Flatness between two parts

[Dragon7]

I have an aluminum plate that will have a smaller, but thicker aluminum block mounted to the top of it. When the block is mounted to the plate, I need the bottom surface of the plate and the top surface of the block to be flat within .0005. Do I just show the assembly of the two parts with leaders pointing to both surfaces and show the flatness symbol with the tolerance?

 

[drawoh]

MechNorth,

I agree with you.

The drawing shows the final state of the part that my inspector will accept. The welder, hopefully, is a clever person who will figure out the warping and the clearances between the pieces about to be welded. I see no difference between this, and not telling machinists what size tap drill to use. Machinists tell me that parts warp. They account for it.

If I don't want clean up machining on my weldments, I specify weldable tolerances and finishes.

JHG

 

[MechNorth]

Joe, I wouldn't dispute either. The problem is that people are often expecting one drawing to serve two purposes in this situation. There's also the matter of scale; a couple small parts with short weld segments ... maybe not too bad. A large fabrication of dozens (or hundreds) of parts, with full-seam welds ... there's going to be problems that can't be adequately predicted by experience alone; you get into thermal FEAs and metallurgy at that point, which is a bit above most fabricators' abilities.
Drawoh, your last statement is right on the money. That's knowledge gained by experience as opposed to just a welding standard.

Jim Sykes, P.Eng, GDTP-S
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[KENAT]

Hang on, I thought I was the one saying we document the desired end item and MechNorth was arguing (what could be interpreted as) the opposite.

I'm lost now.;-)

Posting guidelines faq731-376

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

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

 

[MechNorth]

No, Kenat, I favor explicit communication. Most weldment drawings (that I've seen) deal with final weldment only, but expect you to make it using normal fabrication techniques applied to components to be surmised from the fab drawing, without adequate information to plan for realistic distortions. Others I've seen have separate component drawings without compensation for welding distortions. Neither will economically help you get the final fab per the drawing; both would require post-fab machining, which is cumbersome & expensive vs component compensations.

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

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TecEase, Inc.

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