Tolerancing a simple weldment consisting of sheet metal parts.

[HvdL]

Dear all,

at my company we are trying to add tolerances on certain envelope dimensions of sheet metal weldment parts to make sure the parts will cause no issues in assembly. (Procurement can switch suppliers as they see fit.)

For your information:
We have drawings for the sheetmetal sub components, completely dimensioned and toleranced and also ISO9013 is call out for the laser cutting.
And weldment drawings showing how to position and weld the reinforcement plates and paint the weldment.

We thought of adding ISO 2768-xx on the weldment drawings. But we think a sub contractor can say with reason that ISO2768 is not meant for weldments, and therefore is not accepted.

For weldments there's ISO 13920, but those tolerances are extremely wide for our kind of product. It serves no purpose putting that on drawing.

2 questions;
Is there another standard for weldments with tight(er) tolerances?
Can you advice another method or should we add ISO 2768 anyway?

Best regards,
Hans.

 

[CheckerHater]

You can (and you should) use direct tolerancing - that is simply show on the drawing which dimensions are acceptable and which dimensions are not.

The general tolerance standards are intended to control less critical dimensions that do not affect fit.

 

[drawoh]

HvdL,

As far as I am concerned, a weldment is an indivisible part. 3D[ ]CAD programs like SolidWorks provide intelligent resources for modelling these, but your documentation should reflect that a single part shows up at your receiving dock.

You care about the final product. You do not care how the welding shop assembled them. You do not care what tolerances were followed on the individual pieces. Possibly you do not care where the welds are. To some extent, welding is a black art. Maybe the welder will ask for your CAD[ ]model.

Your drawing should show the final, assembled piece. Specify the welds. Specify the final, assembled tolerances, and remember that these need to be loose, ±1/16" or ±1.6mm on a good day. There is no point doing a tolerance stack on a weldment specified to ±.005".

--
JHG

 

[MintJulep]

You are the designer.

Put the tolerances you need to obtain an acceptable product.

Since it is unlikely that there is ISO-nnnn "Acceptable tolerances for Hdvl's sheet metal weldment that becomes part of a larger assembly" you might just need to figure out what the tolerances are for yourself and call them out explicitly.

 

[HvdL]

Thanks for your replies.

I agree with what you are saying.

In my role as drawing checker I'm asking my colleagues to add mentioned tolerances on the final part drawings. They see the benefit and agree it should be added.
But then the question arises WHAT dimensions, form and place tolerances have to be added.
As you know a sheet metal part can have multiple flat surfaces, bends, edges and so on that can have an effect on how it fits, and how it looks in the eye of the end user.

We were hoping to find an "overall" tolerance standard similar to ISO 2768 that you can simply put above the title block, add some dimensions and be done with.
We like this because it's little work and people are often unknown with realistic, cost effective work shop tolerances. To be honest with the constant change of suppliers, it would give me some ground to build from and leverage in discussions with suppliers too.

Okay, this exercise shows we have to work on our very basic engineering knowhow. No excuses.
Secondly determine what quality we deem necessary for our product. And design and tolerate accordingly.

Thanks to all,
Hans.

 

[CheckerHater]

You made a good point about making suppliers and/or customers familiar with standards on customary workshop accuracy.

Those standards usually apply to dimensions without tolerances, which doesn’t mean your drawing shouldn't have any tolerances.
You apply direct tolerances to dimensions you consider critical and let title block tolerances do the rest.

As you identified yourself as "Hans", I assumed you may like this little PDF - see example of title block on the bottom of the last page:

http://www.tmf.de/bilder/Toleranzen.pdf

 

[MintJulep]

we are trying to add tolerances on certain envelope dimensions of sheet metal weldment parts to make sure the parts will cause no issues in assembly.

We were hoping to find an "overall" tolerance standard similar to ISO 2768 that you can simply put above the title block

Why to you believe that some standard writing committee composed of random people who do not work for your company and did not design your parts could possibly have any idea what the necessary tolerances for your parts are?

Determining tolerances is a design function and needs to be done by the designer.

 

[drawoh]

...

We were hoping to find an "overall" tolerance standard similar to ISO 2768 that you can simply put above the title block, add some dimensions and be done with.
We like this because it's little work and people are often unknown with realistic, cost effective work shop tolerances. To be honest with the constant change of suppliers, it would give me some ground to build from and leverage in discussions with suppliers too.

...

Drawing and tolerancing weldments is difficult. Usually, you can get away with not thinking through your machined parts. Welding requires looser tolerances. Apply tolerances to each feature of your final weldment drawing. Think the tolerances through. Make sure they are fabricateable. Do your tolerance stacks.

Completely tolerancing a drawing actually is not a whole lot of work, unless you detect assembly problems. This is a whole lot better than your assemblers or your customers detecting assembly problems.

A standard title block note allows your designers to not think about tolerances.

--
JHG

 

[Tunalover]

I see it as a waste of time to delineate each little piece part that goes into the weldment. Just show the end-item with the tolerances you can live with. Let the fabricator worry about how to meet those tolerances! He's more experienced with that sort of thing.

Tunalover

 

[BillPSU]

I believe the amount of attention given to weldment detailing depends on the product you are making. I would have great concern if someone was making a nuclear reactor pressure vessel and did not have every weld documented and all detail components detailed with tolerances. My current employer makes construction equipment and we make about 13,000 of them a year. We weld our chassis loader arms and cabs and have more then 10 robot welding stations. All of our components have detail drawings and then very detailed weldment drawings. Holding tolerances are critical to maintaining quality.

Bill

 

[HvdL]

Thanks for your input.

Think we are all on the same page.

We can conclude that for each part we need to balance the need for tolerances verses the amount of work (cost, time) we put in the designs/ drawings.
With sheetmetal parts we normally see that they are in clear sight of the customer. Therefore require besides good fit a reasonable finish and surface condition. So we need to determine what flatness for instance is needed to achieve this.
And put that information on the weldment drawing also.
With an "all bases covered ISO standard" we'd hoped to make life easy for us.

With the example shown by CheckerHater of an extensive title block, we'd gain insurance that not much can be forgotten by the engineer. And it simplifies their job. What has the downside that Drawoh describes.
There seems to be no suitable standard for acurate weldments so this is a dead end for this particular issue.
We have internally been discussing the benefits of applying similar extensive title blocks though. It shows the engineer what real-world tolerances are. But there are other ways to gain that knowledge too.

Perhaps I let myself to believe it's far easier to add a standard in the title block then to add the required tolerances. It may not be that much work.
The feedback I got was that people didn't know How to put it on drawing anyway. Hence my remark we need to improve our basic knowhow.

(CheckerHater, I'm not German myself, but am only 6 miles from their border.)

Regards,
Hans.