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A blend of art and science

Spoil boards are an important tool for digital fabrication, but they require a mix of know-how and proven techniques for best results

I have a vivid memory of the first time I saw a vacuum clamp grab a sheet of MDF. It was at the community college where I now teach, and our instructor was demonstrating how to use a nested-base router. He slid the panel onto the bed and opened the vacuum valve. He then challenged the group of students to move the panel, and of course it wouldn’t budge. It seemed like magic!

The basic element of a vacuum table is a bed with grooves milled into it and holes drilled through it to distribute airflow (it looks like a huge waffle iron). The holes are connected underneath to a vacuum pump. A spoil board is placed on top of the grooved vacuum bed and a sheet good panel is placed on top of the spoil board.

The name spoil board comes from its role in absorbing any damage done by machining through the sheet stock. The spoil board is a consumable item protecting the bed, and it is intended to be replaced when necessary. It’s typically made with MDF, which is sufficiently porous to let some air flow through it but still dense enough to restrict that flow. Wherever a sheet good panel is on top and in contact with the spoil board a vacuum is developed that pulls the panel against the spoil board. Under the right conditions, the force developed by that vacuum holds the panel securely while it is machined. If the vacuum isn’t strong enough, the force of machining will shift the panel or parts, and the job is ruined.

Getting the mix right is a blend of art and science.

Dealing with leakage

Let’s start with a little science. Vacuumed air moving past the workpiece and through the spoil board is wasted. Producing that waste air flow costs money because a shop needs to invest in more or larger vacuum pumps and the electricity to drive them.

High volume is one way to reduce wasted air flow, where duplicate sheets of nested parts can be cut directly on the bed one after another with no special setup. But if the part sizes and shapes vary, a spoil board can deliver more convenience and productivity.

In processes using a compression bit, the pass depth will be slightly greater than the thickness of the material to assure that the up-cut facets of the bit completely engage the bottom surface of the material. As a result, the bit will cut a small amount into the spoil board. Over time, the tracks cut into the spoil board contribute to leakage and reduced clamping effectiveness. At this point the spoil board is resurfaced, and production continues. After numerous resurfacing operations the spoil board becomes so thin that it must be replaced (at about 3/8”).

When sheet after sheet of the same parts is being cut, a fixture board with gasketing may be practical. This is a special kind of spoil board made from a sheet of impermeable material with one set of grooves and holes that are cut to enable distribution of vacuum, and a second set to locate gasketing. This fixture acts a bit like a large, custom vacuum pod: leakage is very low, and clamping forces are maximized. If this is the only approach that will be used on the machine, then the vacuum system could be optimized for lower cost and lower energy usage. However, it would be unusual to have such a consistent workload. More often, this technique might be used to provide more secure clamping when a standard, ungasketed spoil board system is failing to provide adequate clamping.

A brute force technique for securing material so that it will not shift on the spoil board is to use mechanical fasteners such as screws. This comes with its own challenges. Even if the bit doesn’t collide with a fastener, screws can create unseen volcano-shaped bumps on the bottom of a spoil board that lift it off the bed. One solution here is polymer composite (plastic) nails that a tool will machine through with no damage to the bit.

Spoil board prep

So much for science. Now, here’s a little art.

I was taught to begin with 3/4” thick MDF and then remove the skin from both faces by removing a small amount of material. The factory surface is more dense and less porous than the core. After removing about 0.030” of material from each face with a surfacing operation, the new surfaces are less shiny and appear more porous.

Sealing the exposed edges of the spoil board will eliminate leakage in that area, and this can be done with a coating such as paint. I hear of some shops doing it with edge banding, which is quick and needs no time to dry. Sealing edges will eliminate about 200 square inches of surface area – that’s about 4 percent of a 4x8 sheet. Some shops seal the edges, and many don’t.

Any MDF seems to work about the same. Low Density Fiberboard (LDF) might be another choice, but maybe not. Here in Colorado Springs, Stiles Machinery installed a Weeke CNC router at The MiLL school and what we thought was MDF was used as the spoil board. With a bit of fanfare, we set up and began cutting the first full sheet job. About three quarters of the way through, the machine stopped abruptly and reported a low vacuum error. It took a while to get to the bottom of the problem. The first suspicion was that the vacuum system was not powerful enough for the 6,000’ elevation. So, Stiles generously supplied a fourth vacuum pump for the system, and we re-ran the job. But the same low vacuum error appeared. And then by chance, one staff member lifted the spoil board and noticed that it was surprisingly light. The LDF was replaced by a sheet of MDF and there have been no more problems. Fortunately, the Weeke router is equipped to monitor the vacuum level and post an error when the vacuum is dangerously low. On a machine without that instrumentation, the job might have progressed until the workpiece shifted.

Properties of the stock

One contributor to leakage is the condition of the stock to be cut. The larger the surface area, the stronger the clamping force required. However, the bottom surface of the stock must conform closely to the top surface of the spoil-board. That is, it must be in contact with the spoil board. In most cases a warped sheet of plywood will not be secured, although when pressed flat against the spoil board the vacuum may develop under the stock and draw it tight. Nevertheless, this is a risky situation because the warping stresses may pull the stock off the spoil board during machining as the process exposes new paths for leakage of air.

The requirement that there be close air-tight contact between the stock and the spoil board is one reason that hardwood stock is rarely secured via vacuum table alone. Pod-based vacuum systems are better suited to securing hardwood stock due to the ability of the gasketing at the edge of the pod to conform to the surface of the hardwood stock. When securing hardwood on top of a spoil board, mechanical clamping or fastening is often the best course, or appropriate gasketing.

When cutting an MDF sheet, there will be some leakage through the material. Most jobs remove very little of the skin and the vacuum clamping pressure remains adequate. However, for jobs where most of the skin is removed, leakage through the part will increase as the job progresses.

Job parameters

How far should one cut through the stock into the spoil board? The best answer is as little as necessary, and 0.005” to 0.008” is appropriate in most cases. This minimizes the size of the channels that will allow air to escape horizontally between the stock and the spoil board, and it also minimizes the amount of material that will need to be removed to resurface the spoil board.

The size of the parts and the order in which they are cut is an additional factor. The larger the part, the greater the force of the vacuum drawing down on the part. Smaller parts are the most likely to shift due to too little clamping force. As the parts in a sheet are cut, the leakage through the cuts increases and the vacuum drops off. Cutting small parts first, while the vacuum is highest, can maximize success. Designing toolpaths with tabs for smaller parts might be good insurance against shifting. Or leaving an onion skin is a bit like designing a thin and wide tab that may also limit leakage and maintain a higher level of vacuum under the part.

Many vacuum tables are divided into zones, and this can eliminate wasted vacuum from unused areas. AXYZ offers a system that tracks where the spindle is cutting and dynamically turns on vacuum to the area where cutting is active while turning off the vacuum to zones that are not being cut.

Another approach is to use cover sheets. These are typically thin sheets of a non-porous material such as melamine that are simply placed on the areas of the spoil board not covered by the stock. They limit the leakage in those areas, which results in higher clamping force on the workpiece.

Most vacuum pumps are fitted with a filter on the inlet side to capture sawdust or debris. Under ordinary conditions the spoil board serves as a kind of filter keeping chips and dust from entering the plumbing to the vacuum pumps. However, if sawdust and chips get to the back side of the spoil board during service or by accident, the intake filter on the vacuum pump is intended to capture that before it can enter the vacuum pump where it could cause damage and wear. If the intake filter fills up, it will inhibit airflow and reduce the ability of the vacuum system. When installing a new spoil board, it’s a good idea to use a shop vacuum to suck up any stray dust that might find its way into the vacuum system.

Ted Bruning is a furniture maker and college instructor in Colorado Springs, Colo.

This article was originally published in the December 2021 issue.

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