Friday, March 30, 2007

Just for a change

Just by way of a change, some pics of my finished locos - not me driving! This is a 5 in gauge B1.

And this is my A3 Pacific, again in 5 in gauge.

Cylinders again

I then turned, bored and reamed the valve chest (sorry, no pics of this) and cut the two recessed passages where the ports were to go, using a parting of tool. Then the ports were drilled and filed all the way through. Accuracy is very important here. The valve chest is to be a light press fit into the cylinder - the end parts of the valve chest stick out of the cylinder, and their o.d. is not critical to the thou.

So I turned this part down until it would just pass into its bore in the main casting - then withdrew the tool by a thou (diameter) and cut a bit more. I tried it against the cylinder casting, and checked that it wouldn't enter the bore. I actually went over the surface with some very fine abrasive nylon (like panscrubs) to keep the interference fit to a minimum. The problem is that there is very little metal at the valve area, and it can easily collapse if the press fit is too great.

(I've heard it suggested that a sliding fit and loctite might be better - as retains its strength to over 300 deg C, and the steam temp is unlikely to be more than 150 deg C, then this should be ok. I've also heard it suggested that to make the sleeve a sliding fit - then both surfaces are tinned, and the sleeve is sweated in place. )

I was now ready to assemble the valve chambers into the cylinders.
I actually used a press fit for the first cylinder, and was concerned about collapsing the ports (I put .125 in shims in the ports to keep the dimensions right, and had difficulty extracting them. So I used the soft solder method for the second side. On balance, that's worse. The reality is that, once the surfaces have been tinned, the sleeve - which was a sliding fit - can only be inserted once the solder is molten. Not good for the fingers !!

The next step was to drill the internal steam passages right into the valve chambers, and to fit plugs at the access holes. All easily done using the vertical mill as a drill, with the casting set at the appropriate angle; without the facilities of the mill, it would have been much more tricky to get the angles right.
Then the cylinder end caps were machined up, and the fixing holes set out. I use a dividing head; I've made up a mounting plate which attaches to the lathe cross slide, and sets the dividing head at lathe centre height. It wasn't a lot of work to make, and it makes setting up the dividing head extremely easy. I just put it on loose and fit a length of bar in its 3-jaw chuck. The carriage was advanced until the other end of the bar entered the lathe's 3-jaw chuck, which was then tightened. Finally the dividing head table was locked in place, and the dials / dro were zeroed. ( Its quicker to do than to describe).
I always drill fixing holes at tapping size, then clamp the component in place. Its then trivial to drill through into the fixing surface, at tapping size. When I can, once I've drilled the first hole through, I'll put a tap through both surfaces, followed by a bolt. I do this a second time on an opposing hole, then the location is properly defined, and all the holes can be drilled.

The end caps were removed, and drilled through clearance size.
The back cover was fitted, complete with gasket. Also, I measured the crossheads to determine the exact distance between the slippers and how central - or otherwise - was the piston rod.
With this knowledge, the assembled cylinder was set up in the mill, and the guide bar facings were machined.

The guide bars are gauge plate - (I find that J&L ( offer an excellent range of sizes )
These needed tapping 8 BA for the motion plate fixings. Gauge plate is in many ways lovely to work with, but it is tough, and I was beginning to doubt if the tap would survive. So I put in a new tap, and plenty of cutting oil. It picked up the original thread, and very quickly, to my amazement, the new tap was right through, with practically no effort - and, more importantly, no chance of breaking the tap. I tried an 8BA bolt, only to discover that it wouldn't go in. I checked - I had used an 8 BA tap. I then ran through with one of my older taps. Again, it went through easily, although it was cutting slightly. The end result was an accurate thread in tough material with no risk of breaking a tap. I can only guess that my new tap (again J&L) was slightly undersize - by accident or design, but it made the job just so easy. I've seen serial taps in metric sizes, but not in BA before. The difference was amazing.