There are more than a few ways to clamp a piece of material down
to your workbench, and the majority of these methods involve
some kind of hold-down clamp. While many of these clamps do a
perfectly fine job of keeping material held down fast to your
bench, they have a couple of limitations... you often have to
physically work around them, or move them constantly while you
are working on your project, especially when planing,
routing or sanding tasks are involved, and they can mark your
project pieces if too much clamping pressure is applied.
we are going to take a look at a clamp that removes such
limitations and surprisingly, provides clamping force via the
use of compressed air! I'd imagine there is a little physics
The product is the Vac-Clamp, and the model we are looking
at is the VC-4. It measures 8" x 6.3" (200mm x 160mm) and is
roughly 7/8" high (22.5mm). On each corner of the Vac-clamp is a
mounting assembly to screw your Vac-clamp down to a solid
surface. You can either screw it down directly to your
workbench, or find a larger thick piece of ply, MDF or hardwood
even and mount it to that. If you use the latter option, you
will need to clamp that piece of wood you are mounting the Vac-clamp
onto down to your work table to keep everything stable. I
mention the latter option in case you do not want to drill holes
directly into your workbench. A mounting kit is available providing you
with 4 heavy shank screws and a hex-head key to insert them.
Alternatively, you can use panhead screws of suitable size if
more appropriate to your setup needs.
Once secured down using the mounting screws, you attach the
open end of the 1/8" supplied hose length to the Vac-clamp port.
You simply push it in with a small amount of pressure. Once you
have pushed it in as far as it will go, you verify the connection
by attempting to remove the air line. If inserted correctly, you
should not be able to pull it back out. This was the case in our
test, so no problem there. The other end of the air line is
pre-fitted with a 1/4" BSP connector (male thread). I hooked
this up to my existing air supply line running from my
compressor with a quickfit attachment (you may need appropriate connection fittings if your
air line ends are also of the 'male' variety).
With the above completed, you are pretty much ready to go.
Air supply from your compressor must be in the range of
80-120 PSI to get the best clamping power on the unit. My
compressor cuts out at 120 PSI and begins refilling the tank
once it drops to 80 PSI, so I do not need to do anything further
in the way of altering air supply pressures. The instructions warn
not to exceed 120 PSI or damage to the Vac-clamp may occur.
Obviously, if your supply reduces below 80 PSI, clamping
effectiveness is reduced. This can actually be beneficial if you
are requiring lower pressures to clamp lighter, or more fragile
materials like dense or rigid foams. An on/off valve is
recommended somewhere along your supply line to allow you to
easily "switch" the clamp on and off. One can be ordered through
the Vac-clamp manufacturer or resellers. I have an on/off valve
at my compressor outlet and this worked fine. I also installed
and tested the on/off valve that is sold by the Vac-clamp
designers and this was equally effective. If you use the Vac-clamp
regularly, purchasing their valve and installing it near your
workbench (or perhaps even on one side of the bench) might be a
good idea. You can use these same switch valves to control air
supply to several Vac-clamps in association with two-way, or
It is recommended to keep your air supply as "clean" as
possible. i.e. free of dust or oil. A water trap is also a good
idea, however, small amounts of condensate should not adversely
affect performance. I have a water trap on my compressor and on warm days, I do get a little condensation from the
supply. In using the Vac-clamp, this small condensate quantity
did not cause any noticeable issues.
Air consumption in use is quite reasonable - around
23-28L/min (1CFM at 80PSI). If you have a small compressor with
a tank smaller than 24L, it might be fair to say that your
compressor will be turning over fairly regularly if you are
using the Vac-clamp for more than a minute or two. Obviously, a
larger compressor with larger tank will not need to be refilled
The only other major component of the Vac-clamp is the
rubber seal. This is actually what creates the vacuum when you
apply your workpiece to the clamp. The seal is responsible for
closing the gap between the workpiece and the clamp, and hence
causing the vacuum/clamping effect. This seal can be moved
around the clamp in various configurations for use with smaller
workpieces that do not cover the entire surface area of the
clamp, however, any configuration must include the single
suction hole on the first 'square' on the unit (see photos).
Naturally, the seal must be fully contained underneath your
workpiece so there are no gaps for outside air to destroy the
Venturi effect the clamp utilizes to draw is clamping force.
Taller seals are also available to further expand the capability
of the clamp. An important item to note is that the Vac-clamp
will not work terribly well with porous materials. The more
non-porous a material is, the better the clamp will function.
Holes, cracks or other defects in the material will have a
degrading effect on clamping power, as you would expect.
How it works... Venturi Effect
The Vac-clamp works by taking advantage of something called
the Venturi effect. Without going into a detailed scientific
analysis, the Venturi effect causes a pulling/clamping force. As
compressed air hits the workpiece on the clamp, it is sucked
through the small space between the clamp and the workpiece and
out the exhaust port on the clamp. The sucked air pulls the
workpiece down onto the clamp due to a pressure difference
between the upwind (supplied) air and downwind (exhaust) air.
When supplied with high pressure compressed air with a small
volume of space between clamp and workpiece, the force generated
is quite large. This allows a clamping force of around
12PSI at 80PSI supply, or a holding force better than 800gm per
square cm. This is around 7-8 times the holding power of a
household vacuum cleaner!
Holding power is 170Kg (full face) at 1 ambient atmospheric
Thankfully, the Vac-clamp is far easier to use than it is to
understand the physics behind how it works. Simply, you turn on
your air supply to the clamp, apply your workpiece and away you
go. It will take a second for the clamp to engage and 'grab'
your material, but once that is accomplished, it's extremely
difficult to remove the workpiece unless you switch off your
compressed air supply. In regards to air supply and compressor
tanks, I actually have a small 2HP, 24L tank compressor. I
filled the tank up to 120PSI (which is cutout for this unit). I
set the stopwatch and opened the valve and allowed the Vac-clamp
to go to work on the workpiece. It was exactly 80 seconds before
the compressor kicked in again at 80PSI, and the compressor
happily refilled the tank back to 120 while continuing to supply
the Vac-clamp with air. So even with a smaller compressor like
mine, the Vac-clamp isn't going to run it into the ground air
supply-wise. Naturally, a larger compressor with a larger tank
would mean less refilling and compressor kick-in, but I was
satisfied with the benefit the clamp provides over small
Re-configuring the seal for smaller workpieces was a simple
affair... you just need to ensure you close off the seal with a
tight fit between the rubber seal ends for best clamping effect.
I tried the clamp in routing, hand planing, pocket hole
drilling, random orbit sanding and even belt sanding operations
and the workpiece held fast. I was expecting the belt sander to
send the workpiece into orbit (I took safety precautions etc)
but surprisingly it battled hard and won. Admittedly, my belt
sander is not the most powerful going around but it still has
some kick to it! The Vac-clamp really shines in routing
operations and is probably what I will use it most for in the
future. The Vac-clamp does have a limitation, and this is that
it becomes less effective when trying to clamp larger materials.
You do really need 2 or 3 Vac-Clamps if you want to work with
long boards. The reason being is that using a heavy machine on a
board end far from the Vac-clamp obviously results in a force
down on the board end. This acts as a fulcrum and could cause
enough force to break the clamp seal on the workpiece.
Other materials that can be clamped using the Vac-clamp
include MDF, Chipboard Glass, Perspex, Aluminium, Stainless
Steel and some cardboards.
I think the best property of the Vac-clamp is that it uses no
moving parts. This means very little maintenance and a long
expected product life. If you happen to block the air supply
hole with wood dust or shavings or any other small materials,
you can simply place your finger briefly over the exhaust port
and the compressed air will blow out any foreign material
through the top supply hole. It is certainly one of the easier
tools to maintain in my shop.
If you happen to work a lot with router templates, the Vac-clamp
has some handy applications in holding both your jig/template
and workpiece down at the same time. The Vac-clamp developers
have provided a PDF file detailing a similar application which
is worth reading about. You can find it here -