Modelling the Kerr-Stuart Brazil Class 0-4-2ST locomotive in 16mm scale. - Keith Bucklitch
Regulator and Blower Valve
The regulator or throttle controls the flow of steam to the cylinders. This is based upon the design used by David Watkins in Idris. (SMT Nos 71 - 77). This is a safety type designed so that the spindle can not be completely unscrewed. The spindle is stainless steel, all other parts are of brass. For fine control, aim to get the taper on the spindle as long as possible.
1. Regulator Body.
Chuck a piece of 1/4" brass in the lathe. Drill a 1.5 mm hole for a depth of 15 mm. Open this out to 5/32" diameter for a depth of 7 mm. Replace the drill with a 5/32" D-bit and cut the seat for a further 1 mm depth. Tap the bore 3/16 by 40 tpi. Part off at 14 mm from the end. Replace in the chuck and turn the other end down to 316" diameter for a distance of 5 mm. Thread this 3/16 by 40 tpi. Drill a 4 mm hole in the side. The side piece is made from some 3/16" diameter rod. Cut a 3/16 by 40 tpi thread on this, centre drill and drill 1.5 mm for a depth of 7 mm. Make sure that there is a taper formed by the centre drill for the pipe cone to fit against later. Part off at 7 mm, place the side piece in a screwed chuck and turn a stub to fit firmly in the side hole of the barrel. Silver solder the two pieces together.
2. Inner body.
The inner part of the body is intended to prevent the spindle unscrewing completely. The bore is threaded part way through. The outside is threaded at both ends. Chuck a piece of 1/4" hex bar, face the end, centre and drill 2.3 mm for a depth of 9 mm. Tap 6 BA for a depth of 3 mm. Turn the outside to 3/16" diameter for a distance of 3 mm. Thread 3/16 by 40 tpi. Part off at 8 mm from the end. Mount the threaded end in the screwed chucking piece, turn down to 3/16" diameter for 4 mm and thread 3/16 by 40 tpi as before.
Turning the spindle requires some care. As it gets thinner, there is increased risk of catching and bending. Take light cuts (2-3 thou) and use a cutting fluid. I also find it helps to cut only short distances, say 6 - 8 mm emerging from the chuck at a time, repeating this until I have a shaft long enough. Polish the shaft with emery and cutting oil. Thread the spindle 6 BA and part off some 5 mm from the end of the thread. Grip the spindle in a screwed chuck and turn/file the taper. Get this as smooth as possible by burnishing the taper with a piece of hard steel.
4. Gland Nut
Make the gland nut from 1/4" hex bar, threaded 3/16" by 40 tpi. Pack the spindle with some graphite string or PTFE tape before screwing the gland nut in place.
A lever for the regulator can be made from some 1/4" by 1/8" steel strip. A hole is drilled to fit closely over the shaft. A slit is sawn with a saw, and a cross hole drilled and tapped 10 BA for a clamping screw. A piece of 1.5 mm silver steel forms a handle. This latter can be extended out through the rear of the cab if you wish for easier control of the loco. Alternatively, you can turn a disk handle from bar, and thread both this and the shaft 8 BA. Secure in place with a locknut.
6. Blower Valve.
The blower valve is identical to the regulator valve. Here, a longer taper is more important, as we should only need a very small amount of steam fed to the blower. This is screwed into the end of the hollow stay, and a steam supply fed from the bush on the boiler by a short length of pipework. I usually fit a handwheel to my blower valves as distinct from the lever on the regulator. Small brass handwheel castings (12 mm diameter) are available from suppliers such as Blackgates Engineering, Bruce Engineering and others. Leave a short side arm on the casting for indication of the open position. Thread the valve spindle 8 BA, tap the handwheel similarly and fit on the spindle with a locknut for adjustment.
Next Water Gauge and Safety Valve.
If you have any questions, or comments or find any errors in these notes please contact me by email. Email Keith Bucklitch.
Copyright © 1998, Keith Bucklitch
Last Edited - November 1998