Motorcylce Frame Jig!
A frame jig for your motorcycle or chopper frame is what holds the tubing lightly in place, at the correct angles, while you are welding it.
A chopper frame can be built without a frame jig and some top custom builders do just that. But they are top custom builders, not beginners. Most builders consider a jig vital - and it probably is an essential for a beginner. They can be quite basic - if you were building a bicycle (which is flatter and lighter than a chopper frame) a marked out piece of plywood with a few bits of shelving bracket would be more than enough to do a great job.
If you want you can spend thousands of dollars on a shiny adjustable frame jig that will do the job. It's also possible to build your own, which we'll talk about below. The problem is that this takes time and money - more of both than the building of the frame itself. So if money is no object, you might want to buy a jig. You can get professional, original jig plans here.
There are many types of frame jigs available - what is usually just referred to as a 'frame jig' is designed for holding all the metalwork of the frame in place for given bike. In other words it's useful for the frame only.
A 'custom builder's' frame jig usually takes all the components (wheel, motor and transmission) as well, so the frame can be built around these bits. This helps in ensuring that they all fit well because you have all the parts in front of you as you fabricate the machine. And finally there are the parts jigs, which are simply used for bits and pieces like fabricating the wishbone and downtube before bringing them together with the rest of the frame.
There are many different names for a jig - basic flat plate, modified flat plate, parallel beams jig, compact plate jig, bottom up jig, external jig, to name but a few. Here we'll just talk about the basics that you will need to complete the job. The building of a jig is a chance to stretch your ingenuity; not all jigs need to be the same!
The first thing that all jigs need is a solid base that is true (perfectly level). This doesn't in itself have to be hi-tech, and can be metal girders stacked on blocks or set on a perfectly level workbench. Some jigs are vertical and you build the bike as though it was balanced on its back wheel, but I find it easier to visualize the bike in the position it is going to end up being ridden.
Next you will need some fixed points of reference for putting the bike together, such as fixed tubes for sliding the neck tube onto (fixed at the correct angle) and for mounting the axle points. Also you need to remember that once the frame is welded, it needs to come off the frame jig to become a street going vehicle - don't jam it in place with an immovable axle mount and an angled neck support strut!