A little more progress on the Steam Railmotor ...
Most of the motion has now been completed on the RHS of the drive unit. Obviously, much of this is slightly over scale as in 2mm scale this stuff is flipping minute!! Effectively, each of the links has been drilled and cut to size based on what looks and works, rather than a scale version based on any drawings available. The main reason for this is the amount of throw provided by the generic 7.5mm wheel set available from the 2mm Association - I think that the scaled down crank throw of the prototype is somewhat smaller than that provided by the wheel sets used. This means that the for and aft movement of the crossheads in the slide bars is greater than it should be for example.
However, I am perfectly willing to accept such compromises so long as it fulfils my requirements of "waggling about and looking busy". This is after all a layout model
not a museum piece!!
For future reference, the various linkages have had their pivot holes at the following centres :
Union Link - 2.5mm
Combination Lever - 3.5mm + 1.5mm
Radius Bar - 4.5mm +1.5mm
(These were the sizes that I eventually settled on as I had to make 2 of both the Combination Lever and Radius Bar as the originals were either slightly too long or short to provide the movement that worked smoothly).
The photo below shows the motion as it is at the minute - I have still got to make the Return Crank and the Return (or Eccentric) Rod :
At the bottom of the image are 3 failed attempts at making the Quadrant, the one for the LHS motion lies slightly above these 3. As can be seen the one for the RHS motion is in place and free to rock about a central pivot hole. For simplicities sake (and my own sanity), the rear end of the Radius Rod shares this pivot point.
The Union Link, Combination Lever and Radius Bar were pinned together at their respective junctions with small pins made from 0.6mm nickel silver wire - a short stub of wire was left projecting from the chuck of a mini drill, and a fine file used to reduce the wire until it fitted in the holes drilled in the various linkages. The chuck was released, wire pulled out a about a mm, chuck tightened and a fine piercing saw employed to cut off the wire against the chuck jaw. The resulting pin was popped in a pin vice and the head of the pin filed down to leave a little motion pin. The pins were placed in the relevant hole, a piece of cigarette paper used as a barrier, the other rod slipped on, and a flash of solder applied before the excess pin was filed back flush (or nearly so) with the rear of the linkage.
For the Quadrant and rear Radius Rod pivot, a 0.4mm hole was drilled through the bit of meat that I'd left for this pivot hole on the bent up motion bracket slide bars. For the outer bracket of this pivot point, a small piece of 0.018" nickel silver was cut and bent (having had what will be the front face thinned to about 0.010") and soldered in place suitably spaced from the slide bars by a piece of paxolin. Once in place, the previously drilled hole was used to sight the drill so that the pivot point could pass through both front bracket and rear. Then an enjoyable few minutes ensued where I tried to line up all the bits and pass another pivot pin through. Once happy, a flash of solder was applied to the rear of the pivot pin, and satisfied that I hadn't soldered it solid the same was done at the front supporting bracket (again checking that everything was still free to move!)
The reversing rod and crank were bent up from a single piece of wire that passed through the middle hole in the Radius Rod, and the crank pivot point drilled in the space left for it on the bent up Motion Bracket Slidebars (any one would think that I had actually planned how all of this lot was going to hang together reading this!!) Once happy that the bit of bent wire was the right size and shape, the bit where the crank would be was popped in a vice and squashed flat. A small piece of 0.010" was soldered onto this flat, then filed to a tapered crank shape. As it happened, the wire when inserted into the Radius Rod slightly fouled the very slight movement of the latter, so I elected to snip off the projecting wire just past the bend so that to all intents and purposes (to a casual observer, who's squinting and probably not looking too closely anyway) it appears to connect to the Radius Rod.
A further small turning provides the cylinder front face, and when all these little sub-components are joined up and hung on the chassis it looks like this :
So far I'm pretty happy as it seems to fulfil those criteria of "waggling about and looking busy". As can be seen, I've even managed to get a working valve spindle crosshead (if that's what it's called).
Thank you for looking.
Ian