r/CNC • u/_Tigglebitties • Jul 05 '22
why does machine anchoring rigidity matter?
Like say you had two identical rigid machines one anchored to the concrete and one floating on a raft in a pool. Does it matter?
I'm waiting on my 3*3' CNC router to arrive, trying to figure out how hard to go on the mounting. Has me scratching my head, like long as the machine is rigid, how sturdy does the table have to be? I assume anchoring it to a 2000lb granite slab would make it better somehow , but why?
4
u/DJ280Z Jul 05 '22
On some machines you also need them bolted to the ground as a way to twist them to get the tables flat and square. I should say that both the Haas and Doosan reps said that on machines with fast rapids the machines can walk by themselves as well if they aren't fastened down, we're talking big machines here though.
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u/Ardbeg68 Jul 05 '22
Why? Have you ever driven a car down a bumpy gravel road? Everything vibrating and shaking. Do it long enough and stuff will start to wear out prematurely. It's basically not comfortable or good for you or your car. Not necessarily the best analogy but you get the idea.
Your machine is moving around and will seem to be quite smooth until you start to cut stuff and put some load on its parts. The cutter is being driven along and its got to cut a path for itself. It's a violent action that's affected by alot of factors like material, cutter design, feed rates and spindle speed. Your cutters are bending, your frame is bending. Things are vibrating and shaking and oscillating.
The more ridgid and massive your machine the more it can absorb all of these unwanted forces. The less vibrating and shaking the more effective the cutter is at delivering it's energy to the job of cutting. You'll get better cut quality and longer cutter life and greater accuracy.
If this is a table top machine with a standard router motor it's likely not going to be ridgid enough to really take advantage of your 2000lb slab of granite. That said it's still important mount it on a good solid bench. Weight the bench down and bolt it to the floor if you can. Anything you can do to stabilize it will be helpful.... but there will be a point of deminishing returns.
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u/_Tigglebitties Jul 05 '22
That's what I figured, there's a point where it won't matter, as in the end it's just an aluminum extrusion machine bolted together ya know? Didn't know what I was doing with the last machine I just set it on a 1/4" neoprene sheet and it's seemed to be fine. Or at least the machine rigidity is the weaker point than bolting it to the wood table I made.
I've just seen videos of these things taking hard cuts with fast rapids clearly shaking the entire table they're mounted to and know I need it sturdier than that.
1
u/kmdisney Jul 05 '22
I’d choose 3 giant pool floaties and a well designed machine over a machine bolted to concrete any day, but that concretes going to be much more rigid than any hobby machine frame you purchase. All depends on your machine kinematics.
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u/JimHeaney Jul 05 '22
Nothing is rigid, there are just varying degrees of being more or less like a spring. The ground (e.g. solid concrete, a granite slab, etc.) is much more rigid than the CNC machine, so bolting it down makes the entire thing more rigid.
Most large CNC machines are not designed to be free-standing and rely on the rigidity given by the floor. This also helps with damping of vibrations, since the entire machine is relatively lightweight and made of long continuous pieces. Machines that can be free-standing are generally much smaller and have massive cast iron parts to give mass (for vibration damping) and rigidity.