Metal Sheet - Laser cut tolerance

ivan_boudjema

Hi everyone,

I am working on a special frame deign, made of plenty of metal sheets and stainless steel tubes, and I lack knowledge on the design tolerande. I will take as an examble this tube and flange that I would like to weld together.

https://cad.onshape.com/documents/7bdba13371ad3ab631722f84/w/4d571a15cbe007361a44e66a/e/c011fe808dac7e6414fb9d90?renderMode=0&uiState=66792ed4acb0164e9376df88

The tube OD is ø50mm, and the flange's ID is also ø50mm. My question is, if I laser cut my flange to 50mm, will it "slide" on the bar (my first guess is NO), and if not, what would be the flange's ideal ID so that when I weld it to the tube, the tube and flange Z axis are perfectly aligned?

Thanks for your help
Ivan

james_aguilar160

I don't know that much about this but I would guess from my little experience with metal that:

1. The flange with an inner diameter  equal to the outer diameter of a tube will not slide easily. I don't think you'd be able to fit such parts together without damaging one of them.

2. The bore diameter for different kinds of fits (clearance vs interference) varies by metal. There are tables you can look up for the proper bore diameter. 

3. If perfect alignment is critical, intuitively I would not expect the tightness of the fit to be sufficient. I would want some kind of a jig to hold the pin to correct alignment and position before welding. 

A metal machining forum would probably be a better place to ask this question. 

rob_relva

Hi Ivan,

Fabricator here.

You could go round and round in circles chasing tolerances for snug fits, and all you will end up with is a grumpy fabricator who has to die grind the holes out and smash the rings onto the tube.

Cut the faff and allow for ~1mm tolerance. 51mm in this case. That way if you find that the tube is 2" (50.8) it will still go and 1mm is nothing to weld up.

nick_papageorge073

I checked your design and it looks like its made to hold large tanks, and thus very low tolerance overall. If the tubes were shifted 10mm, for example, it would still hold the tanks well.

That said, to answer your original question, there are a few things to consider. The tube you are buying is not precision made. Do you have it in-hand? If so, measure its diameter at several place along its length, AND, rotate your calipers/micrometers to measure the diameter at several angles. Pipe is rarely round, and if it has a welded seam, it will be even more non-round. Whatever the biggest diameter is that you find, must be allowed for in the design.

Laser cut parts: There is a draft angle with laser cut parts. The thinner they are, the more accurate they are. .5mm is about perfect. .05mm range tolerance in my experience. But 2mm and 3mm, which is in your design in various places, has a much bigger tolerance. It might be off .2 mm. It's best to order a small sample from your laser cut vendor in all the thicknesses and materials you are using, with a few different sized holes. Then measure them (assuming you have equipment to do so).

If you do all of the above, have the tube in hand, have laser cut samples in hand, and measure both accurately, you can make an informed decision on how much clearance to leave in the CAD files.

In the absence of both, go with 1mm clearance on the diameter as Rob mentioned above.

nick_papageorge073

Also, go to the learning center and learn the replicate command. It will simplify your assembly at least 10x.

ivan_boudjema

rob_relva said:

Hi Ivan,

Fabricator here.

You could go round and round in circles chasing tolerances for snug fits, and all you will end up with is a grumpy fabricator who has to die grind the holes out and smash the rings onto the tube.

Cut the faff and allow for ~1mm tolerance. 51mm in this case. That way if you find that the tube is 2" (50.8) it will still go and 1mm is nothing to weld up.

Thanks a lot for your help!!

ivan_boudjema

Thanks everyone for the insights!