Sculpture by Jeff Powell
Original artwork in stone, metal, and who knows what else
Air Compressors for Stone Carving
Comments on this page are always welcome. Use the "Contact" button
above to send me any suggestions or corrections.
So you want to buy a compressor?
I hope these notes are helpful. They are targetted at people buying
compressors specifically for stone carving, but they are general enough
that they will help anyone buying a compressor for most any reason.
As with all my other tool recommendation pages:
- I am not responsible in any way for what you do after
reading this page.
- Know your equipment and use all the required safety gear.
- Do lots of research before buying anything. I have only
one point of view, and it might not match your own.
As you review the text below, this little glossary may help things
make more sense.
- CFM -- Cubit Feet per Minute of air
delivered by a compressor or consumed by a tool. Usually written
with a pressure associated. As in "5.4 CFM @ 90 PSI". On a
compressor, the larger this number, the better. The more CFM the
compressor can deliver, the more kinds of tools it can drive.
Bigger and heavier duty tools generally use more air. On a tool,
though, a smaller number may be better, as a tool that uses less
air can be used with smaller compressors. Be prepared to see
this in Liters per minute if you buy a tool made somewhere that
uses the metric system, like Italy.
- PSI -- Pounds per Square Inch. A measure of
pressure. Most of the air tools I have operate at 90 PSI. A few --
particularly Cuturi air hammers -- operate at 70 PSI. Check with your
tool and regulate the air pressure properly for the tool(s) you are using.
Be prepared to see pressure in Atmospheres instead of PSI if you
buy tools made somewhere that uses the metric system.
- Regulator -- a device used to limit air pressure in a system.
Compressors generally pressurize a holding tank to something like
175 PSI. That's far too high for most tools, so a regulator is used
to reduce the air pressure down to something like 90 PSI in the
hose or lines that connect to the tool.
- Filter -- a device for removing "gunk" and water from the
hoses and lines in an air delivery system. When air is pressurized
by a compressor, water condenses inside the tank and can get pushed
down the line to the tool in use. That's bad, as most tools are
made of steel and will rust. In addition, other "gunk" also appears
in the lines. Mineral deposits, etc. These come from impurities and
dust in the air, rust in the tank, chemical reactions going on in the
water condensed in the tank, etc. This stuff can also gum up a tool,
and that should be avoided. A filter is installed inline between the
compressor and the tool to remove some or all of this stuff and thus
prolong tool life.
How a Compressor Works
The simplest typical compressor is pretty easy to explain. A motor drives
a belt that spins a crankshaft, very similar to the crankshaft in a car.
As that crankshaft spins, it raises and lowers a piston in a cylinder --
again, just like a car engine. As the piston drops, air is let in through
a valve into the cylinder. As the piston rises, the air is compressed, then
a second valve opens and the air is emitted into the holding tank. An air
filter keeps dust and gunk in the air from going into the compressor. Oil
is used to lubricate the piston as it moves in the cylinder. A unit having
only one cylinder is called a single stage compressor, and this is about as
simple as they get.
The next step up is a two stage unit. This works in the same way, but
there are two cylinders instead of one. The air is compressed in the first
cylinder, then it passes to the second cylinder, where it is compressed
even more. The cylinders operate on opposite sides of the cycle; while
one is compressing, the other is either getting air from somewhere or
pushing it out. This is more efficient overall, and most of the larger
compressors you can buy are two stage machines.
If you were in some industry that needed huge volumes of compressed
air there are other things available at great expense. Assuming you're
a starving artist, though, buying a simple compressor is going to be a
major expense, and weird technologies aren't even worth considering.
If you're that curious, find a good compressor shop in your area and
make friends with a sales guy there. Come to think of it, that might
be a good idea regardless.
Once the air leaves the cylinder(s), it goes into a holding tank. That
tank is usually pressurized to something high, like 175 PSI or so.
There will be one or more emergency pressure relief valves on your
compressor too, so that even if the motor doesn't shut off for some
reason, the tank won't get over pressurized and explode.
Coming off the tank will be a pressure regulator you can set to some
reasonable pressure for your tools. (You might have to buy and install
this separately depending on what you get. Keep reading.) There will
also be some sort of valve to shut off the air, and some sort of quick
connector to couple a hose to your unit. That hose carries the air
to the tool. Air tools use some sort of finned, spinning contraption (an
air driven motor really) to convert the incoming air into a spinning burr,
disk, wheel, or whatever. Air hammers use the air to move the hammer
back and forth.
As air is pushed out of the tank to drive the tool you're using, the
pressure in the tank drops. When it falls below a certain pressure,
the compressor turns on and pumps the pressure back up to the set point
for the tank you have. Thus, the compressor cycles on and off as air
is used. The larger the holding tank you have, the less often the
compressor will have to turn on. The more CFM the compressor is
rated for, the faster it will turn back off again once it starts.
That's it. Now you know how a compressor works. Loosely anyway.
To purchase a compressor you'll need to figure out several things.
How to Buy a Compressor
Buying a compressor is moderately complex. However, with some research
you'll find something that works for you and stays within your budget.
Compressors are available in all sizes from tiny portable units a
contractor can carry up a ladder and onto a roof all the way up to huge
industrial monsters that would dim the lights in all of the surrounding
county if you had one installed in your garage.
Below are the things I think you need to know as you look at compressors.
- Determine Your Air Needs
This is the most important first step. If you're going to
use one tool at a time, find the tool with the highest rated
air consumption (in CFM) and note that number. If you're going
to have people using tools at the same time, add up the CFM of
the various tools that will be used at once. Two die grinders
is probably about 10 CFM (5 CFM for each) for example. Unless
the pressure differences are extreme, just ignore them. (A 5
CFM @ 90 PSI die grinder and a 5 CFM @ 70 PSI air hammer is about
10 CFM. It's really a bit lower, but how hard do you want to
make the math?) Once you know the CFM you need, you should
purchase a compressor capable of delivering at least that much air.
Yes, this means you need to know
what tools you'll be using before you buy the compressor.
It also means that if you buy new tools after you buy the
compressor, and your compressor isn't big enough to drive
them, you'll be unhappy. As a result, you may want to buy a
compressor that delivers more air than you need now in order
to handle your potential future tools. On the other hand,
if you know your tool needs won't change, you can minimize
your compressor expense by buying something just big enough
to do what you want.
In my own case I have tools like these:
- 2 die grinders that use about 5 CFM @ 90 PSI
- a Cuturi air hammer using about 5 CFM @ 70 PSI
- A die grinder using about 12 CFM @ 90 PSI
That last die grinder was a shock, as I bought it before I knew what I
was getting into. 12 CFM is a huge number, but as I was planning to
buy a compressor large enough to let 2 or 3 people use air tools at
the same time, it's not too large a problem for me. Others might
find it problematic though.
In my case I decided that I needed a minimum of about 15 CFM @ 90 PSI,
and I could probably use 20 CFM or more occasionally. If you're the
only one who's going to use it, and you'll be using a simple die grinder
and an air hammer, you'll probably need a minimum of 5 or 6 CFM, which can
be found in many compressors. As I said before, however, there won't be
much extra capacity in a compressor that can only deliver 5 CFM, so it
will run nearly constantly while you are using your tools.
A friend of mine tells me that based on his experience, compressor
and tool manufacturers rate the air production and consumption
of their products in the most favorable light, much like car manufacturers
measure gas milage. If he's right:
- A compressor rated to produce 5 CFM at 90 PSI may do so only
at 150 feet below sea level when the temperature is 40 degrees
- A tool rated to use 5 CFM at 90 PSI may do so, but only with
no load, or the lightest load, etc.
If you combine those two effects, bad things can happen, like your
compressor might run constantly while you're using your tools. So,
it's best to take any numbers you read on tools or compressors with a
grain -- or boulder -- of salt. It's also probably a good idea to
overbuy -- at least on the compressor side -- to be sure you can drive
the tools you'll be using, now and in the future.
- Power Source: Gas or Electric
Compressors need a motor to do their work. Motors require a power
source. Traditionally, electric motors are used, but if your compressor
is going to be located outside -- where exhaust fumes won't be a problem --
you can consider a gas powered unit. Gas engines, though, require more
maintenance, are louder, run all the time, and you have to feed them
fuel, which may make them more expensive to operate than an electrically
driven compressor, depending on many factors. For convenience, most
people use electrically driven compressors, unless they have to mount
it on the back of a truck and drive it to a job site regularly.
The issue doesn't stop there, though. There are at least 3 choices in
electrically driven compressors:
- 110 volt units
These plug into a standard home outlet to get the power they
need. They are limited in capability because 110 volt, single
phase power isn't enough to drive a larger unit. These are
light duty compressors, and are often equipped with wheels so
they can be moved around.
- 220 volt single phase power units
Larger compressors suitable for home use are generally in this
category. These require a separate circuit supplying 220 volts
and enough amps to support the compressor. This is the type of
compressor I have. These units typically have 5HP or larger
motors on them, and large tanks, 60 gallons or more. Note that
a compressor of this type will have a set of starter capacitors
attached to it to provide additional electricity to get the
motor moving. If you find a 220 volt compressor without starter
capacitors (which usually are contained in a small metal box attached
to the electric motor) then it requires three phase power, and is
probably not what you want.
- 220 volt, three phase power units
These are probably not going to work in your home or a typical
studio. Three phase power is used in industrial settings and
provides more "oomph" (a technical term) to get motors spinning.
Unless you are both wealthy and lucky, this sort of compressor
is probably not for you.
If you don't have a lot of space and need to move the compressor
around, you'll need one on wheels. This limits the size and power
of the motor, the size of the tank, and (consequently) the CFM the
compressor can produce. Tanks up to 40 or 45 gallons are possible
in portable compressors, but not much beyond there. CFM is usually
limited to 5-7 in my experience. These typically operate on standard
110 volt household AC current, which is convenient.
If you have the room, air needs, and budget, though, a stationary
compressor is the way to go. Remember that you can -- and will --
use hoses to get the air to wherever you are working, so if you have
a permanent mounting location for the compressor, you'll be able to
get a larger tank, possibly enclose it to keep the noise down, etc.
Since stone carving is usually done in one location, a stationary
compressor is usually a fine choice, but if you're going to be using
it for other things, you may need to make other choices.
- Horsepower Doesn't Matter
This is odd, but you really don't much care what horsepower the motor
on the compressor puts out. Almost all manufacturers make a big deal
about it in their brochures and advertising, but this is the one case
where you really don't care. What matters is the CFM that the unit can
produce. If some manufacturer could find a way to produce 20 CFM with a
3 HP motor, that would be great.
As a rule, motor horsepower increases as CFM goes up, obviously. And
most stationary compressors will be 5 HP or more. However, at least
some electric motor manufacturers also rate the HP their motors produce
under the most favorable conditions possible. My compressor has a
huge 5 HP Baldor motor on it. I've seen smaller motors listed as
6.5 HP on units in some hardware stores. Baldor is a good name, and I
trust them given what I know, but I still didn't particularly care about how
many HP the motor has. Again, what mattered to me was how much air it
- Nice Features
Here are some features to look for in the compressor you buy:
- Big Tank
As mentioned before, the bigger the tank, the less often the
compressor will run, reducing your power consumption and the
noise generated. My compressor has an 80 gallon tank.
- High CFM
Also as mentioned before, the higher the CFM the compressor
can output, the more capable it is. Know your tools and pick
a compressor with at least enough capacity in CFM to drive them
- Automatic Removal of Water In Tank
Water condenses inside the tank and must be removed to keep
the tank from rusting. All compressors have some sort of
manual valve for doing that. Some units, though, have an
automated system that sucks much of the water out of the tank
as it stops running. This is a nice feature if you can get it.
Compressors generally come in two arrangements. The motor is
nearly always mounted on top of the tank, but the choice is
how the tank itself is oriented. In small, portable compressors
this is less of an issue -- though a 40 gallon tank with a motor
on top of it can get top-heavy in some designs. In stationary
compressors, though, the difference can be large. An 80 gallon
tank with the motor on top can take up perhaps 9 square feet of
floor space if the tank is oriented vertically, and something like
21 square feet if it is oriented horizontally. The horizontal
orientation is what I chose, but I had space available and I live
in earthquake country. The vertical orientation may work better
for you, which is why the big manufacturers give you the choice.
- Quiet(er) Operation
Compressors are noisy. If you can find sound ratings for the
models you're comparing, be sure they are taken the same way.
That is, be sure the measurements were taken from the same
distance, etc. This is probably particularly difficult if you
are comparing products from different manufacturers.
Assuming you have no way to compare compressors for quietness,
the best guess you can probably get has to do with overall
size and the RPMs the compressor -- not the motor -- turns at.
In general, larger compressors are noisier than smaller ones,
and compressors that run at low RPMs are quieter than those
running at higher RPMs. These are just guidelines, though.
Unless you build a cinder block room to enclose your compressor,
it's going to be loud. You'll be using loud tools too, so it
probably isn't worth too much effort to hide or quiet a noisy
compressor if you don't or cannot quiet the tools too. Still,
buying a quieter unit in the first place cannot hurt, all other
things being equal.
- You Get What You Pay For -- or Do You?
My compressor cost something like $2,000, and that didn't
include the cost of the regulators, installation, etc. I, however,
bought a large unit and I tend to fall into the "you get what you pay
for" school of thought. Others might argue that I could have bought a
compressor from the local hardware super store and saved a bit of money
-- several hundred dollars or more. And they might be right.
You have to make a choice about what you believe in this area.
Only research will help you here. Consider how you will get support
if it stops working, where you can get parts, etc.
- Where to Buy From
You have several choices here. I have some comments, but some are
not based on personal experience, as you will see.
- Compressor specialty shop
This is where I went. I found a place that specializes in
compressors. As a result, I found a unit made by Champion
that is well built durable, and does what I need. Champion
and other industrial brands aren't sold in hardware stores,
so purchasing from a compressor shop is a good option if that
is what you want.
- Tool specialty shop
Some tool specialty stores carry compressors. Which brands
and the quality of the equipment varies. If you find a tool
store that specializes in air tools, they should be able to
give you good advice on what to buy and avoid, even if they
don't sell compressors themselves.
- Hardware store
Most of the big chain hardware stores sell compressors now,
often up to 220 volt single phase units. My personal opinion
of these units so far hasn't been particularly high, but then
again, I've already admitted to suffering the "you get what you
pay for" school of thought. These compressors might work well
for light duty.
If you get catalogs -- particularly some of the more obscure
industrial tool catalogs -- you'll find compressors available
from big name companies. You'll find choices from small portable
units all the way up to monster things for use in major industrial
plants. You can have your compressor shipped to you, assuming
you can work out how to get it dropped off, etc. Installation
might be more problematic, though, if you're not up to doing it
yourself. I honestly don't know if you can save any money by
going this route.
- Installation Permits
Be aware that installing a large, stationary air compressor
may require permits where you live. It did where I live, but
that was for the electrical work. Some residential neighborhoods
have limits on the horsepower of motors in use on the premises,
and there are probably other, stranger restrictions. Personally
I recommend doing the right thing in this case. I hired a
professional electrician who also does industrial work to install
a new sub-panel in my garage, provide 220 power, and wire up and
test the unit. The county did the permit inspection once he was
done. It cost a few bucks, but it was worth the piece of mind.
Of course, if you're just using a 110 volt portable compressor
you have nothing to worry about unless it starts blowing breakers
or fuses on the line you've got it plugged into. In that case,
move it to a breaker or fuse all by itself and consider calling
an electrician to resolve the problem by supplying more power,
splitting lines up into more circuits, or whatever will get rid
of the issue.
Like all machinery, compressors need occasional maintenance.
With luck, all you need to do is change the air filter and
lubricating oil every so often. Do so. Just like changing
the oil in your car, it will make your compressor last longer.
Check the maintenance schedule on any compressor you are
considering buying, just so you can compare units for required
Using the above information, anything else you can learn, and
some time, dig around looking for compressor options. You'll find
something that works for you.
How to Install a Compressor
If you've chosen a portable compressor of some sort, then installation
means plugging it in. Pretty simple. If you use an extension cord,
be sure it is a very heavy gauge wire, as specified in the owner's
manual of your compressor.
If you've decided on a stationary compressor, then installation
will be more complicated. Here are the steps I took to accomplish
it, and what I did:
- Purchase the compressor and bring it home. In my case, the unit
weighs over 500 pounds, so I had it put into the back of my truck
on it's shipping pallete with a fork lift. When I got home, I
got a couple of friends to help me slide it down a ramp and into
- Bolt it down. The unit I bought must be bolted to the floor, and
there are some rubber pads that it sits on to isolate it from the
ground as it runs. I had to drill holes into my floor, drive in
concrete anchors, set the compressor over them, and tighten nuts
down onto the anchors. Not too hard if you are tool savvy and
have some way to lift a 500+ pound device.
- Call in an electrician who knows what he is doing. Get it wired
up, with an appropriately sized breaker. I also had a shut off
wired in next to the compressor, so I can turn it on and off
without actually flipping the breaker itself.
- Get the permit signed off for the electrical work. Your
electrical contractor may be able to help with this.
Now the fun begins...
- Using simple lumber, I built a frame on which I mounted a pressure
regulator and filter. Since my intent was to allow multiple
people to use the compressor at once, I also made sure I had room
to mount several quick connects and another pressure relief valve
in case there was a problem. Your situation may not require this
much effort. You might, for example, want to use a flexible hose
to connect the compressor to a regulator and filter hung on a wall.
I didn't have wall space handy as things were arranged, so this
was my solution.
- Using heavy walled copper pipe and sweat fittings, I plumbed in
the line from the compressor's outlet to a pressure regulator and
filter. A ball valve allows me to shut off all air leaving the
compressor if I need to work on the system, and a union in the line
provides a way to disconnect the system in that case. I also built
a line for the outlet side of the regulator with four quick connects
and the pressure relief valve.
At this point, your compressor is installed and ready to run. All
you need to know is how to connect your tools, and a couple of odds
How to Connect Your Tools
The first thing to note, here, is that there are many ways to do this.
I describe two ways, the simple approach and my own -- paranoid -- approach.
Either will work, and there are others, no doubt. Read this information
and see what makes sense to you. With that disclaimer aside...
The Easy Way
Either you have a portable compressor with a quick connect on it
already, or you've got a stationary compressor interconnected to
a regulator and quick connect. To connect your tool(s) to the
compressor, you'll need an air hose. Air hoses are specifically
designed to carry pressurized air. They typically have pipe threads
of a standard size on each end, to which you need to add whatever
fittings you want to use.
Since you have a quick connect on the compressor end, you'll need
to purchase and attach to one end of your hose the proper fitting
to interlock with your quick connect. Which reminds me...
A brief discussion of air couplers and fittings
Air fittings come in a variety of shapes, and they are not all
interchangeable. There are at least 3 or 4 different shapes
that the male portion of the fitting can look like, and correspondingly
there are different quick connects (the female portion) that accepts
them. To make matters worse, there is no consistent naming convention
for the various types of connectors. One might
call a particular type "automotive" while another calls the same thing
type "D". As a result, your first concern is that all of your fittings
and disconnects are of the same type. If you wind up buying from different
manufacturers, be sure to take a male fitting to the store with you so
you can compare and be sure you've bought the same type of connector.
Your second concern may be that friends with air tools cannot use
the setup you've so carefully arranged. You can minimize -- but not
entirely eliminate -- this risk by installing "universal" quick connects
wherever you might have someone else connect up their own equipment.
To the best of my ability to tell, they aren't exactly universal, but
at least the 3 or 4 major connector types will work with the universal
quick connector. You can use these everywhere you need a quick connector,
obviously, but they cost more than the single type quick connector.
If you're just going to connect directly to an air tool, you're almost done.
Your tool will have come with a pipe thread connection on the inlet end. You'll
need the proper fitting to go on there, and you'll put a quick connector on the
end of the hose that will connect with the tool.
Use Teflon tape or pipe thread compound (both can be purchased in the plumbing
department of your local hardware store) to seal the threads on the pipe thread
connections. Do NOT put anything on the air fittings or inside the quick connects.
Just use the sealer on the pipe threaded connections.
To get your tool to run:
- Turn on your air compressor.
- Open the valve on the compressor to let air into the regulator.
- Adjust the pressure on the regulator to whatever the tool needs.
- Connect your hose to the regulator. Note that no air will come
out of the hose because of the quick connect on the other end.
- Insert the air fitting on your tool into the quick connect on
the other end of the hose. If you're using an air hammer, it
should start working right away, as stone carving air hammers
do not have a shutoff. If you're using a die grinder or something
similar, squeeze the trigger and it should run. Note: always
insert and lock down the disk or burr in your air tools with the
air hose entirely disconnected from the tool. Anything else could
cost you fingers, eyes, or worse.
That's the easy way, and it works. But it's not entirely how I do things.
Jeff's Paranoid Way
This is much the same as the easy way, but instead of taking the first
hose and directly connecting it to my tools, I have a whole additional
rack of stuff hanging on the side of my carving bench. So, the air hose
connects to a valve. Out of there comes more heavy wall copper pipe
that goes into a second regulator, filter, and inline oiler. I have
quick connects available after the regulator (but before the oiler)
and after the oiler. And that reminds me...
Oil and air tools
Where were we? Oh, yes...
Air tools need oil. Even with the best filters, some
water and gunk gets through. The oil helps protect
tools from freezing up with rust and related problems.
In addition, since the tools contain moving parts, they
need lubrication to make them last.
There are two ways to use air tool oil:
The inline oiler is more effective as it keeps oil coming
into the tool all the time you're using it. However, it
means that the inside of the hose between the oiler and
the tool winds up with oil inside it. That's not so good for
some things, like blow guns and tools for putting air into
your tires. Thus, you probably want 2 hoses... a longer one
to go from the compressor to the inline oiler, and a shorter
one to go from the oiler to the tool.
- Manually, by just putting a few drops of oil into the
air inlet of the tool before you start using it.
- Automatically, by using an inline oiler to continually
drip oil into the air stream and thus into the tool
There are also two kinds of inline oilers -- wall mounted
ones and ones that just hang in the middle of the line.
The former are about as large as a filter unit, have a
large oil reservoir, and are easily adjustable as to how
much oil they put into the line. The latter are smaller,
difficult to fill, and difficult to control. However, they
cost less and will do the job if your needs are simple.
Finally, you need special air tool oil. At least, that's
what the various manufacturers say. Maybe 3-in-1 household
oil or 5W30 motor oil would work fine, but air tool oil is
of a particular consistency that looks thinner than motor
oil but thicker than household oil. I'm no expert, but I
decided to buy the real stuff. It's not expensive.
So, as you can see, I have a set of connections at my carving
bench. A short, coiled hose is used to connect my tools to the
quick connect after the oiler so they get air with oil. A second short
hose connects a blow gun to the quick connect between the second filter
and the oiler, so that I can blow the dust off my work without spattering
it with oil.
Why the second regulator and filter, I hear you ask? Good questions.
The answer is in two parts:
- The second regulator is unique to my needs. I can set the first
regulator -- near the compressor -- for 90 PSI and leave it there.
Other people using tools can thus get 90 PSI on their hoses.
I, however, want 70 PSI when using my Cuturi air hammer. Thus,
I regulate the pressure down to 70 PSI at my carving table.
This lets everyone do what they need at the same time.
- The second filter is paranoia, but it's proving valuable. Water
condenses inside your air hoses, just like it condenses in the
compressor tank. That water gets blown into your tools too.
so, a second filter is a second chance to remove water and gunk
that either got through the first filter, or that were in the
hose when it was disconnected from the entire system. My second
filter does need periodic cleaning and emptying, as it does collect
I could operate in the simple way with the equipment I have, but since
I have the extra items on the side of my carving table, it makes sense
to use them and thus protect my tools a bit better while I am doing so.
So there you have it. Everything I know about air compressors in a
single web page. I hope it has been useful to you!
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