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.

[Boyan Silyavski]

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.

You have got it wrong here. If it was a sheet or just box section profile, yes, its like you say. But not in that case. As it happens that i make various musical instruments and trying to make some from steel, all my designs of machines are made so that sound is canceled/ no vibration.

I am telling you here not abstract theory. I have already mounted that UPN 140 profiles and definitely there is no vibration. As the C form is doing just that-cancelling vibrations. There is no ringing whatsoever.

I am sure in UK also the UPN 140 is the cheapest way to go compared to all other variants. Just check a supplier, not local welder. Cause for example if i go to a welder he would sell it to me for 2-3eur kg instead of 1euro kg . Just cause he cut it. hense the Rage saw you see on the pictures of my build. It payed on the first machine i made. Just from ordering directly the 6m material. The funny thing is that the supplier transport of a whole truck of steel costs only 10 euro.

You could fill that C at the back with cement and here you have a winner. Dont over complicate things. Its right here in front of me, tested, super rigid, cheap and does not ring. What more?



Michael,
a simple aluminum plate is a nice solution, however if the hold down of it is not designed properly and not braced where necessary it will ring like a drum skin. Know from experience as i had a machine that did just that because was not well thought.

[cropwell]

For the sacrificial bed part of my machine, I an considering Apollo Slabtech (from Sheridan Worktops*). It is a cast and machined stone filled acrylic. I have the offcuts from a kitchen refit. It machines easily and is waterproof. You could easily machine a coolant gutter. On my small machine I have an MDF sacrificial bed with a matrix of holes which have furniture T-nuts in recesses underneath, so I can hold work using M6 screws (I use nylon ones, they are kinder to cutters if you crash into them). I plan to replicate this system in the Apollo Slab.

*If I can get a 700 x 900 mm piece at a reasonable price.


Cheers,

Rob-T

[cropwell]

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.

D.C.

What would be the magnitude of these vibrations ?

[D.C.]

What would be the magnitude of these vibrations ?

Tiny, but enough to cause chatter, wear tools and affect a finishing pass I think?

[D.C.]

You have got it wrong here.
As it happens that i make various musical instruments and trying to make some from steel, all my designs of machines are made so that sound is canceled/ no vibration.

Thanks Boyan, do you have any links I can read for how to design to minimize vibration?
I don't have access to academic papers at the moment and 95% of the interesting stuff on google scholar is behind a paywall. :(

[Robin Hewitt]

To counter sympathetic vibration think stub and unequal helix tooling.
Obviously nobody can talk you out of this so I say go for it, build this monster and be sure to tell us all about it

[Sven]

There are a few concepts and definitions in this thread that in my mind make the discussion a bit clouded, as it often does when it comes up.

Hope I can point all of it out, here it goes.

First and foremost, a cnc machine needs to be stiff (rigid). Everything else comes second.

It can be good if things are strong but it is very hard to build a machine that is stiff but not strong enough.

Then people often say it needs "dampening" and/or "weight" and then go on using either in the wrong way, in real life and in conversations.

If you add bitumen within the construction you will add "dampening" to the structure. It will make any vibrations die out quicker.
But there is a big risk if you add bitumen or similar within the construction.
Chances are you will reduce stiffness.
I am unsure if adding bitumen to a cnc construction is totally useless but I am sure it is extremely hard to do it so that it does dampen vibration without compromising stiffness.

If you add sand to a gantry you will also add dampening but as described above would be missing a great opportunity.

Pouring loose sand in a construction will indeed reduce vibration, in that it helps vibrations to die out.

However, if you add sand with epoxy mixed in, it will become part of the structure, adding stiffness and at the same time adding dynamic stability to the construction, making the amplitude (size) of the vibrations smaller, giving a better finish to the product.

[D.C.]

Obviously nobody can talk you out of this so I say go for it, build this monster and be sure to tell us all about it

Actually your comment about screwing up and still having a sheet of aluminium to sell for scrap did alter my thinking somewhat, don't give up on the subtle prods informing me that I'm an idiot, I might eventually get the message.

I've finished, clearing, cleaning, painting, realising that brushes don't work well on crapcrete blocks, spraying, spraying again, cleaning, etc ordering steel for a worktable and testing.

I'll post back when I've gotten round to ordering the other bits and testing some beams as best I can and then mull things over.

[Robin Hewitt]

don't give up on the subtle prods informing me that I'm an idiot

I'm good at that, maybe there is hope for me yet