Drawing 2 - Pony Frame and Pump
1. Pony frame    
The next parts I made were the two arms of the pony frame and the associated stretcher. Both of these items were very easy to make but I am including all the parts I've made for completeness. The pony frame arms are just an exercise in marking out followed by lots of arm-ache and some swearing (or was it the other way round?). I started with two plates of 3/16" black steel sawn to 11.1/8" x 3.7/16" and marked out and drilled all the holes while the plates were still rectangular. Next I marked out the profile and and then it was sawing, filing, sawing, filing, sawing.....At least I managed to get the holes correct where they connect to the mainframes. I don't know why I didn't use the CNC mill to profile these at the time because it would have made light work of them. An ideal job also for laser or waterjet.
2. Pony frame stretcher    
The pony frame stretcher is four pieces of 1/4" x 1.1/4" flat mild steel which I welded together and then milled to be a snug fit between the two lots of frames, the width being set by clamping one of my spacers close to the end of the mainframes. Then the fixing holes were marked out and this time the holes are tapped 2BA so that bolts can be used to pull the whole lot together. There is also a 3/8" reamed hole at the bottom to take the pony truck pivot pin. The two cutaways at the front are to clear the rear spring hangers on the mainframe.
3. Pump Stretcher    
The pump stretcher is just a bit of 10 swg mild steel plate and a couple of bits of 1/2" x 1/2" x 1/8" m.s angle. The plate was marked out in the normal way and all the holes drilled, the only awkward ones being the two 1" holes. The six pump mounting holes were done using the co-ordinates from the Zeus book, a most useful reference book in the workshop and a mine of information. I had a 1" blacksmiths drill on a 1/2" shank but didn't fancy drilling this size from scratch so I used a 24mm hole saw instead and used the 1" drill to open up to size. This was done on a bit of 12mm MDF and I did make sure that the plate and MDF were well clamped down on the table. I wasn't too worried about accuracy here since the top hole is just clearance for some pipework and the lower hole is for locating the pump body - I can machine the O/D of that to suit.
My steel angle had a bit of a ropey finish so I cleaned up one outer face on the belt linisher and marked out one of the rivet holes on that face, making sure that, once assembled, the angle would be about ten thou wider than the finished size. I also marked out, drilled and tapped the three frame mounting holes on the other face of the angle. After dropping a rivet into the one drilled hole and clamping similar to how I did the top stretcher, the other three rivet holes were spotted through and drilled with the rivet going in each time. After messing up some riveting once before by getting the holes slightly out of position and having to open them up I always follow this procedure of drill, rivet, drill, rivet etc. Once the assembly was completed, it was clamped to an angle plate and the outer edges were skimmed on the mill to clean the faces and end up with a snug fit between the frames. I may leave the pump out when I see how well the injectors work but the pump stretcher can stay, though, as it helps to stiffen the frames.
4. Drag Box    
The drag box came as two identical castings which are machined all over but, strangely, not bolted together. These could just as easily be made from cast iron or mild steel billets which would be a lot cheaper although a lot of pocket milling would be required. It acts as both a stretcher for the rear of the pony frame and a pivot point for the drawbar to connect the tender. It also contains the cylinder cock bell crank assembly fixed to a machined area on the lower part and with a drilled hole on the upper part. The first thing I did was to clamp each casting to an angle plate, squaring up as previously shown, and milling the four edges to final size.
Then they were loaded to a milling vice and the top was flycut and, as can be seen, my flycutter was smaller than the workpiece. It matters not because these surfaces have no function and cannot be seen once built up. Having a cleaned-up surface makes marking out of the holes easier, though, so I felt it worth doing. Also, the inside face of the boss of each casting was flycut to finished height and the area where the bell crank assembly mounts to the lower casting was machined flat. After this, all the holes were marked out and then drilled and tapped as appropriate. The drawpin holes I drilled 15/64" and will drop a 1/4" reamer through them, by hand, after assembly to get them in line. I chose to make this assembly a little different to the drawing.
I made the crank arm as per drawing but didn't bother to make the fancy pin and braze it to the crank arm. The picture shows the simplified parts that I made and the pin, which is just a bit of 3/16" unhardened silver steel, just pushes through the side of the casting, through the spacer and the crank arm and into the bearing. It needs nothing to keep it in place because the whole assembly is trapped between the frames. I made sure that the parts were a reasonably accurate fit between the pony frames and also made sure that they were square otherwise they would probably put a twist into the frame when I bolted it all together. I also noted that the tapped holes in the sides of the top half are at different positions to the lower half. It would have been easy to assume they were symetrical and just mark out both halves the same.
5. Pressure Brackets    
The two rear pressure brackets help connect the rear beam to the pony frame and also carry the hemispherical pressure pads which sit in the sliding cups on the rear crossbar of the pony truck. I used gunmetal castings but I think I would make these from mild steel bar another time. They didn't need much fettling and it was just a case of milling them square and doing the holes.
Although the picture shows all the holes drilled, I didn't mark out those for connecting to the rear beam. After bolting the dragbox to the pony frame and the rear beam to the dragbox I spotted through the rear beam (which will be made next) to the pressure brackets behind.
6. Rear Beam    
The rear beam comprises the large full-width plate from 18 swg mild steel and the main beam from 10 swg mild steel. The main beam has two rubbing plates for the tender buffers riveted to it, and the central stand-off bracket which the drawbar passes through is bolted to it with 6BA bolts. Both plates were first marked out and cut to shape by hand, except for the platform support cutouts which will be done later, and were then co-ordinate drilled using the DRO on the vertical mill. I could have marked out and free-drilled the holes with confidence but it was easier on the mill.
The drawing suggests marking off eight of the holes from the pressure brackets but I have chosen to do it the other way round, spotting through the beam to the pressure bracket. The central stand-off bracket is a fairly rough gunmetal casting and a little thought went into the machining sequence. I decided to first mill the top and bottom edges square to the sides but left twenty thou up on finished size and then face the back to the nominal size.
To get it level, I set four screws into the rear beam and adjusted them to the same height. This then went into the bottom of the milling vice and the casting settled on top. There is no size given on the drawing but scaling from the drawing, which is normally inadvisable, worked out at about 125 thou. It wont matter as long as I remember to make the drawbar to suit, not just to drawing. I could have made this bit in the 4-jaw chuck but it's a bit quicker in the mill. Then it was back onto the angle plate to finish the edges all round, a load of filing and sanding, followed by marking out and drilling the 6BA clearance holes. Be aware that the hole positions are different on the tender pivot block - 1.1/8" apart instead of 1". I had to modify my tender front beam because I didn't check this detail.
It is essential that the holes for fixing to the pressure brackets are drilled and tapped before riveting the rubbing plates on since they get covered by these plates and tapping would be nigh on impossible after. The photo shows the rubbing plates riveted on and the central stand-off bracket about to be bolted on. Finally the two plates are fixed together with nineteen 10BA nuts and bolts
7. Main Horns    
The main horns were supplied as gunmetal castings and need machining all over, although there was not a lot of metal spare so a little thought was needed to ensure a good cleanup. The important bit is obviously the fit to the frames so I decided to try holding on the inside edges. How I solved it was to use my self-centering 4-jaw chuck on the mill which I first set up square to the table. By expanding the jaws, I was able to get a good grip on the horns which allowed me to mill the tops and outer edges of the flanges and the seating faces. These in the picture have been finished but this is how I held them.
Once I had a flat face and square edges, it was easy to set up in a normal milling vice to machine the opposite side and drill the holes. The trapezoidal edges were formed by using a sine bar and bolted to an angle plate. The inner faces of the horns have yet to be machined. There is only about twenty thou per side to come off and these may be milled to size whilst bolted to the frames to ensure the axles are all parallel to each other but I will need access to a larger machine for this. It was at this point that the extra shaping was done on the frames to ensure a snug fit for the horns. Here they are just held on with a couple of bolts in each.
I needed to think about milling the internal bearing surfaces of the main horns and I considered it easier to make the axle boxes to fit the horns rather than making the horns fit the axle boxes. In an ideal world, I would plonk the mainframe assembly, complete with horns bolted on, onto the bed of a Bridgeport, true it all up, load up a long-series 1" dia end mill and finish machine the whole thing in one visit. However, using my little Cowells mill for something of this size is stretching the envelope somewhat. I decided to try and use the precision milling vice to grip the horns on the frame locating flanges but first I had to find a novel way just to hold the vice because I ran out of y-axis travel if I mounted it the usual way. Once I had worked out the best direction to work in, I then loaded the first horn casting to the vice. They are all a good fit to the frames and I would have no trouble with repeatability although to help keep them square, I dropped a couple of large dia rivets through two of the mounting holes to locate against the side of the vice. However, as I tightened the vice the feel was wrong - too spongy - so I measured the gap at the bottom of the horns, released the vice and measured again. I was springing them closed by about seven or eight thou so this was no good. I was also getting a slight lift from the moving jaw which was affecting the squareness.
I decided to treat the vice as a fixture and just lightly nip the jaws up and then use clamps to hold the workpiece down. It's getting a bit busy on here now, and it's going to take quite a while to machine the six horns. I was able to clock off of the outer edges of the external flanges to find the centre line and had to be quite accurate on this because, even though they are symetrical, three of the horns are mirror-imaged (left and right handed) but I can't reverse my fixture so any offset from the centre line will be doubled once the horns are reassembled.
Now it was just a case of taking multiple passes up one side, across the top and down the other side until I had about twenty thou left to come out side to side. I used a 6mm carbide end mill for this as any of my smaller diameter cutters have too short a flute length and I was taking about ten thou per pass. The top didn't need so much material removed and was finished during one of the earlier passes. Before making the final cut on each, I re-checked my centre-line position and calculated the offset either side. The finish size is 1.500", the cutter is 0.236" so the offset is 0.632" either side of centre, not forgetting to allow for backlash. And to finish the job off the corners were filed out once all machining was complete and the horns bolted back into the frames. I checked them with 1.1/2" worth of mild steel and they fit OK but they can be eased to suit with some emery on a stick once the axle assemblies are made if neccessary
8. Pump Body    
Although the drawing calls for the pump body to be made from a gunmetal casting, I decided to machine mine from 2" diameter brass since I had some in stock. The first alteration I made here was the smaller diameter of the pump body. The drawing calls for a 1" x 32 tpi thread on the end with a 15/16" relieved dia along to the 1" diameter locating spigot. To me, this is a nonsense and I decided to make the diameter 24mm with a M24 x 1mm pitch thread on the end. I will also change the plunger to 16mm and use a 4mm section O-ring. After facing my billet to length, I held on the last half inch using soft jaws in my 3-jaw chuck and did all the maching in one visit, starting with centre- and drilling the bore, drilling the small hole at the bottom, turning the two outside diameters, reaming the bore and finishing with screwcutting the thread. For this, I used a standard screwcutting tool with a 1mm pitch insert. The advantage of using an insert is that as soon as you see the outside diameter getting a light skim then you are close enough to size to stop cutting and you have a full-form thread. When I originally made this part, I had the luxury of an Ainjest high-speed screwcutting attachment on a Colchester Triumph 2000 so there is no discernable run-out point but if making it on my simple centre lathe today, I would just create a decent length of undercut and run into that. The length of thread is unimportant as long as it is long enough to allow the nut to screw on fully.
To form the back section, I held the part in a 3-jaw chuck on the mill and machined the four faces to size. The suction and delivery connections were made by holding the work in my normal milling vice and drilling and tapping the first hole 3/8" x 32 tpi and then turning the job over and doing the same again from the other side, bearing in mind that the two holes are different depths. I also drilled the through-hole at this point, being careful because I was breaking into the cross hole that was drilled to depth while on the lathe. Final job was to mark out and drill and tap the six mounting holes which are on a PCD of 1.1/4". The drawing shows a distance of 7/16" between holes but this is incorrect, the distance should be 0.442". When I made the pump stretcher, I used the proper co-ordinates and have to do the same again here if I want everything to fit!
9. Pump Nut    
I made the pump nut, in one visit, from 1.1/4" diameter brass held in the 3-jaw chuck of the Colchester Triumph and, once again, had the benefit of high-speed screwcutting. First, I drilled at 9/16" diameter to about 1" depth and then bored it out to 0.635" to clear the plunger diameter of 0.629" (16mm). Next I bored it out to 23mm (the core size of the thread) and finally created a 24mm diameter undercut, 3/16" long, at the bottom for the thread to run into. Using a boring bar with a triangular tip meant I could do this all with the same tool. The "O" ring will sit in this undercut. Then I screwcut the thread using a 1mm pitch insert and using the pump body as a gauge. Finally, the O/D was given a light skim to clean up and the nut parted off. I haven't put the 6BA tapped holes in yet, they're only for locking the nut to the body.
The plunger is just a bit of 16mm stainless steel with some simple machining applied to it. Here is a picture of the pump assembled on its stretcher and fitted between the frames. The pump strap and eccentric will be made after I have made the main axles. If I was making this assembly with the machines I have today, I think I would make an M24 (or imperial equivalent, say 40 tpi) tap out of a bit of silver steel and make the nut first, tapping the thread. Then I would make the body to suit the nut. This is because screwcutting upto a shoulder in a blind hole can be a bit daunting.
10. Pump Eccentric    
With the pump eccentric my first instinct was to face, turn and form the groove as one operation followed by loading to the four-jaw independant chuck and machining the bore and boss as a second operation. However, I felt that I would likely damage the shoulders of the eccentric with the amount of work needed on the second op so I decided to do it in three stages instead. Starting with a 1" long billet of 2" diameter mild steel, I faced the back to clean up and skimmed the O/D to 1.7/8" diameter by 1/2" long, then turned it round and faced it to 7/8" overall length. This meant I could load the billet to the four-jaw chuck and tighten it up firmly with little risk of damage.
Next, I marked out the middle of the offset and centre-popped it followed by loading to the four-jaw chuck and, since I didn't need a high level of accuracy here, true-ing up with a hard centre in the tailstock. Then it was a straight-forward matter to drill and bore the 3/4" diameter hole and turn the boss to 1.1/4" diameter by 7/16" long. I don't have a 3/4" reamer so boring would have to do. It's locked to the centre axle with a couple of grub screws, anyway, which will pull it square to the shaft. The last job on the lathe was to hold the boss in the four-jaw chuck and true up the outside before machining the bearing area of the eccentric. For this I used a parting tool slightly modified with a notch ground out of the middle. I did, however, play around with two saddle stops to control the width as I didn't want the fit to be too sloppy. To finish, the eccentric needs to have the two drilled and tapped holes in the boss, which I have made M6 because I have grub screws that size. I decided to mount a piece of 3/4" diameter bar in a vee-block and clamp it on the drill table.
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