How to measure parallelism btw top surface and bottom surface?

B

bmatsell

#1
I need help on the best method to measure parallelism btw top surface and bottom surface of a machined plate. No CMM available.
 
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Stijloor

Staff member
Super Moderator
#2
Re: How to measure parallelism?

I need help on the best method to measure parallelism btw top surface and bottom surface of a machined plate. No CMM available.
Please describe the call-out on the drawing (including the parallelism tolerance) to us. Is the bottom surface the datum feature? Does it have a possible flatness requirement on it?

If nothing is specified, then it becomes an implied flatness situation. That can lead to different measurement results depending on what surface is used as the (implied) datum feature.

The quick and dirty way :mg: is to place the part on a surface plate, and place an indicator on the top surface. Move the part under the indicator and note the indicator value FIM. (Full Indicator Movement).

Hope this helps a little....

Stijloor.
 
A

arios

#3
If you are going to produce several units of the same array of plates then you could produce a go-no-go fixture that can be used as a mating part between the two parallel surfaces, one "good" and at least one "bad" sample. Your go-no-go gage could be used for regular inspection purposes.

If you choose this option you will need to confirm the accuracy of your go-no-go gage either by performing an in-house documented verification or "calibration", or even be better by having your customer approving your test fixture for its equivalence to a mating part.

The results however will be attribute and not variable data.
 

Paul F. Jackson

Quite Involved in Discussions
#4
Arios,

Technically you cannot verify parallelism with an attribute gage (go-nogo) because a thin part has the same parallelism requirement as a thick part.

There are controls however that allow Maximum Material Condition modification that could be verified with those parallel plate attribute gages.
In ASME pre-2009 you can use straightness at MMC applied to the feature size and ASME post-2009 you can use flatness at MMC.

Bmatsell, To check parallelism lay the datum feature surface on a flat surface plate and use a test indicator attached to a height gage to scan the surface... recording the full indicator movement.
 
B

bmatsell

#5
Yes, the "bottom" or one side of the plate is the datum feature with a flatness spec of 0.75mm and parallelism spec of 0.15mm. Since the tolerance is greater with flatness and the product has high variablilty in flatness, it is difficult to rely on the surface plate as a true indication of the plane.

Otherwise Stijloor methods (which is what I have tried) is all I can come up with. But I feel my results are not accurate beacuse of the flatness.:frust:
 
D

DrM2u

#6
Yes, the "bottom" or one side of the plate is the datum feature with a flatness spec of 0.75mm and parallelism spec of 0.15mm. Since the tolerance is greater with flatness and the product has high variablilty in flatness, it is difficult to rely on the surface plate as a true indication of the plane.

Otherwise Stijloor methods (which is what I have tried) is all I can come up with. But I feel my results are not accurate beacuse of the flatness.:frust:
Let's do some simple thinking here ... A part placed on a (certified) surface plate is going to locate on the three highest points that come in contact between the plate and the part. If the part is larger than the plate, then how you locate the part can significantly affect the parallelism readings. If the part is smaller, then the location is less critical to the inspection because a good calibrated/certified surface plate is flat within microns.

Now, to trully check the parallelism between the two sides of the part you would have to sandwich the part between the surface plate and a certified parallel plate then use an indicator on the parallel plate. The other higher accuracy method/option is with a CMM but you don;t have one.

For the simplest method, I would follow the suggestions posted by Stijloor & Paul. If you don't like the results then it is possible that the part is not as parallel as you would think and/or there is too much variation in part roughness. A flat tip for the indicator will eliminate some of this variation.

And this is my :2cents:, right or wrong. :D
 
#7
I feel my results are not accurate beacuse of the flatness.:frust:
This makes me wonder: How thick is that plate? Will it adapt to the shape of mating parts when assembled?

I guess what I'm really asking is whether the tolerances are in line with the intended use. Could you give us some info about that? It is not uncommon to find parallellism and flatness requirements on parts that will easily flex to the correct shape when assembled. Could it be that thickness variation is the real worry?

/Claes
 

Paul F. Jackson

Quite Involved in Discussions
#8
Chances are the person that specified parallelism was intending to control material thickness variation... considering the small value for parallelism and the large value for flatness.

Unfortunately there isn't a standarized control for that parameter of a feature in either ASME or ISO standards.

If that is what the design is intending to control it is going to have to be spelled out in words.

Talk with the designer.

Paul
 
M

Mandell

#9
Doesn't the datum place in contact with surface plate (or something else sufficiently flat) allow you to use the surface plate as a datum? Or did this change?

In that case, the highest three points on the datum plane ARE the datum, and measuring against that measures the parallelism. Pretty simple actually.
 

Stijloor

Staff member
Super Moderator
#10
Doesn't the datum place in contact with surface plate (or something else sufficiently flat) allow you to use the surface plate as a datum? Or did this change?

In that case, the highest three points on the datum plane ARE the datum, and measuring against that measures the parallelism. Pretty simple actually.
Nope! No change!! Datum terminology confuses some folks who have not received GD&T training. :notme: (Come see us! See my personal profile.)

The datum feature is identified with the datum feature symbol. That means that this feature has to contact the datum simulator, in this case the surface plate. The datum is understood to exist in the contact between the datum feature and the datum simulator. Simply stated: place the part surface on the surface plate and all parallelism measurements are taken from the surface plate.

Stijloor.
 
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