The outside diameters of the remaining three axlebox brasses have now been turned using the horizontal boring machine. The process is fairly straightforward, hopefully the explanation and photographs will make things clear.
The casting before setting up on the machine. The milled end-face described in the text can be seen as can the rough nature of the casting. (Copyright John Dunn Engineering).
The brasses were previously milled to establish faces on both ends and on the bottom “joint” which butts against the keeps when everything is assembled; these faces are all flat and at 90 degrees to each other. The brass is bolted onto a large box angle plate on the borer using these faces to set-up to. Various parallels, clamps and packing pieces are used to make the set-up as rigid as possible. The facing head is then set up so that it is in the centre of the finished diameter.
The machining process is fairly straightforward and can be broken down into a number of stages. The central portion is roughed out first, this involves using left and right handed tools to remove the material up to the inside faces of the end flanges. Next, the flanges are roughed out.
Rough turning the central portion of the brass. (Copyright John Dunn Engineering).
Finish turning the central portion, the flanges have been rough faced and will also be finished at this setting. (Copyright John Dunn Engineering).
The central portion is then finished from approximately the centre line back to one flange using a right hand knifing tool, the first flange is then faced to finish using the same tool. The central portion is next turned to finish in the opposite direction using a left hand knifing tool and the second flange faced to finish. The distance between the two flanges is critical as this affects the end float (or lack of it!) on the brass when it is inserted into the ‘box so this dimension was made about 0.003″ small to allow for fitting.
Turning the outside diameter of the first (inside) flange. (Copyright John Dunn Engineering).
The next stage involves machining the outside diameter of the flange nearest to the facing head. The centreline of this is pitched 5/8″ higher than the centreline of the central portion so the borer is adjusted accordingly. The flange is then roughed and finished to diameter.
The brass has now been set up to allow the second flange to be turned. (Copyright John Dunn Engineering).
The casting is now taken off the machine and de-burred before being mounted back onto the box angle plate, this time it is turned through 180 degrees to allow the second flange to be turned. The facing head is set on centre, raised by 5/8″ to achieve the off-set, and the second flange machined in exactly the same way as the first, a series of roughing and finishing cuts.
One awkward bit of this job was deciding on a method of determining the radius of the crown of the axlebox on the face which mates with the brass; because it is a semi-circle there is nothing to use as a surface to measure from if an inside micrometer is used. We took a machinist’s ground parallel of known length and thickness and placed it across the crown of the ‘box. Using a depth micrometer, measurements were taken from the top of the parallel to the top of the radius at the outside, centre and inside of the axlebox. With some maths and a formula it was possible to work out the radius using the measurements we were in possession of. It was then a simple case of deciding the best radius to machine the brass to in order to achieve the correct fit between brass and ‘box.
All turning finished , the brass is ready for trying in the axlebox. (Copyright John Dunn Engineering).