Drawing 6 - Pony Truck
1. Pony Truck Horns    
The pony truck horns are made up as left-hand and right-hand assemblies and include the front and rear horn plates, the horn blocks, horn stays, spring anchor blocks and spring guard amongst others. The only castings used in this part are the horn blocks and the horn stays which are made of gunmetal, all the rest is fabricated. I started by splitting the castings into the four horn blocks and machined these all over in the milling machine, all very straightforward work with the flycutter. The base of the horns were drilled and tapped 4BA and the holes in the sides to enable the riveting together of the assembly were marked out and drilled 1/16". The horn stays only need the mating surface machined and this was flycut in the same manner although I also cleaned up the side faces and the bolting face. These were then marked out and the 4BA clearance holes drilled.
I made the front and rear horn plates out of 16swg mild steel sheet and have added extra fixing points for a bit more robustness. Because I am using the front casting, I have also drilled holes for screwing the backplates to it. The spring anchor blocks are just a couple of lumps of brass milled to size and their top and bottom plates made from some 16swg brass sheet. All the rivet holes are 1/16" and the home-made rivets are just some 1/16" dia brass bar cut to length. Assembly has been a bit tricky because of the need to get the horn blocks in the right place and keep everything square. The holes on all of the parts were marked out and centre-popped with extreme care and then drilled and countersunk. To make life easier, I decided to solder the spring anchor blocks and the respective top and bottom plates together.
For this, I used solder paint and then riveted the parts together, using a 4.8mm drill shank in the spring pin hole to aid alignment. Once riveted, the drill was removed and the assembly heated until the solder flowed and finally left to cool for a while. Next I riveted the axlebox guard, just a piece of 3/16" brass angle, to the top of the front plate. Although not on the drawing, I added an 8BA hole in the back of the spring anchor blocks and a corresponding countersunk hole in the backplate so that I could also bolt them together and then riveted the front plate, backplate and spring anchor block assembly together.
The final job on this sub-assembly was to rivet the horn blocks into place. A couple of the holes needed clearing with the 1/16" drill so a rivet was pushed through the top part and the horn stays bolted on to help keep position prior to running the drill through the other three holes. Then all the rivets were hammered up and the whole lot treated to a bit of filing. I also need to dress the horns to suit the axle boxes because, during marking out, I deliberately set the horns to be a few thou tight
2. Front Section    
The pony truck can be fully fabricated but there is a cast iron casting available to make the front part which I have chosen to use. There is very little machining on this but it does need filing or milling to the correct width to allow the back horn plates to be affixed. The hole for the pivot bush was marked out 5.9/16" from the flat back face of the casting but the side-to-side position was just judged by eye. This was then drilled using a 3/8" dia drill followed by a 3/4" diameter drill, although the drawing calls for 7/8". There is also the pivot bush to be made and I have made this from 1.1/8" dia brass, turning the O/D to a touch over 3/4" diameter for a press fit into the casting and then drilling and reaming a 1/2" hole through the middle prior to parting off. This was then pressed into casting.
The side faces where the back horn plates bolt to were machined on the lathe by mounting the frame as shown in the photo and the holes were spotted through from the horn plates prior to drilling and tapping 8BA. I shall use countersunk screws to fix the horn plates to the front frame. The final section that needs attention is the cutout where the centre brace fits to and this was left for the moment as I will file it to depth after I have made the centre bracing bar.
3. Rear Beam    
The rear section of the pony truck is fabricated from 16swg brass sheet and forms a hollow box section. The top plate carries the pressure plates which allows the truck to move laterally and the inside of the box section carries the return spring bar which is used to return the truck to a central position. I have chosen to make mine differently to the drawing and made two side and end pieces, a top plate and a bottom plate. I have also made the centering bar bushes more like the actual rather than what has been drawn. After cutting the two strips of brass that form the sides and ends, I marked out and drilled all the holes in the ends before bending to right-angles. These were then dressed for length and soldered together to form the section shown below. The top and bottom plates were also finished to size with the slots drilled and filed to size, the bottom plate having the longer slot.
I have soldered the top plate into position but have left the lower plate for now because I need access to the inside for setting the springs either side of the centering pin. I also made the centering bushes with their more authentic shape, just a bit of simple turning followed by milling the shape of the base and drilling the four 10BA bolt holes and the spring return bar to fit them which I have modified to suit. I have made the pressure pads from a length of 1" x 3/16" flat brass bar which I hacksawed off at 1.3/8" long and then milled down to 7/8" wide. Next, I marked out the mounting holes and drilled them 6BA clearance and also made up a filing button to help me form the outside semicircles at each end. I had started to mill out the inside form but then realised it would be smarter to get the outside finished first.
The inside was just milled out freehand using a 1/4" dia cutter and nibbling away at the ends to get an approximate curve of 3/4" diameter. It's not that important since, once assembled, they cannot be seen, and the pad cups that ride in the channel can never reach the ends anyway. They were then placed on the rear beam and spotted through, the rear beam was drilled and tapped 6BA and the pads were then bolted down with 6BA countersunk brass screws. Offering up the whole assembly to test the fit, I found that the cups were fouling the sides of the pressure pads near the extremity of travel and riding up. The simple answer was to skim twenty thou off the outside dia of the cups and after that, they slid side-to-side perfectly.
4. Centre support stay    
Another part I have made is the centre support which bridges the axle and is fabricated from 16swg brass sheet, soldered and riveted together. It isn't exactly to drawing but is close enough as it will never be seen once the loco is assembled. I made a tracing of the inside shape directly from the drawing and added an extra 1/4" each end to allow for bending. I also marked out and drilled the 10BA tapped holes in the end sections since these will be spotted through to their mating parts. After the ends were bent over, making a left-hand and a right-hand component, they were clamped together and the rivet holes drilled prior to riveting together.
I didn't get the overall length exactly right but it doesn't matter as it is easier to make the adjustment on the front casting. The top and bottom strips were now formed over and under the bridge and when they were a nice fit, I clamped the ends and soldered the whole lot together. Finally, the ends have been given a very light pass with a milling cutter to make sure they are parallel.
5. Wheels & Axle    
The pony truck wheels were made in the same way as I made my main wheels (see drawing 3 for a full description), that is using soft jaws in my 3-jaw chuck. First op was to hold on the tyre, face the back, drill bore and ream the axle hole and skim the O/D of the flange. All subsequent opeations were done holding on the flange until the wheels were finished. The axle was also done in a similar fashion to my main axles and again I used EN8DM material. However, they don't need quartering so there are no keyways to worry about. I had made them a light press fit but one side was a tiny bit undersize and the wheel went on far to easy, so I employed a trick from my production engineering days which was to knurl the bearing diameter with parallel knurls. I only use caliper knurls on my lathe because they don't put anything like the stress of pressure knurling on the headstock bearings.
6. Axlebox and Covers    
There are gunmetal castings available to make the axle boxes but, as with the main axle boxes, I have made mine from 2" diameter mild steel billets. The back face was first cleaned up on the lathe using the 3-jaw chuck and a 7/16" hole drilled through before the billet was then reversed and loaded to soft jaws with all other turning operations completed in one visit. Using a small boring bar and using the compound slide for varying the depths, I faced the front to length, finished the bearing bore to size and depth, bored the bearing relief at the bottom and finished the 17/32" hole through. I aimed for a light press fit on the bearing bore. Then they were milled to size the same as before
I have made the axlebox covers from some mild steel flat bar. The edges were milled to size on the Cowells mill and then the front faced off in the lathe with a small hole to locate the "Timken" covers followed by reversing and turning the spigot for the bearing bore and the bearing relief. Finally, the edges were rounded off on the linisher and the 8BA clearance holes marked out and drilled. The "Timken" covers are just held in with two-part resin adhesive.
7. Suspension    
Realising that I had yet to complete the pony truck suspension, I decided to get that finished before continuing elsewhere. As with the bogie springs, I will replace these with proper springs at some point in the future but, for now, I will use the supplied castings. There are only a few parts needed to make the suspension - the cast dummy springs, this time in gunmetal, a couple of spring bolts with cross-pins, the spring plunger to carry the coil spring, the spring sockets that rest on top of the springs and the spring guard for over the top of the springs. The springs were held in the milling vice and cleaned up the top and bottom using a 16mm dia end mill before moving to the drill and putting the hole in them to guide the spring plunger. After drilling 5/16", I ground a spare drill and made the hole flat-bottomed as drawn, but it probably wasn't neccessary. The spring bolts were made from some 3/16" mild steel, threaded 2BA at one end and parted off. The drawing calls for 3/32" cross pins but they looked too chunky to me so I used 1/16" material instead - panel pins, actually - and I made a simple drill jig from some spare hex bar to help put the cross-holes in.
The spring plunger was some 5/16" brass, drilled 6.4mm to accept the coil spring and parted off at 5/8" long, the spring guard was made from some 24 swg brass sheet and the spring sockets were made on the lathe by drilling a 4.9mm hole through some 3/8" square mild steel and parting off at 3/16" followed by milling on the side chamfers with the Cowells. I also put two together in the small milling vice and drilled a 1.9mm hole to produce the recess for the cross-pins and then just linished the underside of each so that they sat nicely on the springs.
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