Making a squared right angle bend (L shape) hasn't been a problem. I would like to know of a formula that gives a starting length so that when the iron is bent the legs work out to approximately the correct length.
EXAMPLE:
Use 1/2" x 1/2" square stock.
I need one leg to be 11" from end of iron to outside end of bend.
The second leg should be 15" from end of iron to outside end of bend.
What length of iron do I start with to get these dimensions?

Please point me in the right direction if it happens to be posted somewhere already.

I am also interested in any other formulas for forging to size.

Thanks

I was showen to mark the side of the stock w/ a center punch mark as a reference as to where the center of the bend should be kept. This helps out alot during the forging to give you an idea as to where your bend is going, and once you get one done(or a few for more accurate approximations) you can measure the leg and make adjustments accordingly. - JK

without the math bend the first piece a little long and measure the results and you are good to go. note the length of the bar and the start point of the bend.

when you have bent it you will know the gain/loss for the next pieces.

i have bent a lot of tubing and always found a single test piece more accurate the the math. ime

If you're not making a square corner bend then you need to know the radius of the bend. Length is pretty darn close to the centerline length. The radius makes a huge difference because it takes a "short-cut" across the corner.

I usually use a little math and a test piece like John. But I also use CAD for most things.

Edited July 29, 200916 yr by nakedanvil

I don't remember the formula but Metalbender , who is a member here is really good at that stuff. Try emailing him

I'm Sorry Rutterbush - I was thinking you were doing a forged square corner bend - JK

My shot would be to mark the piece at 14 3/4" with a prick punch, and shoot for that being the center of my bend guessing that I'd pick up the other 1/4" from the thickness of the stock. Test piece is key in this situation I'd say.

I work in the sheet metal design field, and predicting a sheet metal blank before it is bent is a similar challenge. You "gain" a set amount of material for each bend (for the reason that Grant mentioned, among other things), but this relies on press-brake tooling which is consistent. We have had to use experimental evidence to develop standards for each different thickness of material.

Hence: use a jig to keep your bend parameters consistent, and bend one (or three) test piece(s) to learn what the difference between start and end will be. Start with a test piece say 12" long, bend it in the middle, measure the result. Do this three times if you are a patient person, and use the average as a CONSTANT of increase per bend. Keep in mind that measuring across the bend (from outside to outside) will add about

There are a lot of these formulae in here:

The Blacksmiths Craft

hey rutterbush, i have two tricks i do for right angle bends. if using 1/2" square, 11" long, I would put a center punch mark at 10 3/4". put it exactly in the middle of the stock. then make the right angle bend with the center punch mark on the side. It should be lined up in the middle of an imaginary diagonal line drawn from the inside corner to the outside corner. this should give you 11" from outside corner to bottom. the idea is subtracting half of the width of the starting stock from the total length needed. check out "elementary forge practice" by John Lord Bacon. It has some great ideas on forging.
the second trick i do is to forge a corner with plenty of stock to spare and then cut the piece to length. is that cheating?

O.K. Now I think I understand the question better. I thought he meant he had no problem when doing a sharp bend. But i think he just meant he has no problem doing the bend, but now he wants to do the same in an accurate position. Test piece, punch-mark, go for it.

if you are working with inside dimensions then simply add up your segment lengths. If you are working with outside dimensions then you can subtract a thickness for each outside bend or simply convert all your dimensions to inside dimensions.

there is also a formula :

Ba = A/360 X 2pi(R+kt) where

Ba = bend allowance (in) subtract off total length for each bend.
A = bend anglen (deg)
R = bend radius (in)
t = metal thickness (in)
k = constant neutral axis location (a value of .5 places this in the centre of the material)

brad

Neat Guy Thanks! i just copied that to my file of stuff I should know but forget every time. Excellent!

When you bend metal it compresses on the inside and stretches on the outside. There is a place where it neither compresses nor stretches. This place is 44% of metal thickness, so if you add the bend radius to 44% of metal thickness and multiply by Pi you will have the amount of material in 180 degrees of bend. 90 degrees is what you are looking for so you divide by two. In a formed piece you have flat places and bends, so you figure how long the flat places are and of course the bends. In this case you have 11 inches by 15 inches, both outside dimensions. So lets figure using a 1/2 inch radius. First we subtract one material and a bend radius from 11 inches, so one flat leg will be 10 inches to where the bend starts. We subtract a material and a radius from 15 inches and we get 14 inches of flat leg. So now we figure the material in the bend.

.44x .5 material thickness equals .22 plus .5 bend radius equals .72 times 3.14159 equals 2.262 divided by 2 gives us 90 degrees of bend which is 1.131. So 10 inch flat plus 1.131 in bend plus 14 flat gives us the developed length of 25.13. This simple formula has served me well for over 40 years

I don't remember the formula but Metalbender , who is a member here is really good at that stuff. Try emailing him

Hello to my good friend Bruce. Craftbender just got back from Texas.

I've gotten by with centerline (50%) for many years, close enough for a blacksmith. Otherwise I just fit things together with Vise-gripCAD.

Using centerline I get 25.1781. Less than 1/16" different than using 44%. I know you are right with 44%, but when you got bends going left, right and up and down it's easier to keep track of the centerline.

Edited July 29, 200916 yr by nakedanvil

You guys have WAY too much time on your hands..... :-)

I also use the 50%. It ain't rocket building
Ken

The difference between a blacksmith and an engineer is about one-eighth of an inch.

Engineers I have to deal with think 1/8 .125 is .120 out of spec. This on a large welded frame. Every dimension was a 3(dismal) places. Anybody got a .409 drill? Hole centered
on a 13.137 piece of sq tube. Its done can I retire now?
Ken

Well, I appreciate every answer.
What I asked about was the squared corner. Bend, hit end of one leg of the bar to drive it into the corner to upset, hit the opposite outside of corner to square up, repeat until 1/2" x 1/2" looks like it could have been cut and welded into 90

As far as I'm concerned drills come in three sizes:small, medium and large.

As far as I'm concerned drills come in three sizes:small, medium and large.

what is that thing a drill you speak of?? hmm most be how you call what we call a hot round punch ;)

Drill! Drill!!!! We don't need no stinking drills!!!! Well maybe now and then.

Finnr

Hey Riley! Welcome back!