CMM Inspection of Profile Tolerance

Michael_M

Trusted Information Resource
#11
I have not read ASME 14.1 in a long while, but working with prints for a long while, you have to assume tangent points. The reason the 2.38 length must be a reference dimension is because this number will change based on the .6615/.6585 dimension. Using AutoCad real quick for the math, the 2.38 reference will change as much as .217 depending on the actual diameter of .6615/.6685.

I am attaching a very quick and dirty screen shot of my AutoCad to give you a better view.
 

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T

Thetree

#12
I understand the point you are trying to make.

However:

It stands to reason that IF tangency to this non-basic dimension is assumed, the true position of this feature is considered to be dependent on the actual measurement of the .6615/.6685 diameter?
Is this incorrect? Should we assume it is tangent to the mean of the diameter?
 
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Michael_M

Trusted Information Resource
#13
In this case, I believe you will have to get with the maker of the drawing. You don't know what specific diameter the 2.38 length was made to.
 
W

Wilderness Woody

#14
I have a request from a customer to inspect the following (note:units are inches)

:evidence:
------I have a picture for this but due to forum rules I am not allowed to post it. Can anyone help?-------
:evidence:

Datum A is the face of the .305 diameter and there is a .285 hole through. There is no true definition of the location of the 16.000 R. Is it acceptable to assume tangency? If that is the case, does it allow for any step between the 16.00 radius and the .6615 diameter? I would assume it would allow for .0015 step. Also, would you consider the .305 diameter to be a part of the profile? There is an abrupt change in direction here, but the .305 is only shown basic and is not dimensioned anywhere else.

I am being asked to inspect this on our starrett manual CMM with apogee software and I am not confident at all in my ability to get an accurate reading due to short chord length of large radius and small datum face.

Any help would be appreciated.

Moderator Note: Added in edit: See picture in post #4.
Technically speaking, you can't assume tangency.

1) Since the adjacent feature does not have a BASIC dimension from which to design, you cannot determine what value the original design may have used to create a tangency to IF indeed they actual did!

2) There should be either a center point pegging the R16.000 location OR a note on the drawing to refer to the CAD model. Without either of these, the designer/draftsman was negligent and you need to ask for clarification!
Since this is not referred to as a "Controlled Radius", then step at the adjacent diameter feature .6615/.6585 is certainly allowed.

The Profile .003 is not limited to the R16.000, so unless another tolerance supercedes it somewhere else on the drawing, that tolerance is for the entire part (which includes the .305 dia.)
:2cents:
 
T

True Position

#15
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M

MikeSeymourAtl

#16
Profile is a good tool to use. It keeps you from having to measure a large radius from a small arc length.

The optimal measurement device (outside of a custom-built gage) would be a CMM equipped with CAD. Then, you could measure any number of points against the CAD model.

If you don't have a CAD model, then do this. Set up on the centerline of the workpiece (up/down) and on the end of the workpiece (left/right). Create your origin there. Now offset your origin to the theoretical center of the R16. Measure points along that radius, and report back in polar co-ordinates. A bit of a pain, but without CAD, that's the only way. And, for that, you may need to go back to the designer to get that theoretical center. I don't see that defined on the drawing fragment. To assume tangency is a stretch.

Whether you have a CAD model or not, your measurement report should not include any description of the R16 or its location. You would use those features to create a theoretical nominal, then check the workpiece against that.

A side note: The draftsman, or the designer, has put the cart before the horse. No 3" long part will ever be oriented with a .3" surface. Plain physics. The length of your workpiece will end up functioning as the A datum, and you should be checking the squareness of the end.

I do understand, though, that the design may not change, and you may need to satisfy the requirements as drawn. Fine. Measure the form of the workpiece, and add in a check of the squareness.
 
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