Smokebox

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

Smokebox

 

 

The smokebox is fabricated from 20 gauge material. You can use brass, nickel-silver or even steel if you wish. The petticoat pipe can be made from a piece of copper (or brass) tube. This is available from the model shops in 7/16 OD. (11 mm) All the joints need to be silver soldered because as this is an internally fired locomotive the smokebox can get very hot. Ordinary solder would melt. It is also necessary to make the smokebox airtight in order to maintain the draught through the firetubes.

 

Start by cutting out all the components. Mark out the position of the various holes.

 

 

 

To make the holes for the boiler and smokebox door, I firstly fret these out with a jewellers saw and then mount the plates in the four-jaw chuck, carefully centred and bore out to the finished diameter. I suggest that you cut the smokebox wrapper one millimetre longer than shown on the drawing, and remove any excess after fitting. Also, only drill a small (1 mm) hole for the chimney in this piece initially (or only centre pop it deeply) before bending it to shape. Curve the wrapper around a piece of bar about 40-45 mm diameter and adjust to fit the front/rear plates closely. Cut out the base plate, clamp onto the front footplate and spot and drill through the holes for the steam and exhaust pipes and fixing screws. Carefully note which is thus the front edge of the base plate.

 

For the soldering you will need Easyflow No 2 silver solder, flux and some acid pickle solution. The safest acid to use is Citric Acid. This can be purchased from a home-brewing shop as crystals. As we will need it to build the boiler, I recommend you buy a 500 gm bag as this will work out much cheaper than the small 100 gm tubs. Dissolve about 100 gm of citric acid crystals in about 1 litre of water in a suitable container. Commence the soldering by cleaning all the edges to be joined with a fibreglass brush, paint with flux paste and align the rear plate and base plate at a right angle to each other. Heat to redness and apply a touch of solder on the inside of the joint. Move the flame along the joint so that the solder follows it. Allow to cool to black, then dip into the pickle solution. Leave for a few minutes then remove, rinse and examine the joint. There should be a thin line of solder all along it, without any gaps. If there are any gaps, reflux, reheat and apply more solder. Reclean the edges of the base plate and the front plate, apply flux, align the plates and solder the front plate in position. Clean in the acid as before. Apply the wrapper and carefully align the hole for the chimney with the centre of the front plate. Also align the front edge of the wrapper with the front plate. I use a small toolmakers clamp to hold the wrapper in position for soldering. Clamp at the top, apply the flux and just a touch of solder to the top 5 mm to hold the wrapper in place. Clean and examine before proceeding. Adjust the alignment of the wrapper if necessary, reclamping to close any gaps and proceed to solder both front and rear plates to the wrapper, taking several steps of heating/soldering/cleaning if necessary. When both front and rear plates are satisfactorily soldered with no gaps, apply solder to the base-plate/wrapper joints.

Mount the smokebox in the four-jaw chuck with some soft packing to protect the smokebox and centre the position of the chimney hole. Centre drill and open this out to about 9-10 mm, then bore it carefully until the petticoat pipe is a good push fit in the hole. Clean up once more, flux, fit the petticoat pipe and solder in place. Use only the minimum of solder so as not to prevent the chimney seating flush on the smokebox. File off any excess solder and clean up the smokebox.

 

Chimney

 

 

The drawings show this made from brass, but if you intend to paint the whole chimney then it could be turned from steel if you wish. Cut a piece of 25 mm bar about 70 mm long. Hold in the chuck and turn down to the 24 mm diameter for 55 mm. (I suggest that you turn the chimney on the premise that the lower end is towards the chuck.) Turn the main barrel portion (30 mm long) to 17 mm diameter. Set the cross-slide over to 3 degrees and turn the taper of the barrel. Now, by careful use of both saddle and cross-slide controls, turn the bevel portions of the chimney. The curves can be formed by either a round nose tool of appropriate diameter, or by using a round file. (Take great care when filing in the lathe. Make sure that your hands and especially sleeves are well clear of the chuck and the spinning workpiece.) Before parting off, polish the chimney with emery cloth to remove any turning marks left. Centre drill and drill the chimney for its full length to just push over the petticoat pipe. With a boring bar, you can open out the bore to the same taper as the outer diameter if you wish or leave it parallel. Part off the chimney from the chucking piece.

 

Bore a hole in a piece of wood about 16 mm diameter. Cut the wood block along its length into two pieces. Grip the chimney in a vice with the wood block and file to fit the curve of the smokebox. (Alternatively the chimney base may be fly-cut to the desired radius.) If the chimney is a firm push fit on the petticoat pipe it can be left as is, otherwise, it will need to be secured to the smokebox with some epoxy adhesive or a couple of 10 BA countersink screws later.

 

Smokebox Door

 

 

This can be turned from brass or steel. Chuck a piece of suitable bar in the lathe. Turn down to 38 mm diameter for a length of 7-8 mm. Face the end, centre and drill 1.8 mm (8 BA tapping size.) Turn the register for a length of 2 mm until it just fits the hole in the smokebox front plate. Part off the door at 4.5 mm from the shoulder.

 

Grip the register in the chuck, face the end down to 4 mm thickness, then by carefully using the saddle and cross-slide controls, shape the slope of the door. Finish the shape by filing and emery cloth. Tap the centre hole 8 BA. Turn the knob from a piece of 4 mm bar to the shape shown on the drawing and screw into the centre hole.

Hinge

 

 

The hinge brackets are made from a piece of steel bar, make one hole 1.6 mm diameter, tap the other 10 BA. The peg is turned to fit the holes in the smokebox front. Fit these in position with a short length of hinge pin material to maintain the alignment and silver solder to the smokebox front.

 

The hinge is shaped from a piece of brass (or steel) and riveted to the smokebox door.

 

Turnbuckles

 

 

Make these from 3 mm square mild steel. They are secured to the smokebox with 10 BA hexagon headed screws, just sufficiently tightly to allow them to be twisted to release the door.

 

Next Boiler.

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
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