Crime Scene JIG, How can I minimise welding warpage?

[metri]

Thanks for the suggestions.

[hogtied]

I guess this is a little late... I asked Joe how he managed. He said he had to go at it a few times, and possibly he mentioned shimming it. At least in my experience there were two problems. Standard warpage, and the fact that the CR bars are pretty straight, or straightenable, but the 2x3 steel was like a propeller. So shimming and tacking was necesarry. And I had to cut and redo.

A better way that I would probably use another time would be to make the backbone a pipe, orient the curve in the vertical axis, attach brackets and weld CR to it. Basically the Oneway lathe body. http://www.oneway.ca/ Might seem like overkill, but jigs aren't really necesarry, so if you want to use one, it might as well do something, and being stiff and accurate is part of what they can do.

[beast]

Wouldn't thicker tubing help against too much distortion after the welding?
I guess I'll try to make short welds, alternate them in order to compensate their constraints and go for the thickest tubing I can get,
After all, weight is the jig's least problem...

[MechWerks]

Don't overthink the jig. It's sole function is to hold parts in place so they are ready to weld. Only the contact points of the jig need to be accurate. Set up those contact points accurately with shims or whatever so the parts are held correctly where they need to be. Using structural tube, it will be found that the stuff is not very accurate. Even using better CRS will find various defects and tolerance variation. Making room for adjustable features and shimming will make accurate placement of parts.

Here is an article from my website: http://www.mechwerks.com/Frame_Jig.htm

The article explains some of the problems a person may encounter in jig building and some solutions. The use of set screws or capscrews for "jacks" is something that works well for me. The "jacks" are installed in parts at usable locations, adjusted to level parts of the jig and then the normal fasteners for those parts are tighened down locking the setting in place.

The jig is just an end to a motorcycle frame. Unless you plan to build many frames don't waste too much time on the jig. Make the jig simple, adjustable, build a frame and ultimately a motorcycle.