unilateral tolerancing 5

[mmolt]

Friends,

In a meeting today one of our manufacturing engineers stated he was going to call another meeting to state "we should never use unilateral tolerancing". As a design engineer, I am against forbidding use of unilateral tolerances as I feel it is removing a communication tool for use on our drawings. He claims we should always model the parts so that the nominal dimension can utilize a bilateral tolerance. He is saying this because they are using the CAD models to generate CNC programs, and then they have to go back the CNC programs and edit the numbers so that they are in the middle of the allowable tolerance. I hope that makes sense.

What are your thoughts?

Thanks in advance for your input,

-Mike

 

[mmolt]

Well, I would never put that many places on the drawing, more or less it was for completeness. Regardless, .253 +/- .004 is still pretty ugly in my opinion. I just think our manufacturing engineer is picking the wrong battle, and didn't think it through. Our meeting is tomorrow, and we'll be debating it in front of management, and ultimately, they'll make their decision.

-Mike

 

[KENAT]

mmolt,

So basically your manufacturing guy wants all features in the models at mid tolerance range with symmetric bilateral tolerance applied. This is so that he can plug it straight into his CAD to CAM/CNC convertor program without having to do any manufacturing Engineering.

Does he realize that in many cases this may mean either tightening tolerances and/or requiring more timely inspection (for instance changing a threaded hole from ".25 Min Full Thread Do Not Break Thru to something like .250 +- .005 plus some kind of note or convention to deal with the pilot hole not breaking thru)?

While automation is generally a good thing as it reduces labour/speeds things up etc I have concerns that sometimes it means people put less thought into things and that hence the design, or in this case process, isn't optimized to the same extent. Maybe the speed with which the CAM program can be made compensates in this case, I don't know, but generally I have concerns.

It's a small step from not wanting to massage the automatically produced from the model CAM/CNC program to match drawing tolerances to completely ignoring the tolerances and just trying to produce the part dead nuts and hope for the best. This seems to be what a lot of our suppliers do, and we often find parts out of tolerance.

I had more to say when I first read this post but lose track now.

On the slightly off topic but realated issue of Model Based Definition (which implicitly the manufacturing guy is going toward) you need to control your models very carefully, especially if you're going to outsource the manufacturing. You need to make sure the model being made to doesn't get changed (intentionally or accidentally) between the initial ECO release (or equivalent) and when the CNC program is made from it.

Based on your description of your situation if not already done I suggest making it clear somehow that the models are for reference only and the drawing is the controlling document.

Good luck in your meeting.



KENAT, probably the least qualified checker you'll ever meet...

 

[fcsuper]

ASME Y14.41 does allow the models to be the driving document, if it is implemented. However, even on ASME Y14.41, tolerances must be clearly stated. Often, the nominal is not a the center of the tolerance range. This is particular true of molded parts, which can vary over time, and from tool to tool. It's my opinion that it's really not a good engineering practice to disallow non-symmetric tolerances.

Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group

 

[mmolt]

To put a cap on this from my perspective, the meeting with management to discuss this issue went well. In the end everyone agreed we would try to use symmetric tolerances if at all possible and only if the design requirements can be met in doing so. That said, I was pleased in that it was agreed it is good to have non-symmetric tolerances at times, and that they should not be "outlawed".

-Mike

 

[fcsuper]

Cheers to you, Mike!

Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group

 

[mmolt]

Thanks Matt! I truly appreciate your and everyone elses comments and help.

-Mike