Design Best Practices - Rolled Cylinder

[AMontembeault]

I have a rolled, right cylinder, approximately a meter in diameter (inside) and 1.5 meters in length,with a wall thickness of about half an inch.

The problems with dimensioning this are:
1) its not really rigid in the free state
2) To constrain it adequately, on a mandrel, for example, would make the ID surface (which is critical) uninspectable
3) Due to its size, measuring the ID in the center of the part is very problematic without substantial investment in tooling and measuring equiptment, both for our supplier and for our incoming inspection team.
4) Wall thickness tolerance is a relatively large range (depending on material) versus nominal thickness, so measuring OD might obscure a true, local IR surface profile.
5) There is a longitudnal weld seam, which adds welding distortion/shrinkage.

Now, I can think up a dozen different ways to dimension this, each with pros and cons to the dimensioning approach as it would pertain to engineering, manufacturing and inspection, so my question is not so much how to dimension these parts, but rather to ask if anyone has also found themselves in a simelar situation (where you can't guarentee everyone will be happy), and what sort of best practices/compromise did they come up with?

 

[pylfrm]

AMontembeault,

I'd say the drawing requirements should be based as closely as practical on the functional needs of the part, even if proper inspection of those requirements is currently problematic. Your inspection capabilities may improve in the future, eliminating the issue. In the meantime, you can probably contract it out to a third party with the proper tools.

You might also consider whether some sort of functional testing could reduce the need for dimensional inspection. This could be given as an allowable alternative on the drawing.


pylfrm

 

[CheckerHater]

You didn't mention actual tolerance. Sometimes measuring circumference produces good enough results

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[3DDave]

This seems to fall into the general category of problem as described more directly:

A part is such that it cannot be adequately fabricated with confidence in its dimensions nor confirmed to meet a set of dimensions; how can the drawing describing this part be dimensioned so that these fabrication and inspection problems can be overcome?

The answer is, obviously, changing how it is dimensioned cannot make up for inadequacies in fabrication and/or inspection capabilities.

At this point comes the 20 questions that seek to determine how to avoid the lack of capability, but all of those revolve around spending money which is, reading between the lines, something that is unavailable.

The alternative to spending lots of money is to find someone with either fabrication and/or inspection capabilities, as suggested by pylfrm. Creating such items is a fairly routine task in certain segments of industry and turning it over to a company with experience is the best alternative.

 

[CheckerHater]

Unfortunately "company with experience" is charging money for the services.
I was once asked to use calipers on part with the structural integrity of a gummy bear.
When I was trying to explain that part will need different technique, like scanning, they thought I just escaped from the insane asylum.

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future