Air Compressors in Gunsmithing

Air compressors are useful on a household or workshop capacity, however they're one of the tools which the average gun hobbyist is capable of getting along without. That having been said, certain things - primarily related to refinishing - are best done with the aid of air tools. A limited list of things one would use an air compressor for with guns is below:

Abrasive blasting - for refinishing
Air gun - drying wet parts or blowing grit and debris out of tight spaces
Air tools - with a powerful air system, it can be cheaper to buy air-powered angle grinders, etc vs their electric-powered counterparts.
Cleaning welds - needle scalers do a very nice job of dressing up welds. This is important for anyone doing a large receiver reweld project.

Required Definitions

PSI: Pounds per Square Inch. If I were to draw an electrical analogy, this would be "voltage".
SCFM: Standard Cubic Feet Per Minute. This is roughly analogous to "amperage", and it's a measurement of the speed of air flow. Higher ratings are always better. Just about any air tool you buy will be rated for a maximum safe SCFM, and a minimum required SCFM. If your compressor outputs more SCFM than your tool is rated for, you will need to use a regulator to prevent it from overworking the tool. If you're going to run a tool on a compressor, some websites recommend multiplying the tool's minimum SCFM by 1.5 in order to 'right-size' it for the air compressor.

Reading Air Compressor Labels

Duty cycle: This is how long the compressor can safely run, expressed as a percentage or fraction. Generally speaking, it's an indicator of how much time the compressor needs to spend cooling down for every minute of operation.
Horsepower: You shouldn't use horsepower to differentiate, except perhaps in a negative sense. It's entirely possible for a less efficient compressor to just throw horsepower at the problem even though it's leaking air. If at lower horsepower engine can give you you the same amount of airflow, then it is superior to the higher horsepower pump with the same rating.
SCFM: This is a rating of how quickly the compressor can pump air into its connected storage system. You'll see "SCFM delivery at 40 PSI" a lot, and it's lower than the "SCFM delivery at 90 PSI" number. The reason for this is that what's being measured, is the flow rate of air into the storage system. As a compressor fills its tank, it gets harder and harder to add more air into it and thus it slows down. It takes a lot longer to take a tank from 0psi to 60psi than it does to take it from 60psi to 120psi.
Tank size: The available volume (usually expressed in gallons) to fill with air. Higher is almost always better. The only time when a small tank may be better, is when you're only going to do something small and you're faced with the prospect of having to fill up a whole tank just to do a few seconds' work. In practice, this doesn't come up as often as one may think.
Oilless Pumps: While there's a bit less maintenance and there's no risk of leakage if they fall over, oilless pump designs typically can't be serviced when they fail, and are generally considered lower quality than oil-driven pump designs. Most retail store mid-grade compressors are oilless designs these days.

Getting an Air Compressor

Used air compressors are typically available via Craigslist and eBay at a huge discount vs buying them in store. They're large and heavy, and are one of the most common shop tools that people don't choose to bring with them when they move. If one looks carefully, one can also frequently find commercial grade air compressors for less than the price of a new consumer-grade unit at Sears, Home Depot, etc. The one thing to watch out for with commercial grade compressors is that they are typically designed to take 220v input, as opposed to the readily available 120v plugs one finds in garages.

Achieving More with Less

Air compressors start and stop based on what their pressure switch is set to. On most compressors, this switch is not one the user is able to adjust. To prevent running for too long, usually the switch is set to engage when the compressor is 20-30psi below their rated maximum and disengage when the maximum is reached.

In my own personal case, I own two air compressors. The green one is an old Craftsman from the 1970s, made by Campbell-Hausfeld. It's rated for 100psi and has a 12-gallon tank, but recovers pressure very quickly (and is much quieter). The other is a newer Craftsman, purchased in '06 or so, with a 33gal tank and 150 PSI - but it takes a long time to build pressure. I've connected them both with a T junction, which ensures that both compressors feed into one system. When running two compressors with different maximum pressures, it's important to ensure that the higher-pressure compressor's regulator is set not to exceed the output of the lowest rated compressor. Otherwise, for one thing you risk blowing out the lower-rated compressor(s). For another, you won't be pumping air into the system any faster than if you'd just hooked up additional motorless tanks.

Even if you never run the other compressors (or if you just hook up extra tanks), the compressor can build moderate pressure in a large system far faster than high pressure in a small one. For my sandblasting cabinet, I run air at 80PSI. To ensure that my better compressor handles the "real work" and that the weaker one only provides backup when, I set the weaker one's output regulator to 80PSI. This ensures that it only supplies air when the pressure of the overall system has dropped below 80psi. In practice, this ensures exactly what I want - the older compressor takes most of the work, with the other occasionally turning on when I'm doing some heavy blasting. And the overall system pressure remains at a rock-solid 80psi.

Preventing Excess Air Use

You can use a regulator to reduce the PSI going through it. It's basically a two-way valve which clamps off when pressure on the reactive side reaches its (variable) cutoff. For certain applications, such as using a blower gun to dispose of dust, you really don't need a lot of pressure and there's no reason to overtax your compressor.

Condensation and Emptying the Tank

It seems that the humidity in the air which was kicked into the compressor, condenses into water at the bottom of the tank and can lead to rust. This is why compressors have a bleed-off valve on the bottom of the tank - so that you can blast out the water and such. I've also heard that if you have a screw-type bleed-off valve, it's advisable to replace it with a ball valve so that you can bleed off water readily without having to unscrew it all the way and empty your tank just to get the water out.

Performance Degradation in Compressors

Air leaks are probably your #1 problem when first setting up a compressor. If your compressor has leaks, then the efficiency is going to degrade. Leaks can be found anywhere the tank has an interface to the outside world. If you aren't sealing all of your connections with plumber's tape or pipe compound, then you should expect plenty of performance degradation and a general inability to maintain peak pressure without the compressor running a lot (or even constantly).

Drain valve: The drain valve is one place where leaks are commonly encountered.
Tank outlet: The tank's main outlet is another place where leaks are commonly encountered.
Compressor system: The connection between the pistons and the tank can also leak.

Quick-Disconnect Fittings

QD fittings are an excellent way to prevent yourself from having to screw air tools together. I personally use QD fittings on all my blow guns, and on my hoses. I'm sure it does reduce the amount of pressure that actually gets through.

-- SeanNewton - 25 Sep 2011

Topic revision: r6 - 07 Jan 2012 - SeanNewton

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