Tolerancing of member prints (weldment/riveted assembly members) 1

[tw_0407]

We design quite a few parts that are welded or riveted parts/permanent assemblies(is there a better name for this?) and I'm wondering how to differentiate between what should be toleranced on the member prints vs. on the end product print.

Ultimately, the end product is what will be inspected and used, so any necessary tolerances there should be on that print as well. If the individual members won't be inspected and they don't really have any requirements in and of themselves, do they even need tolerances? Or should the tolerances be determined by the fabricator such that the end product can be manufactured and meets the requirements specified on its print? Thinking specifically of member lengths, diameter and position tolerances of hole patterns for rivets, etc.

Alternatively, the member prints could have all reference dimensions, or perhaps easier, a note saying that all dimensions on the print are for reference only. With the latter, you could have tolerances that are contained within the member(for example, a hole pattern that required a diameter and position tolerance on a sheet), but it would also need to be reflected on the end product print, and it's placement on the member print would only serve as a reference for the fabricator.

Curious what others are doing for weldments/rivetments/permanent assemblies, and their member prints.

 

[MintJulep]

Do you fabricate also, or only design?

Will you always purchase the "end product" from a single supplier, or will you purchase various bits and pieces from multiple suppliers and give them to a welding contract to product the end product?

Who do you want to pay to figure out how long to cut each part, and who do you want to be responsible if that figuring is wrong?

 

[Burunduk]

If the individual members won't be inspected and they don't really have any requirements in and of themselves, do they even need tolerances?

Won't be inspected doesn't mean don't have any requirements. Your requirement is a specific geometry, within some limits, for each component. What if you had different vendors manufacturing different members of the same "inseparable assembly", and a separate one making the final product? Would you not be tolerancing the member prints?

 

[3DDave]

What industry is this for and what standards apply?

 

[tw_0407]

Do you fabricate also, or only design?

Will you always purchase the "end product" from a single supplier, or will you purchase various bits and pieces from multiple suppliers and give them to a welding contract to product the end product?

Who do you want to pay to figure out how long to cut each part, and who do you want to be responsible if that figuring is wrong?

Only designing. All fabrication is sourced, and assembly is done through a CM.

This assumes that we are sourcing the complete item from a single supplier. The assembly is quoted and purchased as a single line item and delivered as such.

That does not necessarily mean that supplier will cut all the tube, laser and bend all the sheet parts, etc themselves, and in fact I know they often do not. Which perhaps makes the question more relevant - what if the member prints are toleranced in a way that isn't friendly to how they want to make the final product?

For example - say you have two members that are welded together, but for whatever reason, the welder would prefer to have some small gap between them to aid in their process, instead of butting them together. However, if we dimensioned and toleranced the parts in such a way that wouldn't allow that, they may request a deviation to adjust the tolerances/dimensions of the member, when it does not affect the function or mating of the final product. Not sure how realistic that specific example is but I hope the concept is still clear.

Won't be inspected doesn't mean don't have any requirements. Your requirement is a specific geometry, within some limits, for each component. What if you had different vendors manufacturing different members of the same "inseparable assembly", and a separate one making the final product? Would you not be tolerancing the member prints?

As mentioned above, this is assuming we are sourcing the end item from a single supplier. They may source the members from their own suppliers but could potentially want to adjust dimensions to help their process.

In some sense, I think this is somewhat similar to discussion on whether a flat pattern for a bent sheet part should put on a print and toleranced. From what I understand, this is generally bad practice as you're really concerned about the final condition of what you're buying and the supplier may need to modify the flat pattern based on their own process. Perhaps the supplier will contract out having the sheet cut and do final bending themselves, in which case they may need to modify what is actually cut based on how they plan to bend it, etc.

What industry is this for and what standards apply?

Automation.

 

[MintJulep]

This assumes that we are sourcing the complete item from a single supplier. The assembly is quoted and purchased as a single line item and delivered as such.

Then do you care about the details of cut tube length and flat patterns?

Somewhere along the supply chain, someone needs to figure out how long to cut things. You, or your tier 1 supplier, or lower level suppliers. Do you care who does it?

You have the option of asking for a design review of theses details, as decided by your supplier, or their sub-suppliers. If you used that option, are there any details that you would reject?

In general, specify what you need, prohibit what you won't accept and let the supplier figure the rest out.