Over the past year I have been machining the cylinders for my traction engines. First job was to machine the faces square and to size. I used an 80mm face mill and the horizontal mill:
It took a number of different setups, and some repeat machining, to get everything square. The front to back dimension could not be machined to nominal size, so they're 15 thou short. It was important to take the time to get these faces square, as they will be reference surfaces for all subsequent operations. The cylinder machined square:
The bores were then cleaned up to give a reference circle for the real machining:
The bores were not finish machined on the Bridgeport as my boring head is a low quality import, and the quill travel is not great enough. There are a lot of cavities to machine. The regulator cavity was machined with a 10mm endmill and finished with a 10mm endmill with a corner radius of 1mm. These give a much better finish than square endmills when facing:
In similar fashion the valve chest cavities were cleaned out and taken to size, using a long series (100mm) 1mm corner radius cutter. Despite the long stick out I experienced no chatter problems:
The bores were machined on the horizontal mill. Each bore was centred with a co-axial indicator and the boring done with a Wohlhaupter boring and facing head:
I had all sorts of problems with the boring. The first was chatter due to the long overhang needed to clear the flange on the cylinder. i started with a home made boring bar (from EN8 medium carbon steel) and HSS toolbits. Once chatter had developed the cutter seemed to follow at each pass. I tried all sorts of combinations of speeds, DOCs and feeds without any real success. Oddly the boring head was happy taking a 0.1" DOC and there was no chatter noise (as with a lathe) but the finish was awful, consisting of helical bands. I then bought a commercial 25mm insert boring bar and machined a short length down to 7/8" fit the boring head. Carbide inserts seemed to be slightly better and tended not to follow any previous chatter. I used CCMT inserts, the CCGT sharp inserts were much worse and were the only cutter that produced audible chatter. After a lot faffing about and honing I eventually ended up with bores 10-15 thou oversize and a finish of 4um Ra. On the plus side the bores were round and parallel to a few tenths and in the right place.
The other major problem was to do with the boring head. The head has coarse and fine feed knobs and I couldn't get the fine feed to work reliably. I ended up with a DTI on the head and twiddling the fine feed until I got the increment I needed. After discussion on another forum the feed problem turned out to be due to some missing parts in the coarse feed that prevented it from being locked. I stripped and cleaned the head and made the replacement parts after I'd finished the cylinder bores. More on that in another thread.
The underside of the mounting flange was machined with a home made flycutter:
Roughing cuts were 80 thou deep and 8 thou per rev feed. The feed was done by hand - listen for the end of cut after a clonk, advance the handle two big divisions (0.2mm) and wait for the next clonk. Finishing cuts were done at 4 thou per rev feed. As one might expect the setup is pretty rigid so I had no issues with chatter and the motor note didn't change at all during the cut. The piece of sheet steel behind the cylinder at the top of the angle plate is to tilt the cylinder forward by about half a degree so the bores are not quite parallel to the boiler. This is needed so that the vertical centreline of the bores intersects the axis of the crankshaft and makes any angularity symmetric. The slope on the cylinder is also a feature of the fullsize engine.
The steam ports were cut using a 6mm end mill loctited into a home made silver steel (aka drill rod) extension fitting directly into an R8 collet. After machining the first set of ports using numbers from the drawing and the DRO the ports were a thou or so over width. So the numbers were tweaked and the remaining ports are on size, as checked with gauge blocks:
The ports are to my own design, based on scaling from full size, steam flows, pressure drops through the engine and Reynolds number. Note that in the picture the low pressure exhaust port is longer than the inlet ports. There's nowt that says the ports have to be the same length. Making the exhaust port longer means it breaks into the exhaust hole out to the chimney. That avoids the need to drill, and blank off, a connecting hole in the underside of the cylinder. Ideally I would have made the inlet ports longer too, but one of them would clash with the exhaust outlet hole. According to my model of the cylinder there should be 0.07" of cast iron between the inlet port and exhaust hole. Fortunately this is confirmed by measurement.
The final job was to machine the exhaust steam port from the HP cylinder to the LP valve chest. After a lot of messing about in CAD I came up with a slanted rectangular hole. This neccessitated using the Quillmaster and right-angle attachment for the first time in 15+ years since I bought it:
The right-angle attachment only takes 3/16" cutters, not so common in the UK. Even then I had to modify a atub drill and endmill to get the precise length that enabled access while still being able to machine to depth.
The next post will deal with machining the liners.
Andrew