bending small bore copper pipe

I am about to start bending the first tubing for my traction engine. It seems a good point to fabricate a mini tube bender & i wondered if there was a rule of thumb for the minimum radius, or a typical working radius, that one would use for bending copper pipe from say 5mm upwards on a model traction engine.

This would give me the size for the diameter of the primary wheels of the bender.

I believe there has been mag articles on the subject but I do not subscribe

Thanks

Hello Sam,

Head off down to your local plumbers merchant and look at their tube benders for micro bore copper pipe for central heating systems, these cater for 8 and 10 mm pipe, are well made and easy to use and may save you a bit of time instead of making a dedicated tool. At the very least they will give you ideas.

The other alternatives are pipe springs that fit over the tubing which control the shape of the pipe as you bend it. They are tricky to use as you must over bend the pipe and then ease it back to the angle you want so that you can release the spring and judging that nicely is brought about by experience. They also need to be a close fit on the tubing to start with and may nor be available in all sizes.

Regards Brian

Go to a motor spares place and look at brake pipe bending kits too. Brake pipes are typically 1/4". It may be possible to machine wheels for smaller diameter tube you can incorporate.

or more often 3/16" which is even closer to 5mm. The pipe itself is typically available in soft copper or Cunifer, depending on what colour you'd prefer on the engine.

I already have pipe springs & a pipe bender, which goes up to about 40mm diameter. But not below half inch.& I know how to make a small bore bender - Dubro make one for model plane fuel pipe which is very small diameter.

I just want to know if there was a rule of thumb for very small tubing. Many forumites must be bending pipes for their trains & traction engines & it seems odd that there isn't a rule such as - say- 3 times diameter as a guide line

Edited By Sam Longley 1 on 10/11/2017 12:45:04

Sorry Sam, I misunderstood what you were trying to establish; I don't know but perhaps others have an empirical formula they use.

Regards Brian

There are rules for the tooling needed for CNC draw benders that allow selection of necessary tooling. The larger the OD to wall thickness ratio the more likely you are to need support inside the tube. Also the tighter the centre line bend radius is to the OD the more likely internal support will be needed. I have a cardboard computer (think slide rule made of cardboard) for this purpose. You find factor 1 which is wall thickness to outside diameter figure. Then factor 2 which is the centre line bend radius to OD. On the reverse side you set the slide to the factor 1 position and look down against a window to see what is against factor 2. This tells you if you need a wiper die, plug or ball mandrel and if a ball mandrel is required how many balls.

For example 3/4" 16swg tube has a factor 1 of 15 (they are coarse figures for the factors). With a bend radius of 3D (2.25" the factor 2 value is 3. The result is the table says this bend needs a plug mandrel but no wiper die. Reduce the bend radius to 3/4" and it says a one ball mandrel with wiper die.

This works for stainless steel but I don't know how applicable it is for copper tube, the bending tool selector does not say anything about material.

Martin C

Re. your "It seems a good point to fabricate a mini tube bender" and "mag articles on the subject but I do not subscribe".

You could wose than look at David Carpenter's FREE ! website **LINK**

There have been two designs over the years, the first being **LINK**

But the easier one to build, producing accurate, tight curves in small Cu pipes was published by "Artisan". Unfortunately David seems to have moved the article to a DVD now (requiring payment) but I have the pdf and this image is from that, hopefully not infringing copyright:-

This is mine and have always felt it worth the effort of making in that my pet aversion is seeing poorly-formed bends in wavy pipes on otherwise excellent models.

Photos of the cardboard computer I have for selecting CNC pipe bending machine tooling.

Martin C

Thanks for the second link. That is a lovely piece of kit & worth stopping the build just to make it. I did intend to make something more akin to yours but with a smaller fence to allow closer bends.Looking at the instructions there are quite a lot of bends to make in an engine & one can get better seating on the joints if everything lines up first time rather than being a tad out & trying to force something into line.

I also fly RC planes & bending piano wire etc is easier with a neat bender

Looks like I will have to make the bender & just make a few wheels & see how the first few bends go

Cheers

Sam L

Hello Sam,

For copper tube a pipe centerline radius of two pipe diameters is fairly easily achieved. With smaller radii you need internal support and are likely to have tearing in the outside of the bend.

Martin's table illustrates the requirements.

If you Google this link you will find a mine of information http://www.pines-eng.com/pdfs/H&HBendGuide.pdf

Regards, John

That's an excellent document, John

http://www.pines-eng.com/pdfs/H&HBendGuide.pdf

Thanks for posting

MichaelG.

Edited By Michael Gilligan on 11/11/2017 07:22:09

I​ made a small bore tube bender from a set of drawings, just can't recall from whom without looking in garage but ...





The two examples are 1/4" & 1/8" copper tubing.

George.