Hi - I am a metal sculptor, with LARGE gaps in my knowledge of metal fabrication processes, so I have a basic question: How would I take steel bar stock, or even square tubing of 1" or less and bend it into circles? The size of the circles would range from 6 inches to 3 feet in diameter.
Is there a tool/machine that does this? Is there a clever millwrights trick?
In general, you use a machine called a "slip roll" to roll circles out of flat stock. There is a bit of technique involved with this, post back if you need specific information.
To roll other stuff into a circle, you'd normally use a machine called a "ring roller". It takes a lot of power to form square tubing into a ring with small diameter, as there is a lot of metal stretching going on.
Very generally, you roll the material into a ring, weld it, grind the weld, put it back in the roller and roll it again (welded) to get it as circular as possible.
That's the way I used to do it anyway.
There is another way, which involves blacksmithing. This is to heat the stock to forging temperature and bend it (by hammering) over the horn of the anvil, and repeat until you have a true ring. This would not work well with angle, tube, etc. just flat or bar stock.
The total poor man's way to roll strap aka flat bar into a circle is to first draw an arc of the circle on a piece of paper, then cut the strap to length (use geometry, circumference = PI * DIAMETER) and put the end of the strap in a bench vise and bend it manually ("bump it") a bit, then move the stock down in the vise and bump it some more until you have a curve started. Check the curve against your drawn desired curve and correct as needed, and continue bumping until you have a complete circle. You won't be able to do a whole circle this way, but you may be able to do 2 semicircles which you can weld together. This requires only a bench vise for tooling, and some time. And skill ..
Bending tubing requires some method of keeping the tubing round. As it tends to flatten out, the tubing is usually constrained in a grooved tool to keep the width close to normal. Another way is to fill the tube with sand and plug the ends so the sand can't come out. Then the tubing will stay round through the bending process. It may take more force to accomplish the bend, though. Also it may require annealing steps to be inserted in the bend sequence to avoid wall failure.
This is kind of an urban legend. No metal tube will stay round when filled with sand or whatever. It will always come out oval because compression-strength is higher than when expanded. To come out round, the material must have a Poisson's number of 1. And that material doesn't exist.
In addition to all the other answers, which all seemed reasonable to me, I would note the existence of powered machines that will roll even sizable bar stock into a radius or circle. I want to say that the machine that I saw was made in Italy. The one I saw was a pretty good sized machine, roughly 2 by 2 by 3 feet. It would even take flat bar and bend it the hard way. That is a pretty nice shape. I don't know any other way to do that. I know a local blacksmith who has one, if I needed anything that I could not easily bend with an affordable ring roller or a forge I would go to him. I know a sculptor in Washington state who has one also. I cannot think of the name of that machine.
A practical approach is the following:
Avoid tubing, as it is harder to bend. May be OK with larger bend radius. I never bend tubing myself.
Figure out the largest bar stock that you might want to bend.
Look for a ring roller that will handle it.
The Harbor Freight ring rollers are limited to pretty small stuff, maybe less than 1/2 inch round bar, I have seen sturdier ones for $200 or so. If you have a burning desire for a sturdy one, contact me and I will see if I can find who sells them.
For the larger ring diameters you could also consider building up the cross-section with laminations of thinner bar stock that you could wrap into hoops by hand around a plywood or MDF form that you have cut out with a jigsaw. Then drill and rivet the laminations together.
If you require solid cross-sections or tubing you might look up 'ironwork' or 'steel fabricators' in the Yellow Pages and call around until you find one with a hoop rolling machine to roll the rings for you. Might be a lot cheaper than buying a rolling machine unless you have a very large number of rings to be formed. Businesses who do 'architectural ironwork' and might have such a machine are the folks who fabricate things like spiral staircases, iron (steel) gates and fences, etc.
Should you be tempted to rig up some sort of fixture to bend solid steel stock by hand, I calculate that something on the order of an 8-foot lever arm would be required to initiate bending of a 1-inch cross-section of A-36 structural steel, assuming that one could apply a 100-pound force to the end of the lever. The required force would increase somewhat as strain hardening takes place. That rig could result in a fatal smack up side the head (or similar collateral damage) if it gets away from you.
======================= Actually not if it saves someone from ruining a project.
What you can use are some of the ultra low temp melting alloys such as woods metal or cerrosafe. Fill the tube, make the bend, and put in boiling water to recover the alloy.
As Nick points out, the tube will still be oval, but most likely will still be round enough for what you want, and in any even won't collapse with any normal bend radius. Water with the ends capped or ice is also suggested.
Losts of sources on the web but for quick info see
Note that these alloys contain cadimum and should be treated with respect. Also these alloys may interact with some tubing.
Unka' George (George McDuffee) .............................. Only in Britain could it be thought a defect to be "too clever by half." The probability is that too many people are too stupid by three-quarters.
John Major (b. 1943), British Conservative politician, prime minister. Quoted in: Observer (London, 7 July 1991).
I saw an interesting set up to roll rings. My friend has a large engine lathe and he rigged a steel drum and a set of rollers so he could feed a 20 foot section of 1/2 inch tubing and wind it around the drum.
When he got the end of the section, the last loop was clamped in place and a Sawzall was used to slice through the coil to make rings. The rings would have a little spring in them and were then tweaked flat and set in a fixture to weld the ends together.
This was a lot of effort to make the tooling but I think some where on the order of a thousand rings were required so it was feasible to do it this way.
A different friend made up sets of rings out of 1/16 X 1/2 inch bar stock. This job required that the three rings nest inside one another. He cut the strips to length and rolled them, welded the ends and dressed the welds.
He then set the rings in a fixture he made. The fixture was turned on the lathe with three steps for the diameter of each ring. He made a tapered hole in the middle, and then cut the fixture into 4 quarters. The three rings were then placed around the fixture and a tapered pin was then driven into the tapered hole in the fixture and the rings were stretched slightly and the end result was a set of three rings that fit snugly together.
These rings were then assembled to make a sort of sphere, welded together, polished then plated, and mounted on a base for some sort of a trophy. I though it was a rather clever way to get the job done.
Unfortunately I never saw the final product, only the rings in their sized state prior to the assembly and polish stage.
Back in my Puget Sound shipyard days, we used to work on oil tankers. The tanks in oil tankers have what are called "double bottoms" i.e. there is a bottom layer a few feet high. Running throughout the double bottoms are steam pipes. Crude oil flows easier when it's warm - a LOT easier. At Lockheed Shipyards, long gone now, they had a giant old lathe. I mean GIANT. I remember it as being about 40 yards long, but maybe my mind is playing tricks on me.
They had a big (~12" OD) mandrel on that lathe maybe 25 feet long, and they fed
1-1/2" pipe into it at an angle. Very very slowly. When the pipe was nearly used up they'd butt weld another to it still moving. When the pipe approached the mandrel, they had a couple of guys there with giant rosebud torches, heating the pipe to cherry. It was fed on at an angle, and fed along using gears like winding a helical spring. They'd make long coils of pipe that way, and it was really fascinating to watch. I got to watch them all day once when I was 18, working as a laborer (aka 'shipscaler") because like all hot work in the shipyards, it requires a laborer to sit there with a fire extinguisher Just In Case. And I got firewatch duty watching the above one time when I was hurt.