Tuesday, February 20, 2007

Cylinders

The cylinders on this loco are gunmetal. Simply because that's what came with the bits I bought! I much prefer gunmetal anyway. I don't run my locos a lot, and worry that cast iron cylinders will rust. The other side of the coin is that you can use conventional piston rings with CI cylinders.


The first thing I did was to make up the boring bars. Mine were cross drilled bar, of as big a diameter as could pass through the cast bore - within reason. I have quite a few carbide drills intended for drilling glass fibre printed circuit boards. They do have their uses -but are very fragile. They all have a 3mm shank, which is ideal for making small cutters. They're easily ground using a diamond wheel (but nothing else will do !) The tip is held in the boring bar by one grubscrew which presses on the shank of the cutter, and another grubscrew behind it, which is also used to control the advance of the cutter. I needed to make more than one cutter with different lengths, or a spacing piece to pop in, as the range of adjustment of the cutter is small.



So onto machining: I cleaned up the castings and to establish some working surfaces and datums. Only then could any meaningful measurements be taken. I've seen it recommended to plug the cylinder and valve chest bores with wood so that centres can be established. Personally, I prefer to work from the datums which I've already established, and anything else is just a check that it looks right (which sort of validates the calculations. All I did was to put broad masking tape across the bores, so the centres can be marked -its only for a visual check.

You'll see the datum milled on the outside front of the cylinder casting. Also, the machined surface of the bolting face.



I fitted a 'boring table' to the lathe cross slide, and mounted an angle plate on that. My angle plate is actually a cube, which is much more rigid - and versatile. The cube was set parallel to the lathe axis, by mounting a test bar between centres and running the cube up against the test bar, and locking in place. I now had a surface which was parallel to the lathe axis, at a known distance (the radius of the test bar) from the centreline. Also, by measuring from the surface of the boring table to the bottom of the test bar ( using gauge blocks) I now kånew the vertical distance from the lathe centre to the boring table.
With that information, I now knew how much packing was needed to set the cylinder at the correct height to bore the piston chamber. Also, by moving the cross slide back using the dial scale, I moved the casting to the right position in the 'Y' direction.
I locked the cross slide, then re-checked calculations and positioning. By marking the bore centre with masking tape, and with a centre in the tailstock, I was able to check that the casting was correctly in position. (just a rough check, but it avoided any major errors.

These particular cylinders - typical GWR - had a very pronounced offset between the bores, making them difficult to pack. You'll see from the photo that I used the second cylinder as a packing piece to avoid packing at extreme angles.

Now I fitted the boring bar with a tailstock live centre to provide support. Again, the fact that it fitted was an indication that the casting was correctly located. The cylinder was now bored - a few thou at a time. By making the cutter from carbide, it wasn't significantly damaged by the remnants of casting sand in the cast bore.




Before the first cut, I rotated the chuck by hand, and wound the carriage (and casting) past the cutter to check for any drastic irregularities on the casting bore - the cutter can be damaged by intermittent cuts and severe shocks. Once all was well, the self act moved the casting past the cutter and out the other end. Once complete the carriage was wound back and the cutter advanced by 5 or 10 thou. and take another cut. It took a few cuts before the cut was continuous through the bore.

One of the difficult things is to measure the bore. Its at this stage that I realised that, before I started, I should have made a go / no-go plug gauge or two, so that I would have known when I was reaching the final diameter. So, do that before you start setting up !

Also, keep an eye on the boring bar. The cutter is roughly in the middle of the bar -its weakest position. So the bar will deflect with the cutting forces. After two or three cuts, do a second pass without advancing the cutter. Whatever cut you then achieve is as a result of the deflection of the boring bar. Keep doing this until no metal is removed during the run. Check the bore diameter before and after this procedure - You'll want to do this for the final few cuts -by doing it earlier, you'll get a good ides of how much to allow for the final cuts.

Once the bore was to my liking (parallelism is more important than getting the bore to an exact diameter) I removed the boring bar, and moved the packing to set the cylinder to the valve chest height One of the packing pieces was the same dimension as the offset), and I adjusted in the 'Y' direction with the lathe saddle index. I repeated the boring as before.

Once complete the boring bar was removed. This was a good time to face the end surface of the cylinder, as it had to be exactly at right angles to the bore in both planes. So, I cut it using a huge endmill held in the lathe chuck. I needed need to be very careful doing this, as the cutting forces could easily have pulled the cutter out of the chuck, resulting in an inaccurate surface at the very least. So I took very light cuts, and locked up all slides not in use, and upmilled only.

<I then drilled the transfer passages between the valve chest bore and the cylinder. The mill was set up at some wild angles to achieve this; it took a lot longer to set it up than it did to do it. What I actually used to make the cuts was a small slot drill - much more rigid than a normal drill, but I had to hold it in a drill chuck as the collet chuck got in the way.

Then I had to drill and tap for the bolts mounting the cylinders to the frames. The drawing showed bolts, but they're going into shallow, blind holes. If you get the bolt length wrong, the threads would be damaged. So, instead, I fitted studs, retained with Loktite threadlok; I think this makes a better job.
So my next session will be to assemble the cylinders and related bits.

Saturday, February 3, 2007

Frames



I started with a set of frames which had already been made. I was going to say - which saved a lot of sawing - but in fact I would probably have cut them in the mill.
The important thing is that the two blanks are riveted together and treat as a pair until complete.
As I had no idea how mine had been done, I started with dimensional checks. I soon found that the axle box spacing was marginally different (of the order of 20 thou) from one side to the other. And the horn slots had been finished individually and were different (hence the axle box spacing). Apart from that, they seemed ok.
I set the frames on parallels standing on a surface table.


The structure was not truly parallel, with a rock of at least 1/16 in in one corner. I slackened fixings and kept rechecking, until I found that the central cross braces were the cause of the problem - one was a few thou out of parallel. I milled a few thou off to make it truly parallel, and made up a shim (only 5 thou) to keep the spacing correct. I doubt if it was worth it - but at least its accurate. I kept rechecking as I re tightened fixings, and eventually it was dome.

The next thing was to put in the axleboxes and check the axle centres. For the completed wheel sets, there was only one way to fit them because of the differences in hornblock and axlebox dimensions. I actually checked these by mounting the wheel sets between centres on the lathe, and measuring the offsets from the axle to the hornblock slots.



Again, all were different. I guess that the frames had been set up as a pair, with the blank axleboxes fitted, and then the axlebox bearings had been drilled and reamed in situ. So the wheel centres do end up the same on both sides of the frame.

If I was starting from scratch, I'd have done it differently. Like, I'd have kept the frames as a pair until after the hornblocks ha been milled.

For the axleboxes, I's have squared up the axlebox material, and drilled and reamed - or bored - the axle bearing. Then I'd set it up in the mill, with a stub axle through the journal, and resting on the jaws of the milling vice, and the upper surface horizontal. Then I'd have milled one of the recessed for the horn block.
Having done that and set a stop on the milling machine quill, I'd have rotated the axlebox through 180 degrees,(a small parallel and a couple of jackscrews will do the job) and milled the other horn block bearing. That way, they all end up identical, and with the axles central.