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.
3. Smokebox Rivets    
The drawing shows that there are two circular rows of rivets at the rear of the smokebox and one row at the front, all with a spacing of 5/16". Since my smokebox is 6.562" diameter that means there should be 6.562 x 3.14159 / 0.3125 = 65.97 rivets, so sixty six rivets in each row. Although I have a rotary table this is a particularly poor choice of tool here because angular displacement is 5.45 degrees recurring. I don't have a dividing head but a quick check of my lathe changewheels found that a 66-tooth wheel is amongst them. Research on the internet reveals that this particular size is commonly used in metric conversion from an imperial leadscrew. First, how to hold the smokebox tube and I chose to use MDF end-caps to fit in each end. Standing the tube on a piece of 20mm thick MDF, I marked out the O/D for two pieces then cut them out, removing the corners at the same time. Finding the centre of each piece, I drilled a half-inch hole through in preparation for the next operation. Now I transferred to the lathe and turned the O/D and a spigot to locate inside the tube.
To hold the billets, I am re-using my wheel-turning mandrel that I made for the tender wheels which has a half-inch diameter boss. Once these were made, I found a length of 1/2" diameter mild steel rod which I faced and chamfered at each end. This was a tight fit through the half-inch holes in the MDF which was just what I needed. With the end-caps fitted in and the rod through the middle, it was time to sort out the next stage. The hole through the changewheel is 3/4" dia and I made a sleeve to fit this to the 1/2" shaft, along with drilling a couple of holes in the changewheel to enable the fixing of it to the MDF with woodscrews.
Once I had this built up I started on the mounting and for this I made a base out of 20mm MDF with a pair of risers from 2" x 2" timber and a pair of 2" x 2" feet to clear the base of the drill. A half-inch hole was drilled and reamed through a piece of 2" MS box-section tube and then cut up to make a pair of pivots. All these parts were then assembled and then I turned my attention to making some sort of indexing arrangement. I opted for a spring-loaded detent and this was made from a piece of 1" square MS box section for the carrier and 1/4" dia MS rod for the pin. The end was linished like a screwdriver blade until it was a good fit in the changewheel, then cross-drilled 1/16" for a spring retainer pin. .
A suitable spring was found in the "come in useful one day" drawer and a knob made to fit the other end of the detent pin. Also made was a spring arrangement to apply pressure to the opposite end to keep the linear position stable. After marking a few positions on the indexing end and setting to the 180 degree position, I set the scribed line on the smokebox to the centreline of the fixture. The table was taken off from the drill and the machine screwed to the bench with plenty of clearance either side. The fixture was placed over the base of the drill, the position of the first hole found using a point in the drill chuck and then clamped to the bench.
The first hole was centre-drilled and drilled, and a rivet inserted to keep everything registered. Then I went round the smokebox using just the centre drill, then round again with a 1/16" drill to complete the first row of holes. A similar procedure was adopted for the other end although the offset for the second row was set by eye, moving the fixture in a little and along by 5/32". The slight deviation from true centreline makes no difference at this diameter. This fixture was cobbled together in just a couple of hours and the drilling of the 198 holes took about twenty minutes. I could have done this job on the mill but I had something else set up on there that I didn't want to disturb. That completes the rivet holes but I shall leave the smokebox on the fixture and set it up on the mill for the other holes; chimney, blast pipe, regulator etc.
4. Other Holes    
After setting up my fixture on the mill to cut out the larger smokebox holes it soon became obvious that, although satisfactory for drilling, the setup was not rigid enough for boring. A few variations of the setup were tried but the main problem was locking the work solid in the rotational direction and finally a solution was found. Because the jaws on the chuck wouldn't open wide enough to hold the 66-tooth wheel, a smaller one was used instead. All subsequent operations were suited to the rotary table, anyway. One of the jobs on my "to-do" list is to make a tailstock to match the rotary table but, in the meantime, this cobbled together arrangement works just as well. A lot of faffing around is needed, though, so this job has moved further up the list.
With the smokebox set at 180 degrees, the blastpipe hole was drilled and then bored out to 7/8" diameter. No size is given but the 3/4" dia blastpipe connector needs to pass through and this should be enough clearance. A lead washer under a collar acts as the seal. The saddle had eight 6BA clearance holes made by opening up some of the existing 10BA holes to 6BA clearance and these were then spotted through to the smokebox and eight holes drilled and tapped to suit. The steam inlet holes have also been put in. Rotating round to the 0 degrees position, the opening for the chimney was drilled and bored to the finish size of 1.5/8" but the fixing holes for the chimney were left for now. The two holes for the regulator lever and the dummy whistle have to be created parallel to the horizontal axis and the respective positions of these were calculated and made using slot drills.
5. Regulator Cover    
Because I'm following the drawing, rather than changing the regulator arrangements, the slot above the regulator block needs to be created to provide easy access to the valve plugs. The regulator cover, as drawn, appears to be left for the model engineer to make as they see fit and provides basic dimensions only. Two angular dimensions are given and everything else can be worked out from this. First I cut some 20swg brass sheet to size to create the base of the cover, adjusting the overall length to suit my smokebox diameter. This was then clamped to some MDF and the mounting holes drilled 10BA clear using co-ordinate drilling on the mill. One of my general-purpose fixtures was set up on the mill and, using the same co-ordinates, four 10BA holes were drilled and tapped to facilitate mounting of the base for milling. Using a 1/8" slot drill the middle section was removed, a clamp being introduced on the return leg to stop the waste piece flapping around in the breeze.
After deburring, the base was bent to fit the smokebox. The smokebox was once again set up on the mill, the centreline found and the two angular positions, 29 and 39 degrees respectively, scribed by dragging a centre-point across the work. This allowed me to align the base in the correct position and spot through the first of the holes which was subsequently drilled and tapped 10BA. The base was then screwed to the smokebox and three more holes drilled and tapped. After drilling four pilot holes, one in each corner, a 1/8" slot drill was used to cut the regulator slot using the base as the guide. I haven't used the rotary table a lot yet and have marked the direction of rotation to match the winding handle so I don't inadvertently wind the wrong way. At completion, the waste dropped away without problem leaving the area ready for cleaning up.
Attention now turned to finishing the regulator cover and I chose to make mine as a three-piece fabrication. The neccessary sizes for the upright section were calculated and transferred to a length of 16swg brass sheet. The section was carefully cut out and the various bends put in until I had a closed loop. The top was made from another piece of 16swg brass sheet and cut to fit inside the loop. These two pieces were soldered together and finally soldered to the base. There was much manipulating neccessary here and I didn't take any interim pictures but my soldering is not pretty anyway and better hidden from sight. I've done some filing and polishing to get the basic shape but there is quite a lot more to be done.
6. Chimney    
I have chosen to use the cast gunmetal chimney for my project and, at first sight, it appears to be a reasonably good casting without a lot of flash. The two halves of the core box have been aligned quite well and the casting is symetrical. Because of the saddle shape of the lower section, it is not the easiest casting to hold and I started by getting it to run at true as I could in the 3-jaw and getting the bore to size. However, I am going to deviate from the drawing in a small way later and bored out to 1.500" instead of the given 1.625". Sorting out a way of holding on the bore came next and I opted to weld up some more soft jaws rather than make a mandrel. An expanding mandrel would be the ideal workholding tool here but would be far more work than making the jaws, and a fixed mandrel would have limited use.
As you can see, my stick welding is on a par with my soldering and should be hidden from view. A neat trick for locking the jaws in the correct direction when skimming to size is to use a chain around the outside. One of the hanging baskets is a little lop-sided at present but I'm hoping she won't notice. I then ground up a piece of 3/8" square tool steel to form an all-round 11/64" radius, which was done freehand on the grinder. The work was loaded with a small amount of overhang at the front and the top section formed first, skimming the O/D, facing the front and finishing the upper curve with the same tool.
I had slight movement just at the finish of the upper curve so reversed the workpiece on the jaws to form the lower curve. I was able to get further back and a better grip this way. To create the radius for mounting to the smokebox, I set up my home-made lathe fixture on the toolpost mount. I don't have a vertical slide so this does that job. The lathe centre-height is already scribed on both sides of the block so it only left a mounting boss and a couple of clamps to make.
To cut the radius, I sharpened up a piece of 3/8" square high speed steel as a left-hand tool and mounted it in the 4-jaw self-centering chuck, itself mounted in the 3-jaw chuck. This was needed to get the neccessary travel of the carriage. To set the tool, I used a rule against a live centre to measure off the 3.9/32" distance required since plus or minus 1/32" is going to make negligible difference at this radius. After checking to ensure the tool would not foul the cross-slide at any point, multiple passes were taken until I had a full clean-up and then extra passes to get to the required 11/16" height. I'm running at about 200 RPM and using a four-thou per rev feed in the next picture. The live centre is being used purely as a safety device - if I get a snag, this will prevent the 4-jaw chuck or tool flying across the workshop.
This is the component as machined so far and here it is sitting on the smokebox. I now need to get the shaping of the outside correct but because it is impossible to create compound curves with 2-axis tooling, this will be done freehand with sanding drums in the dremmel and with files. The bore will be modified later also, as mentioned at the start.
7. Regulator Stuffing Box    
The regulator stuffing box is an awkward little thing that sits on the side of the smokebox but not square to it. It needs a curved mounting plate with the brass bush silver-soldered into it at an angle. I started with some 16swg brass sheet which I milled to 7/8" wide for easy mounting in a machine vice. The mounting holes were drilled next, remembering to modify the formulae in the Zeus book to suit an elipse
The bend was put in next and the work placed back in the vice, attempting to match the angle that it sits at on the smokebox. The hole for the bush was then machined using successively increasing sizes of slot drill until final size was reached. The first end was sawn off and then shaped, followed by a similar operation with the other end. I've been using a reground cutter for the smokebox and plate holes, which is approx 0.550" so the bush was turned from some 5/8" dia bar to match. The drilling and counter-boring were all done in the mill, however, and the tapping of the 10BA holes using my little tapping fixture. I have not broken a single tap since I made this fixture so it was well worth making.
After checking that everything fitted together and sat in the right place, the parts were silver-soldered together. Keeping them held in the correct orientation proved to be a bit of a head-scratcher but a solution was found in the end. I managed to fill one of the 10BA clearance holes with silver solder but was able to drill it out freehand.
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