Tools required to make this bender, a metal working drill press, saw, arc welder and dial calipers.
Optionaly it helps to have a lathe to make the mandrel and a mill or shaper to square up blocks or cut the slot.
The hole size needs to be carefully determined.
The pin or mandrel wants to be an even stock size for what ever system you are using.
Add to that double the thickness of the stock.
Then add .010" (.25mm) to the total and find the closest drill size to that number using at least 1/64" (.4mm) drill sizes.
When using gauge sizes be sure to use actual decimal size of the stock.
The tricky part of making this bender is the slot.
My first one of these was sawed.
My second one cut using a 1/16" (1.5mm) lathe cut off tool in a shaper.
The width of the slot is not critical, however it must be tangent to the edge of the hole.
An alternative method for making the slot is to fit pieces together as shown.
The small piece is the slot.
The pieces must fit together very snugly or the drill will hang on an edge and possibly break.
Cold drawn steel or ground precision bar is recomended.
The slot can also be made without the fourth "slot" piece. See below.
There are two ways to use loose pieces.
One is with a piece that is removed for the slot.
The other is with three pieces.
When using three pieces the third piece makes the inside of the slot.
After drilling the third piece is removed and cut off square then replaced flush to the edge of the hole.
When using loose pieces they must be clamped together tightly and welded before drilling.
When using a fourth piece for the slot it should be lightly welded so that the the welds can be ground off and the piece removed.
The other pieces should be permanently welded together.
IF using the three piece method the third piece needs to be held tightly but not permanently welded.
After drilling it is removed, cut off and then replaced by either bolting or welding.
The hole center must be very carefully laid out. Then a center drill or pilot drill used to position the hole.
Note that a normal small pilot drill nay be too small for drilling across the joint surfaces.
1/4" (7mm) should be satifactory.