Tuesday, January 8, 2008

Tender 1

The frames were already cut out, saving a bit of sawing. I checked that all dimensions were correct, and cut out the buffer beam and drag beam, from angle section. It never seems to be truely square, which shows up on the insides, when the angle fixings are mounted. So I lightly milled them all over, which got rid of the mill scale at the same time. Also the angle sections to lock it all together. I clamped the angle iron in place, and drilled through from the fames. Only then, I marked of the length of these pieces, taking the dimension from the frames rather than from the drawings - at least this way, they all fit accurately. I worked on a surface table, checking that all was square as I went along.

For the horn blocks, I milled the mounting surfaces to size, and riveted them to the frames, slightly overlapping the horn openings. then milled the horn gaps to size. This made sure that the horn slots were accurate and truly square. If I'd milled them completely to size before fitting, the chances are that riveting would have introduced at least slight distortion.

I then had to make up the spring hanger brackets. Twelve of them amounts to quite a lot of work. With small components, holding them during the work can be tricky. It took me a long time to realise that, by far the easiest way is to make up the part as far as possible before cutting them off the stock. So I made them as a 'production run', setting up only once, and drilling all at once - with a cutting allowance between components. Rather than trying to mark out and centre punch the fixing holes (always difficult on such a small area, I set them up the milling vice a fixed length from the end, and drilled them in turn - I still needed to start the holes with a centre drill - without a centre pop, drills, especially small ones, will wander quite a lot.
The photos show progress. I took less than two hours from starting to having them all fitted to the frames.

Before fitting the wheels, I tried the axles and axle boxes in the frames - any small mis-alignment would cause the axles and / or the boxes to bind. A small amount of filing of the axlebox / horn surfaces was needed on one axle to get them to run freely. I also filed a slight curve on the side faces of the axleboxes to allow a rocking movement, to allow for any irregularities in the track.
I then mounted the wheels. I'd been aiming for a slight press fit - some were ok, but others were more of a running fit! A spot of Loctite sorted that. Also, I trimmed a 1/32 off the end faces of the wheels on the centre axle to provide a bit of end float -this will help the 6 wheels to negotiate tighter curves.

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