[gabdab]

Watching epoxy granite big heavy plates makes me worry about cnc machine parts mobility problems.
Something like epoxy granite bricks kept in place by steel structure, instead of full slabs, would ideally be beneficial, but precision?

[joeavaerage]

Hi,
Why?

CNC machinery is first and foremost very rigid, if its not rigid then its junk. You just cannot make decent parts in a flexy machine.

Being super rigid requires large volumes of stiff material, most commonly steel, cast iron, but as in your case epoxy granite, but other materials as well, aluminum, high performance concrete,
laminated and engineered wood among others. All of those materials weigh, and even with some optimised geometric distribution of those materials the resulting rigid machine will be heavy.
Fact of life....rigid and heavy go together.

Heavy is the exact opposite of mobility.

Seems to me you can have rigid and heavy OR light and mobile but lacking in rigidity but not both.

My opinion is make a machine out of steel and/or cast iron in parts that can be assembled. I would not describe the assembled machine as being 'mobile', being a heavy and large as it is
but the individual parts and assemblies that make up the machine might better be described as mobile.

I built my machine in that general vein. It has three near identical cast iron axis beds 700mm x 250mm x 140mm weighing 115kg each. Each axis bed has the linear rails/cars, ballscrew, servo and saddle attached.
The resultant axis beds end up at about 150kg each. The axis beds are then bolted to a steel frame, two 'L' shapped pieces of 32mm thick med tensile steel. Then the entire construction sits on a welded steel
frame, a recycled pick-and-place machine base, and that is on castors. The total weight of the machine is just over 800kg.

I added the castors because I thought it would be useful to move it around the workshop......but really that idea is a bust. I actually put it in place and have to wedge it in place otherwise it starts rolling around once
I really get the machine up and 'rocking and rolling'. To be honest the castors are more a PITA than useful, but they are there and in place. I do ocassionally roll the machine out to sweep up in behind it......so they do serve
that purpose.

The point being here is that I want and need my machine to make parts. It has to be rigid and therefore heavy to make parts of an acceptable quality and accuracy. I don't need it to be mobile, in fact being firmly fixed in place
is an advantage. It would be 'nice' to have it mobile, but not at the expense of rigidity.

Craig

[awerby]

If that's what you want, why not just use blocks of granite? What does the epoxy do for you?

[joeavaerage]

Hi,
as you may well know that peteeng, a very regular Australian contributor on this forum has made extensive tests on epoxy granite combination in his 'milli' thread
and the best has has been able to achieve with epoxy granite is 18GPa. I think you'll agree 18GPa is atrociously poor, by comparison to UHPC (50GPa),granite (70GPa),
aluminum (70GPa), cast iron (110GPa) or steel (205GPa).

https://www.cnczone.com/forums/cnc-d...11248-cnc.html

If anything using granite as opposed to epoxy granite will increase stiffness at comparable weight.

Craig

[gabdab]

Granite surface plates are too pricey from a quick search.
18" 12" 4" 308.27

[catahoula]

if we're talking about high precision structures, i would say that granite surface plates are wildly cheap. compared to buying steel, welding, thermal stress relief, machine shop flattening. or having a plate cast aluminum endoskeleton, building mold, pouring a bunch of EG, then taking it to machine shop for flattening.

surface plates do make for some design limitations though and you have to either through-bolt the linear motion components or drill holes and glue in inserts. drilling granite is easy though with the right bits and a water dam around the whole.

[joeavaerage]

Hi,
to be honest I'd say they are cheap too.

If I got a piece of 32mm med tensile steel 450mm x 300mm plasma cut I'd have to pay $200NZD. To slap that in a surface grinder to get it flat to within a few um would cost around $180NZD.
Total is $380NZD or $230USD. Cheaper than granite....but not by much! The steel would be a few kg lighter.

Craig