Rubber layer between plates

Hi everyone,

I designed my own cnc mill to mostly cut aluminium, plastic and maybe a little bit of wood.
The spindle is a 2.2 kw watercooled chinese spindle, that i know is by far not the perfect for metal, but i think it will do the job. The gentry is fixed and it has 10+ mm (thickness depends on the local manufacturers stock) steel plates that will be welded together. I decided that for the sake of easier assembly and the deformation i will separate the gentry and i will bolt them together. I looked at a lot of videos on milling aluminium on routers and i think this will be rigid enough for aluminium, but i might need to cut some steel and i am concered about vibrations.

What i was thinking is to add a rubber layer between the 2 plates and maybe that would help dampening the vibrations.
My other thought was to seal the bottom of the frame and fill it with some fine material like sand, but i am not sure if that would help if it moves freely in the bottom.

What do you think about this?
I would also appreciate comments on the design(The spindle, motors, ballscrew and the aluminium frame is fixed, but other stuff is not)

A drawing would help clarify what you're talking about doing, but it sounds like you're making a sandwich out of 2 steel plates, bolted together with rubber "jam" in the middle, to use as the main beam of your gantry. I don't think that will be as rigid as a solid assembly, although by using bolts you won't risk distorting it with the welding process. Is the point of this to make a quieter machine? If your goal is rigidity, a box beam would be better than the sandwich, since it gives more protection against torsion, the tendency of the beam to twist when confronted by cutting forces leveraged by the extension of the Z axis. If this is a fixed gantry design, weight isn't a problem, so the bigger and heavier the beam is, the better.

People here have experimented with putting sand into the hollow spaces of their machines in hopes of deadening vibrations; they report some success, but loose sand doesn't seem to work as well as sand consolidated with some kind of adhesive and packed in firmly.

My point with separating the gentry into 2 parts are to make it easier to assembly and to probably it is cheaper to machine out 2 pcs from the same plate size what i use everywhere, than make them cut a bigger plate just for that purpose. Putting a rubber pad in between was an idea to reduce vibration that i have seen in many videos online.
I made some finite element analysis and the maximum dislocation on the gentry was around 0,005 while putting 100 N on the linear rails. It might be more since it was on the rails and was not leveraged much, but i am going to test it later in a more real condition.

Are you suggesting to attach the linear rails on the side of the beam? I am worried if the flatness is going to be really bad on it. Maybe machine it flat?

I tried to look for cutting forces on these spindles while machining metal, but did not find anythig useful. Is there a magic number that i can use for calculation?


Thank you for the comment, now i guess i can forget the sand :)

Hi,
in order to use a substance like rubber to dampen vibrations in a steel structure require that strain be transferred from the steel into the dampening
material where the vibration energy will be dissipated. Steel, being very stiff, will have very small strain displacements and even if the rubber is bonded to the steel
the displacement in the rubber will likewise be small and therefore of limited utility for dissipating energy.

Rigidity trumps vibration dampening every time. I would save the money that you might use on rubber by getting double thickness steel. If vibration is a real
concern to you use a good vibration dampening material like cast iron or epoxy granite.

I had the axis beds cast in iron, approx 115kg each, for my new build mill. I had hoped to use a cast SG iron frame of about 300kg, but I've maxed my
budget and have to consider making the frame out of steel. Steel is plenty stiff enough, and because it cuts and welds so well is highly versatile and modestly
priced.......but I still really want a cast SG frame.

I think I'll have to compromise, make a 'cheap and cheerful' steel frame now and in a year or so, when my budget recovers get an SG frame cast.

Craig

Thank you for the answer, I was looking for cast iron, but I think it would cost way more make that.
I guess since steel is very cheap I will try to get bigger plates like 20-25 mm thickness.
The way I wanted to do is to cut every part with laser, then machine every side that contacts to something flat and weld it together.

I made the screw holes that attach to the bottom frame slot holes so I can tilt the gentry if it the spindle is not perpendicular to the table, but if I make it heavier that wont be an easy job. It might be better to machine the X axis's plate perpendicular after welding. Or could it be enough if I machine the table myself, hence every error of the spindle goes into the table and it will be perpendicular?

What I was also thinking is to get a wide vertical plate for the side of the gentry, that would be basically a cover for the side of the machine. Or maybe not in the full height, but cover the bottom frame with the gentry's vertical plate. If I cover the whole side it would add 40-60 kg to the machine. Would that weight also matter, or it would not matter that much?