Position tolerance, radial dimension tangent to semicircle slot

[ScottEng21]

Need some thoughts on a positional tolerance.

The part is essentially a bolt circle pattern of holes with a center hole that has a diameter that cuts through the radial holes leaving semicircular slots around the bore. (see attached sketch for reference) There is an existing drawing that has a radial dimension from the center of the part to the edge (tangent) of the semicircular slot.

Is it possible to use this dimension as a basic dimension for the positional tolerance? If so what exactly does it mean? Would it be a cylindrical tolerance at that point (edge of slot) or still about the center of the semicircle?

Never mind on attached sketch, thought I could attach it directly to the post.

Thanks for any input.

 

[SeasonLee]

No attached sketch.

Season

 

[ScottEng21]

I didn't attach it. I thought I could attach it directly to the post, but it looks like you can only put in a URL for it.

Can you understand the configuration from my description?

 

[CheckerHater]

From your description it looks like you are trying to apply Position tolerance to the feature that is not full circle.
This is not the best idea from both standard and practical points of view.
Profile is a better choice.
Enclosed picture is not exactly your case, but philosophically similar.

 

[academix]

Hello

The radial dimension would have to be to the PCD and would be basic. The position tolerance call out would be to the x pattern of semicircular holes. These could be conventional cylindrical zones or, bidirectional position tolerancing using polar coordinate method, as provided in ASME Y14.5 Std. Hope this helps,

Best Regards
Natarajan R.


Natarajan Ramamoorthy
Design Engineering Consultant

http://www.egsindia.com

 

[CheckerHater]

Dear academix,
Would you be so kind to provide reference to where exactly in Y14.5 standard Position control is applied to "semi-circular holes"?

 

[ewh]

¶ 7.4.5 Noncircular Features of Size
Wouldn't it work if the semi-circles are >180° (having opposing points thus features of size)? Hard to tell in this situation without a sketch.

“Know the rules well, so you can break them effectively.”
-Dalai Lama XIV

 

[CheckerHater]

I think Para. 7.4.5 is about slots, tabs and other features having center plane.

Technically yes, if you have opposite points, you have size. Really want to see the sketch as well.

 

[ScottEng21]

This is my first posting. Is there an easy way to post the sketch? I will put it up if I know where to post it. Doesn't seem you can post directly in here, but have to post somewhere else and put the link on here.

 

[CheckerHater]

You should see the field named "Attachment"
Then click on the link "...upload your file.." and follow the instructions

 

[ScottEng21]

Thanks so much CheckerHater, I was looking at the top tool bar with the "picture icon". Here is the attached sketch.

 

[CheckerHater]

Doesn't look like Position to me.
The way I see it, you make your R.13 basic and apply Profile to semi-circular part or the entire outline, whatever your function demands.
Profile will create tolerance zone (associated with theoretical outline) that will control both position of the semi-circle and its radius.
You can keep your .73 dimension or create dimension to pitch circle; as long as dimension is basic, it's OK

 

[ewh]

Would position work if the dimension was to the opposite tangent instead of the centerline?

“Know the rules well, so you can break them effectively.”
-Dalai Lama XIV

 

[CheckerHater]

You mean like a "virtual" obround slot with only ends of it visible?

Theoretically yes, but we would still have problem controlling radii (you agree that there in nowhere to apply diameter, right?)
The way radius is controlled in Y14.5 there is no way to know where the center of it is.
It was discussed many times on this forum and agreed that if you really want to control radius you use profile.

Naturally I expect to see other opinions too.

 

[ewh]

I agree, profile would be the way to go if form is important... no way to control the radii with position as you noted. I was just looking at the theoretical possibilities.

“Know the rules well, so you can break them effectively.”
-Dalai Lama XIV

 

[ScottEng21]

Thanks guys for all the dialog and input, it's been helpful.

 

[pmarc]

I am not sure if anything else needs to be said in this thread, but the link below may help to understand why for features having no opposed points profile of surface is the correct choice.

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

 

[3DDave]

The Tec-ease diagram isn't correct - there is a large section of the second semi-circle that has no opposing surface. Take an extreme version where the arc is just slightly over 180 degrees, and the feature is on the high side. The motion of the MMC feature-simulator may not be constrained towards the outboard direction.

The difficulty is more obvious if the feature is then used as a datum reference at MMC. If it allows the datum simulator to escape, it isn't much of a constraint.

 

[pmarc]

3DDave,

I think it is important to remember that the Tec-Ease graphics are intentionally incomplete. I am pretty sure the complete version of the lower drawing would show something clearly controlling the width of the opening and its location relative to datum center plane B in order to avoid the extreme scenario you just described. I think this would also help to solve the problem of "poor" interaction between the hole and its simulator, if the hole was assigned as a datum feature and referenced at MMB (MMC using '94 terminology), wouldn't it?

There is, however, one aspect that makes me say that the approach presented in the lower picture of this Tec-Ease tip isn't the best option. It is the fact that form and the LMC size of the portion of the hole that has no oppposed elements can't be controlled properly. In other words, the surface of this portion can be of any weird shape, and as long as it does not violate Rule #1 (MMC) envelope, this will not be discovered during inspection. That is why, if I really had to apply position tolerance, I would probably go with profile of surface/position combo. Profile would deal with size and form of the hole, position would make sure that location and orientation of the feature relative to datums A, B and C was properly controlled.

On the other hand, when I look at datum feature B from fig. 4-16 of Y14.5-2009 for example, I see a feature controlled by perpendicularity tolerance (similar concept to position, but without locational constrains) that isn't full cylinder too, yet its size is controlled by directly toleranced dimension, leaving the portion of the feature with no opposed elements totally uncontrolled for form and the LMC size. In the light of this figure, I would not say that the Tec-Ease picture shows anything being in clear violation of Y14.5's approach to locating or orienting features of size (Side note: as the tip is based on Y14.5M-1994, I would love to use a figure from this version of the standard, but unfortunately was not able to find a single picture showing non-360[°] FOS controlled by position or orientation tolerance).

 

[Belanger]

I think the Tec-Ease tip is correct if we understand the second picture's semi-circle as an "irregular feature of size." Recall that this is something that can be simulated or bounded by a cylinder, etc., so in that respect it's OK to use position. It needn't be true that every minuscule point have a direct opposite point.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems