Using 3 Basics to find Position - Customer says to use the X, Y and Z Basics

[jager]

Have you ever had to use 3 Basics to calculate Position? Our customer says to use the X, Y and Z Basics. I think one of the Basics should be Profile of a Surface instead. What formula would you use to calculate using 3? Any ideas? Thank you!

[Stijloor]

In Reply to Parent Post by jager

Have you ever had to use 3 Basics to calculate Position? Our customer says to use the X, Y and Z Basics. I think one of the Basics should be Profile of a Surface instead. What formula would you use to calculate using 3? Any ideas? Thank you!

Before we get too carried away with GD&T/Inspection guesswork..can you attach a drawing? Just the pertinent info is helpful.

Stijloor.

[jager]

I'll sketch something up. It is position of a hole, which is at an angle. The point in the center of the hole has 3 Basics going to it. The print is very large so I'll just have to wing it with a little sketch.
I will need to figure out how to post a picture....

[Paul F. Jackson]

I think I know what you are asking…. You have a hole that is not square to the datum features that… its position control references for location… but the basic dimensions are specified “square” to that datum reference. Is that correct?

The designer has chosen a reference point along the hole’s axis (probably where it intersects another feature surface a one end) to detail its location. Is that correct?

The customer wants you to figure the hole’s position deviation at that point using all three deviations from basic using the formula 2*(Xdev^2+Ydev^2+Zdev^2)^0.5. Is that correct?

If those assumptions are correct and there is a position control applied to the hole’s size tolerance… then the position control applies for the entire hole’s axis, not just one point at its end where it intersects another feature. To inspect the hole for its position to the datum features the inspector or inspection programmer has to orient and displace a measurement reference normal to the hole’s specified axis location from the datum features. To do that he must first translate the basics specified into equivalent basics that are normal to the specified hole’s axis. When that orientation and reference move is completed he can probe or inspect the hole’s surface for its position location along its axis.

Here is the problem!

The designer, who has never inspected a part, arbitrarily chose to specify basics square to the datum reference frame not realizing the additional work that his choice will cause the inspector and he will argue if pressed to change his specification that “Basics describe the theoretical exact size form orientation and location” and may be correctly specified equivalently in many different ways.

The quality reviewer, who must comply with ISO requirements, has circled the basic as quality requirement and wants a measured result compared to its tolerance value to discern whether the measured basic is within its limits and the probability of a defect is sufficiently remote. He cannot excuse the basic from having individual limits and from being transformed by the inspector… it is all too confusing for him so he will just declare a non-conformance.

The inspector who has diligently transformed the basics into equivalent basic displacements and orientation from the datum reference and discovered the size of the cylindrical tolerance zone required to contain the measured axis deviation throughout the hole’s depth is being asked to report the equivalent X, Y, and Z measured displacements of the axis at the intersection of the hole’s axis and the surface so that the inspection report will match what has been circled for quality scrutiny.

In a perfect world the designer would specify the basics normal to the feature being controlled just like the inspector has to check them and the quality reviewer would not circle basics but rather just the position control.

Paul

[Jim Wynne]

In Reply to Parent Post by Paul F. Jackson

Here is the problem!

The designer, who has never inspected a part, arbitrarily chose to specify...

That is indeed the problem, I'm sure. Practically everyone who's had to inspect a part against GD&T requirements could complete that sentence with some sort of horror story.

[bobdoering]

Any MMC called out?

[Stijloor]

In Reply to Parent Post by bobdoering

Any MMC called out?

I am anxiously awaiting to see what the OP has in store for us.

Stijloor.

[jager]

I believe Paul has it right the way he described it. No, there is no MMC! From what I have heard, the customer had an outside programmer program their cmm for this and whatever result the operator gets is what they go with. They tell us that all three Basic dimensions are involved in the result.