Internal mandrel material

[AndyStobbs]

Hoping to pick the brains of those people who have had experience of tube bending.
For sometime now I have been wanting to construct my own mandrel bender for exhaust tubing. I have the basic bits but its an unstarted project as yet. Having looked into commercially available options the mandrels appear to have been made in brass. Has anyone experimented with aluminum or other materials?

 

[BrokenBiker]

forgive my ignorance, but i know nothing of mandrel benders. How is the mandrel situated with regards to the tube/pipe and what is its function?

 

[AndyStobbs]

It's inside it. They are shaped to bend like a spine and support the tube. I'm referring to a rotary draw operation by the way.

 

[BrokenBiker]

so on a basic level it provides the same function ad a spring a plumber would use on copper pipe?

 

[AndyStobbs]

Exactly that yes.

 

[hotrodder]

The ones that look like they're brass are some flavour of nickel aluminium bronze (££££) which is used with stainless, inconels and Ti. The theory goes that a cheaper aluminium bronze actually works out more expensive in the long run 'cause it wears out faster apparently. At the other extreme plastics can used with aluminium sometimes and inbetween that various tool steels also get used, probably hard chromed or nitrided? I think the deal with tool steel mandrels is that while they can be used with mild steel they aren't as cost efficient but this'll be about commercial settings i.e. downtime = lost money

Depending on the bend radius and wall factor you don't always need segmented ball mandrels. Initially at least a solid mandrel that supports the tube at the bend tangent is enough to stop the tube collapsing. He doesn't post here anymore but if you search the site for 'rotary draw bending' and username Blackjack you should find a bunch of threads on the subject and a youtube vid of his homemade rotary draw machine (made from bits of a Ford fiesta IIRC) being trialed with a mandrel. As bend radius and/or wall factor increase bend difficulty the tube needs more support to stop it collapsing hence segmented balls. The solid nose of the mandrel still does the majority of the work though. I think it's at this point wiper dies are usually added which support the tube where it enters the bend die to stop it from wrinkling in the ID of the bend.

 

[BrokenBiker]

a bit of searching but...

this is blackjacks bender

 

[AndyStobbs]

Interesting video, and a pretty good end result all in all, given what he's made it from. Good info on the mandrel materials, thanks HR - just need to start playing with the plan now. There are always so many things need doing before you can actually start doing the thing you're trying to do.

 

[Robotstar5]

Shame you didn't ask this about 12 months ago, I worked for a company that did lots of tube manipulation, mainly Addison benders. The other half of the workshop I was in had all the toolmakers who made the tooling for the benders.

 

[AndyStobbs]

Well they say timing is everything, mines always been hopeless.

 

[BrokenBiker]

Out of interest what benefits are there using a mandrel bender like this over a hydraulic bender

 

[AndyStobbs]

Out of interest what benefits are there using a mandrel bender like this over a hydraulic bender

What do you mean by hydraulic bender? One of those hand pump rams with the form on the end of the ram?

 

[hotrodder]

Hydraulic is just a method of powering the thing. 'Mandrel bender' is additional tooling used on a rotary draw bender to allow the thing to bend tube on a smaller CLR (centre line radius) without the tube collapsing. Or when w Pic nicked from the interweb that shows 4 common tube bending methods...

Ram bending is the style typically used for pipe kinkers and also gets used a lot for 'low end' exhaust tube on all in one machines with flaring/expanding stations. Doesn't offer much support for the material being bent which is fine with soft, heavy wall stuff as you can stretch the outside of the bend a bunch before the wall thins too much and collapses. While the outside of the bend is stretched the inside is compressed. This type of bender typically deforms the tube on the inside radius (necking at the bend tangent) regardless and on purpose as CLR gets smaller. It doesn't so much compress/shrink the material (locally increasing wall thickness) as push it out of the way

Compression bending is the style common with plumbing/conduit benders although sometimes the 'slide piece' is omitted and the roller that usually drives it is scalloped to fit the tube and becomes the die. Supports the inside of the bend better than ram types but there comes a point where the outside of the bend will collapse. This point depends on CLR (tighter radius bends are more difficult) and 'wall factor' which refers to OD of tube divided by wall thickness i.e. as dia increases and/or wall thickness reduces it gets more difficult to bend cleanly.

Rotary draw is pretty similar to compression but provides more support at the bend tangent (where the tube wants to collapse) and does so consistantly as the bend is made. There still comes a point where the tube will collapse which is when internal mandrels and wiper dies are added- initally a solid mandrel that extends just past the tangent is enough to stretch things and provide the material needed. As bend difficulty increases this stretching isn't enough to stop the thin wall from collapsing as the bend progresses so additional balls are added to support the tube. The wiper dies job is to stop the inside radius from bunching up when it enters the bend die as it needs to shrink which isn't something it wants to do

Advantages/disadvantages all depends on what's being bent to what CLR. Large radius stuff and 3 roll bending often wins- for a given size of tube only one set of dies are needed regardless of bend radius or angle of bend. Ram bending is quick, tooling changes are quick and the tooling is relatively cheap but it's limited elsewhere
Rotary draw offers more versatility and very tight CLR with mandrel tooling but the tooling is vast and £££££ by comparison

 

[AndyStobbs]

I plan to make my own tooling as much as possible - ambitious I know but not impossible. Got some 44.5mm 304 that I need to bend to 90 degrees for my exhaust manifolds. At the cost of bought in bends it wouldn't take too long before you were upto the kind of money that if spent wisely would be well on the way to a DIY rotary draw bender.

 

[BrokenBiker]

thanks for the explanation, when i mentioned hydraulic benders i meant the ram type.