Vibration, mass and massive arguments

Hello again! You all thought I'd forgotten about you but it is very far from being the case! Life is throwing a lot of curveballs at the moment but I still hope to have my first machine finished before Jonathan graduates :)

Anyway, at the risk of starting another one of the mass/rigidity/vibration arguments, I have a quick question about the machine and the worktop that it will sit on.

Is it better for the machine to just sit on the surface, or be bolted down to the surface, or sit over the surface on small feet?

I understood that the small feet work as a way of reducing the amount of vibration passing into the worktop. If the worktop itself is quite hefty (and is using smaller feet to isolate it from the floor) would it be able to work as a "vibration sink"? Would this make it better to let the machine rest on the surface, or would physically bolting it to the surface be any better?

I also appreciate that dealing with vibration in the design and use of the machine is best, but every little helps. For various reasons I have to build the enclosure first so I need to know if boltholes in the worktop will be necessary.

Also, I seem to recall a few years ago coming across someone using a levelling compound in their project to try and get a perfectly flat and level surface, though it might have been specifically for the surface where the rails would be mounted. Has anyone any experience with using a pourable levelling compund that would be suitable for the worktop?

I'm interested in this aswell regarding minimising vibration. Also, if the machine is completely independent of the worktop, why does it matter if the worktop is completely flat and level?

It might not matter, but the part of me that insists on lining all my pencils up on the desk in order of size and sharpness is requiring me to ask :barbershop_quartet_

I'm also wondering if the machine should be completely independent - if it is firmly bolted to the surface then having that surface flat and level is presumably a good thing.

I think it is suck it and see. This is the problem with sympathetic vibration...

You can't kill it without making everything out of great lumps of cast iron.

Tweaks will only move it.

A knob to adjust spindle rpm on the fly is good.

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Originally Posted by Rogue

Is it better for the machine to just sit on the surface, or be bolted down to the surface, or sit over the surface on small feet?

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Very much depends on the machine.? If small lighter machine that's going to rapid around fast then you'll want it bolted down to stop it walking off and even heavier machines can and will do this thru inertia when changing direction.! I always prefer bolting down has it keeps things that bit more ridged and safe.

Quote:

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Originally Posted by Rogue

Also, I seem to recall a few years ago coming across someone using a levelling compound in their project to try and get a perfectly flat and level surface, though it might have been specifically for the surface where the rails would be mounted. Has anyone any experience with using a pourable levelling compund that would be suitable for the worktop?

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Unless special levelling compounds like Epoxy resin then they don't level to anywhere near accurate enough to direct mount rails and not use levelling feet.

Unless you have a very very stiff frame and I mean cast iron stiff then some form of levelling method will be required to keep or bring rails parallel so it's always a good idea to start with a level surface and if your wanting accurate parallelism of rails then some form levelling will be needed.!

The "bolting down" question came out of too much time reading about vibration and isolation, but too little time reading it to understand it properly! There's going to be vibration, I need it to go somewhere but I certainly don't want it to go into the building structure.

As for the levelling question, I seem to recall they had made a horseshoe-shaped channel (ie the two mounting surfaces were connected to ensure the liquid level was identical in both) and filled it with some kind of liquid. I think it was for a lathe, if my old creaky memory doesn't fail me too badly. The need to potentially tweak and maintain levels as an ongoing concern completely escaped me. I was quite taken by the idea to be honest, it seemed like a very neat solution to a potentially awkward problem.

I was thinking about the epoxy method the other day after reading through various build logs. As epoxy is doing little to dampen any vibration, I was wondering whether a solvent based moisture cure urethane would be a good alternative, perhaps a two part casting resin.

If not a direct swop, an alternative approach would be to exclude sand from the box section and coat the interior with a thick film of urethane. The method would be akin to making china cups. A mold is filled with slip and emptied, leaving a thin film behind. The actual practical approach could be the targeting only one internal face of the box section and leaving a thick 5mm film i.e. seal off both ends and pour it in from above. As the resonance comes from the motors, the first section I would look at is gantry. An alternative section to consider filling (depending on the urethane being used) is the gap between the supported rail and the box section it is mounted on.

It would take a bit of research to find the right product, its something we manufacture in the UK, used on industrial floors, shipping and oil rigs. Another product to consider would be RTV silicone.

I have a product in my garage at the moment, I'll give it a try and see if there are measurable effects.