Advantages of a Bleeder / Spoil board with a Vacuum hold down system?

[jlawren6]

I'm trying to get up to speed with my PRT96 and have been experimenting with vacuum hold down. To describe what I have setup and make sure I'm using the correct terminology, I have following:

Support board - 3/4" MDF bolted directly to the steel table
Plenum board - 3/4" MDF, glued to the support board, surfaced, an 8 Zone vacuum grid cut into it, and sealed with several coats of poly.

Each vacuum zone is plumbed together with 2" PVC pipe and powered by a single Fein Turbo III vacuum. For now, I isolate the individual zones with rubber bathtub plugs at each port. This initial setup was intended to be experimental and I did not want to lay out the cash for 8 valves until I was happy with what I had. I'm pretty happy with the plugs so far though.

I've seen that several others use bleeder/spoil boards with their setups, so I thought I would give that a try. I finally located a local source for 1/2" MDF and plan to use that. Based on what I've seen here, I plan to surface both sides of it to arrive at a final thickness between 0.3" and 0.4". In thinking about how to use and maintain this board, however, I'm struggling with what exactly its advantages are over laying to material to be cut directly on the plenum board. This is what I have been doing so far and it's worked well. Obviously, I've been careful not to cut into the plenum board to avoid either marring its surface and vacuum capability or to lose vacuum on the pieces I'm cutting. I initially thought adding a bleeder/spoil board would allow me to cut all the way through the pieces I'm cutting and lightly into it and save worrying about messing up my plenum board.. It still seems though that you would have the same issues with lost vacuum on the parts being cut and the cuts into the bleeder/spoil board would create paths for vacuum loses on the next project. A sacrificial mask seems like the only solution that would get around these issues.

So, what am I missing? How do those of you that use bleeder/spoil boards use them and address these issues? Do you cut through your parts into them? If so, do you resurface them? How often? For a Fein vacuum based system, what would be a practical minimum thickness for this board?

Looking forward to your input.

Thanks,

Jon

[joel_schuman]

A bleeder/spoil board on top of the plenum solves several issues for me. YES, the spoil board allows me to cut all the way through parts without scarring the plenum and without losing vacuum. For many materials I often cut lower than the workpiece, especially when the good side is down and I'm using an upcut or compression bit. If you don't have a bleeder/spoil board you would, as you say, lose all or most suck if you cut through the workpiece. But a proper bleeder board provides enough resistance that there usually isn't enough loss to affect hold down. From job to job the spoil board "ghost tracks" may indeed affect hold down power. You can test by pushing the edge of the workpiece - after a short while you'll know by feel when there's not enough suck.

There are other issues. Even if you seal the MDF support and plenum carefully, they will swell variably across the surface. Without a spoil board that you surface often, how can you deal with this?

I have the older 6HP vacuum pump ShopBot sold a few years ago. I use 3/4" REGULAR MDF spoilboard, as I have found UltraLight too permeable for my usage. I skin both sides initially, taking as little off as possible. To resist the spoilboard's tendency to curl, I stick it to the plenum with a very thin bead of silicone adhesive/caulk. When replacing, I scrape off the old silicone with a razor blade and lightly wax the edge of the plenum. I surface with a 1.5" mortising bit ($15 from Grizzly) at 3.5ips (chews up the bit pretty fast, but time is money). I probably resurface 2-3 times per week, depending on what kind of jobs are coming my way. Fewer if it's long runs of panel work, more if it's lots of small jobs and small parts. I routinely resurface 1st thing Monday, unless I am continuing from Friday on a job that isn't depth-sensitive. Two days off almost always makes a difference in table planarity.

Hope this helps.

[beacon14]

It seems that most folks who use a bleeder/spoil board have "real" vacuum pumps; does anyone use a bleeder board with a shopvac or Fein vacuum?

[richards]

I use a bleeder board with my Fein vacuum. It works very well on larger pieces. Normally, I cut almost through, leaving 0.030 or so; however, cutting through the material does not diminish the vacuum hold down. Two days ago I cut 24 parts per 4x8 MDF, cutting clear through but using tabs to hold everything together. Everything worked fine.

Before using the bleeder board, I constantly had problems, especially when cutting particle board. If I left just enough material so that I could easily break the parts apart, the suction would often suck pieces right out of the particle board , which would release all vacuum. The bleeder board solved that problem.

For the bleeder board, I use 1/2-inch MDF surfaced both sides. It is sealed to the plenum board with silicon so that it can be replaced after it becomes too thin from repeated surfacing.

[btk]

I have been using a piece of PVC as Plenum board.
I wanted to make sure that I did not have any air excaping through the bottom of the sheet.
I use 3/4" low density fibreboard (LDF) for spoil board planned on each side.

Brian

[jlawren6]

Thanks to all for the info and tips. That was what I was hoping to hear. Do all of you use the bleeder board in combination with a zoned system? If so, how do these work together? Is it enough to just shut off the zones that are not in the target area or do you have to use masking in combination with that? In other words, how far out from the perimeter of a zone might you expect the vacuum to be effective?

Given the suggestions on attaching the bleeder board with silicon adhesive, it looks like I'm going to have to abandon the plugs and rework my plumbing to incorporate valves for each zone. It's either that or break the bleeder board up into 8 seperate pieces (sealing the cut edges) so that I can access the plugs as needed. Seems like this would lead to a lot of alignment/flatness trouble between zones though.

One last question, how thin can the bleeder board be and still be effective? From what I've read, each system has a specific thickness range that it likes to work in. I'll probably just have to experiment with this, but an idea on starting parameters would be helpful. Thanks again.

Jon

[btk]

When attaching the bleeder board with silicone, run the vacuum to hold down into place while silicone is setting.

I also put a few screws in from the bottom.

I used the cutting file from Shopbot for 4-Zone system. I change bleeder board around 3/8".

Brian

[richards]

I change the bleeder board at about 3/16 inch. Silicone without screws works good for me. (I'm a little leary of hitting a screw. It only happened once, but that ten-cent screw destroyed a $60 cutter.)

The four-zone system from Shopbot works great for me. I usually turn off the quadrants that I'm not using, but things still worked fine on those times that I 'forgot' to shut the valves.

[joel_schuman]

Masking isn't necessary for me when cutting big parts, but can be critical when I open up a lot of the spoilboard with small parts or lots of thru cuts.

The suck does not extend past zones border to ANY appreciable extent.

I have a 60x120 table, and the 4 quadrant zones that ShopBot suggested did NOT work for me. Since I cut a lot of 60x60 Baltic birch plywood as well as 48x96 stuff, I made 4 linear zones, each with x=30 and y=60. So a 60x60 aligns on a zone boundary. For 48x96, I use three zones with a 12" strip of 1/4" melamine on hardboard, which provides a GREAT seal. The un-sucked 6" hasn't caused any problems yet.

I advise against breaking up your spoil board. The edge alignment issues will be a big hassle. The valves will pay for themselves in short order.

I use my spoilboard down to about .25-.3". For me the issue isn't vacuum. When the board gets too thin it won't stay put. It bulges and bubbles unpredictably, which makes skin-cutting too risky.

[jlawren6]

Thanks again for the info. That gives me a good place to start and should save me some frustration. Having just got back from attending my first Shopbot camp, I have a long list of things that I want to try. Getting this setup and installing E-chain and some squaring stops/adjusters are on top of list though. I appreciate the help.