Making Piston Valves

Mark Smith


I had been prompted to approach making my piston valves differntly than intended by Martin Evans, I thought it would be worthwhile sharing how I made mine. The locomotive they are going is a Martin Evans 'Royal Scot'

I have already bored the cylinder blocks for both the cylinder itself and the piston valves. So the next step was to turn the Valve Bobbins. I'm taking the 'ringless' approach, so here goes:

For the first, bobbin, I turned the recess in a lump of Cast Iron in the lathe:

Photo 01

Taking into account the diameter of the tool piece, this equated to 25.7mm on the top slide:

Photo 02

The finish was not particularly good, so I cleaned it up with a piece of emery paper. I then centre drilled, drilled and reamed the spindle hole:

Photo 03

I then faced off the end, taking my dimension from the inside of the recess:

Photo 04

Lastly (on the first bobbin), I turned the outside diameter:

Photo 05

The second and third bobbins I approached differently. First turning the outside diameter, then the recess, followed by facing the end and drilling/reaming the spindle bore. That left me with three of these:

Photo 06

Each then went into the vice, out came the hacksaw to remove the stub and then each went back into the lathe, held just tight enough to avoid marking:

Photo 07

 

Photo 08

 

Photo 09

Having bored all three pieces for the middle cylinder, it's on to turning them to length and diameter. As per my other thread, these are specified as being a 'Press fit', but I'm going to loctite them. As such, the diameter is turned to give a light push fit into the bores. Turning the diameters on one of the end pieces:

Photo 10

First, I had turned a mandrel in the 4 jaw chuck, with a gentle taper on to which to mount the liners for turning. Again, this is a light push fit for the liners. Next, I turned the recess for the ports to the steam passages:

Photo 11

Rather than go by the drawings here, I measured the distance to the steam passage 'break through' into the valve bore. The middle section is much simpler. It will need a recess turning on the inside diameter - I guess this is not critical, rather it's to give more volume in the valve:

Photo 12

The nearly finished liners - here they are stacked together alongside the bobbin:

Photo 13

And another view:

Photo 14

However, I have managed to make some progress on the valve liners by getting the ports machined this week. First, I turned a mandrel from a blank arbor, to fit into the rotary table:

Photo 15

The outer parts of the liner were then fitted onto the mandrel, which has a slight taper at the bottom to 'hold' them. For security I also lightly clamped the lot together. I eventually decided on 10 ports, 36deg apart and 3.8mm wide. The 'depth' (in this view) being the specified 0.156" (3.96mm):

Photo 16

Here are all three parts of the liner for the middle cylinder:

Photo17

 

Photo18

Bearing in mind the good Dr's experiences, I will no longer be adopting Don Young's method of 'beating' the bobbins through, rather I will revert to bobbins with cast iron Piston Rings.

I fixed the liners into the bore using loctite and this time without the 'cotton reels' and then cut (drill and end mill) through for the exhaust ports and inlet port:

Photo19

I was going to put grub screws in at each end of the liner to give some additional strength but of course forgot all about them. I'm confident the loctite will do its job though.

I then ran through the bore with a 7/8" hand reamer. It took a bit of effort (spanner on the end) but went through quite nicely and left a nice clean finish:

Photo20

Photo21

If you look closely in the 2nd photo you can just make out some swarf at the end of the reamer.

Now, I'm not one for ignoring the great advice that I normally get from this forum, but I did chose to ignore the good Dr's advice and did follow the Don Young method of beating the living crap out of the bobbin through with some molybdenum disulphide grease. My thinking here being that the reamer removed any hint of a join between the respective pieces of liner. It took a while, in fact it took a long while, but the result does seem to be very good. Unfortunately, in my enthusiasm, I did manage to nick the end of the bobbin, but hopefully not enough to be a problem. Here's the bobbin, part inserted into the liner and covered in the grease:

Photo22

And here's the bobbin itself:

Photo23

The 'nick' being on the left hand, right in the middle.

Time will tell as to whether or not this will work.


Author: Mark Smith (UK).
This article is based on a forum posting in Oct 2010 by the author, and the content is used with permission.