Cylinders

Modelling the Kerr-Stuart Brazil Class 0-4-2ST locomotive in 16mm scale. - Keith Bucklitch

Cylinders

The first diagram shows the cylinder assembly and cross section.

 

Drawing 2 shows the cylinder block and port block.

1. Take two pieces of gunmetal (or brass) and face them in the lathe 4-jaw chuck down to 27 mm by 27 mm by 20 mm. Mark off the appropriate face for the centre of the cylinder bore, centre drill, open out by drilling and boring until just large enough to admit the tip of a 12.5 mm hand reamer (This is slightly tapered, so when the tip enters, there is just enough metal left for the reamer to cut a smooth finish.) Ream the bore at slow speed, taking the block out of the chuck and finishing in the vice by hand if you can not pass the reamer right through in the lathe.

 

Mark the top face of the cylinder block and mill and drill the steam passageways as indicated. Mark each block R(ight) or L(eft). I mark all components to ensure that they are a) correctly assembled, b) easily identifiable as to front/top etc., especially if it is necessary to disassemble them. Drill and tap 6 BA the front bolt hole on the frame face of each block and put the cylinder blocks to one side for now.

 

2. Port Block.

Shape the port blocks from a piece of brass 6.4 mm thick. Mill or file the angle, ensuring that you finish with a fine, smooth face for the valve to slide on. Mark a centre line, and mark the position of the ports. Drill right through, perpendicular to the port face for the steam ports, and half way for the exhaust port. Drill the exhaust passage from the side of the block to meet the port hole. Counter bore 1/8" for 4 mm. Smooth the port face with a piece of fine emery paper on a small sheet of plate glass (or a surface plate) after drilling to remove any burrs or raised lips to the port holes. As before, mark them Left and Right.

3. Steam Chests

 

Make the steam chests from a piece of 6.4 mm (1/4") brass. Mark out the cavity and mill this or chain drill, saw and file to shape. Mount the steam chest in the 4-jaw chuck, centre with a No 0 centre bit and drill 1/16" for the valve rod. Turn the stub to 3/16" diameter and thread 3/16 x 40 tpi, before removing from the chuck. Mark the position for the studs, and drill 4 1.4 mm holes right through the steam chest. Drill and tap the 1/8 x 40 tpi hole for the steam pipe, making sure you drill the correct side of the steam chest. (If you do drill the wrong side, simply turn a tapped plug, screw it in with a touch of Loctite to secure and seal it. File off any excess.) Again, mark. Left or Right as appropriate.

Using a small amount of superglue, carefully position the steam chest on the port block (make sure that you are on the port face side). Using the holes in the steam chest as a jig, drill 1.4 mm holes through the port block. Tap these 10 BA after splitting the steam chest away from the port block and cleaning off the adhesive. Open out the holes in the steam chest to 1.8 mm (10BA clearance).

 

4. Steam Chest Cover.

 

Make the steamchest cover from 1.5 mm brass, and using the steam chest as a jig, drill through 1.8 mm for the fixing studs. Make the studs as indicated from 1/16 silver steel, tapped 10 BA at each end, Insert into the port block before soldering the block to the cylinder block. This will prevent solder tracking up the screw holes and blocking them. It may also fix the studs in place but this is no problem. Use tinman's solder to join the port block to the cylinder block, making sure that you match the pairs.

5. Cylinder covers.

Chuck a piece of suitable brass bar in the lathe and turn to 20 mm diameter for the front cylinder covers. Turn a register to fit the cylinder bore carefully with minimum shake. Part off from the bar. Mark each one for the cylinder it fits.

6. The rear cylinder covers are turned from the same brass bar as the front, but special care needs to be taken to ensure that the piston rod hole is concentric with the bore. The important thing is to ensure that the register is a very accurate fit to the bore before drilling the rod hole. If there is the slightest doubt, throw the piece away and make another.

 

To mark the holes for the bolting screws, I usually make a jig from a piece of mild steel bar. Turn and bore this so the register just fits into it. Before parting off from the lathe, draw the lathe tool across the face of the jig to scribe a line, rotate it 90º and scribe a second line. Mark a circle at 16 mm diameter. Part off from the bar. Centre pop the intersection between the circle and the scribed lines. Drill 1.4 mm.

 

Place the jig on the cover, fitting it to the register, clamp in place and drill 1.4 mm holes. Repeat for all four covers, front and rear. Place a front cover on the cylinder block and spot through. Drill 1.4 mm by 6 mm deep and tap 10 BA. The rear covers require careful placement to ensure the slide bars are at the top position. Again spot through, drill 1.4 mm by 6 mm deep and tap 10 BA. Open out the holes in the covers to 1.8 mm. The gland nuts for the rear covers should be made from bronze or gun-metal. Phosphor-bronze is available as rod or hexagon bar in standard sizes. Mount a piece in the chuck. Turn down to 3/16" diameter for 4 mm length. Thread 3/26" by 40 tpi. Drill 2.3 mm diameter for 10 mm and ream 3/32". If you have used bar then knurl the grip portion of the nut, if hex, then part off now. Part off at a total length of 7 mm. Make two nuts.

 

7. Pistons

Make the piston rods from 3/32" stainless steel. Turn the rod down to 2.2 mm diameter and thread each end 8 BA as indicated on the drawing. The short thread is screwed into the piston.

 

Turn the piston from gunmetal, brass or aluminium until it just enters the cylinder bore and slides freely. Centre drill, open out to 1.8 mm and tap 8 BA. Try to leave a slight taper on the thread. Turn a groove for the 'O' ring. This should be 0.025 mm (less in depth than the thickness of the 'O' ring, so as to compress it ever so slightly against the walls of the cylinder. The groove should be 0.5 mm wider than the 'O' ring to allow it to roll slightly as the piston moves. Put the piston rod into the tailstock chuck of the lathe and screw tightly into the piston. Slacken off the tailstock and check the rod spins without wobble. Part off the piston at the required width, chuck the rod in the lathe and smooth off the edges of the piston with a fine file or emery cloth.

Fit an 'O' ring on the piston, fit the rod into the rear cover and temporarily bolt the covers in place. Place a finger over the front port hole and pull the piston back. Release it and it should fly back to the front with the air pressure. If not, the cause is most likely a leak around the cylinder cover which can be dealt with later. If the piston movement seems tight, check for any binding at the gland. If the gland is tight, try the effect of a half turn on the gland nut. If this frees the movement, ease the hole very very slightly with a round file or reamer. If the gland is free sliding, then you probably need a deeper slot in the piston for the 'O' ring. If you need to deepen the slot, it is important that you have the piston running truly in the lathe. I suggest you make a split chuck to grip the piston rod. Take a scrap piece of 6 mm rod, (about 12 mm long) chuck in the lathe, face, centre, drill and ream 3/32" for the piston rod. Mark the end of the rod by the number 1 jaw (so it can be replaced) and remove from the chuck. With a hacksaw, cut a slot along the rod, through into the centre hole. Replace in the chuck with the mark alongside No 1 jaw. Insert the piston rod and tighten the chuck. The piston should now run truly.

Assembly

At this point, assemble the cylinder components, with a smear of Gasket compound (obtainable from car accessory shops) between the cylinder covers and blocks. Wrap a 2" length of PTFE tape (from plumbers merchants/DIY shops) around the piston rod, and screw in the gland nut until the rod is tight. Slacken off the nut until the piston rod moves freely without undue tightness, but the tape should be slightly compressed to seal the gland against steam leakage. Place a finger over the steam port and pull the piston out. When you let go, it should snap back sharply. If not, check for leaks either at the covers, gland, or the joint between the cylinder block and the port block.

 

 

Copyright © 1998, Keith Bucklitch
Last Edited - October 1998

1