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Bending 10mm copper tube

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Proud Mary

Sun Oct 25 2009, 12:47AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Hi guys,

does anyone have any basic suggestions about the best way to bend 10mm copper tube into fairly tight turns for use in a high current inductor?

Fill it with sand to stop kinking while winding it round a mandrel? Heat it and fill it with sand?

Thanks for any suggestions.

Carbon_Rod

Sun Oct 25 2009, 02:26AM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155

metal spring,
Removed while creating a continuous bend.

Cheers,

klugesmith

Sun Oct 25 2009, 02:36AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Tube-bending is an advertised application for low-melting soft metal alloys such as Woods Metal / Cerrobend (m.p. 158F = 70C ).

I have a bit from fusible plugs in a big diesel fuel-tank cap. Easy to melt under hot water. Not hard to find the stuff on ebay. e.g. search for 158 Alloy, or for more money in industrial catalogs like

or smallparts.com.

I don't know whether it tends to wet the inside of a copper tube, which would be a waste, but an initial wetting with oil or paraffin wax might help.

[edit] found a couple of how-to articles:

and

. I like the picture of a guy pouring silvery molten metal out of a steel crucible held with garden gloves.

Z28Fistergod

Sun Oct 25 2009, 02:37AM

Registered Member #2040 Joined: Fri Mar 20 2009, 10:13PM
Location: Fairfax VA
Posts: 180

I'm wondering just how tight you want to wind these inductors? Sand sounds like a good idea, as long as you could somehow compact it within the tube. If it wasn't packed in there tightly I imagine it would still kink. One way that comes to mind is to solder a cap onto one end, fill it with sand and then crush the other end flat and roll it up like a tube of toothpaste.

Or, maybe you could crush the pipe flat to begin with, that would make it easier to coil tightly.

Whatever you decide, a piece of pipe to wind the tubing around should make it easier.

Proud Mary

Sun Oct 25 2009, 03:16AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Thanks Guys

IamSmooth

Sun Oct 25 2009, 03:45AM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

Harry,

I just bent some 3/8" copper tubing with no problem. I filled it with fine play sand. I used a solid nylon tube as my form, but you can use whatever you wish. I fastened one end to a heavy vice. I used my hands to wrap the tubing around the form, pressing it firmly to keep it tight. The sand works perfectly to prevent the tubing from collapsing.

Proud Mary

Sun Oct 25 2009, 05:19AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

IamSmooth wrote ...

Harry,

I just bent some 3/8" copper tubing with no problem. I filled it with fine play sand. I used a solid nylon tube as my form, but you can use whatever you wish. I fastened one end to a heavy vice. I used my hands to wrap the tubing around the form, pressing it firmly to keep it tight. The sand works perfectly to prevent the tubing from collapsing.

Is 'play sand' some kind of sanitized sand for kids, Mr Smooth?

I have a good length of the 10mm tube, so I thought to get extra mechanical advantage by winding it from a distance. I want the inductor to have OD = 30mm if possible, which seems quite tight with 10mm tube.

Did you find there was any "spring effect" of the wound component increasing its diameter, Mr Smooth?

Thank you for your help.

aonomus

Sun Oct 25 2009, 06:28AM

Registered Member #1497 Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801

Play sand probably means a medium to fine grit, not too fine to cause lots of dust and silicosis. The easy bit is getting the sand in and bending the tube, the hard part is getting all the sand out....

Herr Zapp

Sun Oct 25 2009, 07:23AM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

Harry -

I've used both the "fine sand" and the CerroBend materials, and gotten good results with both.

I sifted the sand through a fine mesh screen to gen uniform particle size, and strain out any "lumps" that might cause voids. Bake the sand for an hour at 250F before sifting to drive off any moisture, and it will be easier to sift.

You may be right at the limit of formability with your 10mm tube/30mm OD inductor. Is it absolutely essential to achieve the 30mm OD? 40 or 50mm might be a lot easier.

Also, is your copper tube fully annealed (refrigeration tubing)? Springback will be a problem if its not fully annealed.

If you have access to a lathe, you can mount your coil form in the chuck, and support the outboard end with the tailstock. Run the lathe in "back gear" mode to get a very low spindle speed, and wear heavy leather gloves to feed the tubing onto the form. Have an assistant with his hand on the lathe power switch, because in back gear even a small lathe has tremendous torque, and you need to be able to shut the thing off instantly if something gets caught up in the moving coilform.

I've also seen a simple roller fixture made to attach to the toolpost to feed the tubing onto the form; this is much safer than doing it by hand.

Regards,
Herr Zapp

Proud Mary

Sun Oct 25 2009, 08:22AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Herr Zapp wrote ...

You may be right at the limit of formability with your 10mm tube/30mm OD inductor. Is it absolutely essential to achieve the 30mm OD? 40 or 50mm might be a lot easier.

Herr Zapp: There is no electronic reason to limit the inductor OD to 30mm beyond the increasing massiveness of the Rayleigh Pulse Forming Line, and the consequences of that for the size of the case. I agree it will be much easier to wind the inductors on a 50mm mandrel, and if that's what I'll have to do to make it work, then that's what I'll do.

Thank you for your interest in my project.

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