A useful workshop addition for bending steel
by Geoff Merryweather
This project started when a wooden gate beside the house fell apart.
After ignoring it for ﬁve years, I now have to do something about it. I had a few short lengths of 40 mm x 40 mm x 3 mm steel hollow section (SHS) from the scrapyard and a zero budget, so it was obvious what material I had to use to make a bow for the top of the gate.
I originally bought the 2.4-metre lengths of 40 mm x 40mm steel from Omega Metals (New and used steel) in Onehunga in Auckland. The scrappie sells a lot of new and good condition steel all for $1.60/kg and my project wouldn’t have happened at the retail price of steel so he is a great resource.
The steel I am using apparently used to hold big bags of sugar in place in containers at Chelsea Sugar and becomes surplus when the container is emptied. But making a curved top was more difﬁcult than my usual approach of bending a piece of steel around a wooden former.
The solution was to use the offcuts and a cheap bottle jack to make a set of rollers that can bend the material into wide arcs. Since I had to buy the $39 jack, the budget was well and truly blown. But this ring roller will be useful for future projects.
The most common method of bending pipe or box section is where a the pipe is pulled around a former. A former sets the bend radius and allows sharper bends than a roller.
By contrast, a roller allows large arcs which can vary along the length and is commonly used for arches, greenhouse frames, boat rails, etc. There are two main ways of making ring rollers. Either:
• the top roller is both the drive roller and is adjustable; or
• the adjustable pressure rollers and drive roller are separate.
I have made this second model. Most commercial rollers are like the former as it is more compact while the design I have is easier to make and to motorise later.
The principle is the same as a set of sheet metal rolls, where one roller in a pyramid of three rollers is adjustable to put pressure onto the material while it is driven through the gap. In the case of my project here, a cheap six-tonne bottle jack is used to provide the pressure, using a sliding frame to hold a pair of rollers. The rollers are either pipe with bearings or solid steel.
As the material I wanted to bend was ﬂat, ﬂat-faced rollers were okay. But pipe or angle will need rollers machined to ﬁt the proﬁle. Richmond Wheel and Castor Co who once advertised in The Shed, listed pipe rollers to support pipes which look like they could be modiﬁed to suit pipe-rolling. My machine has several positions for the rollers which are adjustable by moving the pin to different holes. Having the rollers wider apart makes it easier to bend the material but the minimum bend radius is less and thin material is more likely to kink.
The rollers I have used in this case are 40 mm pipe although solid bar would last longer.
The 6004 bearings I used are sold in packets of 10 as ATV parts online ($65/10).
These bearings are 20 mm ID with a 42 mm OD and will ﬁ t into 40 mm pipe. (Sometimes you need to grind out the pipe’s welding seam. This is also a handy trick for other rollers in the workshop, say in a conveyor.) There is a lot of pressure on these axles so the bearings need to be robust. The 20 mm inside diameter seems to work well. While the bearings are overloaded, they won’t see much use and I still have the remainder of the packet when the installed bearings die.
Start by buying your jack as this will set the minimum width of the rollers.
My jack is six tonne and just under 110 mm wide. Six tonne is plenty and a smaller one—four tonne or maybe even less— would be perfectly adequate.
The frame that holds the pressure rollers is a robust, open-topped frame made from 90 mm x 6 mm ﬂat steel sides and angle for the ends. A ﬂat plate is welded on the bottom for the jack to bear against. This was originally two layers of the 90 mm x 6 mm ﬂat steel but it bowed in use, so I have since reinforced it with an extra piece of 50 mm x 8 mm ﬂ at inside.
Mark out the holes and tack or clamp the two sides together to drill them as one, so the holes line up. Clamp the side plates and the angle that form the end of the frame together and put a 20 mm bar through each of the holes to keep everything in line before you tack-weld the frame together. It is important that the rollers are parallel and square to the sides when it is ﬁnished, otherwise the metal being bent will tend to corkscrew to one side, so check this carefully as you go.
When you weld this up, chock the top opening so it doesn’t distort as the metal heats up—you need to have the sides parallel and ﬂ at so it can move without rocking. If it does pull in, then use a press or a soft-faced hammer to knock it square so the rollers ﬁt in easily without slop or rubbing on the sides
Next is the ﬁxed frame that has a drive roller and jack.
The moving frame needs to be snug sliding ﬁt inside this fixed frame so it doesn’t tilt and throw the rollers out of parallel.
I drilled a 20 mm hole for the pin through the side plate of the ﬁxed frame. I was concerned that running the pin directly in the hole would wear it oval fairly quickly with the pressure on it, so I used a short piece of the 40 mm pipe and another bearing on each side.
Put the pin through the hole and ﬁt the bearing and pipe, then weld the pipe into place, knock out the bearing and drill the hole oversize for clearance. The proper method is of course a ﬂange-type pillow block or some wide bronze bushes.
I put a pin through the holes with the drive roller to keep these side plates in line and spaced correctly and ﬁtted the moving frame inside. Check that both ends are the same distance apart and it is all square and tack it into place. Make sure everything slides freely after welding it up. Square the two frames to each other and clamp and weld on some 10 mm square bar on the sides so the moveable frame can slide up and down without tilting.
The drive roller is 50 mm solid bar, drilled through with a 20mm hole.
Set screws (2 mm) hold it onto the shaft. I found it slipped unless it was very tight so using either more set screws or a dimple in the shaft to help key it into place would be a good idea. I added bearings to this top roller and a base ﬂange to bolt it down. The addition of a bearing to the top roller doesn’t make it easier to work—the effort is the same in trying to bend 40 mm square 3 mm thick SHS. I put the bearings in as I was concerned about the holes in the frame wearing with the pressure of the jack on them.
I don’t know if it was ever likely to be an issue, but it was easy enough to do.
The handle is 20 mm bar welded to the end of the drive axle.
This handle needs to be strong—it is going to see a lot of use and a surprising amount of effort. I added an extension piece sideways on each end as this will save skinned knuckles you could get from trying to rotate a straight bar close to the frame.
The pins are made over-length so another set of rollers can be ﬁtted on the outside of the machine. This will allow full circles to be rolled in thin ﬂat steel; you weld the ends of the steel sheet together, dress off the join, and then run the whole through the rollers to complete the circle.
To use the roller, secure it very well to a strong bench or a good vice.
Put the material to be bent between the rollers. Make sure it is square and not going through at an angle or it will twist. Nip up the rolls using the jack—don’t put a lot of pressure on it at this stage. Run it through the rolls in each direction to make sure it all feeds through OK, then give the jack a small pump and repeat until you have the bend you want.
You only need the smallest of movements in the jack – my 6T jack is perhaps 1/3 of the handle stroke. If you do too much, the drive roller will put a small dent in the top of the material and it won’t be able to climb uphill. In that case, loosen it off, move the material forward and roll over the area along with the rest of the length to even out the curve. You will ﬁnd that there is a section at each end of the arc that isn’t bent—you just have to have your material over-length then trim it to ﬁt afterwards.
Like any project, there are always improvements to be made if you are going to use it a lot.
The main one if you are doing a big project is to motorise the ring roller with a chain drive and gear motor. While doing it by hand is good exercise, the novelty will wear off very quickly on a repetitive job. The pipe rollers did the job but have grooves from rolling the SHS.
These rollers will need to be replaced at some stage in the future with solid steel ones. Even a well-greased drilled hole should give years of service before either the axle pin or the roller hole wears. Purists can ﬁt some bronze bushes. Keeping the bar that is being bent square to the rollers helps to stop it twisting. If you have a lot of bending to do, a couple of rings that ﬁt onto the rollers to act as guides will make it easier.
Radius of an arc
It is important to know how to measure the radius of an arc of a circle.
At some point, you will need to roll to a known arc radius or measure an existing curve to make a match. The radius of the arc is
R- (4H2 + W2)
where W is the length of the chord—a chord is a line joining two points on the circumference of a circle—and H is the height measured at the midpoint between the arc and the chord (called the sagitta for Trivial Pursuit players). Those who left maths behind at school can also check out http://liutaiomottola.com/formulae/sag.htm which also has some online calculators or http://gicl.cs.drexel.edu/people/sevy/luthierie/software/curvulator/CurvulatorApplet.html for a calculator applet.