Step 1: Introduction
Commercial jigs start at about $1600. For the beginner this could be a huge investment - the equivalent of many bicycles worth of tubing.
There is also the option of building a jig out of 8020, such as described in my Instructable here. However, the cost on this is also not trivial, probably $350-$500, and it does require some machining work that is outside of the common garage tools.
If you ask around on the internet, many people will tell you that you don't really even need a jig, you just need a flat surface and some sort of tooling to hold your tubes at a fixed height above the surface. This is quite true, but for some people using an old pool table or maybe a surface plate is not exactly convenient, due to the size and weight.
This design and method is what I developed over building my first 5 frames. It's compact and cheap, and works by giving you a system of 3 small orthogonal flat surfaces to work with, and several means of holding tubes at the right position while you tack them.
However, since this jig is compact, you can only work on one joint at a time. This is the standard way for beginners to build, as described in the Paterek Manual. If you haven't heard of it yet, it's essentially the beginning framebuilder's bible. You can purchase it from Paterek or Henry James. The main disadvantage on this is that you never see the complete picture of the frame until you tack in the last tube. You can't set it up and then have a sanity check where you can see if it all makes sense.
I no longer have this jig, having moved on to my 8020 design, so I had to reproduce everything in SketchUp. For the sake of speed, I didn't model everything exactly to scale or try to make everything look perfect. But it should get the idea across.
If this Instructable helps you out, you can always show your appreciation through my Amazon Wish List.
Speaking of books, you might also want to check out Atomic Zombie's Bicycle Builder's Bonanza. While it doesn't cover classical framebuilding like the Paterek Manual, it does have lots of good ideas for "low road" building.
The materials are simple steel structural shapes:
1" x 1/4" bar, about 6", though get a foot since you'll eventually use it.
4"x 3" angle iron, give or take an inch in either leg, about 7-8' total
1"x 1" angle iron, another foot or so
Old rear hub axle(s) or similarly sized threaded rod and nuts
Some bolts, about M5 x 40 or whatever you have taps for.
A couple of Welder's magnets, available from Harbor Freight for cheap.
For tools you'll need a hacksaw, a drill and bits, and a tap to match your bolts.
Step 2: Base
A smaller piece, about 2' long, is welded underneath it to form a sort of Z-profile. This piece is used to clamp the jig to the front of your workbench. If you want to bolt it to your workbench you can drill some mounting holes in it.
The final smallest piece is about 8" long as is welded perpendicular to the top piece. This gives three surfaces that are all orthogonal to each other. Measure with a machinist's square to make sure the angle is 90 degrees before you weld it!
Note: when I say "weld", this includes other joining techniques such as brazing, or even drilling and bolting/riveting.
Also shown in the picture are the dummy axle and a tube lift, which are described in the next two steps.
Step 3: The Dummy Axle
Since the inside of the bar is on the same vertical plane as where the right end of the bottom bracket contacts the upright reference piece, you can use it as the reference for you axle locknut locations.
Looking at the dummy axle from the rear, your left locknut will be 34mm plus half of your hub width. The right side will be half of you hub width minus 34mm. This all assumes you are using a 68mm shell.
In use, the piece is just clamped to the angle iron with a C-clamp.
Since this is so simple, you might as well make a new one for each axle size.
Step 4: Tube Lifts
NOTE: Since this references the OD of the tube, you will need to make them for each size of tubing! The height of the lift will need to 34mm minus half of the tube diameter (assuming you're using a 68mm bottom bracket).
Clamp them together at the right height and give them a tack weld on each side. After in cools double-check that the height came out right. If one side is off, knock off the tack and bend it to the right height and tack it again. Since this doesn't need to hold any significant weight, the tacks are all you need to hold it together.
After you finish each piece, don't forget to label them with the tubing size you built it for!
However, this isn't the only way to align the tubes, as you'll see in the next few steps.
Step 5: Joining the Bottom Bracket and Seat Tube
With a big C-clamp and some flat bar stock (not shown), clamp the bottom bracket to the jig table. Then using the appropriate sized tube lifts (also clamped to the jig as shown), line up the seat tube against the bottom bracket over the vent hole. Welder's magnets will hold it place for you to tack the tube. You also might be able to use those big spring clamps (the ones that look like big metal clothespins) to hold the tubes against the lifts.
Of course, before you tack the tube, double-check that the seat tube is parallel to both the jig face and to a straightedge lined up against the face of the bottom bracket.
After you tack it, check for parallelism again.
Step 6: The Alternate Method
Drill and tap two holes in the jig base and thread in bolts from the bottom. When used with a couple of welder's magnet, this will hold the tube in place for tacking.
In this case, just fine-tune the height of your bolts to get the tube parallel to the jig base and the bottom bracket face.
As in the previous step, the clamp on the bottom bracket is not shown.
Step 7: Another Way to Hold a Tube
Clamp a piece of angle iron to your base, and then clamp the tube to the angle iron upright using a metal spring clamp. You could also use a small C-clamp, if you file the flat end of the C-shaped part of the clamp to have a round profile to fit inside the tube without denting it.
Step 8: Tacking the Head Tube and Down Tube
This step has nothing to do with the jig - I'm just including it for continuity. Refer to the Paterek manual for more information.
Step 9: Pieceing Together the Front Triangle
At the front of the jig under where the head tube will end up, drill and tap a set of two holes to hold leveling bolts.
Using the two leveling bolts under the head tube, and either another leveling bolt or tube lift under the down tube, set up the down tube in its approximate position so that it is the correct height off of the jig face. Use welder's magnets to hold this in place
Then lightly clamp the seat tube to the jig and nudge it into place against the down tube. Double check that your down tube and head tube are at the correct height.
Next stand back and sight down the head tube and make sure it is parallel to the seat tube. You can fine tune the leveling screws to get them parallel. If you have a digital angle finder, it will be very easy to get the two tubes parallel.
Now set the angle between your seat and down tubes. Double-check by measuring the distance between your seat and head tubes, where the top tube will eventually go.
When everything has been double-checked and looks good, tack the down tube to the bottom bracket.
Step 10: Top Tube and Chainstays
After that, it's time for the chainstays.
Clamp the bottom bracket to the jig upright so that it rests on the longer face. Pivot it so that it's at the right seat angle, setting it using an angle finder or with protractor measuring to either the face on the upright or the main table.
Since most bottom brackets are 40mm, your centerline of your bottom bracket will be 20 mm above the face. Add that 20mm to your bottom bracket drop and mark that distance on the bar of the dummy axle using a piece of tape. Loosely clamp it at the right height using the mark and measure the chainstay length with your machinist's rule. Adjust fore and aft until you get the right length and firmly clamp it in place.
Now you're ready to add your chainstays. After they're tacked and cooled, double-check the alignment using a true and correctly-dished wheel.
Step 11: That's It!
Hope this jig gets you started, and once you've gotten bitten by the framebuilding bug, consider upgrading to my nicer design based on the 8020 extrusion.