Sunday, January 07, 2007

Brakes - fixing rigging to the shoes

As soon as I cut the rigging from the fret I realise that I should have opened the holes first. It's too flimsy to hold now so I clamp it and use the broach.

Next thing is to lay the rigging over the shoes. The instructions recommend using a piece of thin card to prevent the rigging fouling the wheels, but how thin should it be?

From photos of the prototype the clearance looks negligible but my model's axles will have some sideways movement so I'd better make enough allowance.

Decide it's time to do some calculations to understand exactly what I'm doing.

A member of the 2mm Scale Association has kindly pointed me to a diagram in our Yearbook which shows that the back-to-back (the distance between the insides of 2 wheels on an axle) should be 8.5mm. The total width of the chassis is 7.6mm, so there will be 0.9mm of space between wheels and chassis. So when I put the wheels on I'll need to make sure that there's 0.45mm space on each side.

I put the wheel axles muffs in the chassis and find the greatest movement is about 0.5mm.

So I'll also need to make sure that the muff is central along the axle, giving 0.25mm play either way. If I do that no wheel should ever be more than 0.7mm from the chassis.

Now I can see how this looks.

The shoes and rigging are held in place with wires in some balsa wood, as suggested in the instructions. This took me some time - must treat myself to a proper magnifier soon.

I’ve added some pieces of 0.7mm plastic strip under the wheels to represent the extreme of the wheel movement. The wheels still move freely, and I can see exactly how much clearance is needed.

Turns out some 0.3mm card fits very well, except for the central shoe that seems a bit wider. So removing the strips I use a sliver of card over the two end shoes, keeping the rigging flat with gentle pressure using a small brass rod while I'm soldering.

I tried to use as little solder as possible, but this backfired when I lifted the rigging off and one of the shoes fell off immediately. I re-do them all with a generous blob of solder.

Can't think of a good way to cut the wires really close to the rigging, so I just file them back slowly.

Feel that I’ve taken a long route to get here, but at least it looks reasonably good, and I’ve started to tackle some questions about the wheels and muffs.

No comments: