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

[driftspin]

Hi folks!

I have not designed anything yet as I still struggle to decide which design route to take. Your help here will be really appreciated!

What I want:

- a machine capable of milling aluminium with a decent speed and surface finish
- a cutting area of about 600 x 900 mm
- to be lightweight, with a small footprint. To be able to move through a 80 cm door (partly disassembled).

I was starting from Jonathan and Routercnc designs. Then I tried to find ways to add damping and gantry stiffness without adding a lot of weight.

The easiest and fastest design to build is Jonathan's frame with Routercnc's gantry , all welded RHS steel and epoxy levelled. But those RHS left empty look so... 'not professional'. The stiffness isn't great and they ring like a bell.

For the frame, filling it with sand (after moving/assembling) looks like a good idea.
The gantry needs to remain lightweight. What could be used to fill the RHS to add damping, stiffness without adding to much weight?

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

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.

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

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.

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...

I am just wondering what material is Datron using for the gantry of their M8Cube CNC?

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

What do you think? Any comments are welcomed as I'm lost in so much technical information/alternatives!

Paul

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.








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