MK2: moving gantry router for metal cutting - comments sought!

[m_c]

Hollow sections work well if designed correctly. The original FADAL's were mostly weldments, and performed well.

If you've not seen them yet, check out the David Decaussin (one of the original FADAL owners) videos on youtube showcasing his small mill and lathe, as he mentions a bit about it. His channel is at https://www.youtube.com/user/DAVIDDECAUSSIN

And having just searched for that, I've just noticed he has some newer videos which I'm now off to watch!

[paulus.v]

Thanks m_c!
I have not heard about Fadal and David. I like the vertical toolchanger he has built. And how he explains his ATC drawbar.
But steel remains steel and cannot be compared with grey iron in machine tools :) . When he pushes the machine a bit harder it starts ringing...

Yes the online calculators I've used for the RHS deflection were crap. I have checked with a FEA and it gave 0.0081 mm for the 160x80x5 1 meter RHS with 50 kgf in the middle. Now compared with the 160x80 aluminium extrusion with 0.012 mm deflection, if it had the same weight (18 kg), it would deflect 0.009 mm.
In conclusion the rectangular hollow section steel is a bit stronger than the aluminium extrusion with the same external dimensions and weight.
And I think that steel has a little bit better vibration damping than aluminium but I'm unable to find figures.

[m_c]

Everything rings ;-)
The question is how much is due to the weldment construction, and how much due to the linear rails?
There's a very good reason why lots of the high end machine builders are still using box ways.

Aluminium has better vibration damping than steel. A very general rule is, the more springy something is, the more it rings.
Simple experiment is take a couples bit of steel and aluminium bar, and give them a tap. The steel will ring far more than the aluminium. Even cast iron bar will ring more than aluminium. Which reminds me, only certain grades of iron have self-damping properties.


This is something I've occasionally looked at, as I have a machine idea bubbling away in my mind, but I think to build it properly, would involve a foundry. But then I come back to linear rails acting like springs, and think would a weldment be just as good, with a lot less expense involved?

[paulus.v]

The rails are only passing on the vibration to the structure they are attached to. The box ways are great because there is always a layer of oil between the two sliding metal parts and it is absorbing 20x more vibrations than the linear rails (according to the info in the last link from my first post) therefore less vibration transferred to the structure.

Only the grey iron is very good at damping vibration. And it is due to the shape of the graphite that forms, the graphite flakes.
http://www.atlasfdry.com/grayiron-damping.htm

I've seen no springs in linear rails or blocks :) If you have the rails transmitting all the vibration to the structure it attaches to, then you have do something to kill them in that structure and steel is not good at that.

[m_c]

Linear rails rely on a couple rows of balls with minimal contact, and the balls/rails deform under load acting like springs. Off course, the big benefit with linear rails, is you can over spec pretty easily to minimise the problem.

As with everything, it's a case of picking the best compromise for your needs.

[Voicecoil]

Aluminium has better vibration damping than steel. A very general rule is, the more springy something is, the more it rings.
Simple experiment is take a couples bit of steel and aluminium bar, and give them a tap. The steel will ring far more than the aluminium. Even cast iron bar will ring more than aluminium. Which reminds me, only certain grades of iron have self-damping properties.

I dug out some damping tables (I've long had an interest in this sort of stuff, being a loudspeaker designer) and we have \mu values of:

Aluminium 6061: 5 x 10-3
Mild steel: 0.9...1.4 x 10-3
Grey cast iron: 30...90 x 10-3

Oddly some copper manganese alloys have high damping factors, but sadly I don't see much that for sale, hey-ho.

[paulus.v]

Thanks Voicecoil!
So aluminium is considerably better at vibration damping than steel. Good to know.

I was just reading about Sonoston, the copper-manganese alloy, used for vessels propellers..