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Converting from coordinate-type tolerances to GD & T
- Thread starter dglee
- Start date
CheckerHater
Mechanical
No, there is no easy way. There is a learning curve involved. And your example looks really strange to me. You definitely need to learn some basics first (pun not intended)
"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future
"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future
To get maximum benefit from changing you really need to take a look at function and do some tolerance calculations on matching hole patterns etc.
I would say I don't believe your attempt is particularly good, sorry.
I was going to give some pointers but I'm not sure where to start.
OK, one tip if you're determined to do 'dumb' conversion of +- to position tolerance without looking at function/tolerance stack. Assuming the +- was correct then for hole patterns the position diameter can generally be equal to the diagonal measurement of the 'square' zone created by the +- dims. e.g. if your holes are +-.05 from 'datum' edge or similar then this gives a square tolerance zone of .1. In terms of deviation from the center of the square though if you are in the 'corner' then you are not .05 from the center but around .07 (Pythagoras). So your equivalent position diameter can be .14.
I would say I don't believe your attempt is particularly good, sorry.
I was going to give some pointers but I'm not sure where to start.
OK, one tip if you're determined to do 'dumb' conversion of +- to position tolerance without looking at function/tolerance stack. Assuming the +- was correct then for hole patterns the position diameter can generally be equal to the diagonal measurement of the 'square' zone created by the +- dims. e.g. if your holes are +-.05 from 'datum' edge or similar then this gives a square tolerance zone of .1. In terms of deviation from the center of the square though if you are in the 'corner' then you are not .05 from the center but around .07 (Pythagoras). So your equivalent position diameter can be .14.
Posting guidelines faq731-376 (probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484
What is Engineering anyway: faq1088-1484
dglee,
Your datum[ ]A should be the bottom face of your part. Definitely, you need to read up on GD&T and get trained, if possible. I suggest you purchase a copy of the standard, ASME Y14.5?
My general observation is that you are making things complicated. Unless you have some weird application, you do not care about perpendicularity of the holes. You just care about position.
You have a couple of perpendicularity specifications of zero at MMC. I like to do this with positional tolerances. I strongly recommend placing a sloppy tolerance on the hole size. The zero tolerance plus feature of size, add up. Something has to be loose, or the part cannot be fabricated. If I call up an accurate hole, I call up a non-zero positional tolerance.
--
JHG
Your datum[ ]A should be the bottom face of your part. Definitely, you need to read up on GD&T and get trained, if possible. I suggest you purchase a copy of the standard, ASME Y14.5?
My general observation is that you are making things complicated. Unless you have some weird application, you do not care about perpendicularity of the holes. You just care about position.
You have a couple of perpendicularity specifications of zero at MMC. I like to do this with positional tolerances. I strongly recommend placing a sloppy tolerance on the hole size. The zero tolerance plus feature of size, add up. Something has to be loose, or the part cannot be fabricated. If I call up an accurate hole, I call up a non-zero positional tolerance.
--
JHG
I'm just going to assume all your tolerances are functionally appropriate.
Remember you can make all the holes in a pattern a single datum. Your perpendicularity on Datum X is fine; if you changed it to location, it would mean exactly the same thing. But, rather than specifying X and Y datums separately you could do "2X 10 +/-0.1", attach a location tolerance of 0.2 relative to datum Z (this constrains perpendicularity to Z AND the holes' locations relative to one another), and refine that with your perpendicularity zero @ MMC. The two ø10 holes can then become datum X. The ø12mm holes I recommend something similar: do a single callout with "2X ø12 +/-0.2", attach a location of ø4 relative to Z and then X @MMB, and right below refine that callout with your zero @MMC perpendicularity. The only thing what I've just described changes in terms of actual tolerance is that now all holes will have the zero @MMC perpendicularity, but whoever is inspecting the part benefits because you have fewer datum reference frames and the intent is more clear to all who may read the drawing.
Just remember that zero @MMC perpendicularity does not really give you any (worthwhile) bonus tolerance which is the main benefit of using the MMC modifier - it only constrains the orientation based on size, while the location is RFS; say for instance that hole was made precisely at MMC, it would be a perfect cylindrical surface whose axis is perfectly perpendicular to Z BUT the axis could be located anywhere in a ø4mm zone centered at the basic dimension; this could definitely result in fit-up issues depending on what's mating to this part. Be careful there and make sure it fits with the intent of the design. You're on the right track but you definitely will want a copy of Y14.5 - it's a surprisingly effective learning tool all by itself.
Remember you can make all the holes in a pattern a single datum. Your perpendicularity on Datum X is fine; if you changed it to location, it would mean exactly the same thing. But, rather than specifying X and Y datums separately you could do "2X 10 +/-0.1", attach a location tolerance of 0.2 relative to datum Z (this constrains perpendicularity to Z AND the holes' locations relative to one another), and refine that with your perpendicularity zero @ MMC. The two ø10 holes can then become datum X. The ø12mm holes I recommend something similar: do a single callout with "2X ø12 +/-0.2", attach a location of ø4 relative to Z and then X @MMB, and right below refine that callout with your zero @MMC perpendicularity. The only thing what I've just described changes in terms of actual tolerance is that now all holes will have the zero @MMC perpendicularity, but whoever is inspecting the part benefits because you have fewer datum reference frames and the intent is more clear to all who may read the drawing.
Just remember that zero @MMC perpendicularity does not really give you any (worthwhile) bonus tolerance which is the main benefit of using the MMC modifier - it only constrains the orientation based on size, while the location is RFS; say for instance that hole was made precisely at MMC, it would be a perfect cylindrical surface whose axis is perfectly perpendicular to Z BUT the axis could be located anywhere in a ø4mm zone centered at the basic dimension; this could definitely result in fit-up issues depending on what's mating to this part. Be careful there and make sure it fits with the intent of the design. You're on the right track but you definitely will want a copy of Y14.5 - it's a surprisingly effective learning tool all by itself.
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