Implied Concentricity tolerance on coaxial diameters???
Implied Concentricity tolerance on coaxial diameters???
(OP)
My question regards a shaft with a through bore and several steps of diameters on the OD, two of which are bearing surfaces. I am a fairly young engineer and somewhat new to GMT, and I'm dealing with a lot of old prints made back in the 80's and 90's. Most of them only contain size tolerances for the OD's and no relationship between any of the diameters. I have been trying to find out if there is any sort of default tolerance implied by diameters on the same axis or if they could in theory be misaligned .050" and still be ok "to print" as long as they meet the size requirements(approx. .0007"). Could anyone reference me to any standard that clarifies this?
I intend to designate a circular runout on the bearing surface diameters, but that doesn't take care of the other diameters which aren't as critical(.006" size tolerance)...is it necessary to specify the runout of each of these individual dia's or does the size in any way control the position relative to one another?
The only reference I have for something similar is the implied 90 degree angle that defaults to the title block angular tolerance, but I can't find anything similar for diameters on the same axis.
Thanks,
Blaine Williams
Manufacturing Engineer
I intend to designate a circular runout on the bearing surface diameters, but that doesn't take care of the other diameters which aren't as critical(.006" size tolerance)...is it necessary to specify the runout of each of these individual dia's or does the size in any way control the position relative to one another?
The only reference I have for something similar is the implied 90 degree angle that defaults to the title block angular tolerance, but I can't find anything similar for diameters on the same axis.
Thanks,
Blaine Williams
Manufacturing Engineer





RE: Implied Concentricity tolerance on coaxial diameters???
If you want it, place either circular or total runout on the feature where a relationship is needed. If it is not important or there really isn't a relationship, please do not place GD & T on the feature.
Too many engineers place GD & T on features that have no function or relationshiop driving up the cost.
Dave D.
www.qmsi.ca
RE: Implied Concentricity tolerance on coaxial diameters???
You don't have to have a particularly tight tolerance just because you are using GD & T. The tolerance value should be driven by function/tolerance analysis.
On the other hand if you are using shops that don't understand GD & T then pretty much any you put on, even if it actually relaxes tolerances compared to non GD & T dimension scheme, then the price will likely to up.
RE: Implied Concentricity tolerance on coaxial diameters???
RE: Implied Concentricity tolerance on coaxial diameters???
The "old way" of controlling the coaxiality of concentric features is to write an awkward note. The other way is to not provide a note at all; it is implied that the features are coaxial. Either method provides the same result.
The "Y14.5M way" is to create a local datum on one of the diameters and call out the coaxial diameters relative to that one using positional tolerance feature control frames(of course providing the same result as the first two methods).
Many machine shops will charge more for the second and third methods even though the results are identical for all three!
Tunalover
RE: Implied Concentricity tolerance on coaxial diameters???
RE: Implied Concentricity tolerance on coaxial diameters???
I thought GMT was the inspector's initials.
Chris
SolidWorks 06 5.1/PDMWorks 06
AutoCAD 06
ctopher's home (updated 10-27-06)
RE: Implied Concentricity tolerance on coaxial diameters???
The corrective actions of either training or finding a new supplier would be my preference.
RE: Implied Concentricity tolerance on coaxial diameters???
"People don't do what you expect, they do what you inspect."
And this one regarding "hard dimensions vs. centerline dimensions. "Dimension for inspection---let the machinist determine his own centerlines.
RE: Implied Concentricity tolerance on coaxial diameters???
This whole thing came up because our QC department was trying to reject parts and was asking that a tolerance be added to the print arguing that it had to be held to something.
I was struggling with finding a balance between overcomplicating the part and not giving too much leeway to our machinists in making it. As I said I intend to restrict the bearing surfaces with a runout, but I think I'd rather leave the rest of it alone and trust that common sense will keep it reasonable, as it is not critical to the function of the part. I just wanted to make sure there was no "unwritten rule" that I couldn't find that might satisfy everyone.
Thanks again
RE: Implied Concentricity tolerance on coaxial diameters???
I've used a pretty loose tolerance value that should be very easy to meet but it's still there just incase anyone gets any funny ideas.
RE: Implied Concentricity tolerance on coaxial diameters???
RE: Implied Concentricity tolerance on coaxial diameters???
Yell at the supplier about the parts not conforming, he replies, "... not conforming, to _what_ dimension?"
So much for implied tolerances.
The parts were PVC, which moves around some, but I still have no idea how he managed to make them so badly. I had a lot of time to contemplate the question; I had to buy the lot.
Mike Halloran
Pembroke Pines, FL, USA
RE: Implied Concentricity tolerance on coaxial diameters???
Blaine, GET SOME GD&T TRAINING; it's good that you know enough to ask for help, but it's a steep learning curve and you'll only get there by training. There are a number of earlier posts on this site that give good references to numerous training suppliers...some are even members of this forum. Good luck on the tough treck ahead.
Jim Sykes, P.Eng, GDTP-S
Profile Services
CAD-Documentation-GD&T-Product Development
www.profileservices.ca