Position Tolerance - Urgent Help ! 1

[deepeshmm]

Hi All,

I have little experience with using GD&T. I have a 8mm plate in which there are 12 bushings. The location of these bushings are critical.
In-order to control this, i have made 1 bushing as Datum B, and given position tolerance of 0.3mm. All the other bushings are referenced to this. Datum A is the bottom surface of the part.
I am not sure if this way of dimensioning is correct. Please help me in correcting this. Please find the attached picture of the drawing.

 

[Belanger]

One quick comment about the dimensioning -- you don't need to say 2X or 3X or whatever because it's already understood that all holes/bushings that appear on the same line will be at that distance.

If all bushings are of equal importance, then the best way might not be to pick on one of them as datum B, but rather have datum B be the bottom (or top) surface. Then, there is a special way in GD&T to control the location of the bushings on the part, while controlling their spacing to each other a little more accurately.

It's called composite position tolerancing. If you're new to GD&T this might be a little advanced, but I'm thinking that it may be a good solution for patterns such as yours. The way it's displayed is to show a "double-decker" position callout with one position symbol centered vertically in the first compartment, but then have two lines of tolerance. See attached image for a random example of what one looks like.

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

 

[pmarc]

I would first ask, what exactly does "the location of these bushings are critical" mean?

Is it enough to keep bushings within dia. 0.3 relative to each other without worrying too much about location of pattern of bushings relative to some faces of outer contour of the part (like face identified C) or is location of the pattern relative to the outer contour critical too?

If former, then J-P's (Belanger's) suggestion to use composite positional tolerancing is good choice. If latter, then simple single segment positional callout may be well enough.

 

[deepeshmm]

Thanks to both of you for this feedback. I am only concerned about the Bushings locations relative to each other and not to the outer contours.
I will study about Composite tolerancing and figure out on how to use it.
Last question: For knowledge sake, if my location of the pattern was important w.r.t to the outer contour, is mentioning Datum reference A & C in the feature control frame sufficient? Is it necessary to include Datum B in the box ?

 

[desertfox]

Hi deepeshmm

Well you have a datum C but there is not a single hole dimensioned from it so having datum C is pointless.
I cannot find datum A on your post and thirdly while I can see you have a datum B which is one of the holes it appears to be floating as it isn't dimensioned from any of the other datums, if you want to keep datum C then dimension the hole referenced datum B from it.

 

[pmarc]

No, it is not sufficient to reference to A & C in the feature control frame. This just controls perpendicularity of the pattern relative to datum plane A and location in horizontal direction from datum plane C. However, nothing controls location of the pattern in vertical direction.

desertfox has a good point about lack of basic dimension from datum feature C to at least one center of the hole within the pattern. It has to be there.

 

[deepeshmm]

Ok. I notice this basic error i have made. I have modified the drawing, and it seems right to me now (i.e. considering the outer contours is important).
I have attached the updated pic now. Please tell me what do you think about this. Looking forward to your feedback.

 

[desertfox]

Hi deepeshmm

That looks a lot better now!

 

[Belanger]

Yes, better -- don't forget to drop the 2X and 3X before all basic dimensions.

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

 

[Tunalover]

deepeshmm:
You still have the redundant 2X multipliers on the horizontal dimensions. As already explained, whenever there are extension lines extending to in-line features the multiplier is not needed. Also, nowhere on the drawing is datum feature B used. Lastly, the drawing is not functionally dimensioned. Formally, you're dimensioning scheme is sound because with basic dimensions it doesn't really matter how their placed, but you can always take a step up and dimension the part in a way that shows the spacing between features; if someone wanted to determine those functionally important dimensions they can see them directly on the drawing and won't be required to do arithmetic to get them. The way you've done it is traditionally the way coordinate dimensions (non-basic dimensions with +/- block tolerance applied) are placed to avoid chain dimensioning and minimize the resulting tolerance accumulation. But there's nothing technically wrong with the way you've done it since you are using basic dimensions.




Tunalover

 

[deepeshmm]

I have done the changes. I still have to understand Composite tolerancing. But thank you all for helping me out !

 

[DiegoLGraves]

Tunalover, you mention "whenever there are extension lines extending to in-line features the multiplier is not needed." I follow this practice and I see this indicated in Y14.5M for example in Fig, 1-55, but is this explicitly declared anywhere? Paragraphs 1.9.5 seem to be where this is should be covered, but it's not.

I only have access to Y14.5 and Y14.100 but I'm trying to dot my t's and cross my i's for the new boss who seems to love it when I share chapter and verse.

Thanks for any insight. Diego

 

[ewh]

I don't think it has to be explicitly declared; when you dimension to a centerline with multiple features on it, you are still dimensioning to the centerline, not the individual features. By default, the features are considered to be on that centerline (if that is the way they are presented). A bolt circle is a good example... you never see "#X" appended to such a dimension as only one centerline diameter is being dimensioned, but many holes may lie on that diameter.
If the centerline does not connect to all of the holes in line (even though they do lie on a line), then you would add a qty to the related locational dimensions.
Hope that makes sense!;-)

ôKnow the rules well, so you can break them effectively.ö
-Dalai Lama XIV

 

[DiegoLGraves]

Thanks for the feedback ewh, it does make sense. I wondered if this explicitly applied only when using a basic dimension to the centerline, or if it applies (Per Y14.5M or some other ASME standard that I don't have) when GTOL is not used.

Thanks, Diego

 

[ewh]

It is basic drafting, with or without GD&T.

ôKnow the rules well, so you can break them effectively.ö
-Dalai Lama XIV

 

[tbuelna]

One minor issue, I don't see where datum B is used.

Looking at this particular dimensioning scheme, it seems like you would want to use a composite true position for the hole pattern.