I've tried to show as much as possible in
this one end view drawing of my 72"
press brake. The drawing is fairly close
to what I built but I have included a little
of what I should have done in some places.
For those unfamiliar with press brakes the
objective is to bend a flat metal strip by
forcing the flat metal down into a slot until
the desired bend angle (plus any spring back)
is reached.
There are four 78" steel (iron ?) elements to the brake: Two 2" X 2" angles, as heavy as you can get them. One 2" channel, again - heavy. And one 1" X 1" bar.
There are a total of twelve shorter cross pieces of 2" channel, seven on the bottom (base) and five on the top (punch). Mine are 12" but when I think now about bending a landing gear box I wish I had made them longer.
There are also a total of twelve 3/8" threaded rods 18" long. If I had used a 2 X 6 instead of the 2 X 8 these rods could have been shorter. The wood piece is there to keep the top edge of the bent metal from hitting the top cross bars.
To make the base, clamp the 78" long 2"channel between the 78" long 2" angles. Drop the 1 X 1 bar into the 2" channel, CENTER it carefully, and clamp them both down on a bottom cross channel at each end. Drill 3/8" holes at each end CENTERED between the 2" angles and kinda centered on the bottom cross pieces. This match drilling will keep the holes for the two end 3/8"threaded rods aligned. Drill two more holes at each end thru the angles and the cross pieces and bolt them together. These bolts have to keep the angles from tipping under considerable pressure.
The ten remaining cross channels can be drilled in matching pairs for their 3/8" rods. The 1 X 1 bar and the solid rod are also match drilled for the long flat head wood screws which fasten them to the 2 X 8. The rod is back-drilled to provide clearance for the screw heads. The 78" channel is also fastened to the top of the 2 X 8 with wood screws.
The five bottom cross channels are drilled and bolted to the 2 X 2 angles about 1 foot apart. The rear threaded rods are fastened with 2 nuts underneath, they are under considerable tension during the bending operation. The front rods go into 3/8 coupling nuts bolted to the cross channel so the rods can be easily moved to outboard holes to bend wide pieces.
After all the pieces were assembled I slid the punch down on the end threaded rods, set the top cross channels down on their threaded rods against the 78" channel and clamped them down with the coupling nuts. I then drilled thru the cross channel and the top channel for the 1/4" lag bolts that fasten them all to the 2 X 8.
The key to bending without scratching are the tipping bars. As the punch is driven down into the slot between the angles these tip into the slot and there is little relative motion between the metal being bent and the tipping bar. These bars can be the cheapest you can find. There are small lengths of light angle fastened to the underside of the tipping bars so that they can be slid together and meet centered under the solid bar. I put some Teflon tape I got from a woodworking supplier on the top surface to reduce friction and ®pad" the surface somewhat. The wood blocks (4) are just hot glued to the angles to support the tipping bars during prebend setup.
To set up, the top coupling nuts are turned up and the punch assembly is raised using the lift nuts. The tipping bars are separated, I put a couple of thin wood strips across them and then lower the punch lightly onto the wood strips using the lift nuts. The metal to be bent gets a strip of masking tape along the length that will be in contact with the solid rod. The edges are deburred and it and the tipping bars are wiped clean. Then the metal is slid onto the tipping bars pushing the thin wood strips out. The bars are pushed together and the metal is positioned by measuring from the edges of the metal to the edges of the tipping bars. This is fairly easy to do with light pressure on the metal from the punch. Then run the lift nuts down out of the way.
The final setup step is to set the initial position of all the top coupling nuts. First I finger tighten them, then with a level I check to see that the top cross channels are level. If they're not the front and back coupling nuts are adjusted to level them. Next I measure from the lower flange of the 2 X 2 angle to the top edge of the long top 2" channel near each end and near each top cross channel. In my press this is 12 1/16" with a slight tension on each rod.
To bend, first go back and make sure that each rod, washer pair, and coupling nut has a coating of grease ( I forgot to put that in earlier). You'll understand the need for the grease about halfway through the bending process.
Now the boring part. Each coupling nut gets turned one turn in sequence, then one more turn each in sequence, etc, etc, for 16 turns each or 1" down to 11 1/16", angle to top, in my press. I usually check that measurement several times during the bend for assurance that the punch is coming down evenly. occasionally I have to make a half turn adjustment to keep everything even. It also helps to relieve the boredom. You'll want a fairly long wrench for this. I set a couple of nuts on each end rod after some trial bends as stops for the lift nuts so I won't go to sleep and put in a severe overbend. After the 16 turns I back off 1 turn at a time to check the spring back. It appears to be fairly consistent at 5/16".
Yes, I have been avoiding any mention of the solid rod diameter. I decided early on that I would bend the top and bottom outside longerons to a 3/8" radius. Your choice. I've read that a 3/8" diameter rod will give you a 1/4" radius. Also, it's only six feet long because my shop space is limited and I decided to splice the lower outside longerons. You might want to put the 2" angles a little closer together if you use the 1/4" radius
This all comes with every liability waiver you have ever read or will ever read, in any language, for your entire lifetime, even if any of those waivers have been found invalid by any court, anywhere, anytime.