Position Tolerance - Urgent Help !
Position Tolerance - Urgent Help !
(OP)
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.
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.





RE: Position Tolerance - Urgent Help !
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
RE: Position Tolerance - Urgent Help !
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.
RE: Position Tolerance - Urgent Help !
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 ?
RE: Position Tolerance - Urgent Help !
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.
RE: Position Tolerance - Urgent Help !
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.
RE: Position Tolerance - Urgent Help !
I have attached the updated pic now. Please tell me what do you think about this. Looking forward to your feedback.
RE: Position Tolerance - Urgent Help !
That looks a lot better now!
RE: Position Tolerance - Urgent Help !
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Position Tolerance - Urgent Help !
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
RE: Position Tolerance - Urgent Help !
RE: Position Tolerance - Urgent Help !
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
RE: Position Tolerance - Urgent Help !
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
RE: Position Tolerance - Urgent Help !
Thanks, Diego
RE: Position Tolerance - Urgent Help !
ôKnow the rules well, so you can break them effectively.ö
-Dalai Lama XIV
RE: Position Tolerance - Urgent Help !
Looking at this particular dimensioning scheme, it seems like you would want to use a composite true position for the hole pattern.