If you are experimenting with pneumatics being able to attach an airline or foot pump to your system is useful and to do so is easier if you can use a schrader type valve. In this instructable I will deal with making a standard rubber mushroom type valve stem as used on steel wheel rims into a screw fit valve stem.
Step 1: Source Your Valve Stem
you need either a rubber and brass valve stem as used on steel wheel rims which can often be had for the asking at any tyre depot that does a free valve and balance promotion as they cut the old ones off and bin them. If you have to buy a new one it's probably better to work out how to use it as designed rather than do this modification. If you happen to get hold of a solid brass (or sometimes alloy) stem as used on some lorry wheels or alloy car wheels you can skip the step about removing the rubber and go straight to the thread cutting.
Step 2: Tools and Health&safety
Fire, blades and maybe abuse of electric drills and files caution advised oh and watch out for your wife/mum if you do the burning rubber bit indoors.
Additionally and of Greatest importance you are modifying a pressure item it must be tested to twice the pressure it will be used at after the modification. The easiest way to do this is install it in your project as you are going to pressure test that anyway aren't you!
On to the tools knife, blow torch/gas ring, wire wool, wire brush, emery cloth, pliers, leather gloves,die and die stock (you might get away with a die nut) possibly an electric drill and a file. A valve core tool if you can get one this is a useful little tool it consists of a die to clean the outer threads a cleaner tap for the inner thread and a valve core extractor the tyre depot may sell them if not try a cycle shop or motor parts store. (I got mine from a large national chain that also sells bicycles I wanted a new inner tube so wander down to the cycle section and found one there packaged as a cycle tool for £1.50 less than the one I picked up in the car parts section) If you can't get one a solid hexagonal metal valve cap is useful to protect the outer threads if not that just be very careful with the pliers. vernier calipers/ calipers and a rule/ thread gauge basically anything to measure the brass stem once extracted so you can decide on your thread size on past experience it will be m6-m8 (1/4 to 5/16) range
Step 3: Remove the Valve Core
use the valve core extractor or failing that fine nosed pliers.
Step 4: Carve Off the Rubber
either screw the valve stem into the cleaning die of the valve stem tool or fit the metal dust cap and grip it with pliers or gently in a vice whilst you carve off the bulk of the rubber using your cutting tool of choice.
Step 5: Burn Off the Remains of the Rubber.
Using a blow torch or gas ring or just possibly a lighter ignite the remains of the rubber. It really is best if you do this outside over a non flammable surface (now just what makes you think I didn't the first time I did this? ) if you left too much rubber on it softens as it burns and you can scrape the burning goo off with a screwdriver. Once most of the remaining rubber has burnt off or if you did a real good job of the carving you will need to hold the stem in the gas flame to burn off the rest do this a little at a time you do not want the stem getting overly hot if you can avoid it getting hot enough to melt solder that's great. DO NOT heat it to red heat..and notice in the second burning photo I'm wearing the gloves I really should have had them on in the first as well. Once as much has burnt off as possible scrub off the residue with a wire brush or wire wool and once cool use emmer cloth if required.
Step 6: Decided on Thread Size
measure the diameter of the recovered brass valve stem I used vernier calipers you can just as easily use ordinary calipers and a steel rule or a tapping gauge, somewhere in the house there is a knitting needle gauge this goes from 2mm to about 10mm in 0.5mm steps.
This valve stem proved to be of a slightly different design to ones I have used before it has 2 steps the smaller is 5.2mm the larger 5.8mm it still has a nice tapper at the end which makes starting the die easier. These steps are too large for M5 and too small for M6 threads respectively, so I need to reduce the smaller step to 5mm. If I had a lathe this would be easy but I don't so......
Step 7: Cut Thread
I knew full well I should have reduced the 5.2mm section to 5mm before I tried to thread it.
Some where I have a nice little clamp that fits round the end of an electric drill so you can clamp it to a bench and use it like a bench grinder/wirebrush/polishing mop. It had been my intention to clamp the threaded section in the chuck and carfully apply a file to the spinning shaft to turn it down. This is needless to say not the safest thing in the world to be doing, but as I couldn't find it :-
A) expect an instructable in the future on how to make one
B) I tried to thread it with out reducing the size.
Now 0.2mm might not sound very much it is about the thickness of the side of a drinks can which is apparently how thick the skin of the luna landers were. When you consider the thread depth of am ISO M5 thread is only 0.49mm it is over 20% of your thread depth. I was thinking previous experience would let me feel if it was too tight and indeed I felt it go before it seperated but not the tightness that gave me any warning.
To add to my woes I only had an M5 die nut not a split die. Die nuts are only really meant for cleaning threads not cutting them but you can just about get away with it as you can see the tappered section threaded nicley.
So clamp your cleaned and if needed reduced valve stem verticaly in a vice, grease the section to be threaded the valve stem is conveniently tappered and your die nut or split die has a tapper to aid starting as well, if using a split die you will have it mounted in a die stock if using a die nut I find it is better to start it by hand then preferably use a socket and 'T' bar to turn it or a spanner you only need the lightest of pressure and you only turn 1/2 a turn forward then a 1/4 back to break of your cutting chips. As can be seen from the pictures the full depth of un tappered thread had just bitten when the stem failed.
Step 8: Blue Peter Method
Otherwise known as here are some I made earlier. I made these a few years ago the tube is a 6.02mm which was close enough to 6mm that a proper M6 split die in a proper die stock threaded them without problems. These have been in use to make an air reciver from a 2Lt PET bottle, a foot pump on one end with a shrader addaptor on the other to enable an air brush to be powered by a foot pump. Instructable to follow at some point