T

TThurston

Well Known Member
I'm currently using a very small borrowed Craftsman air compressor (1.5 hp, 3.3 scfm @ 90psi). It's seems adequate for riveting, but when I use it to drill, the pressure drops rapidly, the compressor starts making refill noise, and the drill slows down. After a bit of slowing, the drill doesn't get any slower, but it's painfully slow. I assume this is the point at which I'm just using up the air being pumped into the tank. I'm looking to upgrade to a larger compressor, but I have some questions.

I'm currently on a 15amp circuit, and don't feel like upgrading at this time. Thus, I'm looking at compressors with this limitation. It seems like the best they'll give me is around 5 scfm @90psi, with varying sizes and pressures of tanks. I'm trying to determine exactly what this means.

1. If I have a a tool that states that it uses 4 scfm, and my compressor delivers 5 scfm, then I can use that one tool forever, right? The pump will run 80% (or so) of the time trying to keep the tank full, and it may get hot (and damage the motor). Is this what the scfm numbers mean? In the case above, would the tank size make any difference?

2. One compressor has a 30 gallon tank, and the other 15 gallons. If both compressors have the same scfm rating, I assume that the bigger tank takes twice as long to fill. It will also go twice as long suppying air before the pump starts to refill it (assume the same max pressure and refill point). But then, if a tool is stilll using air at a constant rate, what difference will there be in how the pump runs to trying to refill the tank?

3. I'm guessing that the total amount of time the pump runs is only related to the rate at which air is being consumed by my tools (plus the time to originally fill the tank). Thus, if I start every day with an empty tank, my pump will run for a longer total time if I have a bigger tank. Right?

4. This runs counter to the conventional wisdom that a bigger tank is better. I'm guessing that the main advantage of the bigger tank is that it allows you to consume air at a greater scfm than your pump can supply for a longer time before the pressure drops unacceptibly low. But if your air consumption is below your pump's scfm or above it and sufficiently intermittent, then there is no advantage to a big tank. Right?

5. Related to the tank size questions is one of tank pressure. If tank one is pressurized to 120 psi, and the other to 200 psi, I assume the second tank is has a capacity to supply air that is (200/120) greater than the first one. In other words, it's like having a tank that's 5/3 bigger.

6. When I adjust the pressure on my rivet gun, I understand this means I'm not only changing the rate at which it hammers, but also changing the rate at which it consumes air. I assume this also applies to other tools. For example, if I get an air die grinder, and mount a 1" scotchbite wheel to use for deburring (or whatever), I may find that at 90psi the grinder turns "way fast", but uses air to quickly for my compressor. Could I just put a flow regulator on the die grinder, drop the pressure to something that would give me "adequately fast", and use enough less air that my compressor can keep up?

7. On a similar note, if I see two different versions of the same air drill, one rated at 2600 rpm and the other 4000 rpm, I assue that the faster drill will use air faster (and probably drill holes faster, too). Is this right? (Van's "Tools and Workspace" section of the manual states that a minimum of 1100 to 1200 rpm is required of a drill.)

I know this seems like a foolish lot of questions. It's kind of like of like the flying rule: Obtain all available information before flight. So I try to obtain all available information before purchase.
 
Die grinders "eat" air. Suggest an electric dremel tool instead. A good battery drill can substitute for an air drill and once you are off the workbench and into the "boat hull" stage it can be handier than an air drill. No hose, just grab and drill. I still like the 9.6V stick battery Makitas. The slim handle is easier on the wrist and gets into tighter spaces than the big battery ones. The new NiMH batteries will drill lots of holes. Plus, the controllable speed does a better job on countersinks & deburring. I didn't even use an air drill on my last 2 airplanes. Biggest benefit for me is not having to listen to that stupid compressor or wait for air pressure. In fact, I think an air drill is a dinosaur mostly.
 
I'm assuming a 15a, 120v circuit. That's going to be the biggest limitation of what options are going to be available to you.

You'll want the largest tank you can find, probably 30 gallons. This is for drills, sanders, etc. Yes the motor will run longer, but also the extra capacity will also keep the tool running longer at the appropriate speed.

You'll see a lot of similiarities since the Lowe's and Home Depot oem brands are really Campbell-Hausfield (sp?) compressors. They'll have different color paint and a different part number, but the performance numbers will be similiar.
 
Air tools

Trying to keep this simple:

A small air compressor isn't going to keep up with continuous operation type air tools. It will help to turn down the pressure to the tool.

I think most of your comments on storage and recovery are reasonably accurate. Bear in mind that a compressor advertised with a 200 psi tank won't necessarily recover any quicker that a compressor advertised with a 125 psi tank. Storage volume and recovery rate are two different things. Air storage can be compared using either tank volume or pressure (or both).

The manufacturers are trying every way they can to market their rinky dink compressors as "large".
 
I would be surprised if you could run an reasonable-sized tank out of air using a drill. You would have to have the drill running constantly for quite a while to do so. With a die grinder, more likely to run out of air, but most of the reasonable-sized compressors will do you fine most of the time.

If I had it to do over again (which I just might one of these days), I agree with Aerhead, one can relatively easily build an airplane with electric tools and not use the compressor except for driving rivets (and painting if you do that yourself).

greg
 
I had a similar restriction, I bought a Harbor Freight compressor, $99 with about a 30 gal tank I guess, upright model. It has ran my air drill, die grinder, and a half dozen other air power tools quite well, no complaints, for my whole construction phase. And it runs on a 15 amp circuit.
 
I just bought the new dewalt 12v with the new batterys, 2.4 Lbs and runs forever.
 
I have a 26 gal compressor and even with the air turned down the drill still keeps the motor chugging. Had an old 18v cordless with a spare battery that is much better. Keep one battery charging all the time, never ran out of juice during a build session. The air drill is def better than an electric though if you have a large enough tank. A dremel is also a good tool to have, as has been mentioned. And a drill press. And, and, and.... (you get the idea!)
 
Oil lubricated vs. Oil-less

To me the difference comes down to oil-less or not. The oil-less compressors have some advantages, but are significantly louder and are not really meant to be running continuously. I picked-up a 25 gallon oil lubricated compressor on craig's list for $80. I'm very happy with it - especially with the noise level.
 
one point on regular operation of a tank...

i only drain my air tank occasionally so there is no extra time running the pump to have compressed air available. oiled compressors are MUCH quieter and do not have the same surge current demand that many of the oil-less units have. i have a couple of hangar neighbors with oil-less units and they have to run the tank down to an unreasonably low pressure before they can start the compressor or the circuit will trip... my oil unit never has an issue with the single 15 am circuit provided to each hangar.
 
Duty cycle

One often overlooked spec (mainly because "they" don't want to advertise it) is compressor duty cycle. Just as an aircraft is designed for a certain G load that is safe for continuous operations, a compressor is designed for a specific rated duty cycle.
Many commercial compressors are designed for no MORE than 50%. This means that for the stated pressure and CFM ratings, the compressor is only designed to run 50% of the time...Using air tools for an hour, compressor should only be running 30 minutes.

"But that is impossible, my compressor pretty much runs continuously", I bet it runs hot too. Anyone want to volunteer to go pull 6 G's in their RV? Probably would live through it, 8 G's? Etc...running it over it's duty cycle is plain bad for it...so, either buy a cheap one, run it till it die's, or try to find a higher duty cycle. Some professional (NOT at home centers) are designed for 100% duty cycle.
 
You most likely have solved the problem by now, but another solution that is almost free...

Find an old propane tank (25 lb or so) take the valve off the top and plumb in a T fitting with one side going to to a T fitting on your airhose, and the other going to an emergency valve (auto parts store). This way you can use any compressor, but have the extra reserve (time before depletion) when you need it. Often the reserve tank is not needed and can be unplugged from the system so the compressor does not take as long to charge it.
 
clevtool said:
......Find an old propane tank (25 lb or so) take the valve off the top and plumb in a T fitting with one side going to to a T fitting on your airhose....... This way you can use any compressor, but have the extra reserve (time before depletion) when you need it......
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That is almost exactly what I did...in my case I plumbed a 7 gallon portable air tank into the 20 gallon compressor line to effectively increase total air capacity to 27 gallons. It help keeps the compressor from constantly cycling. The technique is no silver bullet fix but it does work well enough.

Like others, I do prefer using a cordless electric drill instead of an air drill. Cordless drill operations are far quieter, there is no hose to deal with and a cordless drill will be much easier on the compressor too.


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Like others, I do prefer using a cordless electric drill instead of an air drill.
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When I was in school, cordless drills were new inventions (seems like yesterday). We were taught that the biggest reason "airplane shops" use air tools was to cut down explosion risks from fuel vapors, etc. Not because they were the best tool to use. Many times I've walked towards an access panel with my cordless and had to stop myself, turn around, and go back for the spinner handle so I didn't blow myself up. Of course I could also use that trip as an excuse to gulp down half a cigarette before I went back.
 
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