Improving suction on a vacuum table

I have a 5'x12' vacuum table for my CNC router that I generally keep topped with a 4'x8' spoilboard for sheet cutting. I am cutting some 2" wide rails and have been playing with the table to try to get better suction so these pieces don't move. I have made changes that I think have helped, but it's hard to verify if one change made the difference or another so I thought I would ask for advice here.

First, while I understand that suction applied to a piece is a function of the vacuum power and the surface area of the piece, is there a correlation with the size of my spoilboard and the suction? The way I have it figured, I will get twice as much suction from a 2'x8' spoilboard as I would from a 4'x8' spoilboard because there is [roughly] half as much surface area for the air to pull through. Is this accurate? The guys upstairs are telling me that the table has the same suction whether I have a 1'x1' spoilboard or a 5'x12' spoilboard and I just can't see this being true.

Second, if I were to drill a 10mm hole through my spoilboard directly under where a narrow piece will be cut, will this create a localized increase of vacuum suction at that one spot and help hold the narrow piece in place? My thought is that the hole will provide less resistance than the MDF spoilboard and will create an increased point of suction directly beneath the narrow piece I wish to keep in place while also maintaining the same diffuse suction throughout the board and across the face of the piece. Or does it make no difference because the contact surface area of the narrow piece remains the same?

I would really like to have more control over my vacuum table so any help or advice anyone can provide will be greatly appreciated.

Thanks,
Parker

Are you sealing off the un used areas?

Yes I am.

Parker,

I've found the following of hepl to improve overall hold down on my 4x8 spoilboard.
1) I use 1/2" MDF vs. 3/4" to reduce the resistance through the material
2) I epoxy coat all edges with 2 coats of expoxy paint to create a airtight seal that wont pull off during production
3) I surface both sides of the MDF spoilboard to remove the was film which increases the vacuum transfer.
4) For a piece 80 sq. inches or smaller, I perimeter rout in 2 passes with the last pass less than a 1/16" (preferrably 1/32") remaining for the final pass to cut the piece free without applying any x/y pressure on the part.

I also do believe that the further your part is away from the vacuum port in the table, the less draw it has. I Say this because it it typically the small parts at the perimeter edge of the sheet that ever cause a problem.

Hope this helps a bit.

Regards,

Scott Hill
fromscott2finish

You should get more CFM if sealing off unused areas.The major forces are side forces on the slippery spoil board.Some abrasive local gaskets will help with a drilled hole.Check...
http://www.allstaradhesives.com/ for some ideas.With a 5X10 router I assume you have a 20HP vacuum pump.
Larry

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First, while I understand that suction applied to a piece is a function of the vacuum power and the surface area of the piece, is there a correlation with the size of my spoilboard and the suction? The way I have it figured, I will get twice as much suction from a 2'x8' spoilboard as I would from a 4'x8' spoilboard because there is [roughly] half as much surface area for the air to pull through. Is this accurate?

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Yes, and No.

It really depends how much air is being draw through the vacuum pump(s). If they're are already puling their full capacity through a 4x8 sheet, then they can't pull any more through a 2x8 sheet. But, the 2x8 sheet may direct more suction over the total area of the spoilboard, which could get you more suction on your part.

Our Morbidelli 5x12 has a two vacuum pumps(not sure if they are 25HP each or 40 HP each), each with it's own gauge on the table. If I put a 2x8 sheet on the table, I'll get a certain amount of vacuum. If I put another 2x8 sheet, then I definitely get more suction. So I would say that yes, you'll get a b it more suction. But, you'll also get the same amount by just covering half the spoilboard.

My guess is that drilling a hole would make things worse. Without a seal, you'll probably lose more vacuum.


For a 2" part, you're best bet is probably to go to a dedicated pod system independent of the spoilboard.

We're nesting cabinet parts, cutting at about 800ipm, and are having a lot of trouble with smaller parts (4" wide). We started nesting our drawer parts as well, which gives us a lot of small parts, so for now we are just leaving a thin skin on the small parts and using a laminate trimmer to cut them away. It's much faster than having to re-cut parts.

I'm a newbie on this forum but contribute a lot to the ShopBot forum. My CNC is a 96x60 ShopBot PRS Alpha with 2.2hp spindle and air drill.

For the spoil board I use 1/2" Trupan (LDF). My table is divided into four zones with each zone having its own 220Vac vacuum motor.

Each zone has its own spoil board which has melamine edgebanding on all four sides and then glued down to the plenum. I can surface the spoil board right down to the plenum and then replace it with four new spoil boards.

My shop is at about 2500' altitude and I'm able to pull more then 7.5 inHg. I've been very pleased with the hold down capability.

First off, thanks to everyone who threw in their two cents. Much appreciated.

After reading the posts here and doing some research on my own, I finally have a handle on this suction issue. I have two vacuums in my table that pull 40 cmHG through a 4'x8' sheet of 11/16 mdf surfaced on both sides.

If I seal the edges of my spoilboard and prevent unnecessary air loss out the sides, I get about 42 cmHG.

If I cover 24 sqft of the spoilboard's total 32 sqft with non-porous material (I used laminated 1/4" mdf) I get about 50 cmHG on the remaining 8 sqft.

So in conclusion, yes, your suction increases if you seal the edges of your spoilboard and decrease its operable area by either gasketing underneath or by placing non-porous material on top, effectively sealing it.

As for the question about drilling holes through the spoilboard, I think it matters not. The suction doesn't seem to be increased at the hole, but the resistance to the air pulling an object (like a piece of paper) through the hole is decreased. There is no localized change in suction.

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Originally Posted by dlwoodworks

I'm a newbie on this forum but contribute a lot to the ShopBot forum. My CNC is a 96x60 ShopBot PRS Alpha with 2.2hp spindle and air drill.

For the spoil board I use 1/2" Trupan (LDF). My table is divided into four zones with each zone having its own 220Vac vacuum motor.

Each zone has its own spoil board which has melamine edgebanding on all four sides and then glued down to the plenum. I can surface the spoil board right down to the plenum and then replace it with four new spoil boards.

My shop is at about 2500' altitude and I'm able to pull more then 7.5 inHg. I've been very pleased with the hold down capability.

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Very cool. I'm looking at something exactly like this. which motors are these? Link?

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Originally Posted by FS2F

2) I epoxy coat all edges with 2 coats of expoxy paint to create a airtight seal that wont pull off during production

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Hi Scott,
Thank you for the list. I found it very informative. Similar to our process for prepping a new spoil board. The one thing that I have been having difficulty with is finding an effective and easy to apply substance for the edges of the MDF to reduce or negate vacuum leakage out the side of the MDF. we have been using surf board wax. A pain to apply and I don't think it covers the pores very effectively. I'd like to try your method of applying epoxy paint to the edges. Would you mind expounding on that process? for instance, what kind of epoxy paint do you use, how do you apply it? That sort of thing. I would greatly appreciate it. Thanks.

Hi Cormack - Epoxy is expensive for sealing purposes, white glue will do just as well. I'd do two or three coats on edges just smear it on with your finger....Peter

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Originally Posted by peteeng

Hi Cormack - Epoxy is expensive for sealing purposes, white glue will do just as well. I'd do two or three coats on edges just smear it on with your finger....Peter

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Excellent point and great suggestion! Thanks Peter.