Modelling the Igus R48 energy chain in Fusion

What up?

Not been doing a great deal in the workshop recently, partly due to grief / stress at work and partly due to uncertainty over the Bantam cabinet build. I've got most of the key parts now, give or take, but before I start drilling and cutting stuff, I need to convince myself I have got a sensible concept in mind. I'm getting there but haven't arrived yet.

I ordered some Igus "E14 energy chain" from RS recently. This will carry the various cables from the moving saddle assembly back to the control cabinet. Having received it and had a good play with it, it looks like a pretty sensible product in general terms and I seem to have got the dimensions about right. However, the links are open to swarf getting in, which doesn't feel right to me, given where it will be operating. The bend area is most likely to be operating close to where the machining action is happening and the "inside" surface would act as a fine trap for small bits of swarf. I can imagine them worming their way into the cables with time, resulting in intermittent shorts.

So - close but no cigar. Not certain I want to build my machine around this.

What now then, fatty?

Before getting too clever on the detailed design front, I thought I'd better see if there's a better solution out there. Sure enough, there's an (almost) totally enclosed alternative that can be bought directly from Igus UK - the "Series R48 E2 Tube". 

I missed this route, as I expected that I'd need to be looking at the likes of RS, Farnell, CPC etc for ex-stock delivery in the UK. However, I got a confirmation email confirming it's stocked in the UK and (apart from the long bank holiday weekend) available for immediate dispatch. Bargain buckets.

Furthermore, the components are available as CAD models from the Igus website. Not Fusion 360 native format but step and iges, which is fine. 

Once you've specified the length of the chain and specified the mounting brackets (end pieces), you can download the assembly and import it into Fusion. This wasn't too tricky but when I got it imported in there, I was a bit underwhelmed:

Yes, the central portion is a single body ie incapable of articulation. It's not strictly necessary or even enormously helpful to go to the bother of modelling the chain in the Bantam assembly but I thought I'd have a go at creating my own. Bollocks - why not? 

Modelling drag chain in Fusion 360:

The basic parts are available to download including the mounting brackets and the link component, from which the chain is made. So off we go. Here's the chain with a couple of links and the end brackets highlighted for no obvious reason, other than to give some visibility into the assembly:

The key to getting its behaviour right is the joints implemented between the links. The characteristic behaviour is due to the restricted movement, which seems to be 30 degrees in one direction and zero in the other. That and the connections to the end brackets provides most of the constraints. The final joint required is the sliding / offset joint between the end brackets. Once that is in place and the "fixed" nd bracket is grounded, you can grab the "moving" end bracket and everything works as it should.

The (25) links were all manually assembled, which took a while. The angle of articulation of the links is a key parameter when it comes to the minimum bend radius, which in turn is one of the key behaviours of this chain. So I used the parameter table to create a user parameter to control all of the joints used between the links. Then I was able to finesse the angle to get observed behaviour.

The calculations are fairly simple. The bend radius for a chain made of these links is claimed to be 60mm and the link length is 30.3mm (from the datasheet). So the angle of each link to the tangent is 2*arcSin(15.15/60) ie ~15 degrees. After a bit of frigging about, I ended up with 30.5 degrees max on the joint limits:

Once the revolute joints for the links and the sliding joint for the end brackets were in place, it behaved like the real thing.

My workshop PC is a pretty feeble thing, with an i3 processor and integrated graphics. However, if you don't try to move to suddenly or quickly, it just about manages to keep up:

Next, integrate with the main machine assembly in Fusion. Not just for bragging rights but actually to figure out the best place for it to live. Here it is with the saddle in its closest position to the headstock:

And at the other extreme:

I can clearly move the fixed mount slightly nearer the tailstock end by 50-60mm or so, FWIW.

In terms of the end view (ie X & Y position), this looks pretty reasonable when you eyeball the machine itself. Wrong drag chain but it shows the general idea:

Looks workable. The E48 cable won't arrive until Wednesday at the earliest and I will be busy with work during the week anyway. So back to the cabinet for now.