Stock tolerance (especially structural metric)

I should probably be posting this on weldingweb, but I'm assuming machinists are more likely to notice small variations, haha. Here in West Africa I have a ton of trouble getting material that's correct to the name. Sheet metal isn't gauged, it's called out by the mm of thickness. When we buy 1.0mm sheet, it's 0.7mm, if we buy 1.2mm it's actually 0.9mm, etc. I'm pretty sure that's just the resellers playing games (importing .7 and calling it 1.0) because I don't think the russian mill that's producing it can't tell the difference between .7mm and 1.0mm. We just compensate by buying the next size bigger to get what we actually wanted.

However, I've had a ton of trouble finding strong 25mm x 25mm square tube. This week I finally went to the regional distributor for West Africa with a mic, and even there what they're selling at 25mm x 25mm x 1.50mm wall actually mics at 1.35mm. Is this common in metric structurals? I've spent much less of my metal-consuming life back in the States, but when you buy 16 gauge tube, I'm assuming the stock tolerance isn't so wide that they could get away with giving you 18 ga, which is pretty much what's happening to me?

I'm working on a small factory to produce chairs and desk from bent/welded tube, and the difference in stiffness and how quickly rust will get through the wall is very different between .9mm, 1.2mm, 1.35mm, and 1.5mm.

Is this common in metric structurals?

Warm formed structural steel is covered in DIN EN 10210-2

Just looked it up:
Outer dimensions: +-1%
Wall thickness: +-10%

StefanG said:

Warm formed structural steel is covered in DIN EN 10210-2

Just looked it up:
Outer dimensions: +-1%
Wall thickness: +-10%

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Well, sounds like my suppliers are on the ragged edge of -10% then, eh?

I can't speak to the metric standards, but the -10% tolerance issue exists with the standard US tubing (ASTM A500) as well. The standard allows it, so all of the mills make it to the smallest allowable thickness to save money, and the engineering standards have to take that into account.

There is a newer US standard, ASTM A1085, that works to minimize this problem. Wall thickness tolerance is changed to -5%, and there's an overall mass tolerance of -3.5%. There are also strength, toughness, and radii differences.

Thank you for the replies from both sides. It's somewhat comforting to know this is a global issue rather than one isolated to west africa. We'll just accept it and move on, using what we can get then!

JasonPAtkins said:

Thank you for the replies from both sides. It's somewhat comforting to know this is a global issue rather than one isolated to west africa. We'll just accept it and move on, using what we can get then!

Click to expand...

That's not global, That's just fraud.

JasonPAtkins said:

When we buy 1.0mm sheet, it's 0.7mm, if we buy 1.2mm it's actually 0.9mm, etc.

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It ain't no where as bad as that Downunder. One thing sheeties know is gauge / thickness. Your at 30% there. No way that would fly down here.

Regards Phil.

machtool said:

That's not global, That's just fraud.

It ain't no where as bad as that Downunder. One thing sheeties know is gauge / thickness. Your at 30% there. No way that would fly down here.

Regards Phil.

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The explanation about the tube tolerance makes sense, it seems like just the mills playing games.

Our issue with sheet is a local one though, I think. When I buy 1.2mm sheet, it sometimes comes with the mill certificate saying 0.9mm. So that's not the mill cheating, it's local suppliers ordering 0.9 and selling it as 1.2mm. I've come to think of the thickness as more of a trade name than a measurement, lol. I think the prices aren't too far off of reality for the real measurement though. A 1x2m sheet of 1,50mm (measured) HR sheet is 27.07USD. We have pretty high import taxes (which are usually dealt with by the importers by just bribing customs officials), so I don't think that price is too far out of line given where we are. I could be wrong - but on some level it is what it is. I can't pack sheet in my suitcase and bring it with me when I fly over, so...

I've used a lot of RHS over the years. Whenever I checked wall thickness it was ALWAYS right at the bottom end of tolerance.

At my workplace we buy big enough quantity's that it's made by order through mill, we specify no less than 0.05mm or it goes back, sent double trailer stacked with tubes right back once.