There are a handful of approaches to design a bicycle frame fixture. Taking advantage of Henry James knowledge from his website and my own researches I have ended up with this article about the existing possibilities. A lot of information spread all over the bike forums that somehow I wanted to gather and analyze.
Jigs can be classified according to different criterias: professional solutions vs. hobbyists fixtures, horizontal configurations vs. vertical mountings, tack-only vs. fully weld frame jigs, mass-oriented production jigs vs. custom frame geometry units, beam inspired types vs. beamless jig designs, multipurpose and extensible jigs vs. simplified versions, the place where you start to assemble the tubes on the jig and so on.
Professional fixtures will provide full access to each joint for TIG welded and fillet brazed construction without the hassle of tacking and removing the frame in order to fully weld it apart. At the same time is easy to remove and replace the frame during the assembly process, especially important when using lugged construction or when we want the gravity to assist the welds, which is a good thing.
Jigs having vertical structural plates and/or members are superior to horizontal or backbone based models. Their individual clamping components are shorter, located closer to the joints and connections, hence stiffer than the same type of fixture that has to extend all the up from the base plate.
Homemade tack-only jigs are affordable if you are planning to build just lugged bicycle frames. Roomier units that provide enough clearance to fully weld the frame vertically on the jig can be achieved with DIY beam designs at the expense of more complexity when determining, for example, the BB drop as it will be “floating out in space”.
Pure beam style jigs for mass production (the ones used in BMX companies for example) have limited range of adjustment but work well and offer a lot of access for peanuts. On the other hand, jigs that combine compact plates and adjustable arms via extensions that hold the “bearing areas” (BB, HT and rear axle) generate almost endless geometry combinations with good clearance.
Fixtures made of beams are very rigid and provide great accuracy if we go for extruded aluminium profiles and machined parts. On the downside, aluminium can be considered a bit expensive compared to wood or traditional metallic square profiles.
A multipurpose jig will meet more framebuilders needs, for example integrating a fork building feature in the rear axle frame fixture, or offering motorcycle jig capabilities.
Finally choose the jig that makes the tubing logic assembly fancier for you. Building from the BB up is a better alternative in my opinion.
Here is a brief summary of the frame jig types according to their design:
No jig. This goes back a hundred years. Accuracy depends on craftsmanship, not tooling. Because no fixture is used, lugged joints often are pinned together by driving nails tightly into drilled holes in every joint. This holds the frame together, and hopefully keeps it in place during brazing.
Flat surface. The frame is assembled on a flat plate which is larger than the frame, and made of granite, cast iron (steel) or aluminum, or for beginners, particle board or plywood. Fixtures consists of shims, vee blocks, or other holders that locate the tubes on the center line of the frame. Setup takes forever, and access is limited. Used for tacking only. A good choice for building your first frame considering a flat surface is also needed to check the alignment.
Modified flat plate or vertical plate jig. Still intended only for tacking, The plate is shaped specifically for a range of common frame configurations in an attempt to provide better access. It has specially designed holders for head tube, rear axle, etc. but still with limited capabilities. Because intersecting tubes (HT and BB) and rear axle are clamped it can be mounted vertically.
Parallel beams jig. Here the plate is replaced with beams that are parallel to the head tube and seat tube. These beams rest on one or two cross beams that are intended to keep the structure flat. This style of jig is very sensitive to warping of the beams. Most beam materials are not inherently dimensionally stable, so internal stresses and external stresses from torch heat, etc., can lead to a loss of accuracy.
To use this style of jig you move and rotate the beams to set the jig up. The problem is that the beams are parallel to the tubes, limiting access just like the plate jigs. The net result is still a tacking jig without the simplicity of the plate jigs, or the access of the compact plate jigs.
Compact plate jig. On a compact plate jig, the plate is much smaller than the main triangle. Adjustable arms extend out to support the head tube, BB shell, and rear axle. Carefully designed, offers the best combination of clearance, rigidity, fast accurate set-up and versatility.
Because of the compact design, these jigs can be much lighter than other jigs, making it much easier to rotate the jig as you weld, and the jig takes up much less space in the shop.
Sputnik and Henry James professional frame fixture are ample proof of this design.
A rare variation of this style uses no plate at all, just a bunch of arms that support critical parts of the frame. Both Anvil and Bringheli jigs implement this design.
Bottom up or backbone jig. A narrow (4″-6″ wide) four foot long bar or beam is the base of this style of jig. The frame sits upright on this base. Along the base are supports for the BB shell, rear axle, and head tube. Additional supports may be added to hold the seat, top and down tubes.
Only the axle and BB shell which are close to the base are accurately held. All the other supports extend a relatively long distance from a very narrow base, so errors and tolerances are magnified. Set up is usually slow.
An almost jigless version of a frame fixture can be achieved thru this design. The method is proposed here following famous Paterek instructions.
External jig or motorcycle jigs. A rectangular framework structure is sized so that the bike frame fits completely inside the structure. For this reason, these are sometimes referred to as “picture frame” jigs.
A foundation of a wide rail, multiple rails or table are built upwards with the jig fixtures supported from below. Arms extend inwards to support the head tube, BB shell, axle, and seat tube. Because the framework must be the largest of any style of jig, and because the framework members must be heavier to provide the stiffness this large size structure requires, these jigs are heavy and expensive. The inherent problem with this style is that the largest frame you can build is limited by the size of the framework. And, if you make the framework really large, the extensions that hold the bike frames must reach in much farther to build very small frames.
It is stable by itself without the need of mounting platforms.
A variation is a hybrid jig with a smaller framework offset to the rear with extension arms for the axle and/or head tube.