Tuesday, January 8, 2008

Tender Body

The original drawings showed what I think is the Hawksworth tender - I've previously made one in 5 in gauge, and thought the Collett tender would make an interesting change. It has a couple of major challenges - the top of the sides are flared, and also the rear corners of the tand are very rounded, so a tricky compound curve is created where they meet.





I took the basic dimentions from the drawings, and scaled the Collet specifics from a couple of photographs. The fact that the major lines of the tender matched up with the profile of the cab gave me some useful reference points.
Then there's a raised extension which fits on top. I left a tab which I curved to match the lower part of the tender, and riveted and soldered the two together.

The other tricky bit was the rivets -there are several hunders of them, and they're very prominent.

My solution was to make the tender tank out of thinner brass than usual -about 22 gauge. (Another reason for this was that I had a couple of sheets of this size availible!)
It made it easier to form the flared tops - I'd intended to form them around a 1 in dia steel bar, but eventually realised that the front edge of my workbench (originally intended as a kitchen work surface) was about the right radius. So I gently tapped tthe sheet to shape using a rubber mallet. I cut away the flared top in the region of the rear corners, and bent them to shape around a steel bar.

I took a lenght of 1 in dia copper tube and flared it (just like making the petticoat pipe.) It needed frequent annealing to get it to shape. I cut two 90 degree sectors out of this, and soft soldered them into the gaps in flares at the corners. A bit of excess solder and a file blended the curves together and it looked ok.

I worked out the rivet patterns from the photographs. To 'create" them, I made up a jig, to allow me to punch through from the inside. On the anvil of the jig, I formed recesses for the heads, using a ball nosed end mill. The upper leg of the jig had matching holes for the punch, and the two were kept in alignment by two pins in reamed holes. Finally, the punch had a step to fix the depth of the depressions.After a few trials on scraps of the same brass, I was surprised by how well this worked, creating convincing and even rivet lines, and very quickly.

A few real rivets held 1/4 x 1/4 / 1/16 brass angle to the bottom edges and to fix the coal plate. Once this was complete, it formed a very rigid structure, which was attached by 6 BA screws to the soleplate.

I made up the water filters from very fine mesh stainless woven cloth. The drawing showed these to be 1/4 in diameter; I chose to make them a good inch diameter to greatly increase the surface area and reduce the chance of them getting choked or reducing the flow rate. I soft soldered these together! I know stainless doesn't take solder, but it penetrated the mesh easily, and held the filters together, although they probably wern't soldered in the true sense of the word.

I painted the wheels, frames and body before final assembly.

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.

Painting

I recently read the book 'How (not) to paint a locomotive' by Christopher Vine (ISBN: 9780955335907). It's full of useful information, and a council of perfection. I read a lot of the things I'd tried when painting the A3, and although I stopped short of what he'd done, I came to many of the same conclusions.





I sprayed using a Pasche internal mixing air brush, using automotive type paints (NOT two-pack cyano acrylics, which absolutely need an external air fed mask. I used water based paints (sounds odd, but I'm told are the replacement for cellulose paints) , thinned with cellulose thinners.
I started a very light coat of etch primer, just a token rub down, then grey primer. Finally, a few light coats of gloss.

Once its cured - at least a week - I flatted down the surface with 1200 grit wet and dry paper. I then used cutting paste in two grades, and was amazed at the quality of finish I achieved.

I've heard aerosol spray paints being recommended. I have tried them, and been totally disappointed with the results - so much so that I stripped it all off and started again. One of the problems was that the later coats would attack the earlier coats especially in areas that had been well sanded. Now, if you really wanted to create a crackle finish, that would be the way to go.

Its surprising how long it takes, and how little there is to show for the work. Also, there's very little worth photographing. I tried photos of the surface texture as work progressed, but they didn't show anything useful.

I left the completed painting for a couple of weeks to harden before doing the lining. Longer might have been better, but I had no problems. If the paint's well hardened, its easy to wipe off any lining errors and try again.
For lining, I use a draughtsman's ruling pen and Humbrol enamel. I take a few drops of the paint onto the lid of another tin, and add a drop (only) of Ronson lighter fluid - this totally changes the viscosity of the paint, but doesn't significantly dilute it. I made up templates for the cab lining, holding them off the paintwork with bluetak to prevent capillary action from spoiling the lining.
For lining that runs parallel with an edge, I use a set of bow-spring ink compasses, with the point set a long way out to act as a guide. With a little practice, and the knowledge that errors are easily removed, its quite easy. As soon as the paint stops flowing, I clean the pen and re-fill. Occasionally, I get a slight blob of extra paint where I start or stop - If they're only little, I tend to leave them, and trim away the excess with a modeling knife (scalpel !) once the paint's dry.

As you see, I've painted and lined all the loco, but still have the tender to complete building. Not very clever -it's much more efficient to paint it all at once, but I just wanted the loco complete and out of my workshop.