calculation of tolerance 1

[jebedar]

I am looking for help in interpreting the error we are getting on this part in particular on the .086-56 hole positions. the question is concerning the 1.340 basic dimension and the hole position being call out as true position to -D-. Would this not be checked as position relative to D. Both err to the plus so .670 is actual 672-.673 both holes and this makes 1.340 at 1.3455 which is the correct way to calculate the error?

 

[pmarc]

The maximum allowable actual distance between axes of the threaded holes and the datum axis D is .675 (.670+.010/2). So if your measurements are .672-.673, position of the holes is OK for me.

 

[powerhound]

.672-.673 is good as long as the distance from A is no more than .004 from .090 in either direction. Also, datum D should be one radius or the other but not both. Is the note trying to say that Datum D is both?

Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[pmarc]

powerhound,
Why .004 and not .005?
Why both radius should not be assigned as datum features? Do you know something about part's functionality that we do not know?

 

[btrueblood]

"Why .004 and not .005?"

because .003^2 + .004^2 = .005^2

 

[powerhound]

If a feature is off by .003 in one direction and .004 in the other then that puts it within a radius of .005 of the theoretically perfect position, thus within a diameter of .010 which is the limit in this case.

As far as both radii being considered datum D, maybe things were done differently in the 1982 standard but the features that constitute datum D can be displaced from one another by .006. If that were the case, which feature would you use to calculate the positional error of the holes? I never got to know the 82 standard very well so maybe there's provision in it that cover the issue.

Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[pmarc]

Okay, I see it now - .004 in horizontal direction is maximum allowable if actual error in vertical direction is .003.

 

[drawoh]

jebedar,

The 1.340" dimension shows one hole relative to the other. The positional tolerance shows each hole located from datum[ ]D. In other words, each hole is nominally .670" from the centre of the inner radius.

JHG

 

[powerhound]

What constitutes datum feature D? From what I see, an attempt is made at making it both counterbore-type features. I don't think you can do that. There is an OD, an ID, and a counterbore coming in from each side of the part leaving a .050 lip in the center. As I said in a previous post, I'm not completely familiar with the 82 standard so can someone who is familiar with it explain what datum feature D is?

Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[jebedar]

Thank you all! my main concern was we had an inspector measuring the 1.340 dimension and calculating the true position using the error in the 1.340 dimension as the error in that direction. From what I have read here you all agree this is incorrect and the error on each hole is calculated fron the basic of .670

 

[SeasonLee]

jebedar

This could not be your main concern, but I am wondering if you can use the position FCF on the ID R.595, since it is not a feature of size, here is the tip.

http://www.tec-ease.com/gdt-tips-view.php?q=147

SeasonLee

 

[jebedar]

Okay I guess I still need a little help. My customer insists that this true positon is calculated from the result of the 1.340 measurement of 1.3455 and is out of tolerance to the true position of .010 as it would be if you used this measurement Can someone please respond with an interpretation of this utilizing the standard that I can show them why that is not the way this is calculated. ARGH!!

 

[pmarc]

I am not sure you will find anything really useful in the standard to convince your customer.

First I would try to clarify very fundamental thing:
Basic dimension 1.340 should not be measured at all. It is on the print (together with basic .090) to define where, relative to datum reference frame C|A|D(S), the positional tolerance zones for both holes are perfectly located. This perfect location relative to DRF, as defined by basic dimensions, is called true position of holes. So actually the measurements should show how far from the TP the actual axes of the holes are, and should not report the value of perfect 1.340 which itself has no tolerance.

Now coming back to 1.3455 measurement (assuming you have to live with this result without an option of modifying inspection report)....
According to the print the maximum distance between holes axes can be 1.350. So seemingly 1.3455 is within that. But that's true only if both axes are perfectly perpendicular to C and are exactly located at .090 from A. However if the actual distance from A is different (but within positional tolerance zone) the maximum actual distance between axes in orthogonal direction has to be smaller to stay within positional tolerance zone. So 1.3455 may in fact be outside the spec. That being said, in order to be certain whether the holes meet positional requirement it is required to know the actual deviation in horizontal direction (distance from A). Without it, your customer may be right, though relying on incorrect / incomplete data.

 

[jebedar]

Thank you all for your help. through your replies and the use of some of the tips on this site was able to resolve with a win!