Drawing 11 - Smokebox
1. Smokebox Tube    
I have made my smokebox from a length of 6.5/8" diameter CDS2 tube that started life as 1/4" thick wall and which I had reduced to 1/8" wall (10 swg) by the supplier. This was primarily to reduce the weight and I told the supplier that size and finish were unimportant. This made the length of tube much easier to handle but, conversely, a little more flimsy. As one of our collegues noted in a related thread, it's a large smokebox finishing at nearly 10" long. To assist machining this rather large lump I have recently made a couple of special tools, a large fixed steady and a large pipe centre that I have covered in the Tools pages. That just left the headstock end to be catered for (or so I thought) and, after a tentative start and a minor panic when things went slightly awry, I stepped back and had a rethink before I made a pig's ear of the job. The first task was to make up a set of decent soft jaws to clamp down onto the work.
Following my first exploratory cut, I found that I was making the tube triangular-shaped with the jaw pressure, and this sprung back when the jaws were released, leaving the tube the same triangular shape instead of perfectly round and, once again, the fixed steady was unable to handle the shape. To get around this, I modified one of my clamping rings to sit inside the tube and give the soft jaws something to clamp down onto without distortion.
Another problem that became apparent was that the tube was not perfectly round inside either, and when I squeezed the live centre up, that too was distorting the tube which relaxed on release. With the tube trapped inside the fixed steady but not under pressure from the rollers, a small chamfer was carefully put on the inside diameter. To stop the work bouncing around I ran the lathe at about 60 rpm and applied some downwards pressure to the tube with a baulk of timber. Now everything could be clamped up tight and the O/D skimmed for a full clean-up and no distortion when releasing the work at the end. After the final cut I dragged the tool back along the component to leave a lightly scribed line. I shall use this as the lowest-point datum when I set up to machine all the holes. Now able to use the fixed steady effectively, I faced off one end of the tube and cleaned up the bore for about 1/2" long, no particular size - the smokebox ring or boiler packing ring will be made to suit. Chatter was still a bit of a problem but a length of emery cloth wrapped around a stick cleaned out most of it. The tube was then reversed and the same done at the opposite end but finishing the length at 9.15/16".
2. Smokebox Ring    
I have be made my smokebox door ring from some bronze plate, athough I'm not sure what grade it is. This is what I'm starting with, the piece being 6.3/4" (170mm) square and 1/2" thick. First job was to saw off the corners to make for easier holding in the chuck, and three quaters of an hour later, I was able to load the work to the independent four-jaw chuck. Now, I want the lump from the middle but didn't fancy trepanning this so had a cast around the workshop and found my 110mm holesaw. Perfect.
Because I don't want a hole in the middle of my trepanned billet, I set the saw up without a pilot drill, which meant everything had to be pretty tight at the start. Here, I've just got underway and experimenting with speeds, settling on about 60rpm as optimum. On my lathe, the tang on the drill chuck sits in fresh air, and the chuck was creeping round in the 2MT socket, hence the 5/16" bar in one of the keyholes, resting on the tool carrier. You can see by the swarf that it's a bronze, brass would chip. I'm using a home-made version of neat cutting oil to assist the cut - a mixture of Rocol RTD paste and undiluted soluble oil, mixed to a runny slurry.
It took about ten minutes to get through the billet, but here I've backed out from the work on completion. And here are my two billets ready for their respective jobs. The reason I've covered this in detail is because it shows that unusual tools can be used in the right circumstances, or a tool used in a different manner to achieve the desired result. In this case, a holesaw on a lathe without a centre support which wouldn't normally be considered suitable. Did the job, though.
This was subsequently loaded to the 3-jaw chuck, faced off and the O/D roughed out. The soft jaws used for the smokebox tube were then set in the chuck and the ring reloaded, facing off to just over finished length and then getting the bore to final size. The drawing also shows a recess with a radius in the corner and this was finished with a radiused form tool. The workpiece was now reversed onto the inside jaws and the front faced to length, the O/D skimmed to final size and the external radius put on with a form tool, finishing with some emery cloth on a rule to blend. Staying with these jaws, the ring was reversed once more and the locating spigot turned to size.
This was made to be a good, tight fit to the bore because I will want to be able to remove it when in service but I don't want it leaking like a sieve and destroying the slight vaccuum that gets created in use. I'm using an ultra-sharp polished carbide tip for this in the picture above right. I needed fine control of the diameter and these tips are like HSS. Finally, the soft jaws were used again to hold on the O/D and machine the large chamfer that the smokebox door sits in. This was too large to plunge-cut so the compound slide was set to 45 degrees and a boring bar used to create the chamfer. Here is the ring pressed into the end of the smokebox tube.
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