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Air compressor to hose or female QD fitting
QD female fitting or male stud to hose
male stud to tool (such as - air nozzle, nailer, screwdriver/drill/impact driver etc, spray painter)
any semi-permanent 1/4" NPT to 1/4" NPT fitting
To add to the confusion, there's nothing on a 1/4" NPT that really measures 1/4". THe threads are visibly tapered and the overall outside size of the male NPT threaded fitting is around 1/2" to 5/8" diameter.
The QD stands for Quick disconnect; Pull back on the outer sleeve of the female QD and you can plug in or release a male stud. A female quick disconnect and a male stud are usually used in a shop to permit rapid, leak free connection and disconnection of tools and hoses. THe QD has a check valve so unconnected, it won't leak air at the end of a hose. The stud is open through and will bleed air from the tool when the connection is opened. This is usually used on the tool end so it bleeds the tool and makes it safe when disconnected. For convenience you're probably going to want a QD at the air compressor (maybe a Wye with two or more QDs), a hose with a stud at one end and a QD at the other, and one or more tools with a stud installed on the tool.
Since the hose I've described is male to female, you can use one or more in series. Hoses differ a bit from electrical extension cords. Longer lengths add air flow resistance for continuous high airflow tools (grinders, sprayers) leading to loss of pressure at the tool, like electrical extension cords lose voltage. With real high airflow you will need to go to 3/8 or 1/2" ID hoses. But for intermittent tools (like air nailers) that use air in short bursts, the air hose is actually an extension of the air tank and stores air at pressure. So air nailers on long multiple long hoses is usually quite OK.
Example of QD fittings at Harbor Freight -
http://www.harborfreight.com/5-piece...set-68237.html
The above picture has one female QD to female 1/4"NPT, three male stud to male 1/4" NPT, and one male stud to female 1/4" NPT.
Note the male stud and female QD, the "sex" depends on the part that mates with each other. Depending on the hose and or the tool, the NPT fitting is usually a 1/4" male or female NPT. There's actually then four types of fittings:
Female QD with 1/4" male NPT
Female QD with 1/4" female NPT
Male stud with 1/4" male NPT
Male stud with 1/4" female NPT.
they also come in 3/8" NPT for higher flow rates but 1/4" is probably all you'll encounter or need for home shop use.
To add to the confusion there are three types of QD/studs in general use.
there's a industrial/milton, automotive and ARO. By far the most popular is industrial. They're often color coded (orange, yellow, blue) with paper and bin tags and if you stick to the same type you'll be OK, but by far the most popular is the Industrial - orange - that's all i buy. Blue is automotive, Yellow is a universal; the QD coupler supposedly works with the industrial, automotive, and ARO studs but I've never tried it. Physically they all look alike so look for the colors when you shop at HD, or Lowes. I think the HF stuff is the popular industrial and not marked. There's also more types than that in use that look similar but don't fit - Stay away from those not because they don't work but just to make life easier for yourself by using the most popular. That way if you borrow tools or take yours to someone else's shop your stuff will most likely plug in and run.
my do's and don'ts for teflon tape:
its only for tapered, threaded joints - 1/4 NPT and the like usually used on air fittings. Do not use on straight threads. Especially don't use tape on the stud or QD ends of the adapters, just use it on the the NPT threaded ends.
use three layers or so.
Don't cover the first thread... this keeps shredded tape from getting into the airway.
Try and wrap in the direction so that screwing it in makes it tighter, not unwraps it. - That means holding the male end in your left hand pointing right, start wrapping the tape under the bottom and towards you. Pull the tape tight so that you can see the tops and valleys of the threads clearly through the tape. Don't pull it so tight that the tape thins so much it separates. After three wraps I just pull the tape hard it'll stretch and break leaving the stretched and broken end well seated into the threads - no flying end. (actually, the wrap direction is not critical in my opinion if the tape is properly stretched on and the end pulled tight and seated like I described - I never had a problem but it won't hurt to wrap in the direction I described).
Once covered with the tape, thread the fitting into the female until it bottoms. and you encounter resistance. Then keep going another half turn or so. THis will deform the tape to fill the gaps. Don't back it out, if you do this will release the seal the tape made and you will have to strip the old tape off and retape before putting back together. Tape cannot be reused.
I've never observed any thread leaks in my fittings after this.
If you do think you have leaks,
1. Listen for the sound of air escaping (hissssss)
2. Use some soapy water dripped over the threaded joints and look for bubbling. This can find even very small leaks.
If you have a leak, try tightening the threaded joint just a bit. Do not loosen a joint with teflon tape or you have to redo it.
The fittings may also leak when joined or if the seal or check valve in the female QD leaks. Sometimes wiggling a joined QD connection may cause or stop a leak, if so, my suggestion is to try replacing one or both ends involved.
Finally, there's the choice of air hose. Air hose comes usually with 1/4" male NPT fittings on both ends and it recommended you put a 1/4" NPT female to female QD on one end and a 1/4" female NPT to male stud on the other end.
Air hoses for home shop use with 1/4" NPT males factory crimped on each end come in three materials (listed below in order of increasing price and increasing performance):
PVC in 3/8" ID
Rubber in 3/8" ID
Polyurethane in 1/4" ID
PVC is cheapest (maybe $10 for 50 foot of 3/8") but it is flexible when hot or maybe 75°F but when it gets to 50°F and below it gets maddeningly stiff and awkward to handle. It also retains the coil so that makes it worse.
Rubber is around $15-20 for a 50 foot x 3/8" piece and its more pliable when cold but its heavy and somewhat stiff.
Polyurethane (PU) is around $25-30 for a 50-foot x 1/4" piece. Its much stronger than rubber or PVC so the wall thickness is much lower and the total weight is probably 1/3. It's very flexible, does not get stiff with temperature and it does not have a coil memory. However it doesn't like to have axial twist. It'll lay out flat nicely but if you coil it up by making loops it'll be a tangled mess. Best is to reel it and spool it off straight from a extension cord reel.
My advice on extension hoses: avoid PVC. Rubber is OK, but I like PU a lot, but it is weird about twisting.
Shorter pieces (25 ft, 15 ft) of hose can also be bought and used for jumpers where needed to connect compressor to regulator or filter... if its permanent you can connect without QD fittings (a 1/4" female Straight coupling may be needed to connect two male 1/4" NPTs together). And if its permanent, cheaper PVC may be OK, since you are not so worried about flexibility or handling.
[2013 postscript:]
if you ever install some hard piping in your shop, you can use iron pipe, or copper pipe or even flexible hose tied down peiodically. Do not Use PVC plastic pipe. When it fails, it shatters and send shrapnel everywhere. The other types listed fail with a split or a crack and are not known to shatter.
Re: Harbor Freight
I refuse to buy Harbor Freight hoses or fittings. I do not use them at home, but at work we have all HF stuff. I dropped a hose and the brass end piece broke into two pieces, and the blower tool went flying. We have one of their more expensive "heavy-duty" rubber hoses that is developing bubbles in a couple places after just a few months of use, and seems ready to blow at any minute.
My only HF tool at home is an angle grinder, which I bought years ago, and it does see very little use. Last year the side handle broke off, since the bolt was held in the rubber handle with no real support. But it is a good loaner tool for the neighbor. I have replaced some older Craftsman sanders and saws in my wood shop with newer/better tools, so they are now in the garage for outdoor use, and for loan to my neighbor.
dbhost commented
It is messy, it smells bad, but it seals MUCH better... Just keep your nose clear of the can when you open it.
dbhost commented
#1. When assembling threaded pipe fittings, once tight, don't keep turning. I have found that if I snug my fittings down, typically 1/8 - 1/4 turn past when they just start getting snug, the fitting stays together, and no new leaks are created, however if you keep cranking down on the wrenches, the pipe tape or compound tends to get driven from the joint, creating a potential for new leaks.
#2. Just because your fittings are new, and the threads look good, does NOT mean they will seal well. Always buy extra fittings, and be happy if you don't have to use them... I had 2 fittings that looked good, but no matter how I assembled them, would not seal...
#3. "Bubble test" your fittings. Use soapy water, preferably dish soap that generates a LOT of bubbles, and soak the connection once pressurized, and look for bubbles. If your connection is creating bubbles, you have a leak, disassemble and fix the leak...
#4. Leak down test your system. Leave the system pressurized for a given period, the longer the better, but use your best judgement. I like 24 hours as a good number. If your pressure holds steady for long term, you know you have knocked your leaks out...
#5. Don't be a brand name snob. I have had failures among fittings and pnuematic tools from the cheapest of the cheap, and the cream of the crop brands. Everyone makes bad stuff from time to time...