Finding the Right Process for Bending Steel Flats to Parabolic Curve

How about making a template to a "sharper" parabolic curve than needed, pressing the material into this curve, and allowing it to spring back to the desired curve? (You might need to experiment a little.)

Thanks Tornado for the reply..

This is one good option. Another idea I got from your reply is take the desired parabola and press the flat and apply heat so that it stays to the profile (correct me if I am wrong) !!!

But i think this kind of applying heat option will be expensive than sharper than required profile option.

If you bend pipes or electrical conduits through 90°, they tend to spring back about 2-3°. Other degrees of bend (or nonuniform bends) would give different results. But this (or something like it) could give a scheme with which to start. If you can experiment along these lines, you have a good chance of finding a workable approach. Good luck.

I'm curious about the purpose in bending such a small piece of steel.

How big is the parabolic dish going to be? What will it be reflecting? If these pieces of steel are being used as the dish surface (or to support the surface), shouldn't the amount of bend depend on their location on the dish surface, since the local curvature of a parabola is flatter going away from the center.

If the parabolic dish is simply a 'light bucket', i.e. for capturing solar energy or radio signals rather than for precise 'imaging', forming a curve into each small piece might not even be needed.

Lay out the curve with dowel pins on a board and bend the flats around it. You will have to make the curve a little smaller unless you heat it as you bend. If bent cold you will get some spring back.

Another way would be a die to press it into the shape. Two mated halves that form the flats into the curve.

Seems like a natural for grinding a set of matched steel dies and using an arbor press to form the part. Or a hydraulic press or even a hammer.

I'm not clear on whether you are bending in the 25mm direction or the 3mm direction; are these parts that actually reflect the sun or are they the supporting stiffeners? the fixed width makes me think they are stiffeners.

When you bend steel putting a permanent set in it, you stretch the outer fibres through the yield point, but the inner layers remain elastic. Slight differences in rolling tolerances both thickness and metal hardening, will cause your results. There is always some springback.

If these are stiffeners, bent in the 25mm direction, heat straightening techniques could help you fine tune the curve but I don't think it will help if you are bending in the 3mm direction.

If this is a production line, and you are bending in the 3mm direction, there is equipment available for freehand bending from which, with experience, accurate results could be achieved

Thanks for all for your valuable replies.. To make things clear, actually these flats are used as the stiffening ribs for the dish. The length of the flat is around 3m. So i want to bend 25mmX3mm thick of 3m length to the desired parabolic curve.

This should have been in the first post.

Bye.

That's what I deduced and why I asked the question.

Would it not be easier to cut these from sheet metal? you would achieve the exact inside curve each time with very little wastage. The width would vary slightly but not enough to matter.

The other way to fine tune is with a ball peen hammer, hit with the ball, on the flat faces, at the edges that you need to lengthen to get the curvature. This the standard method of truing-up the included angle of carpenter's squares when you buy them.

You can use an adaptive roller to get a parabolic curve, or roll it to a radius=2*focal length of the parabola then adjust out any error by hand, or bend it over a parabolic fence set up on a tooling bed with a focal length less than the target to allow for springback. For C-band reflectors with a long focal length you may be able to just role to a radius twice the focal length and leave it at that.

Hydraulic Press and Die. Same exact shape every time, and only takes a second.

Without knowing how many you need or what facilities you have.....

Predictable repetition requires a jig.

Create a sturdy steel work surface large enough to accommodate the finished rib. This could be a welding table or a some C-sections tacked together side by side.

Mark out your profile onto the surface.

Weld steel pegs onto the surface along the profile curve's inside edge

Weld a another peg or 2 onto the surface at the start of the curve on the outside edge

Slot your preheated material between the pegs at the start end of the jig. Apply force to material and bend it into the curve and hard up to the inside profile pegs. Keep the material hot.

Assist the shape compliance as required with a hammer.

Clamp off and allow the material to cool or quench it.

Repeat this basic iron mongering technique until the run is completed.

If you create a jig with 2 mirrored curves (or have 2 jigs) you can flip the rib into the second curve and beat it into compliance to true up any minor curve irregularities.

Durable, predictable, accurate and quick.

You may have to adjust the pegs a little for the first piece before committing to the final production run.

This probably way out of your expertise but this company uses explosives to form the dish itself. It is referenced in a few textbooks, too.

I would probably use a die and a press, but this could be fun.

A set of rollers spring to mind, because of the small size of your flat bar a relatively small and inexpensive set should do you, once set up you can do many bars and they all be identical,if you cant bend them in one go you just do one run at a lesser setting then keep resetting & rolling a bit at a time and remember to do them all at each stage. the type of roller I'm thinking of has three rollers the top one being adjustable,they can be 6"wide or more & if you are on a tight budget you should be able to pick up a set from a junk yard for the price of scrap value.

Bazzer

Do you have access to steel sheet, instead of bars. If so, cut the shapes from the sheet and use them "at right angles" to the way you were originally intending to use them. The way you are doing it they will probably be too flexible anyway.

You could weld the new shapes to the centre of your original bars to make a very rigid T rib.

Why don't you look for an active blacksmith living in your area? They have to do this every day, often making multiples of some unique shape of flat metal. They build jigs and also have Hossfeld Benders etc. They often use heat but not always. You will very quickly see how they make it happen. Good Luck!

If you load your bar so the bending stress varies in a parabolic manner along the length, the beam will bend into a parabolic shape naturally. You might design a four point bend fixture where the bending moment is uniform, and control the bend stress by drilling a series of holes in the bar, or you may find a method to apply variable parabolic stress to a uniform bar.

I agree with passingtongreen, the easiest and probably most cost effective method would be to have pieces cut either laser or plasma or waterjet from sheet steel. this way you would have not only the benefit of having greater material in the plane of the parabola itself lending greater strenght rather than the flat side along the dish. this would give greater strength and durability and you can have greater accuracy and repetition.

Two ways come to mind. Well, actually three of four. Try these on for size.

1. start with angle iron. Cut slots in one side nearly up to the crease. Cut or nibble as many of these as you need to do the job. Insert simple wedges or scraps of steel into the slots until they bend to meet your paper pattern. Run a bead of weld down the outside to hold it all into position. You can make quite large parabolic shaped pieces that way, quite inexpensively.

2. Cut the parabola pieces out of sheet steel with a jig saw or water jet or laser, whatever you have. Tack weld or braze a strip of thin metal to the inside of the parabola to provide the attachment surface.

3. Cut the parabola out of sheet metal. Cut 3mm slots (or a little more) into the inside of the metal so that they form tabs. Bend each tab alternately up and down to form the nailing strip.

4. Cut the parabola out of sheet metal. GLUE the perforated support strip to the inside of the parabola with a good epoxy like JB Weld.

I am sure you can come up with other ways as well.

Regards.

Blacksmith Bill

aka Yusef1

The OP wants to introduce a permanent parabolic bend to flat ribs in the "H-plane" with repetitive consistency.

He isn't looking for a new design.

Basic jigging and iron work.

Had to look that up. By H plane, you mean the "horizontal plane" for a wave guide or antenna?

But yeah, its basic iron working with jigs and pins driven into work benches.

Ya.."horizontal plane" for a wave guide or antenna

Where does he say, "H-plane", he calls them "ribs".

Nobody offered a new design, OP asked for a faster way of fabrication and that's what we were addressing.

An H plane bend is a bend in the "hard" direction.

There were some excellent alternatives suggested to the flat ribs the OP wanted to bend with easy, precise repetition.

If I had to suggest an alternative, for someone with apparently limited access to fabrication facilities, to defining the parabolic surface of a dish with ribs I would suggest going out and buying a ready made dish.