Selecting the Best Air Compressor for Woodworking

My familiarity with manufacturing hardwood furniture has taught me that about half the time involved in constructing each chair, table or cabinet is taken up with sanding. When you are trying to make a livelihood in the woodworking industry with hourly employees you must cut inefficiency to a bare minimum. This does not mean becoming a tyrant but, instead, removing any and all impediments that may be slowing construction, sanding and finishing.

I started my woodworking career with a quarter-sheet electric sander, quickly graduated to an orbital electric disc sander and finally realized that I could substantially cut sanding time with an air palm sander.  I settled on a 5” Dynabrade sander and Sears 3HP air compressor. It took me less than an hour to realize my faux pas: The small compressor I bought could not begin to keep up with the air requirements of the air sander. It would run out of air pressure almost immediately and the air sander would slow down to the point of being worthless. I would then have to wait for several minutes for the pressure to build up again to get another minute of sanding.

Compounding the problem was the fact that I had three people hired as sanders and so I would need to keep three air sanders running at 10,000 RPM all day long. I did some arithmetic and discovered that I would need a ten horsepower air compressor with a large tank to keep up with the demand. I was lucky enough to find a used one with a reasonable price tag but it required three phase power and lots of it. I had to consider the additional expense of an electrician to wire it up to the building’s 208 volt 3-phase power. The enormous air compressor was so loud it could be heard all over the building and down the block but it powered those three sanders continuously. The good news is that it paid for itself in production efficiency very quickly.

Air sanders are aggressive and effective. They are lightweight when compared to their lesser electric cousins. My sanders took to them immediately and production took off. I was as ecstatic as they were. Soon there was another machine  besides the air compressor that required having large amounts of air available in the shop: An Onsrud inverted pin router. It was also great to be able to blow sawdust from benches and machines while cleaning the shop at the end of the day. The compressor was also used to spray finishes on the completed furniture.

Years later, I built a smaller woodworking shop in my home which only required one air sander running at a time. For that shop, I purchased an air compressor half the size and isolated it in a soundproof room in one section of the shop. I ran ¾” galvanized pipe under the shop floor to three regulators at three different connection locations. The machine I purchased for that shop as a 5 HP Ingersoll Rand model with an 80 gallon tank. At the 80 PSI required by my Dynabrade sander, the compressor would produce enough air from morning to night. I must say that that compressor was very well built. All I had to do was keep an eye on the oil level in the sight glass. At night, I would turn off the master air valve on the side of the air compressor, leaving the electricity on, to silence the compressor until the next work day.

I must assume that, having read this far, you have some interest in utilizing an air compressor to power air tools in your wood shop. Most likely, a 2-stage reciprocating air compressor will fill the needs of a small to medium shop. As a rule of thumb, a 5 HP air compressor will power one sander, a 7.5 HP machine will power two and a 10 HP machine will be needed for three sanders.

The size of the compressor’s air tank is an important factor: The smaller the tank, the more often the electrical motor will need to cycle on and off, This is hard on both the motor and compressor pump over time and it uses more power. I would never purchase an air compressor used to run an air sander with less than a 60 gallon tank and I would feel much more comfortable with an 80 gallon tank.

The type of electrical power required by an air compressor is another thing you will want to think about. If you have three-phase power available at your shop location, fine. Three phase motors tend to use electricity a bit more conservatively than single-phase motors.Industrial-type air compressors will all require 3-phase power but the 5 HP models come either way. If you do not have 3-phase power available, you can create it with a rotary or electronic phase converter as I did in my smaller shop. Whether you use single or three phase power, you will need 230V AC power for single-phase motors and 208 to 220V AC for the three-phase type. Be sure to check the voltage and amperage requirements of any air compressor before you purchase it. Electricians can be an expensive surprise.

A two-stage compressor pump is a must for an air compressorof this size. Two-stage air compressors have two cylinders, one larger than the other. Air is first drawn into the large cylinder where it is partially compressed and sent to the smaller cylinder for final compression into the tank. As air is compressed, heat is produced and so a good machine will always have a finned intercooler built in.

Compression not only produces heat but squeezes water out of the air which ends up in the tank. Tanks can rust internally over time and if this is not controlled, the  rusted air tank can eventually explode causing considerable damage and even death. That is why it is really important to evacuate the tank of water regularly. Most air compressors come equipped with a drain valve at the lowest point of the tank. If you don’t want to spray water all over the floor under the unit, you may want to consider piping it from the valve to another place such as under the floor or into a drain. Piped water will flow uphill into a sink because it is being pushed out of the tank by compressed air.

You will need at least one regulator and a water trap in line before the regulator. These are not costly. A regulator allows you to set the correct air pressure for the tool you will be using (say, 80 PSI) instead of tank pressure (say, 175 PSI).

Air output of a compressor is expressed in standard cubic feet per minute (SCFM) or just cubic feet per minute (CFM). Not all 5 HP compressors put out the same volume of air per minute. This is a function not only of motor horsepower but also the efficiency of the air compressor pump the motor is powering. The higher the CFM, the less your air compressor will have to cycle on and off to keep up with the demands you are putting on it.  A small compressor pump on a huge tank will produce no more air than on a small tank. The only difference will be in the number of times the compressor cycles on an off each hour and the time it takes to recompress the tank on each cycle. In the final analysis, you need to pay attention to SCFM (or CFM) more than you do motor horsepower or tank size. Air flow is the end product of any compressor and the CFM must be sufficient to the job at hand.

All reciprocating air compressors throw out some oil with the air they compress. When the tank reaches it’s designed pressure limit, a switch will interrupt electrical power to the motor. Simultaneously, a certain amount of oily air will be released into the shop environment. You may see oil collecting on the wall behind the compressor and on the pump and compressor as well over time. This is not cause for alarm but periodic cleaning may be needed.

Reciprocating (piston type) air compressors are noisy and this is something you need to anticipate for the sake of yourself, your workers and others who are in the neighborhood. If quiet is an important consideration, you may want to think about spending the extra money for a screw-type air compressor. Screw-type compressors have no pistons or cylinders. Air is compressed in turbine fashion by a large metal screw, turning at a very high speed. These machines just purr compared to the reciprocating type but they are very costly. They sound more like a quiet jet engine than a loud truck motor.

I hope this information has been useful to you. Buying an air compressor for your woodworking shop can be a fairly expensive investment when you consider piping, regulators, hoses, water traps, wiring and electricians. You will want to buy a compressor that is capable of the tasks you will be using it for but no more than that. Purchasing the wrong air compressor can be a very expensive mistake. My motivation in writing this has been to give you the knowledge you will need to select the right one.

Bob Gillespie

Woodworker

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©2010 Robert M. Gillespie, Jr.