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

[paulus.v]

Thank you all for your input!

I'm intending to add a layer of foil-faced Tecsound damping sheet inside my gantry extrusion, I've found it very effective in the past on metal enclosures. I may also try some expanding polyurethane foam, either on it's own or in conjunction with the Tecsound: just need a bit of time to experiment!.

Tecsound looks promising. Not sure how different could be compared with the ISODAMP C-1002 mentioned in the MIT paper. A friend is soundproofing cars for 'dB drag racing' with StP. Not sure how good it is at vibration damping but I'll test it.
I don't think the PU foam has any damping characteristics. If you knock it the sound is very well propagated.

The answer is easy, the implementation of it is not, you set up a factory to make the Aluminium Foam filled parts of your choice, it is not an "Aftermarket Filler" choice.

I see a few books cited on wiki about ally foam manufacturing. I need to get my hands on one of these books.
Here's an experiment but the result doesn't look that great. https://www.youtube.com/watch?v=8FHTK2LZNTY

In the end, for this size of machine, the weight vs dynamics can be managed with appropriate motion component choices.

Exactly here's my problem. I need a lightweight machine, and being light allows me to use steppers and the price difference to servo I can use to improve damping and stiffness without adding to much weight.

Wow, never heard of this! Veeeery interesting. You just added another option to investigate to my list.

About shrinking/retraction, I don't think this such an issue, or at least that it can't be solved easily, as this is a construction method used daily in buildings. Quite sure there should be something in Sika or other product range for this problem.

This is actually my champion ATM! I find this Constrained Layer Damping technique very interesting by its use of multiple materials, smart mechanical principle, and affordable materials and build techniques.
Visco-elastic mat used in the thesis can be found and is not insanely expensive, and I'm sure can (and most probably will) be replaced by other cheaper and/or more easily available products. I'm currently thinking to asphalt mats...
They also use (IIRC) aluminum oxyde in concrete mix as an expansion agent, which should also be an answer to the previous retraction issue.

I doubt the expansion agent will work with foam concrete as it will tend to put pressure on the air bubbles in the concrete not against the walls of the mold. This chart shows that the shrinkage increases as concrete density lowers.

Even if I'm a little surprised by your stiffness numbers... Price remains one of the top decision factors, at least because all cents saved in structure will be to increase quality and performance of other components.

i have checked the deflection only with online calculators, two for each and they gave similar values. But now I tried a lot more calculators fot the RHS and the values were different... from 0.007 to 0.11

Could any engineer here help me with a proper calculation?

Other usual suspects are epoxy concrete/granite casting, and of course, solid granite stone. These are my second and third options.
You'll find tons to read about the first, and high end machines are also built this way.
About the second, you should be very interested by this build series https://www.youtube.com/playlist?lis...BoMfvQ8Ttsq-57 (as well as by all other videos from this guy!)

Interesting granite build.
The epoxy-granite is another story. There's the famous thread on cnczone with almost 5000 replies. But as far as I understand it, the most difficult part is the vibration for compacting. Vibrators are not cheap and not that easy to build and for good results they need to be tuned to the right frequency using inverters...

But a epoxy-granite cnc wouldn't be lightweight so it is out of the question for this build.

I won't even look for pricing infos just to not feel the pain!

They say 5-6 times the price of roller rails. But the price of their rails not the hiwin ones... And requires a dedicated oil pump.

Hi Paul,

Are you sure about the stiffness you describe above.

As a rule of thump, steel is more rigid by a factor of 3 for a given shape at the same weight.

Box section should be more rigid than tslot profile..

You explain above the tslot alu 13.5kg is more rigid than the steel box at 18kg....

I am not saying you are wrong... it just doesn't sound right.
Please check to make sure.

Also weight is your friend preventing vibrations.

I know it is hard to make the choices.

If you have access to tools and skill to go the steel frame / epoxy route i recommend it.

Grtz Bert.

I'm not sure anymore about the hollow section stiffness... If the calculation was wrong and stiffness is not a problem I'll probably take the steel/epoxy route with some sort of vibration damping inside them.

[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..

[Nick]

Interesting granite build.
The epoxy-granite is another story. There's the famous thread on cnczone with almost 5000 replies. But as far as I understand it, the most difficult part is the vibration for compacting. Vibrators are not cheap and not that easy to build and for good results they need to be tuned to the right frequency using inverters...

But a epoxy-granite cnc wouldn't be lightweight so it is out of the question for this build.

That cnczone thread is a mess... I have read through it twice and i'm not sure it is helpful. Ideally it should be vibrated and I was put off due to that thread before. But I have found you can get reasonable results with just compacting it with your hands/tools. If your very concerned about vibration I think something like this might be worth trying

https://www.amazon.co.uk/Nordstrand-...FCCYN1BBXNSFA0

The other discussion topic on that thread is about aggregate size, the members of the "cncecke" forum tend to use pre-mixed fullers approximated aggregate such as Silimix-282 with good results. If your interested there is loads of good information on these forums but you need to register and use google translate.

Filling the RHS section with epoxy granite or UHPC concrete would be a reasonable way of vibration damping. It won't add as much weight as you might think, for example the 160x80x1m section filled with EG would only add 25kg.

You would be using the RHS and just using the EG for dampening so you could use an easy flowing sand mix such as this one

https://www.adambender.info/single-p...e-Frame-How-To

I am currently making a mill from EG so i'm perhaps a bit biased.

[paulus.v]

Hi Nick, I'm following your build with interest. Looking good so far. I like the manual scraped cast iron parts :)

Reading the Silimix-282 description I found that the company did exactly what the forum guys were trying to achieve. To have a very compact aggregate to use as little resin as possible. Less resin means higher stiffness. But if you have an internal metal structure or use the resin to dampen vibration in a cast iron machine then stiffness is not very important and more resin means better vibration damping.
It is good for DIY that such aggregate mix is available on the market but what if you want real granite aggregate instead of quartz? :) You have to go back to that messy thread trying to find answers/recipes...

I'm not sure if/how I'll be using epoxy-granite or cast iron in this build. I cannot add to much weight. For cast iron it should have at least 10 mm thickness for a decent stiffness and for epoxy-granite about 25 mm which translates to roughly the same weight.

Like m_c said, I need to find the best compromise. I'm aware that I cannot have great material removal rates, surface finish and tool life with a lightweight machine.