Sunday, May 27, 2007

Boiler 2

I decided that I didn't want to cut holes in any of the plates until I'd formed the firebox wrappers - so I started on those next. One way to form them is to make a wooden template and beat the annealed copper around them. Or you can form the sheet using bars of the appropriate diameter, which was the way I chose to go ( not being an enthusiastic woodworker !)

Once annealed, the copper is incredibly soft, and needs little more than hand pressure to start to form it. I used various formers, as the pictures show. This firebox has a combination of inside and outside curves (most do) which makes it arkward.
I just bent a bit at a time, checking against the flanged plates. A good fit is important, as silver solder has no gap filling properties.
When the copper stops bending easily, its time to re-anneal. It took me five or six heats to get the inner firebox wrapper to my liking - less for the outer.
Incidentally, I deliberately left the sheets oversize; putting lots of bends and folds in sheet is difficult enough without having to worry about having enough length - the first fold on my outer sheet ended up 1/4 inch from where I'd planned it, so I was glad of the excess. I won't trim the excess length for a long while yet.

Boiler 1

I know several model engineers who wouldn't touch boiler making - it's not for everyone. You need several sources of heat - a lot of it, somewhere safe to work, and you need to be aware that - unlike most aspects of model engineering, it is possible to get into a situation where all your work - and expensive materials - are irrecoverably scrapped.
I've got to say, I like the challenge, and satisfaction that its 'all my own work'. Then there's the lack of waiting time. Whilst its also true that there's a significant cost saving, this is definately not a good reason for doing it.

I bought a kit of materials from Reeves- this comprises all the copper sheet, bar and tubes. You really do need to have the correct materials, all in good condition, and, these days, of traceable quality.
Copper is interesting to work with. After annealing, its amazingly soft, and can easily be shaped - until it reaches a point where it work hardens. Its then time to stop and re-anneal before continuing. This is how the flanged plates are formed. It requires patience, and some very substantial formers . The formers are a lot of work to make unless you have a serious bandsaw, and are of course only required during the making of the components.

The firebox front and rear plates (Inner and outer), and the front tube plate were bought already flanged.
The boiler barrel is tapered - 3/8 inch difference in diameter over 10.5 inches, which works out as a 1 degree taper. As is typical of GWR practice, all the taper is all on the top of the boiler, with the underside lying horizontal. The drawings (and materials supplied ) require the taper section to be made up from flat plate. But there are alternatives - it could be made up from extruded tube, slightly streatched over most of its length. I've never done this, so can't comment on how difficult it is. I did seriously think about abandoning the taper altogether, and using seamless tubing of the right diameter. The small amount of taper could be built into the cladding, and no one would be any wiser - and I doubt if it would affect the operation or performance of the loco. (Can anyone comment?)

To make the tapered barrel, I needed some reference dimensions. I started with the front tube-plate, which I chucked on a 3 jaw chuck, from the inside. I also made use of a tailstock centre to stabilise it. I then took extremely light cuts - like 1 -2 thou. to create a smooth surface ready for silver soldering. Turning copper like this can be tricky - its material properties are such that it can grab, pull out of the chuck, and tear. This is mad much worse by the fact that it cannot be held very securely in the chuck without being damaged. My solution is to turn the chuck by hand. Also, to use a suitable cutting lubricant. Paraffin is suitable - and so is WD40! It makes a big difference to how freely the metal comes off. just take off enough material to produce a smooth, round surface.There was a significant hollow in the plate, so I removed it and tapped it to a better shape before continuing, as I wanted to remove a minimum of metal.

Thursday, May 17, 2007

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 - if they were milled to size before fitting. This makes sure that the horn slots are 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 will cause the axles and / or the boxes to bind. A small amount of filing of the axlebox / horn surfaes was needed on one axle to get them to run freely. I also files 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, with the assembly held in the lathe until set.

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.

Monday, May 14, 2007

A panel saw

I've been trying to buy a panel saw (sheet saw?) for some time, but have been told that they havn't been made for many years. I was lent one a year or two ago, and found it much better for cutting large sheets of brass than the jigsaw I had been using.

As I can't just buy one, I took an old wood saw, and ground off the cutting points. I then made up brackets to mount a fine-toothed 12 inch hacksaw blade, and drilled the saw blade to suit (A solid carbide drill goes straight through the saw steel)

It might look a bit odd - but it works. Its as easy as can be to cut a straight line right through a 4 ft by 2 ft brass sheet. I might get round to tidying the saw up - the real thing is styled like a tenon saw without the tenon. And there seems to be no need for a blade tensioner because of the way the saw works.

Valve gear 2

I initially assembled the linkage with all the joints unpinned, so I could check that it all fitted and could move freely without any obstructions.
The angles that some parts of the motion were moving through were clearly wrong. The pendulum levers - which support the expansion link, and the link hangers were both visibly wrong.
I checked all the components against the drawings - and discovered that the weighshaft -which supports the link hangers had been located in the wrong place on the frames- a dimension of 13/32 on the drawing had been set out as 13/16 in. (Actually, by the original maker of the frames -but I was supposed to have checked it). Everything else was correct - but -adding up the various dimensions on the drawing showed that the centres for the link hanger brackets were in fact wrong -by 1/8 in. Still, it made me read up and understand how Stephenson link gear is designed.

To make matters worse, I'd drilled all the mounting holes around both items,and the new holes would run into the existing ones. I thought about scrapping the frames -a lot of work - and of silver soldering fillers in, or getting them welded in. In either case, I think the distortion would have ruined the frames. As I had space to spare inside the frames, I made up doubler plates for the frames, using the existing holes as the attachment points, and fitted the brackets (shortened) to the inner plates only. You can only tell if you look inside the frames. I don't think the strength is in any way compromised, and the motion is in its original position. All the motion now looks right, including the operating angles.

I then set the expansion link to the mid position, (at only this point, there's no movement transmitted from the expansion link assembly to the valve gear). Then, with the intermediate joints and the hanger brackets at right angles, and drilled and pinned the joints. I used 1/16 taper pins - and was glad when I was finished and the taper pin reamer was still in one piece. (I held it in a pin chuck, and cut very slowly, with lots of cutting oil, and cleaned the reamer frequently.)

Then, with an airline connected to each cylinder in turn, and using very little pressure -only a few psi - and with the valve coupling links and the main connecting rods removed, I operated the piston valve, driving the piston from one end to the other (and through the cylinder end plates if you use too much pressure). Also, its seriously a time to keep fingers out of the way. I used a piece of plastic tubing to listen for air escaping from the cylinder drain cocks, to determine when the valve opened in each direction. I measured the valve crosshead position, and calculated the mid point. Then, without disturbing anything, I could measure the distance required between the valve connecting rod centres. By comparing that with the actual length of the valve connecting rod, all I had to do was to move the valve on the spindle by the required amount -and knowing that the thread on the valve rod was 40 psi, I knew haw many turns and flats to turn the valve lock-nuts by. It was quicker to do than to describe. I finally fitted the valve coupling rod.

Then, with the gear in full forward position, and the crank-axle at forward dead centre, I adjusted the forward eccentric until air could just be heard at the front cylinder drain cock. I locked the eccentric in position, and rotated the crank axle to back dead centre, and checked where the valve opened. It was a few degrees late, but I left it for the moment.

I then repeated the whole thing for the reverse eccentrics, with the reverser in full reverse.
Then repeat the lot for the other cylinder.

I then fitted the coupling rods, and again with only a few psi of air to both cylinders, rotated the crank axle wheels, and could feel that the valve events were about right.
So, standing well clear, and with the wheels clear of the bench, I increased the air pressure to 30 psi or so, and things started to happen! It initially needed about 60 psi to get the engine to run reliably. After only 10 minutes running, it would run on 30 psi, and eventually on less than 20 psi. Its surprising how much difference a few minutes running makes.

Once the engines freed up, I then repeated the valve setting procedure, making minor adjustments. I thought that the valve positions weren't exactly right, as the valve events weren't quite equal for the two halves of the cycle. I was able to rotate the valve rod which is threaded onto its clevis position, to get the valve position just right. (Again, a temporary measure, but easy to do and get it right. Then I found that, as I reduced the pressure, the engine would always stop at the same position - again, small adjustments of the eccentrics and the valve position helped. Once I was satisfied that I'd got it as right as I could, I dismantled the valves, and tightened up the clevis pins -four flats -which I was then able to correct for on the actual valve adjusters.

Now it was running properly, I could lock up the eccentrics permanently - I removed each grubscrew in turn from the eccentrics, and replaced them with one which I'd drilled right through in the lathe. I used this as a guide, and drilled into the main crank axle. I then removed the drilled out grubscrew, enlarged the hole in the crank axle, and fitted new screws, with a protrusion turned to fit the holes I'd just drilled.

Valve Gear

The Hall uses Stephenson link gear mounted internally between the frames, and the cylinders have piston valves.
Most of the valve gear components are straightforeward to make, although the eccentric strap might be worth a comment.
The first step was to machine up a bit of bar to 1 1/8 diameter, with a lead that was a few thou less - as a go/ no-go gauge, and also for use as a jig.
To machine up the eccentric,(from a casting), I sawed it in two, to form the two halves of the strap, then milled the surfaces square. I used a fine hacksaw blade, and was careful to mill off the least possible metal. Then I silver soldered the cut halves together again, and set up in the 4 jaw to bore the eccentric. (To set up work in a 4 jaw chuck, I use a dial gauge mounted on a tool holder, and TWO chuck keys on opposing jaws. This let me move the work together with the clamped up pair of jaws - much quicker and easier.
A simple jig made sure that all four eccentric links were the same length.