Progress has slowed a trifle at this stage as there was some difficulty in deciding how to construct the bogie. The prototype bogie design for James and Sam are quite different in that one has the horns very visible on the outside of the frames whereas the other has the horns hidden from view on the inside of the frames. Axle boxes The four axle boxes for the driving wheels are from gunmetal and are made in matching pairs. There are various methods for making axle boxes and the approach described ensures that they are perfectly matched and balanced when mounted in the frames. However, one needs to decide if the axle box will have a single flange as per the majority of 2 ½ inch gauge designs or a double flange commonly found with Gauge 1 designs. There are three steps to manufacture - creating a gun metal blank for each pair, drilling the axle hole and finally making the horn slot. Gunmetal is easy to cut with a saw and files, but tends to be more difficult to mill than brass. Start by sawing out two blanks about 44 mm long and smooth the widest flat side by draw filing on a large flat file. Cut out with a sharp saw blade to near as possible the final shape of which is 19 mm wide and 9 mm thick. Finish off using the mill and then draw filing yet again until the blank is perfectly parallel, see middle blank in the picture below.
The next step is to drill and ream the axle holes using the vertical slide mounted on the lathe bed. First blue the face of the blank to be drilled and mark out the centre lines. Bolt the vertical slide to the cross slide bed, mount a gunmetal blank in the jaws of the vice. Use a dial gauge to ensure that the blank is in the right vertical and horizontal planes to the lathe bed.
Start drilling with a centre and then use increasing sizes of drills until just before the reaming operation at 3/8 inch diameter. Take great care to only move the cross slide horizontally to position the vice for the second axle box (easy to forget this if you are not concentrating). Repeat the operations above and you have a blank with matching holes perfectly positioned. Repeat all steps described for the second blank.
You now have two gunmetal blanks with accurately reamed 3/8 inch axle holes. We now need to cut the slots for the horns. First of all clean up the faces that have been reamed around the axle holes, with a file, to ensure that the blank is still parallel. Mount each blank in the vertical mill and using a suitable sharp mill, cut the slot for the horn on each side of the blank. Gun metal is quite hard to mill and so I take 10 thou at each cut to avoid juddering of the mill. If your cutter is not quite wide enough, adjust the setting, taking care to note down what the setting is. Take care to remove any burrs when turning the blank over to mill the other side and keep the blued side towards you otherwise you may forget which side is which.
Eventually you will be able to get the blank to fit just nicely in the horn slots in the frames. The final finish for the box must not be tight, nor must it be loose. I finish off with a Swiss needle file, otherwise the mill cutter might remove too much if you are not careful. Make sure that the horn slots are smooth with no burrs. Split the blank in two, mark them with a scriber or number punch so that you know which box sits in which frame slot. Repeat all the steps describe above for the second blank, ending up with four matching pairs of axle boxes that fit smoothly, without wobble, and not loose in their respective horn slots. The final step is to thread them all on a length of axle rod, mount in the mill vice and carefully mill the tops such that they are all identical when mounted in the horns. Total time taken for the axle box set was 5 hours 15 minutes, split roughly 1 hour 30 minutes for each of the three main steps - making the blanks; drilling and reaming the holes; milling the slots. Horn keeps The four horn keeps are made from suitable lengths of ¼ inch brass angle attached to the frames with 8 BA screws. You will recall that the horns are attached to the frames leaving a 3/16 inch gap below them to take the horn keeps.
My approach is to drill the holes in the steel frames first, then drill the corresponding holes in the brass angle. Hold each angle tightly to the frame with the usual tool makers clamp. Next, carefully tap the 8 BA hole in the steel frames using the top hat device described in an earlier episode. File off the surplus material and you have four axle box keeps. Time to complete the four keeps was 1 hour 30 minutes. Axles Turn up two driving wheel axles from 3/8 inch diameter ground steel rod. I do not use the 3 jaw chick for this operation, but rely upon an accurate 3/8 inch collet. I prefer to press my wheels on rather than using Loctite, however that is my choice. Make sure that the cutting tool is set absolutely parallel and with great care turn each axle end dead parallel and smooth at 1 thou greater than the hole reamed in the wheel. Manufacture of the crank axle parts will be delayed until we have decided upon a single cylinder or twin cylinder. Time taken for this simple turning operation was 1 hour. In summary it took a total of 7 hours 45 minute to prepare the axle boxes, axles and horn keeps for Part 10 - Frame Erection. Bogie A decision was made to adhere to prototype practice and make the bogie with realistic horns on the outside of the frames. First of all make up a steel blank from two suitable bits of steel plate riveted together, one side filed smooth, blued up and marked out. Next cut out the axle box holes and file parallel using your eye and a right angle to ensure reasonable accuracy. I did not mill the slots as absolute accuracy is not necessary. The horns were milled from gun metal slab as brass was not thick enough. Possibly one could get away with brass angle, however I have yet to find any that is actually a right angle, bless the Chinese manufacturers. Small bits of brass were silver soldered to each angle to represent the appearance of the horns. Three holes were drilled in each angle and the corresponding hole in the frames and riveted up with small 1/32 brass rivets. This is most tedious work as it required 24 holes in the brass angle and a further 24 in the steel frames.
The approach to making the axle box was a quickie based on turning circular cotton reel shapes from cast bronze rod in the lathe. Phosphor bronze is much too hard for this. First drill and ream a hole in the rod to the required diameter of the axles - in this case ¼ inch. Then using a parting-off tool, cut a slot to the required diameter and width to facilitate an easy sliding motion in the bogie frame horns. Cut off and repeat until you have four circular axle boxes. This method is very quick and accurate. Finally to prevent the box turning in the horn slot, solder a small length of brass bar into the slot at the same width as the axle slot.
The bogie frame is made up from two lengths of ½ inch brass angle and a slab of steel plate 1/8 inch thick. Rivet the angle to the slab at the correct width. This width is defined by the working out the width between the wheel backs of 58 mm taking account of the depth of the horns and allowing a small gap of say 1 mm to enable the wheels to turn easily. The bogie was finally assembled using a flat surface (glass plate) to ensure that the bottom of the steel frames were parallel. Holes were first drilled in the steel frames, then using the toolmakers clamp to hold the brass angle in place whilst spotting through holes into the brass angle. Drill through and tap the steel frames 8 BA, using four screws to assemble the lot together.
The bogie will be finished with a central pivot, springs and guards during the next part. Total time taken for making the bogie was circa 7 hours. Part 10 will be Erecting Shop, assembly of the rolling chassis. Part 11 will concern the cylinder, crank axle and connecting rod.