Frame Fixtures: Let's start by defining fixtures and jigs, as you'll often see the two terms used interchangeably on this subject.
Machinerys Handbook: Definition of Jig and Fixture:
"The distinction between a jig and fixture is not easy to define, but, as a general rule, it is as follows: A jig either holds or is held on the work, and, at the same time, contains guides for the various cutting tools, whereas a fixture holds the work while the cutting tools are in operation, but does not contain any special arrangements for guiding the tools. A fixture, therefore, must be securely held or fixed to the machine on which the operation is performed -- hence the name. A fixture is sometimes provided with a number of gages and stops, but not with bushings or other devices for guiding and supporting the cutting tools."
Since we're holding tubes in place we are using a frame fixture, not a frame jig.
The frame fixture must hold the swing arm pivot and steering head (or front suspension mounts if using a modern front end) materials in the proper relationship while you try to induce distortion by welding on them. The steering head must be on the centerline of the bike and in a plane perpendicular to the swing arm pivot. For an example of a complex adustable production fixture, see page 29 of the March 1996 "Performance Bikes" magazine. You do not need anything this complicated. A fixture can be made out of any material you can fasten together that holds the parts where they are supposed to be.
Machinist and carpenter's framing squares, levels (machinist levels are more accurate), long straight rulers, calipers, string and plumb bobs are all handy items to have on hand when getting the frame fixture and the engine/steering head parts squared up to each other.
My fixture has a base made from a 6" length of 12" x 3" structural steel channel. It rests on a stand that positions the base of the fixture about 2 feet off the ground. I had it surface ground on three sides just enough to clean up the surface and square the sides. I have a 28" high piece of 5" square structural steel tubing to hold the steering stem. Again, I had all 4 sides and one end ground to clean and square the sides. This allows me to set the squared end of the 5" tube on the surface of the base channel and know that the tube is perpendicular to the base. I then square the sides of the tube to the sides of the channel with a machinist's square, and bolt the tube to the base with two angle brackets (which I made from an extra 5" length of the tube that I sawed apart on diagonal corners). At the top of the 5" tube I bored a through hole which accepts a length of bar stock (about 1.5" OD) that is parallel to the ground and perpendicular to the long axis of the base . Into a stepped hole in the middle of the bar stock fits a length of 3/4" bar stock that takes the place of a steering stem. This hole in the crossbar should lie on the centerline of the fixture. I hold the 3/4" bar into the cross bar with a bolt, and have the other end threaded. I can then put large flanged spacers on the stem to hold the steering head. The cross bar rotates to adjust the steering head angle (rake), and the steering stem bar is long enough that I can space the steering head up/down about 3" for fine adjustment. The crossbar has a clamp on one end to hold it in place once a rake angle is established. I made the upright 5" tube as long as it is so that I can mount a motor on a stand on the base of the fixture in the position it would normally have in the bike. I can then build the whole bike, including the rear wheel spacers, etc on the fixture, and have the rear wheel installed while the frame is on the fixture..
I have two milled aluminum plates about 12" tall that clamp to the sides of the fixture base. They have sliding blocks with a 3/4" hole in them for holding the swingarm pivot pin at a wide range of heights. I milled these plates while they were bolted together so that I can square them to the fixture base with a machinist's square, and put a level across the tops of the sliding blocks to ensure the pivot pin is parallel to the leveled base of the fixture.
I have other aluminum plates that I clamp to the fixture base to hold the swing arm parts. The holes are about 1.5" off the surface of the fixture.
If you farm out the surface grinding and can make the rest of the parts in house this fixture will set you back about $800 (in the early 1990s).. This too is more elaborate than needed. I planned on making a number of one-off frames, so I figured it would be worth the investment to have a precision fixture I could readily adapt to a variety of motors/frames.
A big piece of scrap steel is nice to use for the fixture as it is pretty hard to set on fire. If you buy a chunk of plate, try to get it as flat as possible. I think the structural channel is likely to be straighter as found than your average bit of plate. A nice flat bit of 1.5" or so plywood could be substituted. For either the plate or plywood make a stand to set it on, and then bolt it to the stand, using shims between the stand and the surface to level the top (working) surface. You could make a steering head tower etc out of 2" lumber, more sheets of plywood/plate, etc, just make sure that the various holes end up perpendicular/parallel to the base.
I have seen pictures of fixtures that were welded up out of angle iron/square tubing, and made from old lathe beds that had flat ways on them. I know of one racer who just fixtured the frame on his lathe after turning the various parts. I think a nifty thing to make a fixture base would be to find a big old scrapped horizontal or vertical mill and swipe the table from it. It would already be pretty straight/square and strong, and comes with T slots for clamping things to it. Royce Creasey recommended finding the base to an industrial paper shear, as it is a large, flat cast iron part that already has a 1" grid laid out on it.
If you can build the frame upside down, you could have the steering stem coming vertically off the base, clamping the steering head an inch or two off the fixture base. This makes it easier to ensure the steering head is in a perpendicular plane. The swing arm pivot ends up being about the same height from the fixture base when building a road race frame.
I like to have the motor mounted on the fixture, because it seems that no matter how carefull I am with my drawings, I always have to make minor adjustments when some corner of the motor isn't quite where I thought it would be, and interferes with the frame tubes. Also, as long as the entire bike less front end can be installed in the fixture, it is easier to make little fixtures to hold foot peg brackets, etc in place while they are being attached to the frame.
I think many production fixtures are built from a prototype frame that was originally built around a motor. Then the prototype frame is bolted into the production fixture, and the rest of the locating surfaces of the fixture are built around the frame.
Try to make your fixture so that you have good access to the frame while welding it. After an all day session of working on a frame, where I was bent over and twisting around to get to the bottom/insides of tubes, I spent the next week flat on my back with lower back muscle spasms. Physical conditioning is a good thing in the garage as well as on the track! It also helps if you can remove the frame after it is mostly welded, finish the bits you couldn't reach, and then reinstall it on the frame for the rest of the procedure without having to disassemble and resquare the fixture.
I'll attempt to make a little drawing of my frame fixture below:
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Stay tuned for the next exciting episode.
Euro Spares, SF, CA
For more information contact us by usinghttp://www.eurospares.com/frame3.htm