Spoil Board Thickness

A discussion of the fine points of choosing a spoil board material and thickness. January 19, 2011

I am new to CNC and was just wondering what you all run for spoil boards and how thick you like them. I have been using 3/4" MDF, I surfaced off both sanded sides to start and have flipped it every few clean offs. I notice as is getting thinner I have less pull on the meter. The parts are held down fine, I am just reading the meter on the end of the machine. Do you like thicker or thinner spoil boards? Is there a better material than industrial grade MDF?

Forum Responses
(CNC Forum)
From contributor J:
It is not really a matter of if you like thinner or thicker spoilboards. As you surface thinner, your areas of vacuum are going to become very narrow and focused instead of spread out evenly over the full surface of the spoilboard. This may not be a problem if you are cutting large parts, but smaller ones will tend to move. The thinner you get the more the sb will want to bow on you as well unless you have it attached to the matrix table, this can make it difficult to get the sb and your material sheet sucked down. We start out with 3/4 and generally switch it out at about 3/8 to 7/16. Could it go thinner? Maybe, this is just from our experience with our machine.

From contributor K:
The trick is not to use MDF. The light weight MDF is better, much more pores. Even better is Turpan – it’s a totally different type of LDF. It is the best spoil board material you can find.

From contributor U:
It really depends upon how big of a vacuum pump you have, how good you seal up the edges, and how often you want to replace your spoil board. I have used 1.5" thick LDF before and never had a problem. It allowed me to re-surface it multiple times prior to replacing. I just sealed up the edges to avoid vacuum loss. This also depends upon how much Z axis stroke you have. If you leave it on the machine it can be better to have a thicker board and just re-surface it. If you remove the spoil board from your table with the parts on it, a thinner one is easier to move. The choice is always up to you and varies from shop to shop. There is no right or wrong way of doing this.

From contributor M:
First, picture a very thin spoilboard. It lets a lot of air pass through it quickly. It doesn’t allow a hard vacuum to build under the board so you don't get any pressure holding your parts down. It still works pretty good if the entire surface of your table is covered with melamine, but as soon as you cut through you get a rush of air equalizing the pressure below and soon the vacuum can't keep up no matter how big it is.

Now picture a very thick and dense spoilboard. No air flows through it, and so no pressure holds the work down. The pump is creating a hard vacuum below the spoilboard, but it is all held in by the very dense and thick spoilboard. A very small pump will evacuate the plenum below the table, but no holding power. Real life is a balance between these extremes.

Like roger I used to use Trupan very successfully. I started with two layers of 3/4, edgebanded all edges, screwed the bottom sheet down to the phenolic with nylon screws and used up the top sheet bit by bit one almost completely, then switched the top sheet out for another one. I liked the extra distance between my router bit and the phenolic table, just in case. That happened to be on a 4 by 8 table with a 9 HP pump.

Now I use standard MDF, and do like Joe does. I happen to be using 40 HP of pump. I use it from full 3/4, coat the edges with 3 or 4 coats of poly to prevent bleed and skim both sides. I use it down to a little more than 3/8 inch, and then surface it to 3/8 exactly. I put that sheet aside, then use another one from 3/4 down to 3/8. Surface it clean. Put a small amount of regular yellow glue around the edges, put the first sheet back on and let the vacuum clamp it under a sheet of melamine for ten minutes. Now you have a piece of ¾ again with no waste at all and keep your spoilboard within a good range, a little more than 3/4 to a little more than 3/8.

Everyone has an opinion on this and with an understanding the principles involved you will need to experiment with your system and find that sweet spot between all the variables at hand. So much goes into this decision. Pump size, pump configuration (rotary vane pump, regenerative etc.), tooling strategy, part size you want to cut, condition of the surface of the spoilboard, edge sealing, material warp, and etc. Receiving a lot of advice and then choosing your own strategy is all you can do. Keep your mind open and have at it.

From the original questioner:
Thanks for all the advice. I am going to put a new spoil board on this week. The first one I ever put on the router is now at 9mm and I feel it is too thin. I keep a 5x9 spoil board on it although I cut mostly 4 x 8's. I cover up the exposed area with scrap. I am very happy with its performance, just trying to learn from others experience. It is amazing what salesman and tooling guys (at least in my case) will say and how it differs from what you read hear from actual users.

From contributor C:
I am totally new to CNC routing please forgive my ignorance but what is a spoil board? Is it necessary and how do I use it? I have a Masterwood project 316-K router and have jumped in head first so to speak.

From contributor K:
A spoil board is used under your project to protect the machine bed when you are cutting your parts all the way through. Also vacuum tables use this on top of the machine bed to hold parts from moving while you cut them. The vacuum pulls through the spoil board.