vacuum table question

Hi all,

I'm setting up a new vacuum table and was wondering your opinion(s).

I have a 5x12 PRT and want to be able to vac-down anything from small parts all the way up to the 5x10 sheets of mdf.

Are there any drawbacks to having more zones than fewer?

My idea is to have 25 zones instead of the normal 4 or 8. I would make a bunch of approx. 12"x24" tile-like zones with my smaller CNC and then bolt them down onto the bed of the PRT, and then do a thin surfacing run and go from there.

alrighty, lemme know what you think!

-JAMES

I've been running 6 years on a 5x8. I have 5 zones, and cannot imagine using/needing more. However, depends on what you will be making.
For me, furniture parts are done on a non-vac region where solids are clamped/screwed down.
Cabinetry is typically 4x8 sheets = wide open on primary 4 zones 95% of the time.
Partial sheets are when the gates get used. If I have a sheet a bit off size, open more zone and use other scraps to mask around the work piece.

My first vac table attempt was running 80/20 aluminum channel, splitting the table into 6 zones and using pallets in between the aluminum extrusion grids. Sounded and looked like a great idea based on examples shown here. Ultimately, I scrapped the whole thing and ran a solid sheet of ultralight mdf "trupan" with zone division taking place in the manifold underneath. Life got way easier and so did the accuracy of my work.

I cringe at the thought of 25 zones. A very complex project that I don't mind saying will almost certainly cause more problems than they're worth.
I'm sure the common response is going to be, what exactly are you planning to produce on this machine, as that really is the ultimate question before the best guidance can be given.

Jeff

it sounds like a nightmare. just imagine how you would plumb it? Plus I don't think suck a small area will hold well sucking through a spoil board. what are you trying to hold in place? there may be better ways to go about this.

hi Jeff,

Thanks for the input. I did have an 8-zone configuration drawn up so what you said makes a lot of sense. The 12x24 grid just matched up nicely with the 5x10 cutting area so I started to get a little carried away with the idea of focusing only on the exact zones I needed. But what you say is true, you can just mask off the areas that you're not needing at the moment.

In terms of production, I fabricate components for artists and small design companies.. so I'm sort of planning for the most flexibility.

I appreciate the feedback, I find with a lot of these CNC projects, it's REALLY easy to get out into the weeds.

Thanks!
-james

hi Knight_toolworks,

the nightmare would only be when it came down to making the manifold, but I think Jeff (above) was right, in minimizing the number of zones for the most common tasks. Problem with my work is that it's totally random in what people ask me to produce so I just wanted to check with y'all and see what better reasoning there was out there.

As for the small area 12x24 zone, I made a test panel and hooked it up to my FEIN and was able to lift a half sheet of 3/4 melamine just fine, and took a lot of force to move when i pushed it laterally.

I haven't yet tested pulling through a skimmed board.

thanks for the feedback / input!
~ james

I think for small areas your going to need either a high vac setup or some other method.

Here's a pic of 8-zones ... instead of 25

Attachment 31117

Quote:

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Originally Posted by knight_toolworks

I think for small areas your going to need either a high vac setup or some other method.

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Yeah this is actually my first foray into vacuum hold-down. I've made a lot of fixtures in the past which worked out great. Still wrapping my head around the vac-system.

thx again!
-J

Remember a couple rules of thumb...
What size are your small pieces? Remember there will be little or no vacuum 1 1/4 to 1 1/2 " inches from the edge. Subtract a 1 1/2" borders area from the total area of the part. Apply the rest of the area in square inches times your max vacuum /2 (for psi) and multiply it by .6 (60% for cut slots). If that number is less than 50, it probably wont hold, period. Between 50 and 75, it could hold with special attention. 75 to 100, maybe and over 100 most likely

Quote:

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Originally Posted by Gary Campbell

Remember a couple rules of thumb...
What size are your small pieces? Remember there will be little or no vacuum 1 1/4 to 1 1/2 " inches from the edge. Subtract a 1 1/2" borders area from the total area of the part. Apply the rest of the area in square inches times your max vacuum /2 (for psi) and multiply it by .6 (60% for cut slots). If that number is less than 50, it probably wont hold, period. Between 50 and 75, it could hold with special attention. 75 to 100, maybe and over 100 most likely

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Hi Gary,

Can you clarify for me a bit on the math? I've never seen anyone write it out so this is very helpful.

I made a test vac-waffle and tried it out with my FEIN T2 and it held pretty good.

For the math problem: The vac-waffle-board is 12.2" x 24.2", offset inwards (as you mentioned) by 1.5" which gives me 9.2" x 21.2" holding area.

Let's assume the part is the same size as the whole waffle-board. (<-- a highly technical term i just made up! )

The vacuum pulls 90inch of water or 6.6hg ( which do you use for your calculations?)

The slots are only 1/8" on this test piece. Do you have a recommended kerf width?

Is that enough information to finish the math? Am I missing something?


thanks for your help!!!

Attachment 31118

Try not to reinvent the wheel...Size your zones to the size panels you mostly cut. I would do 7 Zones:

1) 24x24"
2) 24x24"
3) 24x48
4) 48x48
5) 12x96
6) 24x60
7) 24x60

This combination will give you the ability to hold down a 2x2', 2x4, 4x4, 4x8, 5x8, 5x10, 5x12 by just turning zones on or off.

6.6 Hg" isn't a lot to work with. You would need a min of 4 Fein vacs to do anything for real and probably more like 6. Lighthouse motors are another option & cheaper.

Small parts with the Fein? I wouldn't count on it unless you are an expert on vac hold down...However there are ways around this. Look for my 'Vacuum Film Technique' on this board.

Remember - there is no replacement for displacement. The aspiration here is limp along using the Feins and other 110v solutions until you can afford a real vacuum pump made for production. This was the idea behind the Lighthouse vacuums - just a bandaid...so keep that in mind. There are no 110v (or 220v single phase) solutions out there made for production duty...so expect whatever you are running to die. Not to be all doom and gloom...but I call a rose a rose.


Knuckle Dragger's Guide to Vacuum Math:

In an ideal world, you would get 100% of what your vacuum was capable of delivering and have zero leakage from your setup. In the real world, this never happens...so trim your numbers off a bit.

The entire weight of the atmosphere is holding down your parts when you apply a perfect vacuum to it - In this case, 29.92 inches of mercury (Hg") OR 14.7 pounds per square inch (psi). These two are equal. If you live up on a mountain, you'll have less pressing down. In Death Valley, you'll have more.

You need to ask yourself if the surface area of your part at a given vacuum value results in enough pounds per square inch of hold down to resist the cutting forces of the router. If it isn't you can cheat by selecting a different tooling profile (downcut, straight) or by cutting with a lighter chipload, with more RPM and/or slower speeds.

If a 12" square is to be cut out with a total available vacuum of 6 Hg" (we'll take off 10%, very optimistically) The calculation is as follows:

12x12 = 144 square inches

6 / 29.92 Hg" = .200 X 1 (14.7/29.92 [these are equal so they represent 1 as a conversion factor to psi]) = 2.94 psi

Total hold down on part = psi value x square inches

144 x 2.94 = 423 total pounds of down force

As the part approaches 0x0" in size, the law of diminishing return comes to play. You aren't going to hold a 1" square down with 2.94 psi - so there comes a point where vacuum just isn't the right tool for the job, or you need to combine methods to make hold down reliable. Furthermore, you need to account for when your cutter breaks through the sealed off material and exposes the cutting kerf where it meets the bleeder board. The surface area of all kerfs can add up quickly and your part hold down will fail in the middle of the job. Gotta small vac? Program for turning on/off zones as you machine, when it is practical. It sucks...but take a portion of your profits and save for a pump. It's a nice luxury when you've been doing this stuff for a while.

Good luck - search this forum. I've dispensed countless tips and tricks involving vacuum over the years and so have others.

-B

J...
Use the 6.6 (inHg) number

Hi Brady,

Wow! very comprehensive! Many thanks!

Everything you mentioned looks to be about what I have discovered so far, and I agree about the zones..

I actually bought this ShopBot PRT 5x12 from a guy who also gave me a 15hp FPZ, but the problem is what you mentioned.. it's 3phase and I only got me some 220v.

Right now I'm looking at my options... The specs for this FPZ blower show that it will actually work with anything between 7.5hp and 20hp so I might look into just swapping out the electric motor and getting my machinist guy to make some kind of adapter plate ( if necessary )

The lighthouse motors is an option that I've considered also.

The Fein that I tested with here is just because it was sitting here this morning and I wanted to try it out. About how long do you think they last?


Thanks for the math tutorial, that was really helpful.

I'll update as things progress.
-JAMES

Attachment 31119

You have a 15hp FPZ, best option for single phase is to just get a VFD to run it. I think I paid around $900 for mine.

For small parts I use a 1" thick piece of MDO with t-tracks. The MDO is cut to 48"x48". Fits over two of the zones in my table. The vacuum holds the MDO, clamps hold the material. Works really well and I kept my 5x8 PRS to 7 zones. 5 of the zones are used to hold a full sheet of plywood. The other two zones are for 5x5 BB and large slabs and blanks that I surface from time to time.

Quote:

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Originally Posted by JMCS

You have a 15hp FPZ, best option for single phase is to just get a VFD to run it. I think I paid around $900 for mine.

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Hi Jeff,

Can you fwd me the specs on the VFD you have? It would be awesome if I could run this FPZ relatively un-adulterated.

all my best,
JAMES

James I just sent you a PM for a convertor Mark Stief

Yaskawa V1000 VU2A0069FAA be sure to double check. All VFD's can run on single phase, they just have to be de-rated for there amp usage.

Quote:

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Originally Posted by JMCS

Yaskawa V1000 VU2A0069FAA be sure to double check. All VFD's can run on single phase, they just have to be de-rated for there amp usage.

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hi Jeff,

This looks like a suitable option. How many amps is the circuit breaker you're using on this? ( I don't want to blow anything up! )

-james

I think 65amps