How to Determine Tolerances for Equipment Verification

E

ermoth

#1
Hello Calibrating Covers,

My job is document control and my manager has asked me to make a record for monthly verification of our Surface Roughness Tester. The equipment is calibrated by an outside company every three years, but we want to verify it internally on a regular basis.
We have a "Precision Reference Specimen" (master) that will be used during verification. When I asked my manager what tolerance will be used he asked me to look it up online. I really don't know what to do from this point.

Any help would be greatly appreciated.
 
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E

ermoth

#2
I'll just go ahead and add the steps I have already taken.

1. I looked on the "Precision Reference Specimen" for a tolerance. I did not see one. If there is one there, I don't understand it.
2. I googled. No luck there.
3. I checked the calibration records for when we've had the "Precision Reference Specimen" verified. They appear to have used two different tolerances on the two different times they've done it.
 

Jerry Eldred

Forum Moderator
Super Moderator
#3
I'm probably not nearly the statistics expert that some are on this forum. But if I were going to have to "arbitrarily" derive a limit, it would be some portion of either the tolerance of the specimen, the RS combination of the tolerance of the specimen combined with the tester, or a critical product limit. Small enough that a drift beyond a given value would tell you something is either wrong with the tester or that something is going out of statistical control.

Just to stir things up and give the more expert members something to (rightfully) disagree with, I might start with 10x tighter than some critical applicable specification. In the semiconductor world, using SPC (Statistical Process Control) there are spec limits and control limits (the control limits being some multiple tighter than spec limits to guardband the process limits).

Maybe I've posted enough to get some further discussion on this.
 
D

David DeLong

#4
Hello Calibrating Covers,

My job is document control and my manager has asked me to make a record for monthly verification of our Surface Roughness Tester. The equipment is calibrated by an outside company every three years, but we want to verify it internally on a regular basis.
We have a "Precision Reference Specimen" (master) that will be used during verification. When I asked my manager what tolerance will be used he asked me to look it up online. I really don't know what to do from this point.

Any help would be greatly appreciated.
The rule of thumb is 1/10th the tolerance in its tightest application. I have a copy of ASME 14.43 on gage making tolerances and they have also used the 1/10th concept.
 

Marc

Hunkered Down for the Duration with a Mask on...
Staff member
Admin
#5
:topic: Jerry, is "guardband" or "guard band" the proper way to 'spell' it, or does it particularly matter? I've seen it written both ways. I'm just curious.
 
E

ermoth

#6
The rule of thumb is 1/10th the tolerance in its tightest application.
I'm really not experienced in this technical area, so could you explain this a little more to me? What do you mean by "tightest application"? Would that mean: Of all the measurements the equipment is used for, use the one with the tightest tolerance?
 
E

ermoth

#7
I'm probably not nearly the statistics expert that some are on this forum. But if I were going to have to "arbitrarily" derive a limit, it would be some portion of either the tolerance of the specimen, the RS combination of the tolerance of the specimen combined with the tester, or a critical product limit. Small enough that a drift beyond a given value would tell you something is either wrong with the tester or that something is going out of statistical control.
Thanks for the input, but I'm afraid I'm going to need this explained in layman's terms. :eek:
 
D

David DeLong

#8
I'm really not experienced in this technical area, so could you explain this a little more to me? What do you mean by "tightest application"? Would that mean: Of all the measurements the equipment is used for, use the one with the tightest tolerance?
Yes, you are correct that the tightest tolerance of application, one would use 1/10th the tolerance range as the calibration range of the instrument. This is a bit easier on a vernier caliper where one is using it on a part with a +/- .010 ". The range is .020 so the range for the calibration of the instrument is +/- .001.

Since your instrument is used where there is a unilateral tolerance. I would take the tightest tolerance where it is utilized and then take 1/10th of that value. Now you have a master sample that has been set at a certain roughness. Confirm it on the sample.

Sometimes the 1/10th rule is not applicable if the application is too tight so go "state of the art" which means that you phone the maker of the product and find out the calibration range is on a particular standard. That is it.

Good luck.
 
G

George Weiss

#9
I offer my view as a calibration tech.
Generally a manufacturer’s service manual and spec sheet would be reviewed.
I viewed the spec-sheet, and a similar item, and found that these devices calibrate/zero-out with a roughness std. No +/- limit tolerance(s) were listed. The device basically is specified to report back 1:1 as the standard material, which was offered for the device auto-calibration.
In the K-mart commercial calibration field, this item is viewed as an EASY-CAL by a calibration tech. It works or it doesn’t. An interesting discussion would arise if you asked the outside calibration vender "what +/- tolerance did you test this device against to achieve a 4:1 TAR and determine it PASSED?" . A calibration occurred because a reference standard, (SRM), traceable to NIST was used, but to make a ISO-17025 data-sheet would be interesting.
GOOD hunting on this one !

NIST roughness standards uncertainty budgets would give a direction to best possible accuracy
http://www.nist.gov/mel/ped/smm/upload/nistsurfcalib.pdf

On the 20 AUG 2009 the question came up.
Calibrating Mitutoyo SJ-201P Surface Roughness Testers

A good spec page for reference
http://www.mitutoyo.com/pdf/1902SJ400.pdf
 
D

Daniel Walker - 2011

#10
The manufacturer's stated accuracy should also be considered. You will be chasing your tail with service and recalibration visits if you try to hold a tolerance that is tighter than the unit was manufacturered to operate within. This is usually not the case but something to definitely keep in mind.
 
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