A member of my club is building a Kerr Stuart which is quite a big lump, and he'd had castings made at a local foundry. He offered me the pattern to see if it would do for the Hunslet. He had allowed plenty of spare metal and I found I could just get mine out of it. The Hunslet's cylinders are at a ten degree angle which is why I was only just able to fit it in the pattern. This has saved me a great deal of time and work, so I took him up on his offer.
They are certainly big lumps of metal. The picture, showing the casting in the mill cutting the datum face, gives an idea of the size. The material, which I believe is meanite, is far superior to the castings I used on the Romulant. There are no hard spots or pits.

Boring the cylinder began well enough, taking small cuts and then, almost down to the last cut, the chatter began. Nothing seemed to stop the chatter and the best I could get was by boring at 40rpm with auto feed of .002. Daft thing was, the second cylinder bored without any chatter at all and in half the time. The resulting finish is very good and really not worth honing. However, the chatter did make me have to overbore to get rid of the ridges. So, instead of 60mm I've got 61mm. Probably won't make much difference and I'm sure the boiler will more than cope with the extra demand.

The DRO came into its own when I bored the ports. I'm not sure whether I did the right thing with the ports. I used the recommended sizes in imperial which gives me awkward dimensions for the slide valve as it's now a mixture of metric and imperial. I suppose I could have made the ports 6mm instead of 1/4" as there is little difference, but not wishing to chance anything, I am now reaping the problems.
The full size loco has quite a shallow steam chest with what I think from the GA drawing, is a slide valve which overlaps raised ports. I've stuck to the usual slide valve and made the steam chest as close in size as I can. To keep the volume of the chest as large as possible, I've removed as much material from the inside as possible. The castings from the borrowed pattern, were grossly over-sized for this engine and I spent a long time milling this away.

As can be seen from the photos, the cylinders are not finished. I still have to make the pistons, the fittings, rings and bore the valve rod ends. I am also considering whether it's worth making an enlargement to the chest by fitting a domed top which would look more like the original, although it's not seen as the whole lot is covered.
The one critical part to make is the slide valve itself. Because of the combined measurement systems, I've ended up with decimals of a millimeter. This might not be that critical but would certainly be difficult to achieve manually. So therefore, I think I'll try making it with my CNC set up. At least I know this will be accurate. The first thing I'll do is cut it in plastic so that I can see the result before I commit to cast iron.

I spent quite a long time on the CNC cutting plastic until I was certain that I'd got it right. Cast iron lumps are not cheap. When I'd bought the slices for the endcaps, I'd ordered two more to cut the slide valves. Bigger than necessary, but cheaper than having another smaller bar cut. As can be seen in the photo, the machine has done a wonderfully accurate cut even down to the 0.7mm. I haven't had it cut right through, partly because of the time factor but also because of the length of the cutter. I can cut off the waste material and then finish the other side on the big mill.
I've included the photo to show the little Sherline CNC mill in action cutting the underside of the slide valve. I haven't quite got the software to do what I want when cutting the outside edge. It wants to do its own thing in the corners and I can't get a perfect right angle yet. In this case it doesn't matter.
The finished valve slides show how I've cut away some material as it all adds to the steam chest capacity. I've not attempted to finish the ends exactly with the CNC end, as I didn't want to risk altering its dimensions. The pocket in the centre could have been a slot right across, but the thickness between the bottom and the inside of the underside is only 2mm so I made it a pocket to strengthen it. I could have done it on the CNC but accuracy isn't so important. The dimpled corners can be seen too, but they don't affect the action.

I've copied the same type of glands as on the full size Hunslet. I'm not very keen on this type as it is very easy to over-tighten one side and thus put a side load on the shaft which binds and becomes very stiff. And as there is so much power the engine still goes and then wear appears in the bush turning the bore oval. The glands are made from phos-bronze with brass flanges silver soldered on. The end caps have the flats on ready for the slide rails. These are off-set because the cylinders are inclined. So it was a question of fitting the end cap and then marking the flat using a square resting against the frame. Machining meant judging by eye and no doubt they are not exactly the same distance from the centre which means careful fitting of the crosshead.

This photo shows the assembly so far with the cylinders in temporary position. I've had to move them higher. I found that the bottom of the cylinder would only allow a small movement of the bogie before fouling. I followed the scale exactly and it might have been quite OK, but I didn't want to risk it. The difference from the original is very small. Had I made my own pattern, I could have allowed for this.
It has created another problem in that the centre line of the cylinder no longer passes through the centre of the leading axle. When discussing this with a friend, he said there was a precedence for this as it was once done on the GWR. So now it has risen by about 5mm which hardly shows given the distance from the cylinder to the axle is around 370mm.

As on the full sized Hunslet, I've fitted oil points to the bogie swivel and side control. These will eventually connect to a box on the footplate.

9 March 2006

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