probe problem

[mig]

Hi,

We using renishaw probe on our 4020 okuma milling center.

Last time I found that center of measuring hole always out by Y on 0.0005-0.0007 of inch.

I redialed ruby if probe and recalibrated probe again but it doesn’t help .

I tested probe on test ring result same Diam . size is perfect , center point is out.

Anyone has some experience with this issue ?

Thank you in advance

[?Mark]

quote:

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Last time I found that center of measuring hole always out by Y on 0.0005-0.0007 of inch.

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That's way too much. The only thing that comes to mind is that the head is out of alignment in this plane. Funny thing is that a ball-bar test might not show it but running an indicator on a precision blank holder (forgot what they call it) will show it.

How is it in x axis?

[Rick Henrickson 65]

are you sure you have the position of the calibration correct? and what time of day are you checking this? where I use to work we would probe the guage ring through out the day and make adjustments for thermal comp!

[mig]

Our machine has termal comp control and i use to set my probe in end of the day, also i checked perpend. of the head , looks ok so far.

[?Mark]

You'd need to use one of those

[chris m]

Does the stylus run out? Does the machine rotate the stylus during calibration but not during part probing, or vice versa? If you calibrate with the probe stationary it will account for the spindle and stylus C/L being different, but if the probe indexes 90deg so that you are touching on the same side of the probe during calibration OR during part probing, but not during both, your numbers will be off. If your ruby is damaged that would also account for some problems with position.

 

I always warm up the axes on my probing machines before starting for the day to take the backlash out; could this be a factor for you?

 

C

[Rick Henrickson 65]

probe to set centerline, then calibrate.

[Psychomill]

Add to that... are you using a spin probe with a real long stylus? Is your calibration speed different than your part probing speed? And as Mark stated, you may have some twist in the machine. Ball-bar will not always show twist. You need to check with a test bar and a machine square for the spindle/axis.

 

Another thing I've seen this happen is a fretted spindle or taper. The probe doesn't repeat its seating position in the hole causing the calibration to shift. As Chris mentioned, recalibrating as part of the machine cycle may help the situation. But I've seen some bad ones too.....

[LT_mechead]

Mig,

We are having a similar problem with our probe. I asked the rep about runout, machine misalignment, etc. He explained that the calibration routine should compensate for all these variations unless they are excessively large. The calibration routine simply records the error for each direction of touch. The error and direction data is later used compensate the values output by the measuring routines. As long as the probe is oriented the same direction, and triggering is highly repeatable, compensation for probe and machine misalignment becomes a matter of arithmetic. Repeatability for triggering on the OMP40 is supposed to be within something like 10s of millionths (IIRC); insignificant as far as the machine is concerned.

 

You might try recording the error values output by the calibration routine. They may provide some clues about the problem.

 

As for our probe I am wondering if the tripod triggering mechanism is damaged. The probe sits at a slight angle to the axis of the body. I can get the runout within specs but doing so puts the body at about .08 TIR.

 

Does anyone out there have techniques for testing probes? It would be very helpful if there were a way to check the probe independent of the machine.

 

Lee

[Guest]

We found with our probes that the ring location needs to be checked and adjusted right before you qualify the probe. Our machine do not have glass scales, so as they warm up we found our Y axis was growing and changing the cold location of the ring by about .0008

 

We do not go back to the ring UNLESS it has been a couple of weeks since the last calibration or the probe tip has been changed and if we do we double check the location first and adjust as needed.

[LT_mechead]

John,

Indeed. The Renishaw rep said calibration should be done when the machine is running similar to conditions when measurements will be taken. If a probe is working properly the repeatability is fine enough to check machine growth.

[?Mark]

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The probe sits at a slight angle to the axis of the body. I can get the runout within specs but doing so puts the body at about .08 TIR.

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Normally it's not an issue as the probe is calibrated in a fixed position, except when you need to get into a tight space where .08" is a difference between clean acces to a part or crash.

I removed ours, realigned to within tenths and re-calibrated.

Mig if you indicate a bore/ring and go in with a freshly calibrated probe and it shows the bore to be off, then the misaligned head is a problem.

Not a big problem though, because you can put a Y-.0007 shift into your macro and all should be good.

 

Please keep us updated, as I've been wrong before...

[Psychomill]

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He explained that the calibration routine should compensate for all these variations unless they are excessively large

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IMO... sort of. Probe calibrations only compensate errors to itself and not to the entire machine. Think of this... calibrations happen only in a relatively small area of the machining envelope. If you have a small twist in the Z axis (HMC) for example, the probe will pick up error in the XY elswhere. The check for this would be to set up the calibration gage in the same area as the feature you're probing. If you're still picking up error, then it might be something mechanical like the probe itself as someone mentioned.... or the toolchange motion, taper seating, changing probe speeds, etc. Or, simply measure the gage location (no need to calibrate for this test), then recheck the location with the probe again. Then tool change out/in and re-measure the position again. Even take the probe out/in by hand at the spindle and measure. If its still floating excessively, I'd have to guess mechanical....

[Guest]

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Mark Banaszak

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What's with the name change Mark?

 

You get into the Beta program?

[?Mark]

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What's with the name change Mark?

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I enjoyed that bit of anonymity.

 

quote:

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You get into the Beta program?

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Can talk about it ?

[LT_mechead]

Psychomill,

I didn't mean to imply the calibation routine was a 3D error map. But, for a given probe length and a constant error in spindle alignment the calibration will comp for it.

 

You were talking about setting up the calibration in an area local to the measurement ... I have read that this is exactly what Renishaw does when making their probes. Probe bodies are checked in process against an artifact mounted on the fixture. In this case the "artifact" is simply a calibrated copy of the item being machined.

 

Mark,

Maybe I am misunderstanding you, but did you actually tweak the stylus back into position?

 

Lee

[?Mark]

Yes. The body of mp10 probe is in a way floating, meaning you can move it by substantial amount in any XY direction (and z angle) off the center of a holder.

Basically you loosen the set screws (not too much or it'll fall) indicate it while tightening the screws to whatever accuracy you're comfortable with and recalibrate...voila

[?Mark]

Check pages 21-24

[LT_mechead]

Oh, that's what you meant. I've done that multiple times, once to the point where the runout turned out to be less than the tool holder/spindle repeatability. There's always a .0003-.0007 position error after calibration.

 

Lee

[?Mark]

quote:

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There's always a .0003-.0007 position error after calibration.

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?

[LT_mechead]

Mark,

Thanks for the link. I'll have to see if I can dig up some similar service documentation for the OMP40.

 

Lee

[Psychomill]

Lee... all of the Renishaw docs are online... everything from set up, parts manuals, usage, software/programming, etc. Here's a quick link to the OMP 40 .

 

As long as someone understands that the "road map" can also cause some grief is where I was getting at. I've seen people try to point the finger at bad calibration or keep buying parts for the probe system only to find out that they had a slight twist in the machine....

 

 

Another good one... had a guy who kept calibrating and calibrating the probe and it wouldn't repeat. Re-installed software, changed about $100 in batteries, attempted to re-write the macro, adjusting probe speeds, wrote a macro from hell to comp .... nothing made it repeat. As it turn out... the stylus wasn't snug on the bread-away..

All he had to do was probably indicate the stylus and he probably would've saw it turn. Still can't figure out how he changed all those batteries without noticing that....

[Psychomill]

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the stylus wasn't snug on the bread-away

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Sorry... "break-away"

 

On another note, had another instance where a loose pull-stud caused the same grief....

 

[mig]

Hi,

Thank you everybody for replay.

I got so mach information , witch I have to go thru.

Unfortunately I can't check again my adjustment because i have huge plate sitting on the machine now.

As far i will have chance to go thru adjusting routine I will keep you guys updated

Have everybody good weekend