Techniques for Measuring Flatness

Paul F. Jackson

Quite Involved in Discussions
#31
I should have given more information in my last post. The customer slides our product into a slot on a trough which metal flows down. They feel that too much warping/bowing will create stress on the filter which could result in breakage when the metal flows through it. What if I made an adjustable "toaster" gauge? I could determine the average thickness for a production run, set the thickness of the gauge to the average thickness plus 1.5mm, and slide the parts through? Since the average thickness of a production run does not vary by more than .25mm, this should eliminate the majority of the warped parts. It would not be perfect but it would improve what the customer is seeing now.

Regards,
Steve
Steve,

It seems to me that there must be a physical limit to the "slot on a trough" as you described. If you "determine the average thickness for a production run, set the thickness of the gauge to the average thickness plus 1.5mm, and slide the parts through?" you would not be constrained by a constant limit for warp... imagine a plate folded over on itself... is that too much?

I think that the adjustable parallel plate "toaster gage" may be a good natural choice for process control as stijloor said... but I think that it (or another attribute gage) should have a fixed upper limit for maximum permissable thickness due to size and warp.

Paul
 
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C

Cullet

#32
Paul,

Yes, we would still use a different gauge to measure the maximum allowed thickness for this part, but we would use this adjustable gauge in addition to eliminate any product that has too much warping. Here is a crude drawing of the issue with our product. Think of when you cook a pepperoni pizza and the edges of the pepperoni start to curl up.

Regards,
Steve
 

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M

M Komarmy - 2012

#33
I think what Paul is getting at here is there must be a maximum allowable size for the filter to fit in the slot. If you automatically add 1.5mm to whatever the average thickness is, it will be possible that the part will not fit into the slot.

This goes along with the envelope principle, where form is assumed to be perfect at MMC. The gauge you use should not allow a part to pass beyond the limit of max filter size (assuming that max size is determined based on the MMC width of the slot that the filter will go into).
 
C

Cullet

#34
I see what you're saying. The adjustable toaster gauge would still have to have a maximum thickness equal to the maximum allowable thickness in instances where the average thickness of the production run plus 1.5mm would exceed that. Thanks for all of your help everyone.

Regards,
Steve
 
F

FBOL63

#35
If you use a CMM to measure flatness, does the unit under test have to be supported on three jackscrews to zero out the top like when using a dial indicator?
 

Miner

Forum Moderator
Staff member
Admin
#36
If you use a CMM to measure flatness, does the unit under test have to be supported on three jackscrews to zero out the top like when using a dial indicator?
The CMM software will compensate for misalignment. However, the probe should still touch perpendicular to the surface to minimize your cosign error.
 

Eredhel

Quality Manager
#37
I can't speak for all CMMs but for Zeiss Calypso CMMs it doesn't matter. You can have the part angled in the air and still get flatness.
 
I

InspectorRalek

#38
Flatness is a surface condition using ASME Y14.5M-94 but there is a new 2009 standard where it can be a centre plane and even at MMC where your method would be appropriate. That will not be your case though. It definitely is a surface condition.

There are a couple of ways to measure flatness of the surface.

First, one would have to have a dial indicator on a stand, adjustable jacks and a granite table. Make sure that the surface that requires a flatness tolerance is facing UP with the adjustable jacks on 3 places about 120 apart near the perimeter on the bottom. Zero off the surface directly above the jacks to create a plane on the surface requiring flatness. Once that has occurred, then sweep the surface with the dial indicator and take the FIM or TIR shown on the indicator. That is your actual flatness of the surface. There is no plus flatness or minus flatness only a flatness value or range.

Should the burr be included in flatness? Most people agree that a burr is a separate condition and should be reported separately. Do not include the burr in your flatness report.

If you have a CMM, then take many, many points on the surface and the CMM will give one the best fit and then the flatness value. This method is not quite as appropriate as the dial indicator since the indicator will sweep or contact more of the surface than the CMM.

Hope this helpsl.
Hi, I was wondering, I have an instance with a rectangular part which by using the three screw jack method I get a flatness reading of .105. This part appears to be twisted. A counterpart who works for the same company has instructed me to set the four corners of the part on screw jacks. Then set the opposing corners to the same height. So top left corner and bottom right corner are at the same height, then make the top right and bottom left the same height. The two sets of opposing corners do not have to be all four at the same height. Just each to themselves, if that makes sense. In my example I have two which I have deemed 0. The other two are both .038 from the first two. I then scanned and measured around .055. This method seems viable in that so far as I can report the surface lies in a smaller tolerance zone than the three jack method. I am just having trouble because the three jack method is what makes the most sense to me. Is there a fundamental flaw in how my counterpart is approaching this?
 
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