Thread: Vacuum bed general questions
Good afternoon all, this is my first post, so please be gentle as I ask what must be obvious questions.
I am totally new to this world of CNC wood routing and am a bit confused by the use of a vacuum bed to hold material in place while cutting, the principle is obvious, but what are the minimum sizes of, for example, plywood, that can be held securely and safely with this system?
Also, if I cut through a piece of plywood, for a hole or an outline cut, how does the vacuum be maintained? what happens to any sawdust / cuttings which are produced? do they get sucked into the table?
The pictures appear to show a compressible junction to make the seal to the wood, how does the thickness of this be accounted for when milling totally through a pieces of plywood? surely it must be variable according to the air pressure? and therefore alter the height of the work piece? or have I just not got the idea correct at all??
As I will be mainly machining plywood and board hardwood into sizes ranging from 100mm square up to 300 x 900, is it a viable way for me to secure my work? is it worth the cost to buy one, I have been quoted £500 for a 600 x 900, or the time to make one?
Sorry for what must appear basic questions.
Good vacuum for every situation is very difficult to achieve without large and expensive high volume/pressure Vacuum pumps.
If the surface area is fairly large and material not porus plus you don't cut thru then even an ordinary vacuum cleaner with relatively low vacuum pressure but higher volume of vacuum can work ok good.
But if the parts are small then you need high vaccum pressure and if these parts are cut thru then you need high volume and high pressure to stop them breaking loose.
I've experimented with vaccum and really the only way to cut small parts is to use pods or jigs. Because of the small surface area you can't afford any leakage or cutting thru so making jigs or pods which can be placed so they don't get cut thru works ok and with relatively low pressure/volume provided non porus. The more porus the more volume and pressure required.
How the holes in the Vac bed are designed also makes a difference. I found small holes with larger counter bore gave better holding than just straight holes. The counter bore seemed to give a high pressure pocket that held better on small parts.
Also placing anti slip mat like used for routing between material and bed helped increase seal on larger areas so much so that even with normal Henry type vacuum cleaner I could cut thru more without part breaking loose.
I did a test on small DIY Vac bed made from board with formica laminate so well sealed with 25mm matrix of holes using Henry vacuum. On this was placed material with same matrix of holes but covered with rubber sheet to form full seal. The rubber seal was then adjusted so holes became uncovered to see how many before material broke loose.
Test was done both with mat between material and without.
Without the mat it only took 5-6 holes before material started loosen but with mat between then 20-25 holes could be uncovered.
My findings and conclusion where that for large non/low porus material with minimal breakthru then Vac works fine. For more porus or smaller material then you need Jigs/pods or some rubber gasket type seal and decent volume and pressure.
If you don't cut thru and material is non porus Ie: Brass etc then even a normal vacuum works. I've engraved Brass using this method no problem on a vertical standing machine so it can work. This was using a Fridge compressor and Fire extingusher size tank for volume.
So it can work but not for everything and the smaller the more difficult without special measures.!!
Last edited by JAZZCNC; 20-10-2014 at 09:06 PM.
Thanks for a very detailed response Jazz, much obliged.
Still unsure about how the compressibility of the gasket material affects the work piece height though.
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