/VAR_2=MIN(ABS(VAR_1-QUAL_ACTION.INPUT))
IF_GOTO/VAR_2<>0,GOTO = QUAL_ACTION
SELECT/QUAL_ACTION.INPUT
CASE/1
CHECK/0.25,1
DISPLAYPRECISION/7
COMMENT/OPER,NO,FULL SCREEN=NO,AUTO-CONTINUE=NO,
Ensure sphere is in calibration hole.

Remove cap from sphere.

Press OK to continue.
COMMENT/OPER,NO,FULL SCREEN=NO,AUTO-CONTINUE=NO,
Ensure sphere is in calibration hole.

Ball facing CMM's Y+ (if applicable).

Press okay when complete.
AUTOCALIBRATE/PROBE, PARAMETER_SET=ALL-TIPS-WITH-DEFAULTS, QUALTOOL_MOVED=YES_DCC,
CHECK COLLISION=NO, SHOW_SUMMARY=NO, OVERWRITE_RESULTSFILE=NO
AUTOCALIBRATE/PROBE, PARAMETER_SET=ALL-TIPS-WITH-DEFAULTS, QUALTOOL_MOVED=NO,
CHECK COLLISION=NO, SHOW_SUMMARY=NO, OVERWRITE_RESULTSFILE=NO
COMMENT/OPER,NO,FULL SCREEN=NO,AUTO-CONTINUE=NO,
Put cap back on sphere.

Press OK to continue.
COMMENT/OPER,NO,FULL SCREEN=NO,AUTO-CONTINUE=NO,
MOVE PROBE FAR OUT OF THE WAY.

Once probe is out of the way, remove
the sphere & put in safe place.

Press OK to continue.
DISPLAYPRECISION/4
GOTO/
END_CASE/
CASE/2
COMMENT/REPT,

CMM Operator states that CMM is not calibrated and has decided to use it anyway,.
GOTO/LABEL_END_CALGROUP
END_CASE/
CASE/3
GOTO/
END_CASE/
END_SELECT/
CHECK/0.1,1
LABEL_END_CALGROUP =LABEL/
ENDPRG =LABEL/