Official Luthiers Forum!

I'm near the end of the building phase on my first guitar and finishing is just around the corner. I've purchased a spray gun and it is being shipped to me and I'm looking at compressors. Haven't yet decided if I will buy new or used.

I have a question about how the pump is cycled on and off on compressors. If I purchase a compressor with a 155psi max pressure rating, it appears the pump will continue to run till it builds to this pressure.......and I'm unsure if this setting can be adjusted. What if I wanted to adjust the kick-on and kick-off points of the compressor, can I do it?

For example; if I adjust my pressure regulator to 30-40 psi to run an HVLP gun, it's possible that a compressor with plenty of output to run this gun might run all the time you were spraying if it doesn't cut off till you hit 155psi. Say the gun is using 5cfm @ 40psi and the compressor puts out 8cfm at 40psi;however, the compressor may only put out 4cfm at 120psi......so once the compressor kicks on, it could run till you finish spraying. If I could adjust the pressure points where the compressor kicks on/off and set them at 60/90 psi (in a pressure range where the compressor operates more efficiently), the compressor may run only in brief bursts during the spray process.

Very seldom (if ever) would I need very high pressure like 155 psi and I'm guessing most compressors are optimized for the 40psi/90psi range where compressors are rated for comparison (and the pressure range where most compressors are used).

So can the pressure points where the compressor motor starts and stops be easily changed? If so, is it done by adjusting a screw or other easy method or would I have to order spring kits for particular cut-on/cut-off points?

Thanks!

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Formerly known as Adaboy.......

Normally, the compressor on/off set points are not user adjustable.

That's disappointing. Seems half the run time is spent getting the last 25% of the pressure.

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Formerly known as Adaboy.......

I have a Kobalt 60 gallon compressor and have wondered the same thing. I've never let it go above 100 psi and have yet to find a need for greater. Even taking lug nuts off of tires! I would love to find an electrical add-on where I could modify the setpoint. Btw, most will say that 60 gallons is overkill, and it might be. What is most important for spraying is constant flow rate (without the motor constantly kicking on and off.) So in many cases, volume can be ur friend. HP can minimize the duty cycle. Mine is 3hp and it's never been a problem. But others would say 5hp for 60 gallons and up. Good luck and search craigslist! Also, look for a compressor with an intercooler. This is nothing but a radiator that the compressed air must pass through before entering the tank. Hot air is no good for spraying

So there you go messing up all my well intentioned plans! Ha. Thanks for metioning the radiator.......I've honestly never thought about it. There's just no inexpensive way to do this is there? I guess these can be bought seperately and added to most any compressor? I'm checking Craig's List......but not sure how common a compressor with radiator will be. It's probably even more important to have a radiator spraying waterborne finishes......which is what I plan to use.

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Formerly known as Adaboy.......

I bought my compressor off craigslist... $300. Be patient, search craigslist. The intercooler is common in larger tanks. My compressor was one year old when I got it. So many for sale out there it's silly. 50% discount is the norm.

Some Factory installed switches may have some adjustability for shut down pressure, but not for restart pressure. Pressure switches are just a switch with a spring in it. When the air pressure overcomes the spring, the compressor shuts off. Increasing the spring tension increases the pressure, and vice versa.

You can get an adjustable pressure switch from a source like Grainger. Shouldn't be too hard to install. Need to figure out if the stock one is NO or NC, wire the new one the same way, and do a little plumbing. You need to make sure you get one with an adjustable differential though. Most switches have fixed differentials, so they only restart at a fixed point. Also check the adjustment range, need to make sure the switch will work at the pressures you're looking for.

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Rob

The CFM is (supposed to be) measured in free air delivery. 120PSI at 4CFM, regulated down to 40PSI, is 12CFM (excepting temperature changes and frictional losses)

Moving down the operating range will make the compressor run more of the time at lower efficiency, so it'll be wearing out the tool without any benefit.

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

Thanks for the answers. I have decided to trust my compressor and let it run to cutoff. I also got some good advice and tutoring in pressure regulators. I will now learn to operate this tool at peak efficiency. I don't know why, but I have this sick fear of a compressed air tank exploding. I have read plenty of articles claiming that the failure mode is quite anti-climatic. It will split at weld points/seams but not explode. Anybody have contradictory stories?

I'm pretty sure they're right about the air tanks. 150PSI is sort of a lot of energy, but not to a chunk of steel.

Aluminum SCUBA tanks, on the other hand (well, any tank at 3000+PSI) truly terrify me. Aluminum has more of a 'chunk and fly' than 'split and peel' failure mode.



image from:

http://www.deir.qld.gov.au/workplace/publications/alerts/alloy_cylinders/index.htm

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services