Designing a better bed

[D.C.]

Thank you everyone for your thoughts.

I'm currently clearing out a garage to convert into a workshop and literally sweeping the cobwebs off the ceiling. In a week or two the paint will be dry and I'll need to put an order in for some steel to start building work surfaces, tool storage etc.

MichaelW - I had thought about levelling a concrete platform, but any cost saving would be outweighed by me explaining to prospective future buyers that yes, this was once a garage but now it is an excellent place to park your lathe. Sorry about the giant lump of concrete, how are you with a hammer drill? The machine will need to be be theoretically moveable, even if that means hiring a forklift. Realistically it will probably never be moved and reused. If I really needed to dismantle it, the expensive bits electronics/rails/etc would be for reuse/ebay and a better shiner machine frame built.

Routercnc - I've been reading every paper I can get my hands on to do with CLD, (so many are pay-walled, grrr) as far as I can tell, a thin layer of steel is not required. What is required is a 'viscoelastic damping layer' sandwiched between two comparatively rigid layers and strongly bonded to them. As long as 'pond liner' can not move in relation to the rigid layers, it has no choice but to convert the vibration into heat, which is where the damping effect comes from.

Boyan - You are right about the channel section strength but in Britain steel costs a lot than in Spain. There is also the problem that when you hit a piece of steel with a hammer it rings like a bell. That sound is massive shockwaves travelling through the material and forcing air out of the way as it flexes. The 'perfect' cnc machine would be completely silent, there would be zero vibrations that you could introduce into it that would result in a measurable distortion.

DaveGrennan - I think 20mm seems to be the standard approach around here because of the z-axis. The vast majority of people have a similar requirement for z-axis movement and 20mm alu plate is recommended for multiple parts of the machine. It thus makes cost effective sense to order one sheet of 20mm plate and use it for everything, including the bed.

Because I need some steel anyway before I start my cnc build, and because I know that I will end up using epoxy levelling for my rail mountings I think some testing is in order.
I will buy a bit of 80mm square steel, some pond liner, adhesive, reinforcement etc, and do some comparative tests. Deflection is easy, vibration can be done on a smartphone with a seismometer app, weight, easy, cost easy.



This is a beam design I have being considering. In terms of machining and CLD, it was first done in the 1960's but still crops up in the 2000's as the optimal design.
Bare metal beam VS sand filled beam VS reinforced CLD/EG beam. I'll post the results and go from there.
Any other tests I can do with peasant level equipment and any advice in setting up a fair test?

Thanks,

D.C.

[Sven]

The sides of the bed are 3mmx80mm steel plate.
The bottom is 3mm steel plate.
The reinforcing is 20mm x 20mm hollow steel square.

After much reading about the wondrous technology of constrained layer damping, it turns out that one of the very best 'viscoelastic damping materials' is butyl and it needs to 0.5mm to 1mm for the sort of things we do.

Which is handy, because the super spiffy visc

£90 in steel
£10 in pond liner
£140 in epoxy
£20 in milled carbon
£25 in aggregate

£285 total

I've been very generous with the epoxy as that is the scary expensive part, £140 assumes an 18% by volume mix but hopefully it will average out lower than that. I could still go to almost 25% epoxy and break even with the aluminium.

Any thoughts or comments on this approach for a DIY bed?

Forgot to reply to the general idea...

Here are some thoughts

- Epoxy adheres well to steel but may not stay bonded due to differences in thermal expansion. That would make the steel tub a bit overkill
- If you use that much epoxy it is likely to form a layer on top of the casting.
- Using milled carbon as well as steel tubing is a bit like suspenders and belt to keep your pants up. And still, 20.-- seems a small amount.
- using tubing instead of bars lowers the mass/volume which I think is a good idea.
- I would use round tubing though as I think it would leave for a stronger EG mass.
- You can probably get away with using pvc piping instead of steel if the plate is thick enough

[D.C.]

Thanks Sven, after Robin's comment about having a big chunk of ALU that can be sold if things go horribly wrong I have to admit that I'm rethinking things a bit...

I'm not too concerned about thermal expansion because I'm building a CNC machine in a garage that is attached to a house and the temperature doesn't won't really fluctuate more than 15C over the course of a year.
I was banking on the epoxy getting vibrated to the top and selected the epoxy especially to do that as it will self level.
The milled carbon amount is based on the best practice mix on the easycomposites website, 10% by weight compared to the epoxy. Obviously the quartz/granite/etc in the mix doesn't count, this results in a surprising small amount of milled carbon fibres that add greatly to stiffness and dimensional stability.
I'm not sure exactly where the optimum lies between tube and square, I selected square so that I could weld it together easily, circular tube would be much more difficult to fabricate.

I've been arguing the toss with a structural engineer and insurance company about the fact that my house has giant cracks in the extension so for the next couple of weeks/month I need to back burner the CNC which is very annoying, I am relying on it to do a lot house upgrades. :(

[Sven]

I' ve been reading up on EG as my own ideas are taking shape.
As far as I' read, vibrating EG does not work unless you use a mix with too much epoxy for a small size casting. Too much meaning it will yield an end result that has some shrinkage and maybe some warp.

The best seems to be a mix that is "very dry" compared to what most people on forums seem to be using: in the 4% area, or a bit wetter where it needs to adhere to another material.
Also, best to use various grain sizes, the next size 1/5th of the previous.

Put a measured amount in a jar, then add the smaller size and shake until the volume increases and you know the ratio.

A dry mix like that will need stamping.

And off course, make sure to do trials :)

[Robin Hewitt]

If all you want is to flow a flat surface, why not use Wood's Metal? That goes very runny and sets hard. You could even reflow it if you moved the machine.

[D.C.]

If all you want is to flow a flat surface, why not use Wood's Metal?

Not a bad idea but Wood's Metal / Cerrobend etc looks to be about £40 per kg, so it would cost a lot more than epoxy levelling I think?
Have you ever tried it if you have some lying around?

I' ve been reading up on EG as my own ideas are taking shape.
As far as I' read, vibrating EG does not work unless you use a mix with too much epoxy for a small size casting. Too much meaning it will yield an end result that has some shrinkage and maybe some warp.
The best seems to be a mix that is "very dry" compared to what most people on forums seem to be using: in the 4% area, or a bit wetter where it needs to adhere to another material.
Also, best to use various grain sizes, the next size 1/5th of the previous.
Put a measured amount in a jar, then add the smaller size and shake until the volume increases and you know the ratio.
A dry mix like that will need stamping.
And off course, make sure to do trials :)

For larger aggregate This place does washed, graded and dried quartz specifically for resin bonding:
http://resinbondedaggregates.com/cat...ound-aggregate

For the smaller sizes I think it would have to be blasting grits:
http://www.stacey-processing.com/gla...ing-media.html

Then Aluminium Oxide powder and milled CF.

[Boyan Silyavski]

Linear or volumetric thermal expansion coefficient is irrelevant in constucting a CNC.

First of all cement, steel and concrete have very similar thermal expansion. Even if the epoxy has higher thermal expansion its irrelevant at 1m and temperature changes. Plus the epoxy cement or epoxy granite will more similar expansion to the concrete




EG is same like all, do it properly and it will work well.


There are better options of course and not so expensive- purpose made epoxy concrete, especially made for the purpose. Problem is finding them close to you or in your country at all.

But those who search will find :-).

Key words:

epoxy concrete cast machine bed, ultra-high performance concrete





These guys http://durcrete.de/ can do machine bed design or cast inhouse for you or as far as i remember 1 ton of the stuff was like ~400euro

Gues what i will be bringing home if i go to Germany :-)