Sprung buffers for an Ivatt 2-6-0

Currently on my workbench I have a part finished scratch built model of an Ivatt Class 2 2-6-0 locomotive. I chose this loco as I thought it would give me a few new challenges for scratch building and I happened to have a nice photograph of one sat at Manchester Victoria station. As I was building up the front of the loco one of the “challenges” appeared, namely the buffers. Now the area behind the buffer beam is quite exposed on these locos. The running plates have rolled edges so there are no deep side frames to hide the back of the buffer beam and the attachment of the front steps is also in this area. The usual practice of buffers with a long stem and a securing nut behind the buffer beam would intrude on the openness of this area, so I decided that the buffers would have to be self contained. By self contained I mean that what ever method I used to secure the buffer head it would have to be internal to the body of the buffer, even when compressed. I didn’t want any nuts or bolts poking out of the back of the buffer beam. The final requirement was to be able to take it apart for painting. After sketching out various schemes*, I settled on the one described here.

General Arrangement

Sprung Buffer General Arrangement

The general arrangement of my scheme is shown here.

Core tube

Inner tube for sprung buffer

The core of the design is basically a tube soldered into the centre of the buffer body. I started with making the core tube from 1/8” brass bar. On the lathe I turned the outside step first (110 thou dia.). Note all dimensions quoted are in thousands of an inch. I prefer using imperial and the small Unimat 3 sized lathe I use is graduated for imperial work. The outside diameter is such that the spring can easily slip over the tube. In my case I am using an 1/8” spring. This is then parted-off to give the top hat shape. I then reversed it in the chuck, drilled out with No.48 most of the way. Finishing with a 12BA clearance drill (No.55) at the end.

Main buffer body

Buffer body dimensions

Next item is the body of the buffer. For this loco I used 1/4″ brass square bar  for the body. A short length was chucked in the lathe and then spotted with a centre drill. I tend to drill the larger hole first (No.15) as brass can have a tendency to snatch. I find it easier to control the depth of drill if there isn’t a pilot hole. I then followed through with the smaller (No.35) drill and turned the outside dimensions as appropriate. Once complete I then part-off leaving a thin web for the square base and the back flush. The body can then be marked out on the rear, drilled and pinned to represent the bolts on the base. The core tube when soldered in place gives the small spigot on the rear of the buffer, the buffer beam is drilled 1/8” for location.

Buffer head

Buffer head dimensions

The buffer head is turned from 3/8” steel bar. Where components are unpainted I always prefer to use the same material on the model. Once again from the rear, No.30 drill to the required depth. This drill size is set to give sufficient clearance over the spring outside diameter. The No.58 drill is for 12BA tapping, again drilled after the larger drill, not because steel snatches like brass but I wouldn’t like to drill the full length of the head with such a fine drill. The back is then turned down to be a sliding fit in the body before parting off, turning round and final finishing. I have drilled the No.58 all the way through the head because I don’t trust my self to tap 12BA in a blind hole. It does result in a small hole in the face of the buffer and in some cases this might be prototypical. The alternatives are either to plug the hole after tapping or tap a blind hole, next time I might be brave (foolish) enough to try this.

Finally assembly

Drawing showing buffer compressed

The final piece is the 12BA cheesehead retaining screw, to fit in the core tube the screw head needs to be turned down to be a sliding fit in the core tube. Assembly is then as shown in the diagram. On the prototype the back of the buffer beam is blanked off, I can’t achieve that with this design but the small access hole is not obvious and it frees up all that space behind the buffer beam for detailing. Even when compressed the head of the 12BA screw does not protrude out of the back of the buffer. Next time I’m going to choose an easier loco to model, although I’m glad I tried this one. There were other “challenges”, actually compromises, on this loco with the open frames and running plate widths that pushed me away from narrow gauge and into ScaleSeven.

Buffer shown compressed

Buffers shown fitted

Buffers shown fitted

The published Scale7 article (sprung_buffers-2) and the drawing (buffer.dxf) are available for download in the side bar. * I used the Feynman Problem-Solving Algorithm as cited by Murray Gell-mann:

1. write down the problem;
2. think very hard;
3. write down the answer.