Welded Frame Horror Stories

[sanddrag]

Here's a welded frame done right. Here's a welded frame done light. The front 1x1x1/8" box tube in the second pic bowed in slightly after a lot of bashing but never caused any problems. Professionally TIGed, only one weld ever so slightly cracked ever.

[KenWittlief]

one of the design criteria you should be using to evaluate different design approaches is servicability / repairability, or MTR (Mean Time to Repair)

simply put, if your frame is damaged, how easy will it be to repair? how long will it take on average?

dont paint yourself into a corner thinking there is no way your frame could be damaged. Bad things happen, crates get dropped off trucks and forklifts from 8 feet up, Robots fall off tables and carts, or down stairs. When something bad happens to good robots you will need to repair it, then the question is: will it take 8 people 4 hours, or 40 hours?

If the only approach to fixing a welded frame is to bring a complete spare frame, then thats what you must do. How long will it take you to transfer everything on your robot to the spare frame? Or it might be better to fasten the frame members together some other way. or maybe have welded subframes that bolt together?

[IndySam]

Team 829 has done welded frames for three years. Here is this year’s frame. This frame was welded by our welding class. No breaks or bends and we were a very aggressive player this year. In the background you will see last years robot made from the KOP. We welded it too.

[KenWittlief]

Originally Posted by IndySam

Team 829 has done welded frames for three years. Here is this year’s frame.

this is an excellent example of a frame that could have been designed with 3 or 4 welded sub-frames, that are bolted together

then if one side got smashed in you only need to replace that subframe section, instead of needing to bend metal back into shape, or to weld on an alum frame (which conducts heat very well) at a regional.

[ewankoff]

is welding any better than bolting? we use extruded aluminum and we had a small problem with bending but if was easily fixable.

could someone tell me what is so much better or worse about the two

[Cory]

Originally Posted by ewankoff

is welding any better than bolting? we use extruded aluminum and we had a small problem with bending but if was easily fixable.

could someone tell me what is so much better or worse about the two

weight. You lose all the weight of the bolts by welding.

You also don't have to worry about bolts loosening themselves, and going around retightening everything

[KenWittlief]

Originally Posted by ewankoff

is welding any better than bolting?

yes, definately!

If metal is welded correctly the metal will fail somewhere else before the weld will break

if you use a bolt you must drill holes in the metal, which makes it weaker at that location. A bolted frame will almost always fail at the bolt holes, or the bolts themselves will break, when the metal is stressed beyond its limit.

It will take more force to break a welded frame apart than a bolted frame.

BTW, this is how you test your welding skills. you weld two pieces of metal together, then you bend the metal or pull it apart till it breaks or shears. If the weld breaks it was not a good weld.

[Cuog]

Originally Posted by KenWittlief

BTW, this is how you test your welding skills. you weld two pieces of metal together, then you bend the metal or pull it apart till it breaks or shears. If the weld breaks it was not a good weld.

So you just yank the metal appart with your he-man-ness?

But a weld is alot stronger then the plain metal itself. If I remember correctly(which there is a 50% chance I don't) it has to do with when you weld you are melting the 2 peices together the same way metal is formed, then the extreme heat changes also heat treat it a little to help make it stronger.

[Andrew Blair]

I was under the impression that, with the exception of the fact that you're adding extra metal around that joint, a weld is actually far weaker in a heat treated alloy. For example, with 6061 T6, the weld is reduced to T0, and therefore, the weld was at a disadvatage to the rest of the material. The only way to restore the strength is to reduce everything to T0, and re-heat treat.

On a side note, what's a good way to heat sink a frame so that it warps far less when welding? I remember 67 talking about doing that when they were welding their 2005 bot. I also thought they re-heat treated it.

[Cuog]

Originally Posted by Andrew Blair

I was under the impression that, with the exception of the fact that you're adding extra metal around that joint, a weld is actually far weaker in a heat treated alloy. For example, with 6061 T6, the weld is reduced to T0, and therefore, the weld was at a disadvatage to the rest of the material. The only way to restore the strength is to reduce everything to T0, and re-heat treat.

I think(again not sure) that this is only true for heat treated metals and not standard metals

Originally Posted by Andrew Blair

On a side note, what's a good way to heat sink a frame so that it warps far less when welding? I remember 67 talking about doing that when they were welding their 2005 bot. I also thought they re-heat treated it.

The best solution would be to take your time and do it right. just use spot welds all the way around allowing it to cool some before starting the next weld. Running a bead all the way around and going too quick will likely warp thin metal.

[Tristan Lall]

Originally Posted by Cuog

I think(again not sure) that this is only true for heat treated metals and not standard metals

Andrew is mostly correct. the -T6 represents a heat treatment called "solution heat treatment and artificial aging". When you weld it, you're removing the heat treatment, but not to -T0 (that doesn't really exist); it's actually the -F "as fabricated" temper, which means that no control of the heat treatment was employed during fabrication. There's more here, and even more in MIL-HDBK-5. It's possible for a weld to be stronger or weaker than the surrounding metal—there's no universal rule. The suggestion to take everything to the -O temper (that's "annealing"), and re-harden to -T6 is good in principle, but probably bad in practice because of the difficulty finding a suitable hardening oven (to fit an entire frame), and the difficulty of dealing with deformation and uneven heating. In industry, it's (very) occasionally necessary to do this, but stiffening jigs and distributed heat sources will be employed if there's any worry about deformation and uneven application of the heat treatment. More often, though, wrought aluminum alloys and similar metals come hardened from the factory to their highest strength level.