Leveling my vac board and other spoilboard questions

[bahed]

I want to level my table. I bought it a couple of months ago and after setting it up and cutting a few files, I noticed it could use it - not grossly, just a definite fine tune up.

I have a nice 1-1/4 bit to use but I am concerened with leveling it because it has been leveled before and the hold screws around the perimeter of the MDF are no longered seated very deeply and I may not have enough screwhead clearance for another leveling effort. Should I take the screws out and try and seat them deeper ?

I also am curious about the spoilboard thing. I have 4 sub-areas cut into my 4'x8' vacuum board that are plenum-ed separately - see the attached picture - and I have a Fein III vac that works very good on each of the sub areas separately, or for the entire area.

I am trying to understand some of the threads where they talk about putting a spoilboard on top of the vacuum board, leveling that board and then seal the sides. Can someone please explain it to this newbie ? I find it hard to believe that my project material would be held down by my Fein III pulling through 3/4" MDF - as well as the MDF!!

I am trying to have the best set up for my system that I can realistically attain !!

Thanks


3353

[patricktoomey]

Bret, I shared your skepticism over pulling air through MDF until I saw it work for the first time. It's amazing and it definitely does work. I have a very similar setup except that I use 2 vacs. I use Trupan Light which tends to let more air through than most other types of heavier MDF. Also, the amount of airflow will vary by brand and even sheet to sheet. I start with 1/2" or surface 3/4" down to 1/2" to increase airflow which helps to hold down warped sheets of plywood that I sometimes have to cut.

I don't have any seal around the edges of mine, I just drop a sheet of MDF on the plenum board and go to town. I have been able to increase holding power by putting several coats of shelac on the plenum board and also on the edges of the spoilboard. This prevents the air from leaking through the bottom side of the plenum board or through the edges of the spoilboard which can really reduce the effectiveness.

I can generally pull 4-5 inches of mercury with my system which equates to about 17% of a perfect vacuum and results in roughly 2.3 lbs per square inch of down pressure at sea level. So for a 1 square foot piece of wood being held I could possibly be putting over 300 lbs of down pressure on it. Of course there are other variables and nothing is perfect but my tests show that most of time this is about what I'm achieving. If I try to cut anything much smaller than 1 square foot it will move on me unless I do more shallow passes or use down spiral cutters.

One of the biggest benefits to using the spoilboard is that it "transmits" most of the vacuum pressure while restricting rapid airflow. This means that you can get enough airflow to pull down a warped sheet and yet once it's stuck on there you can cut all the way through your parts without losing vacuum. I cut cabinet parts and I cut all the way through (plus 0.015 into the spoilboard) and it's fantastic, no tabs, no skins, just turn off the vac and remove your completed parts.

There have been many threads on this forum covering this issue and I would recommend doing a search for "vacuum".

[bahed]

That makes sense. Ok, now I am armed with a little bit of knowledge - a dangerous thing I know !!

How about using a lower dense material like particle board and sealing the edges and glueing it down on top of the vacuum board ? Too porous ? Or is it too susceptible to humidity ? I will be looking around the KC metro area for Trupan and perhaps MDX (Brady recommendation from an older thread), and anything else I learn about !!

How often do you level your table, and do you put something on it prior to leveling so that you know that every part of it has been touched with the leveling bit ?

Did you glue the spoilboard onto your vac table ?

I only have one Fein III, and I don't plan on buying another anytime soon. I have a Shop Vac that I could also use, I suppose, since I have 4 pipes to hook up vac to. I'm hoping to come up with something with my limited amount of vacuum power !!

[richards]

As Patrick suggested, you might try sealing your plenum board with shellac to help direct the air flow. Once you have done that, put a strip of AllStar adhesive tape around each zone to create a seal between the plenum and the spoil board. I'm currently using a 3/4-inch thick sheet of MDF for my spoil board (but I'm using Brady Watson's combination plenum/spoil board design). In the past I've mostly used a 1/2-inch sheet of MDF as the spoil board, since Trupan is not available in this area. I have two Fein III vacuums, but usually only have one hooked up.

There are lots of different opinions about surfacing a spoil board, but I just used Shopbot's directions to start and then created a custom file to suit my needs. In any case, I usually take off about 0.0125 to 0.025 inches per side. Depending on the flatness of your table/plemum assembly, you may have to take off less to get a flat spoilboard. You will need to surface both sides to aid air flow. As the spoilboard gets nicked up, you'll need to resurface as necessary.

Particle board and other non MDF/Trupan material might not work because of the construction techniques used in manufacturings other materials. MDF is porous while particle board is not.

Do not glue the spoil board to the plenum. Glue would act as a sealer and keep air from flowing through the spoil board.

Limited vacuum power is not really a problem. Unless I'm cutting large parts, I normally leave tabs or leave a skin on the parts being cut. A skin just 0.030 inch thick normally holds everything together and is easy to remove with a laminate trimmer.

[dhunt]

Use VARNISH to seal your vacuum plenum board.
In the picture it looks dry as a bone: varnish it.

And when you place the MDF spoliboard on top of your (dried) vac.plenum board, varnish the 3/4" edges of the spoilboard,too!

Sealing with varnish keeps the vacuum from leaking out in areas where it'll do no-one any good, thus directing it to the flat working surface of your MDF spoilboard.

[bahed]

I guess I need to find out more about Brady Watson's combination plenum/spoil board design.

I'd also like to hear about pros and cons with gluing the spoil board directly on top of the plenum board vs. using the AllStar foam adhesive.

Is the edge seal effectively complete using shellac ? I always have a reasonably fresh batch of that around so I would be inclined to use that to seal as well as to glue the 2 boards together and just periodically level the spilbloard down to where you need to slap another one on top.

[dhunt]

Try to get aluminium or plastic screws, to screw your spoilboard down with.

Recess holes into your MDF spoilboard
(1/4 inch down - use a 1/4 in. Forstner bit?)
and seat your alum. or plastic screws down in there,
so you don't nick their heads.