Total Guide to Router Vacuum Tables for CNC & DIY
Vacuum Table with zoned sub-plenum plumbing. Image via Techno-CNC…
Vacuum Tables are often the best solution for the flat sheet work often being done on CNC Routers. You can lay an MDF spoilboard right on top of a vacuum table and it’s porous enough to still pull a vacuum. Granted, unless you have a lot of vacuum available, you may want to seal part of the spoil board so the area receiving the vacuum is limited. Vacuum tables can be set up with zones that can be turned on and off with valves as well.
Vacuum Table Basics
Vacuum Tables generate hold-down force because of the difference between a vacuum under the part and atmospheric pressure pushing down from above. Every square inch of area exposed to that difference can have a force as much as 14.7 lbs pushing down on it (the difference between 0 and sea level air pressure).
The hold-down force is proportional to that pressure difference and the surface area exposed to the difference. A large area on a large part can have significant force. A 10″ x 10″ square part potentially has 100 square inches times 14.7 lbs per square inch or almost 1500 lbs of holding force!
Meanwhile, small parts have much less force holding them down. It’s important to be aware of that difference.
Another way force can be limited is you may not be able to apply the vacuum to the entire bottom surface of the part. Consider a vacuum system built out of aluminum with small vacuum chambers underneath. It looks something like this:
Vacuum table with small chambers…
Now in order to keep the part flat, and not bowed, we want it to sit flat on the vacuum table. We may not get much vacuum anywhere except where the chambers are. In that case, it is the surface area of the chambers and not the surface area of the part that will determine the hold-down force, and that’s much less than the surface area of the part.
Most CNC Router Vacuum Tables avoid this problem by using an MDF board atop the vacuum table to spread the vacuum. MDF is porous, so vacuum goes everywhere. This works well, but it does require a vacuum pump that can pull a greater volume of air through because the MDF will leak wherever there’s not something lying on it.
Just as MDF can be used to spread the vacuum, your part may leak vacuum if it is made of anything permeable (foam board, MDF, that sort of thing) or if you introduce too many through holes into it during the machining.
The last thing to know is that there are two forces trying to upset the part on the vacuum table. One is side-force and the other is up-force. Side force is a function of friction between the part and what it is laying on. Be sure the vacuum table surface is not too slippery to provide holding against side force. Most of the time, the coefficient of friction will be such that it takes at least twice the force to move the part sideways as it does to lift the part.
The up-force needed to overcome the holding force is just a function of the weight of the part plus the hold-down force due to vacuum. If the part is lifted even a little, perhaps not enough you can see, you’ll leak vacuum and unless your vacuum pump has a lot of capacity, the part will soon pop off as vacuum drops. If the up-force is enough, the part could even be flung across the table.
DIY CNC Router Vacuum Tables
Building a vacuum table for your CNC Router (or Mill) is pretty straightforward. In this article, we go over a bunch of ideas, tips, and links to resources that will help you put together your own vacuum table project in short order.
Do I need to buy or can I purchase and fit a vacuum table to my CNC Router?
Don’t have time to build a vacuum table from scratch? No worries, there are a number of them commercially available. Here’s just a sampling to evaluate:
Clampusystems offers a variety of add-on vacuum tables, components, and accessories…
CNCStep vacuum table…
NEMI vacuum grid table…
Pierson vacuum chuck system…
Tormach Vacuum Chuck. Tormach has a variety of vacuum workholding solutions…
DIY Vacuum Table Design
First thing to consider is Z clearance if you’re going to set a vacuum table atop your existing CNC Router table. You’ve only got so much travel in your Z axis and a vacuum table is going to take some of it away. Be sure to think about how to minimize that loss. This is where specifying a built-in vacuum table when you buy your CNC Router Table can have an advantage.
Another important consideration is whether you’re going to create some sort of plenum design or whether you plan to actually machine your vacuum table. Machined tables can be more accurate–flatter and more square. They’re certainly more durable too. But a plenum-style table may be cheaper to build and less restrictive on vacuum passages.
There is a key trade-off to consider and that is the ability of the table to support your part versus the surface area that’s available for vacuum to do its job holding down parts. If the part is resting on a surface that is not permeable to air, then only the passages that pull vacuum are exerting force. On a machined aluminum vacuum table, you only get much vacuum from the surface area of the grid and not the whole surface area the part rests on. If you try to create a cavity under the part to spread more vacuum, you run the risk of stressing the part so it is bowed in the unsupported areas if you let them get too large.
The beauty of having your top surface be something like MDF is that it is permeable, and the vacuum can access the whole surface area of the bottom of the part. However, MDF is likely a less accurate support than aluminum and it will require a vacuum source that can pull more CFM of air out because there is a much larger area that leaks vacuum.
Assuming you don’t have a giant vacuum pump with unlimited CFM capacity, one facility that really helps is the ability to zone the vacuum areas so that areas that leak (perhaps because the part doesn’t sit on those areas) can be shut off from vacuum. Here is a vacuum table that has a zoning system that is very convenient to change just by opening or closing some valves:
Vacuum Table with zoned sub-plenum plumbing. Image via Techno-CNC…
The heart of any vacuum table is the vacuum pump. There are many sources, but one of the big things to keep in mind is to get more pump than you need. Vacuum Tables become marginal in a hurry if they can’t pull enough vacuum because the pump is sized to small for the table.
Rather than go through all the ins and outs of Vacuum Pumps here, we’ve got a whole article on CNC Vacuum Pumps.
DIY Vacuum Table Ideas and Links
Here’s a list of photos and links to various DIY vacuum table projects. It’ll stimulate your ideas as you design your own vacuum table project.
Make Magazine’s Shopbot Vacuum Table
A typical MDF vacuum table has a plenum board with grooved passages and a bleeder board that allows vacuum to seep through its porous structure…
Closeup of the plenum board…
Although both the plenum board and the bleeder board are made of MDF, we don’t want the plenum board to leak vacuum, so we apply wood sealer to close up the pores:
The plenum has to be sealed so it won’t leak vacuum. Easy to do by applying sealer…
A vacuum motor is mounted to power the vacuum table…
RC Groups Vacuum Table
Here’s a neat little vacuum table project from RCGroups.com:
It’s powered by a ShopVac and is designed to simply be clamped down to your CNC Router. The table was made from HDPE plastic. To stop leaks in areas not being used, just use a piece of posterboard cut to fit closely around your part or workpiece:
Romax CNC Build
Here’s a little vacuum table intended to go into a milling chuck. There’s a complete build up described over on the Romax CNC forums.
Neat little vacuum table fits in your milling vise…
This builder likes these “PIAB” vacuum generators that are available on eBay cheaply used…
Another view of the PIAB. The long can is a silencer. These are basically just nice venturi vacuum pumps. Hard to beat the price of a used one from eBay.