Slots & Tabs

[dlloyd]

I'm not sure this is the correct forum, but I can't find a better one.

Is anyone successfully using slots & tabs for locating parts in a welded assembly? We are in the agricultural equipment industry so we use a lot of hot rolled steel. The material thickness variation, which has a direct effect on the thickness of the tab, plays a big part in how effective this is. However, due to our industry, a lot of our tolerances (material as well as location) are more generous that some of you are used to.

I am trying to write a design standard for slot & tabs and am not even convinced it will work on a wide spread basis.

[PinkertonD]

The material thickness variation, which has a direct effect on the thickness of the tab, plays a big part in how effective this is.

This begs the question, how tight a tolerance are you using on the slots? The thickness spread (tolerance) on Hot Roll (HR) sheet is not that great. Surely the slots could be large enough to accommodate that spread and not cause a problem.

If you are relying on slots to position welded assemblies tighter than HR-sheet tolerances then I suggest a new look at the design process, you will be getting into machined slots and tabs territory.

Find yourself a copy of the ASTM specs for the steel you are using.

[Android]

Surely it will depend on the machine you cut the profiles with, Plasma, water jet or laser.....unless of course you are still doing it manually.....in which case the skill of the operator will be the deciding factor.

[jboggs]

dlloyd,
I think I understand what you are trying to do, basically use slots and tabs to turn the various pieces of your asembly into a self-guiding assembly fixture. Neat idea. And yes, I'm sure material thickness variations are a factor. You must make all slots wide enough to accept the thickest variation of the mating material, but that will lead to free play and looseness for any material less than that. You must also consider the poor shmucks who have to assemble these pieces. You cannot expect the fit of a precision machine. Nothing like it. In my opinion, if you are looking for this approach to achieve a level of precision better than the average variation in material thickness, you are whistling Dixie. It won't happen. And if you did make the fit of the slots and tabs that close, the guys in the shop would never be able to just "slip it together". I think you have to accept the fact that your fits are going to have a certain amount of free play in them and find other ways to get around that if its a problem. I think its a good idea, and its do-able. You just have to get creative to make it work really well.

[mda53226]

dlloyd --

I realize I'm writing this long after you asked the question.
Am hoping you found your solution.
This answer may be useful to others.

In a couple of my past lives, we did exactly what you are trying to do.
The weldment tolerances were always large compared to the HR steel thickness tolerance.

However, there is an uncommon trick that will center the tab in the slot.

Consider making part of the tab tall enough to protrude through the slot.
Think about this for a while, and you can design a tapered tab that (when twisted with a pliers) will center the tabbed sheet in the slot and draw the slotted sheet down firmly to the tabbed edge. The welder then tacks the parts together, breaks off the tapered tab, and welds the parts together through the slot. All is ground flush for a finished product.

I've used a design like this and it works very well. The details are considered a trade secret of a big company in the ag machinery business.

Good Luck!