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Air Compressors for Stone Carving

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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:

Terminology:

As you review the text below, this little glossary may help things make more sense.

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.

  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:

    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:

    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:

  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:

  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.

  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 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:

  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 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:

  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

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

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.

Where were we? Oh, yes...

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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