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Bonus Tolerances..

anthoknee

Plastic
Joined
Dec 24, 2012
Location
seattle
Hi there. I'm new to these forums, so forgive me if this topic has been covered (I'm sure it has). I drew up this CAD drawing for some parts that I'm working on at work. As you can see in the drawing, the general tolerance is +/-. 03. I forgot to include that the small diameters of this part are . 6246-.624 and is supposed to hold a true position of . 004 to datum A at MMC. Datum A is the centerline of the inside diameter. My question is, how much bonus tolerance would I get on the . 750 diameter? The inspector at my shop is trying to tell me that I don't get any, but I know that I recently learned in school that with an outside diameter, you accrue a bonus tolerance if the diameter were to decrease as opposed to a hole or something. Thanks for all and any input! And merry Christmas!
 

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What is the tolerance of the .75" diameter? Edit: nevermind, I wasn't paying attention to the tolerance block at the bottom.

The way I see it, you have .002" TIR tolerance, when the .75" diameter is at its maximum size. If you make it smaller, there is "bonus" tolerance.


I shouldn't be posting while full and tired from christmas dinner lol
 
Haha. That's what I figured! For example, if turned down to . 740, I'd be sitting at around . 012 TIR or something, right? By the way, I'm turning the O. D With a cnc lathe and a live center on one end and chucking with a custom 5C collet and am only getting about . 0015 TIR. Before I tried turning these down myself for the first time on Saturday, we sent these parts out for someone else to grind for us and they charge about $65 per part, so I figured it'd save us a good chunk of change for us to do it ourselves. Thanks for the input!
 
By the way, I'm turning the O. D With a cnc lathe and a live center on one end and chucking with a custom 5C collet and am only getting about . 0015 TIR.

Have you drilled/bored/reamed the thru hole first, then stuck a plug with a centerdrilled hole in it in the other end of the part?
 
Yes, everything is bored out for the inside diameter. I purposely made the ends .03 longer and applied a . 020 x 60 degree chamfer so the center aligns the part. The final operation is facing these parts to length and the chamfers should shrink to tolerance. Though I never thought of putting a plug on the ends of them. Do you think it'd be better to do it that way?
 
Oh, if you have a 60deg chamfer on the end of the part, then that should work just as fine. I was just making sure you were locating off of the hole (as opposed to doing the hole last, etc)
 
I don't see any bonus tolerance there; the OD must run .002 TIR to datum A, the ID, irrespective of the OD's tolerance.

If that's not the case, let me know.
 
I don't see any bonus tolerance there; the OD must run .002 TIR to datum A, the ID, irrespective of the OD's tolerance.

If that's not the case, let me know.

Maybe I'm wrong (I don't believe so but I'm open to the possibility), but that maximum material condition symbol means that when that feature (.75 OD) is at its largest size, that is the tolerance. When the OD is smaller, that tolerance "opens up", allowing it to be off a little more. If the symbol was an S (1982 y14.5) or no symbol at all (1994 y14.5, as the print shows), then it would be "regardless of feature size". It is -not- "regardless of feature size"
 
Maybe I'm wrong (I don't believe so but I'm open to the possibility), but that maximum material condition symbol means that when that feature (.75 OD) is at its largest size, that is the tolerance. When the OD is smaller, that tolerance "opens up", allowing it to be off a little more. If the symbol was an S (1982 y14.5) or no symbol at all (1994 y14.5, as the print shows), then it would be "regardless of feature size". It is -not- "regardless of feature size"

You're right; yesterday I looked for and completely missed the mmc symbol, today I almost missed the Y14.5 rev level callout. Maybe it's time to give it up and go be an inspector <g>
 








 
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