The left leading axlebox after the boring and facing operations had been completed. The way this was achieved is explained below. (Copyright John Dunn Engineering).
We were now at the point where all of the individual components which make up an axlebox had been either repaired or replaced, fitted together, assembled and we were ready to machine the bearing bores and thrust faces to size. This is a relatively straightforward operation, the crucial element is getting the bores in the right place and making sure everything finishes to the correct dimensions.
Dealing first with the bore: the vertical centre-line was set by Barclays on the drawing and, because the brasses were new and the dimension achieved the required “160º bearing”, there was no need to alter this in any way. The horizontal position was a different matter altogether; this was influenced by the work undertaken on the horns, the horn cheeks on the ‘boxes, and the need to take into account the bend in the frames. In order to get the two axles parallel and in the correct position relative to the cylinders it was necessary to bore the axleboxes slightly off centre, the actual amount varied depending on the ‘box. I would have preferred this not to be the case but I couldn’t make things work any other way; and I understand it wasn’t that uncommon on BR so I didn’t lose too much sleep over it.
The left leading axlebox set up and ready for boring and facing. (Copyright John Dunn Engineering).
The thickness of the thrust faces was determined by a combination of things. Once again the bend in the frames was a major factor, but the amount of wear on the flange faces of both the horns and the ‘boxes, and the steps taken to deal with this, also had to be taken into account. The clearance between the axlebox and the back of the wheel is set by the thickness of the thrust face so it was important to get this correct because it affects the riding of the locomotive.
Roughing out the bore. The tool is cutting into the replacement face on the keep as well as the actual bearing surface, this gives a true circle which makes measuring easier. The keep will be bored out to give clearance in a later operation. (Copyright John Dunn Engineering).
The various dimensions had all been calculated from the measurements we obtained a while ago (see previous posts), they had also been verified by doing a CAD drawing so hopefully they will be correct. I would have liked to have tried the axleboxes in position at this stage, this would have allowed a further check on the relative positions, but I was put-off by the logistics of humping the four ‘boxes all the way to Murton coupled with the time factor. Time will be the judge of whether this was the correct decision!
The finishing cut being undertaken on the bore. (Copyright John Dunn Engineering).
Each axlebox was placed on the horizontal borer so that one of the horn cheeks was on a pair of parallels. This face was used as the horizontal datum and it corresponded to whichever of the horn faces had been used as the datum when setting up and taking measurements on the locomotive. The vertical datum was taken as the bottom faces of the brass. The position of the centre-line of the bore was set from the datums and it was then just a case of boring the brass to the right size to give the required clearance on the journal. I had deliberately made the replacement faces on the keeps slightly undersize on the bore so it was possible to use these as a reference point to measure the diameter whilst boring the brasses; it just kept things simple!
Roughing the thrust face to size. (Copyright John Dunn Engineering).
Once the bore had been completed the thrust face was machined at the same setting to keep everything square. The critical dimension here was the distance from the outside face of the axlebox, an easy measurement to take.
The ‘box has been turned around and the inside of the brass is being faced to size. (Copyright John Dunn Engineering).
The axlebox was now turned through 180° and re-clamped on the same horn cheek as before. This was to allow the inside face of the brass to be faced to size. These were all a straightforward operation and were finished “to drawing”, i.e. 1/32″ proud of the inside face of the ‘box.
The next stage involves some finishing work on the keeps but I will detail this in my next post.