Monday, May 28, 2007

The sandboxes

The sandboxes are glued into the shaped recesses at front and back of the sideframes.

rear sandboxes

These are the same shape so it doesn't matter which side they go on.

front sand boxes

Took a bit of thought to make sure I got these the right way round.

You can see the small cap on the top of the sandbox here. In my over-enthusiasm I filed the top of the back boxes flat. I'll have to represent their caps with a couple of strips of something.

Thursday, May 24, 2007

The bufferbeam stays

These are the external joints where the sideframes meet the bufferbeams.

bufferbeam stay etch

On the prototype there is a plate and a pair of triangular stays at each of the 4 corners. The kit designer says that he omitted the stays on the front corners (from most angles they are hardly visible behind the front steps). Instead they are represented by etched flat lines on the front plates.

The intention is clearly to match the tabs with the slits in the plates and also in the sideframes. I filed so much off the tabs to make them fit that I decided I would be better gluing them together.

stays glued to rear plates

glued against back bufferbeam

plate with flat "stays" at front

Tuesday, May 22, 2007

The fixing screws

The chassis is held to the main body with screws at each end.

I've already fitted the nuts on the outside frame assembly, now I assume I have to drill holes through the chassis to fix the pieces together.

I hadn't been thinking about that when I filled the areas in question with lead shot recently, so I have to remove most of it to drill the holes.

With the frame fitted onto the chassis I start by drilling through the nut from above, gradually increase the drill size, hoping this way to get the holes in exactly the right place. They have to be increased to about 1.3mm to comfortably fit the 12BA screws.

The first one ended up nearer one side, so I had to "move" the hole to the right by reaming with pressure on that side. Although the chassis now runs, it looks from the photo as though I'm not quite central yet.

I do better on the second hole.

body held with 2 screws

Now I find I've introduced a short-circuit. Turns out, when the screws are tightened, they push the PCB underneath the body assembly against the tabs on the top of the chassis.

I probably don't need the screws absolutely tight, but as a precaution I put some sticky tape over the tabs.

Finally I need to cut back one of the screws to keep it clear of the motor. I use a tip that I picked up from my 2mm colleagues recently.

I wound 2 nuts onto the screw before cutting it with pliers.

Once the rough end is filed off, the nuts preserve the thread as they are removed.

short screw clears motor terminals

Monday, May 14, 2007

Fitting the cranks

I've done a little background reading on fitting cranks and quartering them, such as descriptions of how people align the cranks by eye then "fine tune" them by rotating one wheel fractionally relative to the others.

I'm not sure how relevant this is. Since I've already got the balance weights aligned and quartered (by eye) I want to try to glue the cranks onto the wheels using the connecting rod as a jig. However there is some play between the gears, ie each axle can rotate a few degrees relative to the others, so there is no absolute "correct" alignment, and I expect that for this reason I may end up enlarging the holes in the rods to allow for the play. Time will tell.

Still I must mention one tip that I came across - someone uses a set of "loose muffs" so they can slide the wheels on and off easily for testing during construction. Wish I'd thought of that a few months ago...

"Work slowly and carefully", the instructions advise. I'll check things at each stage by making sure the chassis still runs downhill when I tilt my test-track in both directions.

First I check the fit of the crankpins in the rods, and have to open some of the holes out slightly so that the pins will fit through perpendicular to the rod.

Now I fit the first crank on, opposite the balance weight, making sure the pin is perpendicular. Waiting for the epoxy to set I remember I should have cut back the pins before fitting them.

Managed to cut the pin with wire-cutters. I allowed enough for the coupling rod, the fixing washer and a little extra to work with.

Next the front crank. As the glue sets I make sure I can turn the wheels gently. So far so good.

I've filed the first 2 extended axles back to allow the coupling rod to turn freely.

Then the third crank. The clearances are tight, but once I've removed every scrap of excess glue from the axles it still coasts downhill.

For the other side, I cut back the extended axles and crankpins as much as possible before starting.

Then I fit the cranks as before, with the rod still in place on the other side.

Well I'm happy to say, it didn't go that badly. I still have to fit washers to the crankpins (see the left axle above), but till then I'll keep the rods in place with plastic wiring.

Sunday, May 06, 2007

The marker lights

These are fitted front and back above the bufferbeams, attached together by cable.

They're a two-part etch. There's enough material for twenty lights but I only need four, so I can experiment a bit.

Once cut down the middle the part-etches can be overlaid. The part with a slit goes underneath to make a hole for the connecting cable.

I make a couple and cut them off but find that even the tiny sliver of solder I've used is enough to fill in the holes.

They're about 1mm long and it's beyond me to remove the solder. Perhaps I should build them with the wire in place.

The thinnest wire to hand is overscale (0.25mm). I glue the other half-etch on top of it.

The wire is very obviously too thick.

I wasn't able to locate any 0.15mm piano wire, but I got some gauge 8 guitar wire (0.2mm).

Eventually I find a method that works for me. Some part-etches seem to be larger than others, so I start with the large ones.

Use the jigs to cut the wire to length. Solder it onto the lower half-etches. Allow enough solder to bring it level with the wire and file flat.

Fix the top halves with epoxy and trim to shape.

The lowest set here uses the smaller part-etches. I'm not sure which are nearer to scale but I think the larger ones look better, possibly because they make the overscale wire less obvious.

Finally, and this may be too ambitious for me, once fitted the front cable should extend round one side of the bonnet, down and across the steps. I've got several metres of soft brass wire to practice with.