Last installment! (Actually doing it... ).
So, Arthur, (the guy with the Coke can), could - in the words of another fabricator I knew who was modest about his abilities - "weld the crack of dawn if you could find a way of clamping it down". But even Arthur was a bit confused about using a folding machine.
As I said, there are basically two ways of doing it - a box pan folder or a brake press. Odds on, most home-builders will be using a box pan folding machine - they are cheaper, more versatile, but have their limitations when it comes to doing thicker, or longer, bends. Sorry, here comes the maths:
When you bend a piece of metal sheet, the material on the inside of the bend compresses and the material on the outside stretches. It follows that somewhere in-between is what is called 'the neutral axis', (a line through the material, when viewed in section, that does neither). The neutral axis is not necessarily in the middle of the thickness but, when bending relatively thin sheet, we can assume it is with only a tiny error.
Let's assume we want to fold a piece of 0.050" thick material, with an internal bend radius of 0.150" at 90 degrees, to form an angle with one 1" long flange and one 2" long flange. Here it is in section:
If you are using a box pan folder, (BPF), the first thing you have to do is decide which is the 'held flange'. If you are using a brake press, (BP), it doesn't matter. This distinction is very important - and this is where folks get confused! The 'held flange' chosen is usually the longer one. Or, in the case of a drain tray, it would be the bottom face. The held flange is what you clamp beneath the box pan folder fingers. The section you bend up is the 'developed flange'.
In either case, BPF forming or BP, the piece of flat material we have to cut out will measure 1.80" + the length of the neutral axis around the bend + 0.80". So, we need to calculate the length of that arc.
Inside Bend Radius, (IBR) = 0.150
Material Thickness, (T) = 0.050
We will assume the neutral axis is in the middle of the material thickness.
The length of that arc = (Pi * 2 * (R + T/2)) / 4 = 0.275". But, the distance from one tangent line to what is called 'the outside heel line' and then to the other tangent line is 0.200" + 0.200" = 0.400". The difference, 0.125", is what is called 'the bend deduction". It follows that to make our 2" x 1" angle we actually need to cut a piece of material 2.875" wide.
But we are only half way there! Now we have to position it in the BPF or BP to make the bend - and here is the bit where people often go wrong!
If we are using a brake press, this is how our angle will get made:
To align the workpiece, we need the center of bend line marked on it, and we line that up with the center of the groove and the center of the blade.
But if we are using a box pan folder, this is how it gets made:
Now, we need the outside heel line marked on the workpiece, because that is where the box pan folder 'kinks', and that is a different line to the center of bend line. Unless we are bending very thin material, use the center of bend to position the workpiece and any error will increase in size the bigger the 'bent thru' angle is. (If you sketch out a section where we have pushed the material round though, say, 135 degrees, you will see what I mean). Bear in mind that CAD systems assume sheet metal parts are being made as in industry - automatically, in large quantities, on an automated brake press. If your CAD system has a sheet metal functionality, I can pretty much guarantee it will draw center of bend and not outside heel.
Lastly, box pan folders have an adjustment whereby you can move the fingers backwards and forwards relative to the 'kink' point of the machine - the outside heel line. This is called 'the setback'. It is the distance from the nose of the fingers to where the kink occurs as you operate the handles. Every time you change material thickness, or bend angle, the setback distance will change and it will need to be adjusted. You must measure setback on a box pan folder with material of the thickness you are forming clamped under the fingers. Just clamping the material in there changes the finger setback all on its own.
Very last thing! If you buy a box pan folder it will come with a sharp corner on the fingers. By all means, keep these because they will come in useful if you ever go into business making air conditioning ductwork. But invest in another set of fingers with a radius machined on them. For race car work I suggest a 5/32" nose radius. That is a good compromise for the material thicknesses you will be using. (You can cheat by folding material around the fingers to artificially create a nose radius, but it makes setback adjustment really difficult and the folded piece tends to move around).
Super last thing. If you ever walk into a workshop and the guy has folder fingers with a nose radius on them - he might just know what he is doing...
Oh, and check out www.racecartubs.com !!!!