DY1 diesel shunter - a simple scratchbuilt chassis

Article and Photos by Nigel Cliffe

(An edited version of an article which appeared in the October 2003 2mm Magazine)

The DY1 is a small 4 wheeled shunter, introduced by BR in the 1950's. They were fitted with skirts and operated in East Anglia, notably at Ipswich docks.

Locomotive almost finished, still requiring BR logo and numbering on cabside, plus glazing in the cab windows. Sandbox tops are filed from brass rod. The handrails are guitar "E" string.

Due to the relatively large bonnet and the skirts hiding coupling rods and jack shaft, they offer the opportunity to produce a simple quick 2mm scale loco; I'd suggest it's an ideal candidate for a first chassis scratch build.

Though I possess quite a few machine tools, I built the chassis with a fairly minimal set of hand tools - piercing saw, files, drills, cutting broaches, scriber, dividers, ruler, small hammer, small soldering iron, vice, slide gauge (vernier gauge), 12BA tap, pin chuck. Most holes were drilled using an Archimedean spiral drill, though I did resort to a mini-drill in a "Fonly" stand for a couple of holes.

The body of this loco is from a B&H Enterprises etch. I'll comment on the body towards the end of the article.

The chassis is the main point of interest. Following conversations on the email virtual area group, and photographs of the 2002 AGM, I began to think about the use of printed circuit board (PCB) as a frame material. At the AGM, Bill Rankin and Bill Blackburn showed the prototypes of finescale conversions of Farish steam locomotive mechanisms using PCB to allow the use of split-frame wheels. I thought I could take this approach into a totally scratch built chassis.

My idea was to use brass blocks for frame spacers and PCB for the frames. The brass has a number of advantages over the traditional Tufnol spacer: it is heavy (weight low down in mechanism), easy to work, easy to tap (for screw threads). PCB frames can have its copper surface cut to provide insulation where it is required. Standard bearings can be fitted into the PCB to support the axles.

I obtained a drawing of a DY1 and began to plot a mechanism. I draw mechanisms many times model size, in this case 8 times larger. The first observation was that the bonnet was large enough to accommodate a 10mm diameter motor - Faulhaber 1016 was my choice. The DY1 has a large cab, more than big enough to house a gear train leading to one axle. By gearing the two axles together I could remove the need for coupling rods.

My preference for simplicity is to use 64DP gears if practical. Consulting the gearing tables (Yearbook) led me to propose a simple train of 14T & 13T pinions linking the worm wheel to the axles. The wormwheel could be 30T. The gear linking the two wheel axles is raised slightly above centre line to achieve a correct mesh distance.

My large scale drawing also allowed me to sketch the general shape of the chassis spacers required, one at each end of the frames.

General arrangement of mechanism, showing outline within body.

From the drawings I came up with my parts list for the chassis:

Motor - Faulhaber 1016
Screw Motor mount bush
Motor mount bracket - offcut of nickel silver from an etched kit
Frames - 0.3mm thick PCB (0.5mm alternative)
Frame spacers and outside frame weights - 0.25" square section brass bar
Screws 12BA cheese-head brass. Cut down from 0.5" long. (4)

Idler shaft steel, 1.5mm dia shop 3 3-110
Gear axle muffs (4) shop 3 3-101
13T 64DP gear (2) shop 3 3-340
14T 64DP gear (2) shop 3 3-341
30:1 worm and wormwheel set shop 3 3-362
frame bearings (8) shop 3 3-113
7mm dia disc wheels (4) shop 3 3-054 (note printed version of article incorrectly says 6mm diameter)

My motor and mounting bush came from Branchlines in Exeter. The screws, nickel silver, brass bar and 12BA tap can be obtained from Eileen's Emporium (and many other sources).

The frames were made from 0.3mm double sided PCB. I obtained mine from a friend, but I'm told that multi-layer PCB assemblers (try Yellow Pages) might be able to offer off-cuts. Alteratively, Bill Blackburn tells me that the Association Farish conversion kits will be using 0.5mm PCB, and this may be made available to members as blank material. If using 0.5mm PCB then the frame bearings will need to be thinned a little with a file after fixing to the frames.

I selected two pieces of PCB significantly larger than my finished frames.

Firstly, I drilled two frame fixing holes in one frame, and used that as a template to drill the other frame. The two frames are bolted together before marking out and drilling the four axle and idler shaft holes. Marking out was with a sharp scribing point, a ruler and set-square. The centres for holes are marked with a cross, then lightly centre-punched before drilling. I use a sharp scriber as the punch, gently tapped with a small panel pin hammer. The idler shaft holes were marked from the axle holes centres using a pair of divider points. The holes were drilled about 1.4mm, and then opened up to bearing diameter with a cutting broach. Standard Association bearings were soldered into each hole, with their flanges on the outside of the frames.

The frames were then marked and cut to shape using a piercing saw and files.

Chassis disassembled, showing frames with insulation gaps cut, frame spacers and weights. Motor mounting bracket to the front-left.

The two main spacer blocks were cut from a length of 0.25" brass bar. They are initially approximately cubes. A hole was drilled through the bar of tapping diameter for 12BA (1.05mm), and then tapped all the way through. Tapping a relatively thick block requires some patience and care: make sure the tap is held vertically to the hole and that no bend is applied. Rotate half a turn, then back out, then rotate again Regularly back the tap out of the hole to clear any swarf. . I use a small pin vice to hold small taps, not a tap wrench.

I then filed the blocks to shape to clear the gear trains. The front block is L-shape in plan. The rear is more complex, having a U to clear the larger worm-wheel, plus a bit removed from one side to clear the pinion gears. The photographs show its general appearance. Repeated assembly and checking told me when I'd filed enough away from the spacers.

Having made the spacers, I decided I should cut the gaps in the PCB copper. The gaps were cut very carefully with a file, trying to just remove the copper and not the fibreglass substrate. The photographs show the track cuts (both sides of each frame).

The chassis was then test-assembled with the gear train and wheels. The gears were pushed onto the muffs in a small vice. The test assembly allowed me to check that it ran smoothly without a motor.

Assembled chassis (less motor), wheels not pushed fully in to gauge. Shows gear train between wheels, and motor mount.

The motor is attached to the two upwards extensions of the chassis frames using a U-shaped bracket. The bracket is made from two parts; a motor mount with internal screw thread, and a nickel silver U. The two parts are soldered together. The arms of the U are drilled to accept 12BA bolts which in turn fix the U to the frames. The bolt holes are deliberately over-size to allow fine-tuning of the worm mesh.

In order to increase the mass of the chassis further, I added four small blocks of brass outside the frames at each end of the chassis. These were filed to shape from brass bar, drilled to clear and counter-sink (just drill a hole the diameter of the screw-head) the 12BA chassis screws. Those at the worm-end of the chassis have small corners filed back to clear one of the frame bearings. They all have a slight narrowing to clear the steps in the skirts of the body.

Assembled chassis, wheels not pushed in to gauge. Worm drive visible (top left).

When assembled, I carefully tweaked the various bolts to ensure that all four wheels sat level on a piece of glass, and that the worm bearings ran with minimum drag. Using an ammeter is very helpful to reduce drag in any mechanism; the swing of the needle will show any tight spots.

The final assembly will see wires soldered from the chassis block to the motor terminals.

The body for my loco is from a B&H Enterprises kit. The kit includes basic instructions and a tiny bit of prototype information. I don't know where B&H source their artwork, though I suspect it's photo-reduced from larger scale models. The main components are reasonable, though there are problems with the detail parts. There are no nice features one expects from the best etches, such as location tabs to position items. The smaller parts are problematic; I found the cab doors to be too small for the openings and will be making my own. I also only found two door frame elements in the kit, whereas the doors need four (one pair each side).

To fit my mechanism I had to increase the holes in the chassis floor and the clearance in the cab front through to the bonnet area. Both easily done with a file.

The steps which fit into the skirts are not easy to assemble in 2mm scale (perhaps they work at larger sizes) and I substituted a piece of off-cut bent to form a U (lower step - step rear - upper step).

The cow-catchers are just about possible to assemble. They have to be very carefully cut from the etch using a sharp craft knife to produce two combs (one for each cow-catcher). I then bent the edge of the comb to form the lower shape of the cow-catcher, and attached a piece of flattened wire to the upper tines. The excess of the tines were then snipped off with wire cutters and all was carefully filed back to shape. If that's too daunting, then one could leave them off and argue that the loco was just back from maintenance !

Loco body assembled, less cab doors, and cow-catchers standing unfitted.