|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
||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
|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
|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.
||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.
|8. Next Item...