Title block tolerance for Fabrication 1

[DiegoLGraves]

I work for a fabrication sub-contractor. One of our major customers uses a title block tolerance based on process, rather than decimal place. This customer generally does not apply gd&t, or other direct tolerancing methods. The typical title block call-out reads:
UNLESS OTHERWISE SPECIFIED
FABRICATION ± .06
MACHINING ± .020
WELDMENTS ± .13
FORMED ANGLES ± 2 DEG
MACHINED ANGLES ± 30MIN

Is there a specification that applies to this general process related tolerancing, such as an ASME spec? I've been fabricating since the early 80's and never seen one listed. With better equipment than was available 30 years ago we can hold closer tolerances than in the past, but not in all processes. I plan on contacting our quality engineering contact at the customer this week to have them clarify, but would appreciate your input.

Regards, Diego

 

[pmarc]

ISO 2768-1:1989 ans ISO 2768-2:1989 specify general dimensional and geometrical tolerances for parts produced by metal removal or parts that are formed from sheet metal. I am pretty sure you can hold closer tolerances than specified in these specs. too.

 

[KENAT]

Like pmarc says this is along the same line of thought as ISO 2768 but certainly not the same as - or even a gross simplification of it.

As with almost any 'default tolerance' scheme it's open to miss use.

Simplistically I don't see that it directly/explicitly contravenes ASME Y14.5 though is arguably not quite in the spirit of that spec. Just like I don't think the typical default tol block is explicitly detailed in 14.5.

Do the customer drawings not reference any drawing spec?

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[DiegoLGraves]

Thanks for the feedback. Regarding ISO 2768 we only have a section of it to reference, that was supplied by another customer. It is in the spirit of this but not applicable.

The customer print I'm referencing here, from 1999, references MIL-STD-100 for drawing interpretation standards. I believe this was superceded by MIL-STD-100G, which has no application here that I can see. Based on my experience I would put plate burning, plasma cutting, sawing, manual drilling, rolling, punch press (not stamping) etc type of processes into the fabrication category. In machining, any other metal cutting, particularly cnc based, including laser and water jet cutting, turret punching.

The customer has indicated in phone discussions that the only operations in the fabrication class would be forming, such as press brake, folding machine, and rolling. As they have been updating drawings from an older cad system the default tolerances have been getting tighter also, and with our improved in-house processes we now meet all tolerances or get a deviation approval. Our goal as the supplier is always to meet the customer expectation and specifications, have no rejects, make money and get the repeat order.

Appreciate your input. I'll send the clarification request to them tomorrow, and then post here if there is some helpful lesson.

Kind regards, Diego

 

[drawoh]

DiegoLGraves,

I think your customer is an idiot.

A fabricator should be able to fabricate by whatever process meets the requirements. Your specification implies that tolerances will be opened up if he chooses to weld rather than machine. The only way this system starts to work is if the purchase order specifies the fabrication process.

--
JHG