Is there an instance where you would use more than 2 basic dimensions to calculate Position? For example, there could be 3 Basic dimensions that all end up in the center of a hole. Would you use 2 basics for the Position and the other for Profile of Surface or is it possible that the print could intend for all 3 Basics to be used in calculating the Position?

Thanks

The only instance I can think of is if you have a projected tolerance zone

Not sure what that is...

Think of a part with a 90° angle at the left and two holes in the top of the part. The positional tolerance is in the hole in the 90° angle. There is an X basic to the hole, a Y basic to the hole, and looking down at the top of the part another "the three holes on center" basic. This is where I get 3 basics. Any thoughts? I am ok using profile of surface for the X basic, but the question was raised. Thank you for any other thoughts.

Not sure what that is...

A projected tolerance zone is one that is "projected" into space above the surface where the target feature is. It's denoted by a "P" in a circle in the feature control frame. It's generally used to control the perpendicularity of fixed fasteners or shafts that will be mounted in the hole. It's not used a lot. You're more likely to see the hole itself controlled for perpendicularity.

To answer your original question, features are generally located for position using only the basic dimensions in the x and y axes; the z axis is normally controlled as a tertiary datum which is normally a plane established in fixing the part for measurement.

If you're confused by the datum scheme, as it seems with your reference to a profile tolerance, you need to ask the customer, or whomever "owns" the drawing, for clarification--don't try to guess if you're not sure.

Hi Jager,

In addition to Jim's post, a Projected Tolerance Zone ("P" in a circle + the zone height) reduces the multiplying effect of orientation error of the controlled feature thus limiting possible interference with the mating part.

Do you have access to ASME Y14.5M-1994? That's the GD&T standard used in the USA.

Could you possibly scan part of the drawing with the positional callout in question and attach it to a post? Glad to help.

Stijloor.

Jager,

Basic dimensions describe the exact theoretical location of the tolerance zone that the axis, center-plane(s) or locus of the feature must reside within, no matter whether the zone is cylindrical, parallel piped, rectangular piped, or spherical. The tolerance zone (axis or center-plane) of a two dimensional feature extends indefinitely relative to the features that bound its depth, width, or breadth. These zones have up-to five degrees of freedom constrained by the datum features (according to the ability of the specified datum reference frame to constrain them) recognizing that the feature tolerances that bound the depth, width, or breadth of the axis or center-plane is variable itself. A locus describing the exact theoretical location of a spherical or other three dimensional feature may have up-to six degrees of freedom constrained by the datum features (according to the ability of the specified datum reference to constrain them). Often if the features are shown coaxial or at right angles to the datum reference some or all of the basic coordinate locations and orientations are implied values. They are none-the-less basic and contribute in locating or orienting the tolerance zone.

When hole has an odd orientation to the datum reference frame, designers sometimes (often) define the points where a feature axis intersects other features (planes, IDs, etc.). Those points may have X, Y, & Z basic dimensions describing their end-point locations but those end-points only serve to describe the exact theoretical basic location of the cylindrical tolerance zone that the feature axis must reside within. The layout inspector, CMM programmer, or gage maker typically has to compute from those (end-points) the feature’s theoretically oriented two dimensional coordinate displacements from the datum reference to accomplish the measurement. Reporting a position with basics specified like that then causes confusion for the customer or reviewers of the measurements because they like coordinate measurements to appear consistent with the basics specified on the drawing. A talented inspector can inspector can calculate an X, Y, and Z coordinate deviation at the point where the actual feature axis pierces the actual or nominal plane or other feature to satisfy the reviewer but that information is not as helpful to the process owner trying to monitor and adjust the process parameters.

The point is that basic dimensions describe where tolerance zones exist in terms of location and orientation in relation to feature patterns and/or datum references and alternate basics can be calculated from those specified to accomplish measurement. Manipulating basics to make measurements in relation to the datum reference frame is always permissible so long as the tolerance zone(s) of the feature or feature pattern is equivalently located and oriented from that datum reference.

Paul F. Jackson

Thank you for the replies. I won't be able to send a drawing. We have talked to the customer and got clarification. There is a note on the print that says Profile of Surface 0.8 A-B D(m) ALL FORM, so we are to use that for one of the Basics and use the other two Basics for the Position.

Thanks again

It is perplexing to me that they would qualify the profile callout for “ALL FORM”!

A profile tolerance applied to one or more flat/parallel surfaces will control the form of multiple surfaces simultaneously when no datum features are specified…but…when those flat/parallel surfaces are offset by basic displacements or when it is applied to surfaces that are not flat/parallel by design it constrains the entire contour limiting form and size (displacement) within the boundaries of the tolerance zone.

If datum references are specified in the profile callout the tolerance zone would be constrain location as well as size and form relative to the datum features.

By modifying the callout with the words “ALL FORM” is one to conclude that the profile zone does not constrain the feature surface for size as well as form within the tolerance zone?
For instance:
The cylindricity of a cylinder but not its size?
The cube-ness of a cube but not its size?
Yada, yada, yada.

Profiles can be relaxed for location and/or orientation relative to datum features either by using composite profile callouts, combined profile and position (boundary) callouts or by specifying limit tolerances between the basic profile and the datum feature …but I have never seen them relaxed for contour size.

I am not saying that the callout is wrong; it is just something that I have never seen before. Just because an (extension by notation) of the principles dimensioning and tolerancing is not mentioned or demonstrated in the standard doesn’t make it wrong. It is just harder those downstream of the designer to decipher.

We get some strangely dimensioned prints and we also have to get clarification on a lot of GD&T because we suspect the print wasn't really drawn correctly. I think it is a learning process for the folks drawing prints, so we are sometimes suspicious of what they really mean.....so we just ask. It can be frustrating.

You should get clarification of the term used "ALL FORM". This seems to be a generic term some engineer or engineers have adapted and as previously mentioned can be misleading.

I have many prints with a similar call out but the text is (example) "All Basic Dimensions not toleranced" are "Profile|.010|A|B|C" (Modifiers left out intentionally).
From your original post the text above would have clarified itself.

Side note; 3 Basics for one feature
I believe an instance could be as follows:
A spherical (ball) feature can have an X and Y coordinate for linear location as well as a Z coordinate for its centroid.

Some of our customers have that note about all Basics not toleranced..that helps. I think you are right that engineers sometimes leave a generic note like All Form, etc. Thanks for your reply. We are getting it resolved little by little!