Forming dimension 1

[grunt58]

We are rolling and bump bending some 11GA cone segments. We need to hold the OD as the cones fit into the ID of a rolled cylinder. I put the OD to hold but was told by the project manager that the ID was what they will check with a template and to change it. I believe I'm correct. I don't care how they check it the dimension we need to hold is the OD. Nothing is critical and I'm sure the parts will be fine as they will be fitted and field welded anyways. This is more of an I know I'm right sanity check than anything.

 

[chez311]

grunt58,

What standard are you using? And could you provide the relevant portion of your drawing? A diameter dimension (whether ID or OD) is useless on a cone without an associated distance as otherwise the diameter can be satisfied anywhere along its length.

Without knowing the specifics the general rule is the drawing and associated controls should reflect how the part functions/assembles. Changing the drawing to match how something is measured (doubly so if its being measured wrong) is generally bad practice, unless you have specified something which is impossible/impractical to measure, overly restrictive, incorrect/confusing, or contradictory.

 

[TheTick]

A classic sheet metal conundrum: the process works the inside, but you need to hold the outside.

It puts the result one degree removed from the process, thus further from control.

One thing that helps is to use sheet stock with tighter thickness tolerance. Typical is +/- .004 inches. At former employer's, we would use stock of +/-.002 inch tolerance when needed (more expensivw, special order). Also helps to be consistent with grain direction.

 

[drawoh]

grunt58,

Call up and control the dimensions that meet your requirements. If the shop wants to inspect the inside dimension, let them, but make them understand you will not accept outside dimensions that do not meet tolerances.

--
JHG

 

[grunt58]

Chez that is basically all I was asking; a drawing and its dimensions/GDT should reflect how the part functions not how it is measured. Unless as you stated it is impossible to check.

Tick yup. PM said they bend the ID so give dims for the ID. I gave what I want, the OD as a ref. dim. I understand his point but its not good practice. The bottom line is the part will work even if I don't agree with how it is dim'd. PM is a 30yo former fabricator with no formal education sooooo ya that's for another discussion...

dra. I agree but the part will work as called out. Just wanted reassurance that I approached it correctly, dim to design intent not how it is measured.