True Position

[Adams_Bobby]

Hello,

Wasn't sure where to put this but would appreciate any help.

I am trying to dimension a true position as follows. I have a 1.001" finished bore but I would like to verify the roughing operation to ensure the bore cleans up. The roughing operation will create a .9842 bore. That gives me about .008" to clean up. Would I allow the operator a true position of .016"?

My reasoning is:

sqrt(.008 squared + 0) *2 = .016

Thanks,

Bobby

[stkenitz]

May not be understanding this correctly...BUT;

The diameter really shouldn't affect it, unless there is a modifier. Assume the bore is 1.001" ±.020" on the print for the final part. And the position callout is position of this bore to datums ABC or whatever your structure is.

[Cumofo]

Mathematically almost. Tighten it up to allow for the entrance and exit axii as you may be averaging the cylinder. If they are using a boring bar to finish, the hole must be in position before the boring bar pass or it will drift also.

[Adams_Bobby]

It may be my ignorance with true position either understanding or explaining.

We have a first operation machine that bores the rough hole at .9842. When a tool is changed on that machine I need to give a tolerance to the operator so if it is not dead center. Since we are leaving .008" per side to clean up I would like to give a true position to it so I can be sure it is centered enough that the 1.001 bore will clean it up. Both operations are being referenced on a rail that I am giving the XY coordinates to.

I am basically trying to figure out how much a .9842 circle can move inside a 1.001 circle without breaking through.

[Adams_Bobby]

Ok I think that gets me on the right path. something like the tols we have used in the past +/-.005 on both axes would give a TP of .014

So I could tighten it up and shoot for a true position of .010 and that would allow devs of .0037 max on both?

[gibsonridge]

i also may not understand the question, its just a reference check for position ( at the roughing diameter size)
use the print tolerance !
if there is a concern ,
manufacturing needs to have it checked at much smaller dia. in order to adjust appropriately.

[Peter Fuller]

Maybe I'm not understanding completely but wouldn't it be =COS(.785398)*(0.0168/2) (in excel)

[Peter Fuller]

Basically you're asking how far out can the operator be radially from the original bore location and not leave an unfinished surface. So with my math, axially he can't move more than .00594 in a radial direction before he leaves unfinished surface with the final cut. This assumes he is positioned at worst case of 45°between the two axis.

[Adams_Bobby]

Thats what I was coming up with. So that would equal a true position tolerance of .0168 right? Im going to tighten it up and try to control the centerline much tighter, but I was using this operation as an exercise in creating tolerances for TP. Testing my understanding.

[Peter Fuller]

Thats what I was coming up with. So that would equal a true position tolerance of .0168 right? Im going to tighten it up and try to control the centerline much tighter, but I was using this operation as an exercise in creating tolerances for TP. Testing my understanding.

Technically if my math is correct it would equal a tolerance of .0168. Be warned that I'm very rusty since I haven't actually done any real world programming for almost 10 years now.