Designing and Building a Tube Bender
Tube Bender Designs | Building a Tube Bender | Bend-O-Matic Plans
Of the benders I've seen I prefer the look of the DIY Blind Chicken and Lowbuck machines. The Lowbuck has a really simple operation but the frame looks a bit pissy so my idea started as a mix of the 2 machines to hopefully come up with something good.
I ended up designing something that looks like the Blind Chicken and works like the Lowbuck, with the possible addition of a second small roller underneath where the big die pivots. My idea is that maybe by including this and enclosing the pipe at the point where its bending it might improve the quality of the bend and make it possible to bend tube with thinner thicknesses... This seems to be the idea of the grooved bar that the Pro-Tools dies use.
In the design I've tried to make the parts easy for me to make on my lathe. I want to have dies for 1.5" and 2" tube so looking at what's used with other benders I decided a 7" centre line radius for both would be about right. Keeping the centre line radius the same for both allows me to keep the pivot in the same point for the different dies. The dies are made from 1/3 of the steel blank so in theory they should be good for bends up to 120 degrees, but will be a little less in practice because the tube will spring back.
I was originally going to machine both dies from a single 2.5" thick circular blank but it was going to be just too hard for me to do because I'm still learning the whole machinist bit. In the end I got hold of some 2" thick plate for free and cut two 14" diameter pieces and two 4" diameter pieces while I had the chance. To mount them in the lathe I got some 3" dia steel pipe, machined an end square and welded it to the blank.
I machined the blank down to the right diameter and generally cleaned it up a bit, especially where it had been flame cut. This actually took ages because it was very rough and a bit hardened around where it was cut I think. Then I manually started cutting out the 1.5" groove using a bit of pipe as a guide. I got it pretty good but in the end took it to a friends machine shop to get it finally smoothed. They have a special tool with a pivoting cutter that can cut exact circles so it came out looking good.
Looking at it now I'm not to sure how this die will go as the extreme edges are only 1/4" thick. I'm a little worried that they may bend outward while bending tube but we will have to wait and see. I'm still considering using the second blank for a 2" die by attaching some 1/2" plate on the sides but I don't know if all the work is really worth it. I think I'll be using 1 1/2" tube for almost everything so I may just stick with the one size.
Anyway now that I'm back from holidays work is continuing... albeit at the usual relaxed pace.
16.08.03
Had a bit of a change of mind and have modified the die slightly. Previously I had the groove as a half circle so the centreline of the tube was on the outside diameter of the die. I since had a chance to have a good look at another bender working and I realised that the groove is actually sunken into the die to stop the tube flattening out and overflowing from the die. That is to say there is the half circle groove with a lip on either side.
This time around I made a tool for the lathe to do the final shaping of the groove and did it all myself. Its a bit rougher than before but I don't think it will make a big difference.
Another change was the use of 4" x 1/4" strip for the frame instead of using 1/4" plate as I originally planned. At the time we didn't have any leftover plate the right size and it was far cheaper to use the strip, just a little more work. I changed the design of the frame a little to use the strip and in the end used just about all of the 6m length I got. The pieces for the sides were cut and welded then tacked together first so they could be ground to shape and drilled at the same time.
Next step was to cut 1/3 out of the disc and make the other parts that go with the die. How I did it was cut a third of the disc then two 50mm square blocks from the same remaining 2/3 with the oxy. The blocks were then put in the lathe and turned square before being welded into place on the die. Looking back it would have been easier to cut it all out together in one piece and clean it up with the grinder, but it probably wouldn't have looked as good.
While I was working on the lathe I also turned the pin sleeve part for the die and the smaller die that supports the pipe at the bottom. There was going to be two smaller dies but my other piece of 2" plate I was going to use disappeared so I took it as a sign I could do without it for now. All the pins for the bender I made out of a 3/4" shaft from a submesible oil well pump. I don't know what material the shaft is made from but it is hard and doesn't rust so I guess its a high nickel, high chromium alloy. We have other pieces of the same stuff we use as breaker bars and they haven't bent yet.
The connector to join the jack to the die was made out of 1/2" plate and is bolted onto the bottom of the jack. At the top there is another sleeve with a pin that the jack pushes against.
At this stage it was all more or less working so I test bent some tube (3mm wall I think) to see how it would go. It bent the tube well, but was hard to use because the jack didn't have any return springs and would fall out of the shaft at the top when there wasn't any force on it. Also the frame got in the way, and wouldn't allow the removal of the pin for the tube retaining loop. This loop holds the tube into the die and needs to be taken off when fitting the tube then replaced before you start bending. In this case the length of tube was quite short so I managed to slide it in but it wouldn't work with a long piece.
Next the return springs were fitted and also some bolts to fix the top of the jack to its shaft. I wasn't originally going to put these in but I found the top of the jack tended to want to turn the sleeve it pushes against and kick out when under pressure so this eliminates any risk of that happening. Also cut some holes in the frame to allow the pin for the tube holding strap to be removed.
With the bender finished I got a longer length of tube and had another test. This time it worked a lot more easily but the 2mm (0.080") wall tube I was using wrinkled a little on the inside. So I tried again with the bottom roller two holes in put oil on half the tube I was bending and only got wrinkles on the first half of the bend. Moved the bottom roller in one more hole and put oil on all of the tube and it bent smoothly, with a slight crush at the start because the retaining loop is a little short. Fixed that by making the holes on the retaining loop bigger and now it bends perfectly every time!
First wrinkled attempt on 2mm wall tube
I get the feeling that it's the oiling of the tube more than the position of the bottom roller that prevents the wrinkling so maybe when I get some time I will check it out.
So anyway that's one project about as done as they get... You can find pictures and the plans here.
08.11.04
Somebody emailed me asking about the tool i made to cut the groove in the die. here are some better pictures for those of you who are interested.
This tool is only used to make small cuts when finishing the groove in order to make it circular. You should use more robust cutting tools to remove most of the material first. Because it is relatively weak it is suceptable to chatter and might break if you try and take too much in one bite.