sendithard
Industrial
Datum A is two cylindrical surfaces separated from each other....But what is the mating envelope being positioned to?
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Is this image an error in the 2018 standard
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Burunduk
Mechanical
- May 2, 2019
- 2,582
sendithard, do you mean that the position tolerance references no datums?
It is allowed in similar cases.
The two cylinders are simply controlled for coaxiality to each other without a datum. Since the control is at MMC you can go by the surface interpretation and make a go gage made of two coaxial dia. 35.1 virtual condition holes. Or if you prefer the axis interpretation those would be two coaxial tolerance zones of diameters equal to the departure of each cylinder from MMC to accept the actual axes.
It is allowed in similar cases.
The two cylinders are simply controlled for coaxiality to each other without a datum. Since the control is at MMC you can go by the surface interpretation and make a go gage made of two coaxial dia. 35.1 virtual condition holes. Or if you prefer the axis interpretation those would be two coaxial tolerance zones of diameters equal to the departure of each cylinder from MMC to accept the actual axes.
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sendithard
Industrial
Burunduk,
I'm skimming some chapters just as a yearly refresher and I don't remember this particular issue. I understand the surface method and hard gaging the virtual condition.
If those 35mm cylinders were machined separately and have a decent amount of difference, how are we determining the bonus in your opinion. Take the least bonus measurement of are we doing two bonuses and allowing for a best fit of both?
I'm skimming some chapters just as a yearly refresher and I don't remember this particular issue. I understand the surface method and hard gaging the virtual condition.
If those 35mm cylinders were machined separately and have a decent amount of difference, how are we determining the bonus in your opinion. Take the least bonus measurement of are we doing two bonuses and allowing for a best fit of both?
The "bonus," neglecting sine error, is the difference between the UAME size and the MMC size.
Were one end made at MMC, perfect in form, and the other end at LMC, then the UAME would only capture the MMC end and there would be no way to report the gap at the LMC end, but the LMC end would not be allowed to exit the projection of the UAME of the MMC end.
Were one end made at MMC, perfect in form, and the other end at LMC, then the UAME would only capture the MMC end and there would be no way to report the gap at the LMC end, but the LMC end would not be allowed to exit the projection of the UAME of the MMC end.
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sendithard
Industrial
I can tell this callout is right up Dave's alley.
Burunduk
Mechanical
- May 2, 2019
- 2,582
Sendithard,
It is the latter, as you said: "doing two bonuses and allowing for a best fit of both" - which is pretty accurate.
Each of the cylinders has it's own unrelated AME. You substract each unrelated AME from the MMC size (which is dia. 35.1 for both cylinders), and that gives you the size of tolerance zone for each one (that size is your "bonus" added to the specified zero tolerance).
No matter what, those two tolerance zones are coaxial - that's the thing that controls alignment.
Once you have two coaxial tolerance zones with known diameters, the common axis of the tolerance zones is best-fitted over the actual part. The 2 measured values are the tolerance zone diameters that are just large enough to accept the actual axes in a borderline condition, and should be smaller than the calculated tolerance zones ("bonuses").
It is the latter, as you said: "doing two bonuses and allowing for a best fit of both" - which is pretty accurate.
Each of the cylinders has it's own unrelated AME. You substract each unrelated AME from the MMC size (which is dia. 35.1 for both cylinders), and that gives you the size of tolerance zone for each one (that size is your "bonus" added to the specified zero tolerance).
No matter what, those two tolerance zones are coaxial - that's the thing that controls alignment.
Once you have two coaxial tolerance zones with known diameters, the common axis of the tolerance zones is best-fitted over the actual part. The 2 measured values are the tolerance zone diameters that are just large enough to accept the actual axes in a borderline condition, and should be smaller than the calculated tolerance zones ("bonuses").
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