tubing bender die question

In the interest of celebrating the fact that it's not 120 degrees in my garage anymore, I decided to make a tubing bender for 1/2" square steel
tubing. The bending die is a 6" diameter X 1" steel disk with a 0.475" x 0.550" notch in the outer edge. It works and bends smoothly without kinking the tubing, but it is very difficult to get the bent tube out of the die. The walls of the die are about 30 - 50 mils larger than the tube outer dimensions and they are vertical. They are also not the most beautiful finish.
Should tube bending dies have straight walls or should there be some "draft"?
Should I allow more clearance in the die or will the tube just expand out to fill it or kink?
Should I work on the finish of the inside walls of the die?
Are square tubing benders generally a pain to get the bent tube our of?
Thanks in advance, Bob
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Have the same problem with my cheap round pipe bender although a whack with a hammer make it easy to disengage. I was bending 1-1/2" pipes, 2" OD. I've seen the more expensive round tube bender (not pipe) and it doesn't have this problem. Must be the die. Have you tried packing the tube with sand?
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Frank wrote:

I have not tried filling the tube with sand. In the past, I worked a lot with good quality round tube benders in the 1/4" - 1/2" range and had no problems getting the tube out of them.
Thanks, Bob
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On Mon, 24 Oct 2005 03:07:50 +0000, MetalHead wrote:

I think the problem is that the square tubing is bulging at the sides when bent. I don't think sand will cure that condition. If I had a spare die I'd try angling the sides of the slot, so the profile looked similar to the groove in a V belt pulley. How much of an angle would be up to trial and error.
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You do want to have a little bit of draft to help the square tubing come out of the die. Not much is needed but you do need some. The other thing is that you want to polish the die so that the direction of the scoring of the sides is in the radial direction so that the scoring won't try to hold the tubing back from coming out of the slot.
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Bob May wrote:

Thanks, I think it's time to try polishing the die. Since it was made on a lathe, the tooling marks are concentric to the curve. I will try to get a little bit of draft as well.
Artemia is correct that the inside radius of the tube is bulging out against the walls of the die. There are prominent scuff marks on the tube. Hopefully with better finish, this will work better.
Thanks again, Bob

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MetalHead wrote:

[snip]
...
If you have a larger die, say 9/16" instead of 1/2", you could make an insert out of 1/16" sheet. Put tabs or a handle on it so it can be pulled out to release the tubing after bending. -jiw
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James Waldby wrote:

I polished the walls of the die with a hand grinder last night. There were pretty substantial tooling grooves. It is still far from perfect, but it is a lot better. Since the die is not a full circle anymore, I am reluctant to try to machine it in the lathe. It is now possible to get tubing out without deforming it much.
I had not though about a liner, thanks.
Bob
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Lubrication?
dennis in nca
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rigger wrote:

That was the first thing I tried. It didn't seem to make any difference.
Thanks anyway, Bob
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MetalHead wrote:

I saw a square tube bender in a catalog with a two piece die that bolted together so it could be opened to release the tubing.
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ff wrote:

I am thinking about a two piece die anyway to make it easier to machine and polish the walls. If rev 2 of this gets built, that has real promise.
This project was one of those "let's try it and see what the problems are" type projects.
Thanks, Bob
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