In the spirit of my other framebuilding Instructables, here's a simple design for a jig for building forks. It's such an obvious design there must be others out there like it.
This jig is easily made from 8020 extrusion and connectors. The rest of the small parts and materials are all available from McMaster Carr. For tools you will need at least a drill press, drills, and some metric taps.
Yes, I know that's a carbon fork in the picture. Shame on me. It was close by when I took the picture.
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Step 1: Tools and Materials
1 - 8020 #1530L extrusion, 24" (28" if you're building 29er suspension-corrected forks)
1 - 8020 #1530L extrusion, 6"
2 - 8020 #4350 4-hole 90deg Joining Plate
1 - 8020 #3330 Universal Pivot Nub
4 - 8020 #3879 Economy Double T-nuts, M8
15 - M8 x 16mm button head socket cap screws (8020 #3813)
15 - M8 washers (8020# 3860)
1 - M6 x ~20mm socket head allen screw
2 - M5 x ~10 pan head allen screws, like used on SPD cleats
2 - M5 by ~10 socket head allen screws
1 - 1"x1.5"x1/8"t rectangular aluminum tubing, about 2"
1 - 1" (or close) aluminum round bar stock - about 6"
1 - Aluminum V-block McMC# 20065A8
1 - Toggle clamp McMC# 5126a17
1 - front axle from junked hub
4- axle locknuts from junked hubs
Drill press or milling machine
Layout fluid (optional but helps)
McMC = McMaster Carr
8020 = 8020, but they sell through local distributors. Check their website for the nearest distributor. If you can't find one nearby, I can heartily recommend F & L Industrial Solutions.
Step 2: General Game Plan
Here's the general plan:
1) modify the pivot piece to turn it into the dummy axle
2) tap the end of the shorter piece of extrusion and mount the dummy axle
3) modify the v-block to mount the clamp
4) prepare the Park stand mount
5) drill the access holes in the main extrusion beam
6) assemble the jig
7) align the V-block
Step 3: Making the Dummy Axle
First, drill down along the pivot axis with a 9mm drill bit to make room for the front axle.
Then in the center, drill a 3/16 or 5mm hole down to the axle hole and tap it M6 for the set screw
Finally, you will need to make the old counterbores deeper so that the mounting screws will sit below the axle. Use a drill or end mill slightly larger than your bolt heads. A 9/16" ought to do it. You don't have to be precise about the depth, just make it deep enough to clear the bolt heads.
Next, tap the ends of the short piece of extrusion to M8 threading. You might want to run a 17/64 drill bit down the hole first to make it easier to tap, though it's not needed.
Bolt the dummy axis mount to the end of the extrusion, squareing it up to the sides.
Finally, slide in your 9mm axle, add your locknuts (I only used one set but 2 would be smarter) set to 101 mm spacing, and center it, locking it in place with the set screw.
Step 4: Mount the Clamp on the Spacer
Cut a piece of the 1 x 1.5 aluminum tubing to the same width as the clamp. First, scribe a line 1/8" from the long edge. This way you can see if the mounting holes will clear the walls of the tubing when you drill through. Put the clamp on top of the spacer as shown. making sure the mounting holes clear the lines you scribed along the edge, and mark the mounting holes. Then drill and tap them to M5. I only used two bolts, on the diagonal, as you can see in the picture. Don't bolt on the clamp just yet.
Step 5: Prepare the V Block
First, we'll prepare the holes that will allow us to clamp the V-block to the main beam.
On the underside of the V-block you'll find a centerline built into the extrusion. Scribe a parallel line 3/4 inches on each side of the centerline. These two lines should match up to the T-slots on the extrusion. Then scribe two lines perpendicular to the centerline, one at 1 3/4 inches and the other at 3 1/4 inches from the short edge. At the intersection you've marked, centerpunch, drill, and tap to M8.
Now we'll add the holes to mount the clamp and spacer.
Scribe a line 0.4 (13/32) inches in from the long edge, parallel to the centerline, and then scribe two lines perpendicular at 2 and 3 inches from short edge. At these two intersections, drill through 5mm or 3/16", then countersink deep enough to fit your pan-head M5 screws to be slightly below flush.
Step 6: Finish the V-block and Clamp Assembly
Hold the spacer tubing in place on the V-block as depicted in the photos and mark the location of where the countersunk holes. Centerpunch, drill, and tap to M5.
Then bolt it all together.
Step 7: Make the Park Stand Handle
This step will make the handle so that you can clamp the jig in a Park Stand, and then since it's round you can flip the fig over while you're brazing the fork. This can help when you're brazing a fork crown since it helps you with gravity when you're working on the steerer socket or the fork spigots.
Take your piece of round aluminum bar stock and drill and tap one end on center to M8. Then with a hand file, file a flat down on each side of the hole so that you are left with a ridge that will lock the handle into the T-slot. This keeps the handle from rotating and coming loose accidentally.
Step 8: Drill the Access Holes on the Main Beam
You will need to drill some access holes so you can reach the mounting bolts for the V-block and handle with an allen wrench.
For the V-block, drill the four holes in the T-slot groove. The two holes in each groove should be 1.5" apart to match the holes in the V-block. Mine start out at about 20" from the end of the beam. Depending on what size forks you build, you might want to adjust this. You might even want to drill a couple sets of access holes.
For the handle, drill a hole around 10" from the dummy axle end of the beam. If you're really anal, finish building the jig, put a fork in it, and find the balance point.
Drill the access holes with a 13/32 drill bit.
One other option would be to mount and align the V-block to a short piece of 8020, and then clamp that to the side of the main beam. The third picture shows the basic idea, from my frame jig design. This would let you slide the V-block around without losing alignment, and gives you more access and clearance to the crown area. This would probably be the way to go if you're brazing a lugged fork crown.
Step 9: Assemble the Jig
You can now bolt on the dummy axle portion. This should be self-explanatory from the pictures. Use a double T-nut on each section.
After you snug it together crank down the ones on the main beam, since they'll never need to be adjusted again.
Step 10: Install and Align the V-block
To align the V-block, you'll need a straight piece of tubing, the straighter, the better.
Loosely thread in the M8 bolts into the V-block and slide it into the main beam until you can see the allen heads through the axle holes. Tighten them finger-tight.
Clamp your tubing in the V-block clamp and extend it so it meets up with the dummy axle beam. Using calipers, measure the distance from the edge of the beam to the tubing. Then measure from the other side of the beam and compare. If they're not the same, nudge the tubing until it's centered on the beam. Tighten the V-block bolts down once you have it centered.
Now, as a test, I recommend you rotate your tubing 180 degrees and measure again. If the numbers have changed, that means your tubing isn't straight. So you'll have to loosen the bolts and nudge the tubing over to make up half of that difference.
Step 11: Done!
You're basically done! You can file a notch into the end of one the joinging plates and glue a small piece of tape measure to the short piece of extrusion to show how much fork offset the jig is set to. Don't forget that different steerer tube sizes will sit at different heights in the V-block so you will have to have one notch indicating the offset for each steerer tube size.
Otherwise, set your offset for 1 1/8 steerer tubes, and shim up your 1" steerer tubes to the same offset. Either use a piece of 1 1/8" x .058 tubing split in half lengthwise, or a piece of 1/16" thick flat stock folded over at 90 degrees, sized to sit in the V-block.
Go build some forks!