Following the success of my first scratch built locomotive in 2mm scale (my Small Metro Tank), for my next engine project I wanted another saddle tank - this time an outside framed "Buffalo".
This page has been divided into two sections : Chassis, and Body
Before I could make a start on the chassis proper, I first needed a set of coupling rods. The 2mm Association sell some etched coupling rods for the correct wheelbase, but unfortunately they are straight fluted ones - I really needed fish-belly type. However, since I had the etch to hand I decided to use the Association ones as a jig to drill the axle holes (and to test the chassis later) while I drew up some fish-belly ones in CAD to be custom etched.
To start the chassis, a piece of 6mm square and 1mm flat brass strip were cut over long, and bolted together outside the extremities of the finished chassis. The idea being that the parts could be separated and rejoined at any time with these fixings. The axle centre line was scribed along the length a suitable distance from the chassis top, and a 0.5mm hole drilled where the first axle will be.
While in the vice, the fixing positions for the 12BA countersunk bolts which are used to hold the two halves of the chassis together were drilled 12BA tapping size, the 1mm section was unscrewed from the main chassis block so that the 12BA holes could be opened out to take nylon plugs (which were glued in place and cut off flush with the outside of the main chassis block). The pair were then re-united secured with the 12BA bolts outside the finished chassis, and the nylon plugs drilled 12BA tapping size using the holes in the 1mm thick section as a guide. Once removed from the vice, the parts were again separated so the the 1mm section could have its 12BA holes opened out to 12BA clearance and countersunk. The nylon plugs were then tapped 12BA, and the pair reassembled to check that these new fixings would do their job.
Because the Buffalo class had outside frames, Association wheels with extended axles were purchased, unfortunately the wheels do not come with Fly Cranks so these had to be manufactured. A sheet of 0.018" nickel silver was used to fabricate the cranks, a 0.7mm hole drilled in a piece of the material for the spigot on the extended axle, and a 0.5mm hole was drilled 0.080" (about 2mm) from that hole - For consistency across the cranks, a little jig was made with a 0.7mm peg in it onto which the piece of nickel silver sheet was placed so that the 0.5mm hole could be drilled 0.080" away from it on a vertical drill. Finally, the crank was then filed to shape around these two holes.
Through experimentation, I found that a rivet spacing of 0.020" (0.5mm) gave a fairly good (if not completely accurate) impression of the rivets used to join the panels on the saddle tank.
But first, a pair of tank profile pieces were fretted and filed to shape from some 0.018" nickel silver sheet. These profile pieces form the "end caps" for the saddle tank. Once the profile was established, a piece of kitchen foil was wrapped around the profile and the end points of the saddle profile marked. When flattened out again, it was a simple matter to measure the distance between the marked points so that I knew how wide the sheet needed to be to form the saddle tank wrapper (from drawings I already knew how long the tank was).
A piece of 0.005" brass sheet was cut slightly over-size and mounted in a vice on the lathe vertical slide, and the embossing of the rivets performed. Once complete the resulting sheet was finished to size and carefully rolled until it fit snuggly around the end profile pieces. To complete the assembly the profile pieces were soldered in place.
A footplate was cut from 0.010" nickel silver sheet, and the front and rear buffer beams were cut from 0.018". The buffer positions being marked and drilled 0.8mm ready for the buffer shanks. The front buffer beam was soldered into position before the outside frames were initially tacked in place where I thought they needed to go, and finally the rear buffer beam was attached. Because I purposely didn't cut the hole in the footplate for the wheels, motor and gearbox until after this assembly I did find in the end that I had to drift the outside frames further towards the footplate edge to clear the fly cranks properly.
The next stage was to cut the hole in the footplate to accommodate the tops of the wheels and the motor, etc.
Once the footplate could fit over the motor/wheels, etc., the body fixing holes were transferred from the holes present in the chassis. Once the footplate could be bolted to the chassis, trials were made and small amounts of metal removed from the footplate and outside frames until the wheels no longer caught on the footplate or the extended axles did not catch the outside frames.
The next operation was to add the wheel splashers. For these I turned up some "dishes" which had sides and bottom only a few thou thick. Chords were then marked on these dishes which when cut off with a piercing saw provided the 6 splashers. As each splasher was added to the footplate a further trial was made to ensure that I had not introduced either a short or binding with each new addition - doing this may be slightly more time consuming but if a short is found one knows instantly what has caused it and the offending splasher can be removed or drifted slightly until all is well again.
The buffers are fairly simple turnings, the buffer head and shank turned from 3mm silver steel rod, and the buffer housings from brass. I made more than the 4 I needed in case any were sacrificed to the carpet god. The buffer housings were soldered to a small piece of 0.006" nickel silver sheet, which was cut and filed to provide the square foot of these castings - no fixing bolt detail was attempted in this scale!
To Be Continued...