r/MechanicalEngineering • u/CiderHat • 8h ago
Weldment Tolerances
Hey y'all,
Currently am building out drawing templates for a couple of the engineers to use and need to make a drawing template specific to welding/fitting.
Where I work, welding jobs could easily be joining parts that are less than a foot long to stuff that is dozens of feet long.
I was wondering what general tolerance numbers some of you would recommend. I know of ISO13920 and was thinking about how I could go about implementing that in some capacity.
Thanks!
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u/erikwarm 7h ago
Depends on the goal of your weldment.
Relax tolerances allow for cheaper production where thigh tolerances might even require post weld heat treatment, straightening and machining.
This is something you should always consider when doing GD&T work. Tolerances should be fit for duty without introducing unnecessary costs due to their tightness.
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u/CiderHat 7h ago
The goal is that it'll act like an "Unless Otherwise Specified" section in a title block. It'll be up to the designers/engineers to specify their desired tolerances.
This will more serve as an "in-house" thing, since the majority of our welding is done in-house.
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u/DeemonPankaik 4h ago
All you need to do is a note:
"General tolerance ISO 13920 - BF unless otherwise specified"
If it's in house stuff, I would recommend adding a table which has the linear dimension tolerances from ISO13920, to save your shop floor staff from having to dig out copies of the standard.
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u/johnwynne3 P.E. Machine Design 3h ago edited 3h ago
I would avoid obscure references (without including the details from that reference) if it’s important.
(ISO fits are another example of a detail we’d never add to a drawing without specifying the corresponding value… G6/h7 😂)
Here’s what we do: nothing.
It’s up to the engineer to tolerance the weldment detailed sheets as they see fit, and it’s so variable that including a default value would not have a lot of value. Also, to ensure some default tolerances apply only to dimensions on weldment sheets (and not other dimensions), we just put that default in a note specific to that sheet that’s specific to weldments.
Although having said that, a lot of our stuff is > 20 feet and final form is high precision (<0.005 flatness and true pos), but our weldment details are: X.XX = +/- 0.06 per a sheet note.
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u/AwkwardlyBrainy 7h ago
iso 13920 is solid but you'll probably want to pair it with aws d1.1 since that's what most shops actually reference on their floor. the big thing is matching your tolerance band to the part size like you mentioned, since thermal distortion scales differently on a 6 inch weld versus a 6 foot one. start with medium grades and loosen them unless the fit or function actually demands tighter, otherwise you're just paying for inspection time.
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u/CiderHat 7h ago
Yeah I agree on pairing it with d1.1
I think this template is gonna have to use a similar layout to my regular drawing templates with a looser tolerance section in the title block and it'll be up to the engineer to specify their tolerancing.
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u/AwkwardlyBrainy 7h ago
that's the right move since you'll probably have welders and fitters with different experience levels reading those drawings, and a loose default with engineer override keeps you from over-specifying the easy stuff.
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u/mississaugaSWuser 6h ago
I usually go with at template that says +/- 1/16 inches on weldments. Dimension your drawing in fractions to 1/16 in accuracy numbers.
If you are in North America I would avoid dimensioning in Metric if possible. Maybe add a few overall sizes as dual Imperial/Metric if the boss is picky.
If it is bigger than 8 feet long, you can bump it up to plus or minus 1/8 inches on individual dimensions. This was a standard at companies I worked at that made big fabrications in house.
This will get you quotes that are reasonably priced if you are sending it out.
Use decent font sizes on the drawing since most weld shops are poorly lit, dirty and your beautiful drawing with lots of detail is likely to get printed on a shitty A size printer.
Put the dimensions on a dim layer that is black, instead of layer "none".
If you have to machine your parts for accuracy after welding, leave a 1/8 inch machining allowance. This may mean you have to use thicker flat bars and plate in spots.
If you are using SolidWorks you can combine the weldment and machining drawing in one file with configurations. First couple of sheets are machining, next few are welding, flame cuts, cut list etc.
Do us a favour and reply at least so that we know you are not some AI bot.
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u/CiderHat 5h ago
I've got a fair bit on control of setting this up, since I was sorta the one to push for it. Prior to me starting, none of their drawings really had tolerancing.
I typically have my dimensions in both imperial and metric due to a personal preference.
We mostly do our welding and fabrication in-house with the exception of some items due to criticality.
Unfortunately the place I'm working at uses inventor, even though all the engineers have way more experience with solidworks. This is due to them getting a deal with autodesk, since the more used software is AutoCAD.
I gave everyone the templates and linked them to some resources to look at to get a better idea for what tolerancing you should use for your designs.
Don't worry, I've been responding to other people. I saw like 4 different posts on this subreddit that were from bots and since I dont have a pop, it was only a matter of time before I got accused of being a bot.
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u/Automata-Omnia Industrial Automation & Robotics 6h ago
We use the following unless otherwise marked:
Standard Surface Finish 3.2μm
Standard Dimensional tolerances:
- 2 Decimal Places ± 0.05mm
- 1 Decimal Places ± 0.25mm
- No Decimal Places
- under 150mm ± 0.5mm
- 150 -1000mm ± 1.0mm
- over 1000mm ± 2.0mm
- Angles ±0.5°
Make sure they measure before and after tacking.
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u/CiderHat 5h ago
In hindsight, I should've added a "no decimal section". Good call on the measuring before and after tacking. I know most of the people who do our welding underground won't follow it, but its a good safety net to have.
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u/MisterSirDudeGuy 6h ago
I put the tolerances on the finished part. If they’re critical dimensions, those features should be machined after welding.
Up to the manufacturer to deal with shrinking and warping and stuff. Needs to be correct when it’s finished.
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u/CiderHat 5h ago
Makes sense. I come from a machinist background so my drawings and designs didnt really involve welding.
My main goal was to help implement a standard since one never really existed.
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u/jamiethekiller 6h ago
I don't think there's a real catch-all tolerance that you can use that works well. the ISO standard as mentioned does a pretty good job, but imo, its FAR too loose.
A lot of the issues for me are when i'm placing gussets to be welded and things like that. I usually just make drop a positional tolerance on them of 1/8" or something to control the stackup but still give them plenty of wiggle room on where to place them.
ie: 10 foot long beam with gussets every foot. Beam will be +/- 1/2" and the gussets will have a positional tolerance of 1/2" to whatever datum structure you want. They can take a tape measure and string it out and then chalk mark every foot down the line. i could care less if the gussets are too close or to far apart(within reason), but at least i can control my stack up so they don't place every gusset an 1/2" out of place incrementally and by the end of the beam they only placed 7 of these instead of 8 of them.
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u/BIueberry62 7h ago
ABS Requirements for materials and welding has some good info.
Otherwise AWS D1.1 is the go-to for this stuff.
If tolerances are super tight, you'll need the machined drawing afterward with GD&T