A PVC Vacuum Table

As a relative newcomer to the ShopBot community, let me first say I am honored to be asked to join the likes of my fellow columnists. I am sure that all of you respect them, and their opinions, as much as I do.

Over the next few months I will try to use my woodworking and fabrication experience to hopefully expand your abilities and capabilities with your machine by adding a few “accessories” or by giving you some other construction options for things that many have taken for granted..

I was lucky enough to have hosted a Camp ShopBot in February of this year. There were a number of experienced, new, and future users that were a pleasure to meet. There was one common thread between the less experienced users that surprised me. While doing a short demo on vacuum hold down, I overheard, “Ooooooh, the vacuum goes THRU the Trupan!” This “revelation” by the newbie’s, coupled with a large number of emails that I have received since the camp, and a few suggestions from others, has prompted me to start out with a table construction article. Some of this will seem very basic to you “Old Hands”, but I hope it will help enlighten new users.


Bleeder Board: A material that is porous enough to allow vacuum to pass through it and hold down material being cut on a CNC table.

Plenum: A layer of material that has channels cut into it to allow even vacuum dispersal throughout a vacuum zone.

Spoil Board: A sacrificial (replaceable) layer that allows a CNC cutting tool to cut through material without damaging the table layers below. In many cases the spoil board and bleeder board will be the same.

Table Frame:

Table board:: The first layer in a CNC Table system. It forms the support for the subsequent layers.

Trupan: Specific brand of Ultra Light Weight MDF type material most preferred for bleeder/spoil boards due to its porosity. Also called ULDF or Ultralite MDF

Zone or Vacuum Zone: An area of a vacuum table that is used for vacuum hold down of parts being cut on a CNC router. These can be of any size or shape, usually based on the material to be held down.


You should use the time between the day that you order your machine and when it is delivered to design your table and hold down system. I started searching the ShopBot Forum for every article concerning table design and vacuum hold down going back well into 2005. There is a wealth of information there from many that are more experienced and learned than myself.

If you have the chance to go to a Camp or visit a ShopBotter near you, especially one that uses his or her machine similarly to what you plan for yours, do it! You will gain a lot of information by just looking at a machine in full scale.

It seems that very few of us use our machines in the same way or cut the same materials, nor are we exactly sure before we even have the Bot what or where we need to hold down, I made the decision to make a table that incorporates as many options as possible. This decision, of course, made the table more expensive and heavier. It has also made it very versatile, so that in 10 months we haven’t had to put any screws into the table surface, which was one of my goals.

Material selection…

First let me preface my material selection information with me telling you that we are a commercial shop. We plan to have fairly constant use of the machine. We also are looking for every bit of accuracy that the machine is able to deliver. We will often do an operation on material that may cost hundreds to a couple thousand dollars. If you are a hobbyist, you may want to use methods or a material that is easier to obtain or more budget friendly.

Due to the high humidity here in south Florida I decided to not use any wood products in the table bed or plenum. We have many table surfaces in our shop on various pieces of equipment. They are both shop made and purchased from reputable vendors. I have been using them in various forms since the mid 70’s, here is what I know: Plywood will always warp. Fiber or chipboards will always swell.

I chose Gray PVC sheet, type 1, solid. This material has many advantages over any of the wood products: It is available in over size sheets. (I purchased 2 of the 5 by 10 sheets for the table and 1 of the 4 by 8’s for the plenum, all ½” thick), the weight helps resists vibration, there is little or no dimensional change with heat or moisture, and it is relatively easy to machine. It will glue to itself and other materials well with carefully selected adhesives. I have only found 1 disadvantage, the price, at $350 per 5×10 sheet, $250 per 4×8, it is much more expensive than ply or fiber wood products, but I feel that it is worth it. I purchased the pvc sheet from Piedmont Plastics, but there are many plastic distributors in the US and Canada that may have a similar material.

If you choose a wood based product for your table here are a few tips:

If you use plywood try to use one with a non wood “coating” on BOTH sides. This could be in the form of a factory applied finish (catalyzed) or a phenolic coating such as MDO (medium density overlay, sign plywood), or backer or liner board, made for applying laminates. If using a fibrous product like particle board or MDF use one that is as dense as you can get and laminate it from multiple layers using an adhesive that is not water based. Melamine coated sheets would give you a heavy stable surface, but I have not found an adhesive that bonds it to itself or other materials well. Others may have.

Seal any raw sides and all edges BEFORE assembly, reseal after assembly and/or surfacing. Use a good quality epoxy or oil base product. Try to get it all sealed at one time. Find an adhesive that is compatible with the coating that you use. Laminating epoxy will work for both. You can also add a fibrous thickener when using it as an adhesive. I have never had any good results using any water based finish on sheet goods.

REMEMBER: The material you cut will only lie as flat as your table. This applies to how flat your table STAYS, as well as how flat you can build or surface it.

TO “T” or not to “T”…

This is a short description of how I use the T-track for hold down on my ShopBot Table. Hopefully, it will give you some ideas of how or if you want to add t-track to yours. After reading David Buchsbaum’s article on his vacuum table, I made the decision to add t track into my design. David has many good ideas that you can follow, or build on, as I did.

There are many vendors for t track out there. Most of the woodworking supply chains carry lighter duty versions in one form or another for jig making. I chose a heavy duty version from 8020 inc. with a single track, 1.5” wide by ¾” high. It is available in lengths up to 12’. They have many other sizes and styles. They also have an eBay store where they sell scratch and shorts.

Here are some of the reasons I chose to use T track:

  • Gives us an easy way to repetitively index parts at 0,0 or any of the other 3 zones.
  • Affords us many options to hold material both down and laterally
  • Can be used to help seal vacuum zones from each other
  • Can be used to eliminate horizontal movement when using loose spoil boards.
  • Removable spoil boards can be replaced with plastic or wood sealed special purpose pallets.
  • If bolted down as I did, adds rigidity to table.
  • Track can be machined in to be flush with plenum so that a spoil board can cover complete table
  • Track can also sit above plenum to hold spoil/bleeder boards in place.
  • Track can be used to bolt down vises and various jigs for holding odd size materials.

T Track location options…

If the T Track is flush with the plenum, it allows the use of a single large sheet for a bleeder board. The downside of this is that you will need a mechanical hold down method to keep the bleeder from both sliding around while cutting and to keep it from curling when not. Dabs of silicone every few inches would work for this, but makes removal of the bleeder harder when using the track for hold down. Flat head nylon bolts or screws could also be used. I do not recommend screws.
If the T Track is mounted above the plenum, and bleeder board sections are cut tightly in each zone, then lateral movement is eliminated. Vacuum then holds the bleeder very well while cutting, but there is still no way to control the “potato chip” effect that always seems to happen during humidity changes. We store ours upside down in place and may go to silicone in the future. This curling seems to increase as the bleeder material thickness decreases. The downside of this method is when the bleeder is removed to use the t-track it is not flush with the top of the track.

How many Zones?…

1 zone, 2 zones, 4, 8, 16? Read the many posts out there, and base your decision on the material that you plan on cutting and leave yourself options to go both larger and smaller. Remember, the more zones, the more plumbing. Fewer zones mean more masking to cut smaller parts. We have had good luck with 4 zones using thin Plexiglas for zone masks. They seem to work very well with very little vacuum loss. Trash bags, sheet laminates or visqueene also work well. We picked up 2 pieces of 3’ by 4’ by 1/8” Plexiglas panels from the big box store and cut them into 1 by 2, 2 by 2, 1 by 4, etc. sizes, and use them to mask off the unused part of a zone when cutting. This helps keep the vacuum loss to a minimum. We drilled a ¾” hole in each one and hang them on a peg on the wall when not in use.

Vacuum Plenum Design…

At this stage you should know what your materials are going to be for both the table bed and plenum. You may even have them on hand. You should have decided if you are using T track or not and how many zones that you are going to use. I had all of these, but no machine delivered yet. It was now time to design the plenum and table cuts. I sat down with Brady’s video and spent a few nights learning to be able to make the rectangles, circles and straight vectors do what I wanted. One of my biggest obstacles was learning how to reverse the individual straight vectors in the plenum grid so that the machine would cut them in alternating directions rather than all starting on one end. It’s not hard to do, just hard for me to find. Once I was able to get the results I wanted, I set these files aside to rescale based on my actual table dimensions once the machine was assembled. Both the control and design software have a preview mode that is a very good tool. It gets you used to the machine moves and you can zoom in to see what, where and how you are trying to cut.

Practice, Practice, Practice!!!

Next Month: Gluing and assembling the table


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