How to distribute general assembly profile tolerance to component parts

[RXMetrology]

I need someone to enlighten me on this:

I have a welding assembly with 5 sheet metal brackets welded to a main body, if I have a general profile tolerance 2.0 mm on the final assembly, how can I distribute this tolerance to those 5 brackets more precisely? I have no problem to define the FCF on the welding surface of each bracket with profile to local datum and refinement but wonder if there's any formula to calculate the tolerance zone more accurately, without using any special software? Those 5 brackets are independent of each other so there's tolerance stack only between each bracket to the main body.

If anyone can point me to some resources, I will really appreciate that.

 

[drawoh]

RXMetrology,

Are you preparing the drawings or fabricating the parts?

If you are designing and documenting this thing, you should put the profile tolerance on the final, welded face, and let the welder figure it out.

I understand that a 2mm profile is rather tight for welding. I would not design the individual pieces with locating features. I would allow the welder the jig them in place and grind a bit, to meet your tolerance.

--
JHG

 

[MechNorth]

There is no formula to give you what you seek. There isn't enough info provided to suggest whether you should also control the sub-components more tightly to facilitate the welding accuracy. A graphic may help.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[RXMetrology]

Thanks for your quick reply. I'm helping engineering department to release GDT sheet for those component brackets to outsource tooling, which is typical for automotive company. The idea is they want to make sure with the profile tolerance allowed on each component drawing, they will still meet the final profile 2.0 mm tolerance after welded together (so there will be sort of tolerance stack up involved, but only between each bracket and the main body). I can do a quick worst case analysis on tolerance distribution but then the over tight tolerance on component will raise the tooling cost for sure. That's why I'm looking for some methods to figure out the component tolerance with some back up calculation.

 

[RXMetrology]

I was preparing a quick snap shot, with some necessary information. So attached image describes the basic task: how to specify a profile tolerance on this bracket part drawing to make sure it will meet 2.0 mm profile after it gets welded to the main body ? The key thing is I need to show customer some reasonable calculation to support why I apply this tolerance value, which can be difficult. Of course you might need welding information but I'm looking for some general guideline. For me, without a special software it's almost impossible to get a "accurate" value but I'd like to know if anyone has some good advice other than general estimating/calculation.

 

[MintJulep]

It seems that you need to know the allowed tolerance of the main body at the locations where each of your brackets attach.

 

[MechNorth]

A simple tolerance vector analysis should do the job to see where the tolerance stacks are most significant, but that's a two day class. Sorry, that's about as easy as it's going to get for you.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[RXMetrology]

All of your inputs make good sense to me. I didn't get totally lost on this task, but really like to see what other people think of this project. Since we don't have enough information to start with, I will do a generic "function and fit" GDT review, combined with some tolerance stack analysis.