It can be made with not much more than than an Oxy-acetylene torch, hacksaw, and drill press. The materials are probably all available at your local Big Box hardware store, though you should see if you could buy the angle iron as cutoff remnants from your local steel supplier, as it will be a lot cheaper.
Step 1: Materials and Tools
Angle iron, approximately 2' x 2', 2 pieces 12-18" long
Angle iron, 3/4" x 3/4", about 2'
Steel flat stock, 1" x 1/8" thick, about a foot should be plenty
2" clevis pin and a cotter pin or clip
3/8 bolt, 1 1/2" long should do, or Metric 8, or whatever you find in your junk box around this size.
A nut to match your bolt if you don't have the right tap size
And the following tools:
Drill press and drill bits
Tap to match the threads on your bolt (optional)
Oxy-acetylene torch, and welding or brazing rod
Step 2: Bending the Mandrel
It should be obvious, but the open part of the angle iron should point up and you will be bending downwards to make a curved piece with a v-groove. It's self-explanatory from the pictures.
Put your biggest tip on the your torch and heat up a small section of the angle iron, about as wide as your flame width. As it turns red it should start to soften and you can put a slight bend it. Then move the torch over about an inch and get that part soft and put in another slight bend. Continue working your way down the angle iron until you have a nice curved section about 12-18" long. As you advance the flame along the angle iron, you can slide the material further into the vise.
You're shooting for something between a large dinner plate and 26" rim, depending on how much bend you're trying to get. I like to make the bend a little tighter at one end than the other, to give some adjustability depending on where you end up clamping the fork leg on the mandrel. I'd suggest something around 12" radius for starting out with.
Step 3: Weld on the Frame
Step 4: The Clamp
Step 5: Drill the Frame
Step 6: Assemble and Bend Away
Clamp the far end of the bender to your workbench. Clamp the tip of the fork blade with the bolt, and bend the leg down. You will probably need to slide a piece of larger tubing over the fork blade to get enough leverage. Some designs use two long arms mounted to the frame with a grooved nylon roller to lever down on the fork leg, which would be an easy thing to add to the frame.
If you are trying to do tight bends, large rakes, and are using thin fork legs, you might have to pack the legs with sand to keep them from kinking. The one in the photo below has a pretty tight radius, I'd recommend you try something a little flatter to start out with.