Air Compressors for Stone Carving

So you want to buy a compressor?

• 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.

Terminology:

• 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 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

Below are the things I think you need to know as you look at compressors.

1. 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 below zero.
   • 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.

2. 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.

3. Portability
   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.

4. 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 could move.

5. 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 comfortably.

   • 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.

   • Orientation
     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.

6. 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.

7. 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.

   • Catalog
     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.

8. 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.

9. Maintenance
   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 maintenance.

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 decided on a stationary compressor, then installation will be more complicated. Here are the steps I took to accomplish it, and what I did:

1. 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 my garage.
2. 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.
3. 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.
4. Get the permit signed off for the electrical work. Your electrical contractor may be able to help with this.

   Now the fun begins...

5. 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.
6. 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 and ends.

How to Connect Your Tools

The Easy Way

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:

1. Turn on your air compressor.
2. Open the valve on the compressor to let air into the regulator.
3. Adjust the pressure on the regulator to whatever the tool needs.
4. 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.
5. 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

Oil and air tools

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:

1. Manually, by just putting a few drops of oil into the air inlet of the tool before you start using it.
2. Automatically, by using an inline oiler to continually drip oil into the air stream and thus into the tool

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.

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:

1. 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.
2. 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 stuff.

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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