Unison Tube Bending Blog: Control Features – Multi-Stack/Multi-Radius

Tube bending blog by Alan Pickering.

Why do you need a Multi-Stack Bending Machine?

Well, there are some good reasons and one daft reason, let’s start with the good reasons.

• Bend on Bend

Some parts that you need to bend require a straight less than 2 x Diameter between bends and you’re not allowed any marking on the part. In this scenario you may need to consider ‘Bend on Bend’ tooling.

This is where you have a replica set of tooling set directly above the original tool set, except instead of a straight clamp length you have one that matches the previous bend in the orientation you need to bend the subsequent bend.

It doesn’t take a genius to realise this tooling can get very expensive, if for example you have an exhaust manifold part, where there are multiple bends with little to no straight, this would require up to 10 stacks of tooling.

Are there different types of multi-stack machines? I’m afraid so. Some have fixed pitch between stacks and are typically limited to three positions, or like at Unison, you have a set stroke in which you can define as many pitches as you can fit in the space, so be careful to choose the right one.

Recommendation – Challenge your designer to redesign the part with ‘2D clamp straights’, play around with the ‘bend radii’, they don’t have to be 1D, 2D, 3D, it can be anything in between, it just becomes dedicated to the application.

• Multi Radius

Some parts will have different Radii within the same part, this generally comes about because design aren’t familiar with the manufacturing process, however sometimes it’s “necessary”.

So to do a ‘multi-radius’?part you first need a multi-stack machine, again not all machines are multi-radius.

Why?

To do true multi-radius you need a pressure die that goes up and down and is not a static block to, for example, add three static pressure dies to with varying depths.

Why?

Well, imagine a 400m athletic track race. The runner on the inside lane, although running at the same speed as the runner in the outside line, seems to be going faster relative to the other runners, but it’s just that they are running a shorter distance, hence why they have a staggered start.

Why does this matter in tube bending? This is because if your ‘powered follower’ doesn’t match the Swept Length movement of the ‘bend former’?the pressure die stack will run into the back of the ‘clamp block’?as it tries to match the swept length.

You can cheat by matching the follower to the slowest moving stack, but you are then dragging on the other stacks causing excessive ‘wall thinning’.

Everything is a compromise, but be careful not to compromise on the right tool for the job.

• Putting All Tooling that you bend with on at once

This, in my opinion, is a bad idea for several reasons :

1. You still have to change the Collet, Mandrel, Pressure Die when you change Tube Diameters.
2. You have to match your clamp depths to the tooling you have on the stack, compromising the Tube Grip. This is your number one enemy as discussed in Blogs : Control Features : Carriage Push.
3. Wasted investment – why buy multi-stack when you don’t need it?
4. More collision points – the clamp and pressure die blocks are significantly taller.
5. Tooling is more expensive.
6. By the time you add up all the machining tolerances of fitting tooling together the top stack is likely to have the worst clamp grip, and typically people put the tightest radius on the top, when that actually requires the most precise setting.

With a Unison All-Electric Tube Bender, a tool change can be done in less than two minutes on a 50mm machine – see here – and 10 minutes on a 100mm machine – see here.

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