[diyengineer]

I'm slowly approaching the point of where i need to start thinking of hold down solutions for my table. The table is already fully outfitted for a vacuum hold down system.there is EXACTLY 300 3/8" holes in the top in patterns (which comes out to a whopping 33 cubic inches THEN milled plastic panels go ontop that contain thousands of small holes that i assume add up to 33 cubic inches of flow, i would have to remake these because my old panels were gross and warped, so i tossed them) underneath it already has 40 1" fittings and even has (2) electric valves to control different sections of the table.

The only problem is...

I dont own a vacuum pump!


I can only imagine how much a vacuum pump/s would cost for a table this size. I mainly need to hold down 4'x12' .063" sheets of aluminum, but would love flexibility. I'm limited to a maximum height of 1.5" for my Z axis. I dont want to simply bolt a panel in every time. This would take forever.

I was thinking about using 8-10 small pneumatic pistons that would push on individual cams. The cam could hold a piece 0-1" in diameter against the table with a great amount of force. All the pneumatic pistons would be linked together VIA 2 electric solenoids. 1 solenoid would be the fill, 1 would be the dump and would release the cams (each might need a small spring to retract the cam.)

Mach 3 could control solid state relays that would lock the piece in place, and release the part when the cutting is fully done. then i would just have to Slide a piece in hit go, slide the piece out when its done. No bolting, leaning over the table, and unbolting a million bolts, losing hardware, etc. Very fast and efficient. More automated the better.

For pictures of the machine and specs look below, check bottom link for current pics of machine in huge build log.
Thank You to anyone who contributes!

Machine specs below:
Machine max dimensional footprint:
Width: 89.5" wide (2273.3mm)
Length: 178" long (4521.2mm)
Height to table: 38.75" (984.25mm)
Height to gantry: 46.5" (1181.1mm)
Height to top of Z axis: 62.5" (1587.5mm)

Overall MAX table dimensions:
X axis: 82.75" (2101.85mm)
Y axis:178" (4521.2mm)

Actual "Usable" MAX dimensions:
X axis: 67.25" (1708.15mm)
Y axis: 139.5" (3543.3mm)
Z axis: 1.5" (38.1mm)



full build thread here:
http://www.cnczone.com/forums/showthread.php?t=103291

[ger21]

I mainly need to hold down 4'x12' .063" sheets of aluminum

More important, how big will the parts you're cutting be? Will they still be attached to the sheet with tabs, or will they be cut completely free?

I'm assuming you'll need some type of spoilboard on top of your table?

[diyengineer]

More important, how big will the parts you're cutting be? Will they still be attached to the sheet with tabs, or will they be cut completely free?

I'm assuming you'll need some type of spoilboard on top of your table?

Yes i will probably use 1/8" or 1/4" black HDPE.

Parts are going to be fairly large, 20"x20" squares, 8"x20" etc. Sometimes 24x24 with a 12x12 square cut out of the middle.

Well i was thinking of cutting them all the way out. tabs would mean more finish work for me. (possibility i could leave them micro tabbed in if it made the process easier?)

[diyengineer]

Oh and it looks like it has 6 electric valves, so there must have been 6 zones. Regardless, pretty sure anything vacuum related is out of my financial league at this point on this table. A large enough rotary vane pump i assume probably costs thousands and thousands, and might yield the same performance as a simple automated holding technique?

[thewoodnerd]

Machine specs below:
Machine max dimensional footprint:
Width: 89.5" wide (2273.3mm)
Length: 178" long (4521.2mm)

And this is posted in the "Benchtop Machines" forum?

[mwood3]

And this is posted in the "Benchtop Machines" forum?

Good point, you may get better responses in the router forums

[Geof]

...... A large enough rotary vane pump i assume probably costs thousands and thousands....

Yes almost certainly thousands and thousands. But you can make a very serviceable vacuum pump from a thrashed out compressor:

http://www.cnczone.com/forums/showthread.php?t=53796

This one may be a bit small for your table but you should be able to find a large beat up compressor for not too much money.

Or, as I suggested in a different thread some time ago, get an old V8 engine and run one bank of cylinder under power and use the other bank as the vacuum pump. You could even convert it to natural gas.

EDIT: You should restart this thread in the Work Fixtures and Hold-Down Solutions forum.

[RomanLini]

Ok, if you are determined to do a mechanical solution (intsead of vacuum) then I would simplify as much as possible.

Imagine if you had a long cam shaft the length of the sheet, an inch above the sheet, supported in a few places. Then you just rotate the shaft and it presses directly down on the sheet in many places along it's length. The cam mechanisms can be extremely simple, all that would be needed is to make the entire shaft eccentric by a 1/4 of an inch, then have little blocks with holes along the length of the shaft. The blocks act basically as "conrods" so as the excentric shaft is rotated slightly they press down on the sheet.

Then the turning mechanism can be as cheap as a DC gearmotor, say a windscreen wiper motor from a wrecked car would suffice and work from your 12v relays.

An easier construction might be to rotate the conrods 90' so the shaft is moved laterally to operate the clamps, so now the shaft does not need to be round at all you could use square steel tube etc.

[diyengineer]

I like the idea of a single cam the length of each side, that really simplifies it A LOT! Along with it being made out of simple square tube! then i could just weld the cam fingers on. Very good ideas!!

Ok, if you are determined to do a mechanical solution (intsead of vacuum) then I would simplify as much as possible.

Imagine if you had a long cam shaft the length of the sheet, an inch above the sheet, supported in a few places. Then you just rotate the shaft and it presses directly down on the sheet in many places along it's length. The cam mechanisms can be extremely simple, all that would be needed is to make the entire shaft eccentric by a 1/4 of an inch, then have little blocks with holes along the length of the shaft. The blocks act basically as "conrods" so as the excentric shaft is rotated slightly they press down on the sheet.

Then the turning mechanism can be as cheap as a DC gearmotor, say a windscreen wiper motor from a wrecked car would suffice and work from your 12v relays.

An easier construction might be to rotate the conrods 90' so the shaft is moved laterally to operate the clamps, so now the shaft does not need to be round at all you could use square steel tube etc.

[diyengineer]

http://www.mcmaster.com/#standard-air-cylinders/=8wjw5f

Found the air cylinders. There pretty affordable. Single action sounds like what i want. Then i will only need 2 valves, one to fill and then one too dump the system.

I could fit a 1.31" outter bore cylinder, getting 105lb of force per cylinder.

How many pounds of force would it take to hold down a .063" 4'x12' sheet while being machined? You think maybe 2 cylinders on each side hooked up to multiple cams be ok? That would be 420 pounds of force on the outside spread out on probably 12 cams around the perimeter. It would be cool to make one super long cam that runs the entire length of each side. Then it would give me a lot more contact instead of individual cams. Anyways, i need an estimate on force haha.

[Geof]

Before you get carried away building the long piece of square tube with fingers do some experimenting. I have tried this approach and found that the fingers at the end get a good grip but the tube deflects so all the fingers in the middle do not make the same contact. In fact the firmer the end ones grip the less the middle ones grip.

Really the best approach for holding flat thin material is a vacuum table because the stock is held down over its whole area. When thin stock is clamped just around the edges the clamping itself tends to bow the material up in the middle and if the cutter has a helix this pulls it up even more. A reverse helix cutter can push it down but then chip clearance becomes a problem especially with aluminum.

In addition if you have a table that is very flat, something like less than +/-.005" variation, with vacuum it is possible to leave a thin 'shim' between the parts so the vacuum is not lost. Then the parts can be cut out of the sheet with a box cutter and the remants of the shim taken off with a hand deburring tool.

[diyengineer]

I would love to use a vacuum hold down solution, but currently i don't have a few grand laying around just for a vacuum pump. I'm sure it could be ghetto rigged with 4+ harbor freight dual piston compressor pumps, and a huge tank in vacuum, powered off a 20Horse gas engine brigs and stratton 4 stroke but i would rather save my time and the trouble and go with something a little more economical and automated until i get the money for a big pump.

As far as the single cam idea, yes it would most deff take some trial and error along with a lot engineering in solidworks. the long cam would have multiple equally spaced pistons acting on it, not just a single one on the end. I could also put a rubber foot on it so it actually grips too.

I like the thin shim idea, even if i don't go with a vacuum hold down right away i can still leave the pieces all attached to the sheet for a bit more rigidity so it wont flop around while other pieces are being cut, if i run into that issue.

[RomanLini]

I always use the "thin shim" idea when cutting plastics and aluminium, I just leave 0.1mm not cut out, so it supports all the small parts and also it stops chips etc getting under the sheet from vibration (and stops the helix pulling it up as Geof said). The parts just tear out and a touch of sanding takes off any shim remains.

As for the clamp bar bowing up, what I visualised was the clamp bar supported in a few places along its length. Say every foot or so with a support that lets the bar slide laterally but not bow. Each support just needs a bolt into the table.

Then little "conrods" on the bar so that maybe 1.5" of lateral bar movement produces 1/4" of vertical movement of the conrods. That's a heap of easy leverage already in the design so the device pushing the bar laterally doesn't need that much force.

As for the amount of force holding 0.063" aluminium sheet down it would be surprisingly little, I've had some parts break out with thin sheet and was able to hold them down by hand and finish the cut. The big problem (like Geof said) is how you will manage individual parts that may break out, and are nowhere hear your clamps.

If you are going to leave a shim then just get a spoilboard and route some grooves in it, and use a household vacuum cleaner. They make about 2 psi vacuum so you'll easily get hundreds of pounds hold down and no worries about individual parts breaking loose. And you can always put a large vac pump on it later as cash allows.

[dblox]

Hi,

I regularly use conventional vacuum cleaners to hold polyethelyne foam in a milling machine.
They work great, but the largest area I've held down is 800mm by 400mm.
If you can get a good seal (I make custom fixtures for each part I need out of MDF), and are prepared to wreck a few parts while you work out speeds and feeds, I'd recommend this approach.

Houshold vacuums are not any good for work holding systems - I blew one up before someone a bit older then me explained that a household vac relies on the airflow through the hose to cool the motor.

A wet & dry vacuum (I think American terminology is "shop vac"?) has a separate airflow path to cool the motor, and you can leave it running for hours completely sealed off.

Just buy the cheapest you can find - I buy the $99 NZD ones on special at the local hardware store. One day I expect it to blow up, but I've probably had mine running for 100+ hours total without any problems.

[ger21]

Houshold vacuums are not any good for work holding systems - I blew one up before someone a bit older then me explained that a household vac relies on the airflow through the hose to cool the motor.

A wet & dry vacuum (I think American terminology is "shop vac"?) has a separate airflow path to cool the motor, and you can leave it running for hours completely sealed off.

Most "shop vacs" also need the airflow to cool the motor, although some do not. If you use one, make sure you know the difference, or it may not last long.

[johnd]

Can you mask off the unused areas of the table? This would be inclusive to your final part area or trim sections that you don't want to move. Since you are using vaccum or air pressure down to hold the part do you have sufficent CFM to remove air and hold the part with leakage. A vaccum pump with high inches is usally very poor on recovery. 14.6 " of forces is all you are going to get and you are going to need a large tank.
JohnD