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jhmiii
05-28-2012, 06:31 PM
A project (equatorial sundial) I am planning requires a circle of brass. The bar would be about 3/16" thick and 1/2-3/4" in width. I need to bend the bar into a semi-circle with a 7" radius. The radius needs to be fairly exact. Is it best to bend it around a form? How does one determine the size of the form to deal with the flexing of the brass?

Appreciate any help or pointers.

John

RussZHC
05-28-2012, 06:36 PM
http://www.youtube.com/watch?v=FBNjVZDvXy8

or the generic version...this method would be my choice (since I have the roller already), no reason why a form would not work but then you are left with how do I make the form question

(I would not vouch for the accuracy of the ring roller but with some care, it should be OK)

Edit: to add, this style of ring roller may not be best for brass, it will work but the rollers are knurled and they do leave marks on mild steel so I would assume it would be worse on brass

Ian B
05-28-2012, 06:44 PM
Not sure what machinery you have, or how precise it would have to be, but my thoughts would be to machine a complete ring in a lathe (or a milling machine with a rotary table), and then use a section of the resulting ring.

Ian

jhmiii
05-28-2012, 06:58 PM
Thanks Russ and Ian for the quick responses. Unfortunately, I have very small equipment (Sherline) and can't cut a ring from large stock. The roller looks like an interesting approach. I will look into it and see if a novice can make it work.

John

Mike Burdick
05-28-2012, 07:29 PM
You can shape it like they did in the "old" days - hammer and anvil. Start by hammering the outside edge and it'll slowly take shape.

or...

Cut segments of the circle from flat stock and lap join these together using screws.

or...

Build yourself a "specialty lathe" specifically for this one item. It can be cobbled up from any kind of bearings and be powered by a cordless drill if that's what you got! The bearings can be clamped off the edge of your workbench if needed. Be inventive! Heck - a wood lathe and tools can cut brass!

oldtiffie
05-28-2012, 09:07 PM
Try a Sheet Metal or steel fabrication shop as they are likely to have rollers for that job - but ask them about the additional "lead in" and lead out" material required.

elf
05-28-2012, 10:52 PM
A project (equatorial sundial) I am planning requires a circle of brass. The bar would be about 3/16" thick and 1/2-3/4" in width. I need to bend the bar into a semi-circle with a 7" radius. The radius needs to be fairly exact. Is it best to bend it around a form? How does one determine the size of the form to deal with the flexing of the brass?

Appreciate any help or pointers.

John

Why does the 7" radius have to be exact? I'd make this part first, then make the rest of the sundial to match. Modifying the scale in a CAD program should be easy.

Paul Alciatore
05-29-2012, 01:08 AM
You could make a wood circle using simple tools. Saw it slightly oversized with a saber saw. A pivot at the center would allow you to use a sanding drum on a drill press or even a electric drill clamped down to the work bench to bring it down to an exact size and completely round. Start with it at the desired ID of your ring and bend it over it. See how much spring back you get and sand the wood circle down by perhaps 90% of that amount. Try again. Continue trial and error until you get the radius you want.

You will probably be unable to bend the ends of the brass to the radius this way. You should simply start with a piece that is several inches longer and cut the straight ends off after bending.

jhmiii
05-29-2012, 08:30 AM
Thanks again for the interest and support.

Elf - I will need an exact size, because I plan to put the hour marks accurately on the brass before it is bent. The hour marks define the needed curve.

I like the idea of bending it around an 'adjustable' wood template. Reminding me to retain handles (lead in/out) is appreciated.

Some experimentation is required here.

John

oldtiffie
05-29-2012, 10:19 AM
Drawn or rolled brass bar is normally "hard" - ie not annealed - and if bent on its longer axis is more (very) prone to buckling.

Paul Alciatore
05-29-2012, 01:38 PM
Thanks again <snip>....

Reminding me to retain handles (lead in/out) is appreciated.

Some experimentation is required here.

John

On the lead in/out, I have been there and done that. Learned the hard way and had to go to much trouble to bend them and still not come out quite right. When you learn the hard way, you remember.

As for the orientation of the bend, I was assuming it was the easy way. If you are bending a 3/16" x 1/2" the "hard" way, it may work. If you are up to 3/16" x 3/4" and going the hard way, I would definitely think about some kind of restraint or guide to prevent buckling. You could sandwich the wood circle between a couple of steel or aluminum sheets. But even then, you may get some cracking of the brass. If the brass is hard, I would definitely anneal it first. I am not real up on heat treatment of metals and others here can perhaps help better on this.

oldtiffie
05-29-2012, 04:03 PM
A project (equatorial sundial) I am planning requires a circle of brass. The bar would be about 3/16" thick and 1/2-3/4" in width. I need to bend the bar into a semi-circle with a 7" radius. The radius needs to be fairly exact. Is it best to bend it around a form? How does one determine the size of the form to deal with the flexing of the brass?

Appreciate any help or pointers.

John

Why use brass bar?

Why not use a bit of plate brass and use a mill to trepan first the outer surface/ring and then the inner ring?

With care and a good "machinable" brass it should work quite well.

You can use a "boring bar" type tool,with a rounded and hand-honed nose to finish it off using the same principle as if used on a lathe.

You can use a good boring head for the whole job if needed.