Thread: Designing a better bed
The standard approach to a moving gantry CNC bed seems to be a thick piece of aluminium preferably 20mm or more.
Aluminium is expensive, the cost for my bed in aluminium plate would be £338.
With that budget in mind I've been trying to design something better for less money in materials, assuming that I work for free.
Lots of head scratching and reading up on torsion boxes, stressed skin panels, constrained layer damping etc etc This is where I am at now.
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 viscoelastic wonder material has another name - pond liner. 0.75mm pure butyl pond liner is about £7 per square metre.
So all of the reinforcing will be wrapped in pond liner, sorry, (viscoelastic damping membrane.)
After that the bed will be filled with carbonfibre reinforced epoxy quartz and vibrated, the top couple of mm will be pure epoxy so that it self levels.
The quartz I can get from http://resinbondedaggregates.com/ they wash, grade and kiln dry quartz specifically for resin work. About 20p per kilo.
The epoxy is IN2 infusion resin with a slow setting hardener, £72 for 5kg from easycomposites.
The carbonfibre is £47 for 2kg from easycomposites.
So the original budget of £338 for 20mm aluminium now breaks down to
£90 in steel
£10 in pond liner
£140 in epoxy
£20 in milled carbon
£25 in aggregate
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?
Constrained layer damping sheets need an outer layer of thin steel bonded to them to make them work. I don't think you'll get much from just wrapping something in the butyl material.
They work well on large thin sheet materials such as air conditioning ducting panels etc., but you'll not get much if anything on small beams. If you do it will be at very high frequencies where the deflections are tiny, so of no practical benefit for what you need. Also by the time you flood this with the epoxy quartz that is where the benefits will come from.
I wonder if you'd be better off with a couple of 80x80 steel RHS beams across the width, filled internally with sand, then fill the whole area with sand and topped off with a 10mm ecocast aluminium plate tapped with a matrix of M8 holes . . .?
What size is your bed to be DC?
Please read post #165about my thoughts and price comparison on most available materials
There is no doubt for me UPN140 is the cheapest , strongest, stiffest... It is almost mounted on my machine and i confirm that its brutally strong and was a great idea.
basically for your bed it will cost 100 euro or less , here in Spain / 100kg ~16kg/m /
The Following User Says Thank You to Boyan Silyavski For This Useful Post:
Designs for any machine beds come in two classes :
(a) Those which are set up in place once only and can remain in place for service life of machine .
(b) Those which have to be more regularly re-sited .
Considering only (a) pro tem the cheapest and most rigid bed is usually a deep cast concrete block with all the precision machine components bolted down to it .
Composite construction for machine beds is usually non preferred .
Your proposed design would be better with much smaller section reinforcing rods with grip into the epoxy mix . Common ragged reinforcing rods would be ok .
Personally if I wanted to use epoxy concrete construction I would just cast a solid slab of sufficient depth but with no reinforcing at all .
There are other types of machine bed :
(a) Based on (eg) quarry slabs of rigid slate .
(Think billiard tables and DMM's)
Granite would be super excellent but probably be too expensive for your purpose .
Consideration must always be given to how machine bed sits on the workshop floor .
Concrete block cast in place on adequate sub surface is usually ok anyway but any slab type bed mounted (relatively) loose on floor needs mountings which do not distort bed and which usually need to allow for some levelling .
Mountings in any case have to sit on floor and sub surface of sufficient rigidity .
The Following User Says Thank You to Robin Hewitt For This Useful Post:
Always worth trying a bit of lateral thinking on jobs like this .
Why do you need a bed at all ??
If , just for example , work envisaged was routing out shapes from large sheets of plywood a moving gantry machine could be designed where the main slide rails were bolted direct to the workshop floor and the sheet of plywood just mounted on a few packing pieces also bolted direct to the workshop floor .
Floor would need to be rigid but any normal concrete floor of reasonable thickness would be ok . Slide rails could be levelled on packing if needs be .
This set up would not do for precision engineering but for simple routing out it would do all that was nescessary .
Improve the set up by levelling the slide rails more accurately and installing a grid of accurately levelled embedded work holding positions and very good work could then be done .
Purely for interest :
Some engineering works that I visted years ago had jigging floors specially installed . These were large floor areas which were trued and levelled and provided with large numbers of fixing down positions . Used for trial setting up of large machinery and for all manner of ad hoc machining of large components . Sometimes jigging floor was provided with a pit for turning flywheels and a moveable compound slide .
DC the reason I asked about the size of your bed was your comment about 20mm aluminium being some kind of standard approach, surely the thickness of the bed would be proportional to the size? e.g. a 20mm plate would be overkill in 400 x 400 but not good enough for a 4m bed. I'm considering options for a 800 x 600 bed and one thought is to make a grid, with around 75mm spacing and top it off with a full 800 x 600 drilled and tapped plate. This suits me since I have loads of 15mm plate, maybe a similar approach could work for you?
Tried to edit and failed, ignore this one, read the next.
Last edited by D.C.; 15-07-2015 at 11:52 PM.
By Jazmal in forum Gantry/Router Machines & BuildingReplies: 5Last Post: 09-01-2015, 04:52 PM
By Kevman in forum Electronic Project BuildingReplies: 18Last Post: 14-04-2013, 05:43 PM
By D-man in forum Computer SoftwareReplies: 3Last Post: 04-11-2010, 09:49 PM
By psiron in forum Gantry/Router Machines & BuildingReplies: 13Last Post: 05-07-2010, 08:52 PM