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

[silopolis]

Hi Paul,

I have not designed anything yet as I still struggle to decide which design route to take.

We're all there at some point(s)

For the frame, filling it with sand (after moving/assembling) looks like a good idea.

Indeed, found an interesting thesis on the matter tending to confirm this.

The gantry needs to remain lightweight. What could be used to fill the RHS to add damping, stiffness without adding to much weight?

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

This aluminium foam looks so sexy.. But how is it produced? How complicated/expensive? I couldn't find an answer yet.


And here's an article about it: Vibration Damping Analysis of Lightweight Structures in Machine Tools

Interesting, will surely read the paper, but I doubt this can be DIY, and wonder if companies able to do it can be found easily...

Another idea is filling them with foam concrete.
But in both cases the filler will shrink by cooling/curing and maybe detach from the steel walls.

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.

There are other damping techniques/materials but do we have access to them? http://www.aspe.net/publications/Ann...IS/BAMBERG.PDF

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.

Then I compared the stiffness of rectangular hollow section steel vs. aluminium t-slot profile.
Adding a 500N force in the middle in a supported two ends configuration, 160 x 80 x 5 mm x 1 meter RHS steel (18 kg) will deflect 0.1 mm and the 160 x 80 mm x 1 meter aluminium t-slot profile (13.5 kg) is deflecting only 0.012 mm. I have no idea which of the two is better at vibration damping. The only advantage that I see for using steel for the gantry is the price, 5-6x cheaper than aluminium extrusion.

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.

Most of the 'real' metal cutting machines are using grey cast iron which has 20x or more vibration damping capacity than steel. And it won't bend but break.
Iron casting with lost foam technique isn't that complicated/expensive. But how strong should a 1 meter gantry be? How heavy will it be? I couldn't find any information on cast iron gantry design...

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!)

Here is another way to add damping but the cost is far beyond the budget of a DIY cnc. https://www.machinedesign.com/bearin...on-linear-axes

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


Hope it helps

Cheers
Jérémie

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