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.

[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.

[silopolis]

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

Nice find! Had only found bigger ones. Thanks for sharing

There are also electrical and air powered vibrators of all kind of sizes. Affordable one can be found also.

Finally, good infos on DIY vibrators and vibrating tables can be found on YT


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