|Drawing 8 - Cylinders &
||I have chosen to make my cylinders from the cast iron
casting that are available although I could have chosen gunmetal cylinder
castings or raw material of either type. I will also be using cast iron
pistons and cast iron rings. The castings were a reasonably good shape and
I didn't see any chilled spots although I wouldn't have expected to with
lumps this size, and with no small protuberances which are the usual casualties.
The first job I did was to load them to the independant four-jaw chuck on
the lathe and clean up the ends. If there were any chilled spots, my lathe
tooling is more substantial and better able to cut through them. I wasn't
trying to get anything accurate machined here, just to get a couple of flat
surfaces to help subsequent operations.
||They weren't coming off the lathe anything like square
so it was over to the milling machine and loading to an angle plate, getting
the casting reasonably square to the table and then cleaning these faces
up again. Again, I wasn't after anything accurate here, just a starting
point for clamping purposes. I used my large flycutter for this, 4" diameter
with a carbide cutting tip rotating at about 400 rpm. Once I was happy that
the ends were reasonably square, I then machined the back, clamping to the
angle plate through the two rough bores and removing enough material to
just clean up. These were now measured and checked properly for squareness
and centre-ing which showed there was still about an eighth of an inch still
to come off the length. To reduce the overall length, I opted to hold them
a little differently, using my independant 4-jaw chuck mounted to the table,
adjusting how much I took off each end to get them balanced.
||The drawing shows that the centrelines of the cylinder
and the valve bores are offset 7/16" from each other in the vertical plane
but a close look at the castings revealed that the core for the valve bore
is set about 1/16" too close to the front of the casting so I have made
the offset 1/2" which will allow the cylinder core to be machined reasonably
evenly and the valve core will lose 3/16" at the back and 1/16" at then
front to bring them back to the 7/16" offset. I set them up as shown below.
The plugs are small pieces of MDF wedged in and centered so that I could
eyeball the amount of tilt required. After setting the scribing block to
the centre of the valve bore, I set it on a piece of 1/2" tool steel and
adjusted the casting to set the centre of the cylinder. Then it was bolted
up, the clamp removed and the back faced off again until I had a full clean-up.
||The next thing I did was to go back to the earlier ops
and get the overall length finished to size and to get the centreline of
the piston bore reasonably accurate to the back face. These were just repeats
of the earlier machining. Then I drilled all the bolt holes on the back
of the cylinders but I've also added a couple of extra holes that are not
on the drawing. These have been reamed 3mm and I have also modified the
mounting plate by drilling and reaming two holes and setting 3mm dowels
into them. Although I am going to finish the bores using a between-centres
boring bar, it's a lot of work because these bores have a lot of meat in
them. Therefore I set up to rough machine the two bores and had to make
a holder to carry the boring bar to enable me to load it to the 4-jaw independant
||Here, I've loaded up the first cylinder and set the centre
of the bore before locking the slide in the first direction and taking the
gib screws up pretty tight in the other. Then it was just a case of feeding
through under power, backing out, adjust the bar and go again. However,
after the first few passes on the first cylinder, it became obvious that
the setup wasn't quite rigid enough so I stopped machining and modified
the lower clamp by bolting on a block with an M10 bolt through it. Once
I had clamped the cylinder up again, I then adjusted this bolt down onto
the slideway with just slight pressure and a piece of brass packing beneath
and this took practically all the spring out of the fixture.
||And this is where the dowels came in useful. I was able
to machine the largest bore on the first cylinder, unbolt it and load the
second cylinder and be confident that the two bores were in the same place
relative to the casting. Then l used the same boring bar to rough out the
valve bore before setting up for between-centres boring to finished size.
Because I put a pair of locating dowels in my mounting plate, I was able
to get everything clocked true and then load each cylinder in turn and continue
with between-centres boring getting each bore to about ten thou undersize
and then setting the boring bar to final size and finishing each bore in
turn. I have left all the bores three thou undersize to allow for honing,
which should be ample, but it won't matter if they need a bit more. The
valve liners and pistons will be made to suit.
|| I was able to measure them in situ without removing the
boring bar using a dial caliper gauge but forgot to take a picture at the
time. Here is a picture taken later showing the type of clock I am referring
to. They are very good comparator gauges and I have modified this one slightly
with some pressed-on stainless steel feet. I set the size using a 1" - 2"
micrometer. To drill the various holes, I took full advantage of the new
DRO and used co-ordinate positioning throughout. Each cylinder casting was
squared up on the mill and the centre of the main bore accurately found
using a wobbler. Using the tables in the Zeus book for a twelve-hole setup,
I worked out the X and Y co-ordinates of each hole and worked my way round
centre-drilling and then drilling 3.0mm diameter.
||The draincock holes were also drilled and tapped 1/4"
x 40 and the final work on the mill was to mill the steam port and drill
and tap the holes. The drawing shows that the face of the steam port is
28.5° to the horizontal and recommends drilling the steam inlet hole
after assembly of the valve liners. Using angle gauges, I set mine to 27°
and milled the face flat with an endmill. I also drilled the 4BA stud holes
and the 5/16" dia steam hole at this stage and will drill through again
after assembly. The cast pads were not exactly in the centre of the castings
which is down to me for not getting equal amounts off of each end of the
casting. However, I have made sure that the holes are central to the end
faces. Finally, all seventy-eight holes were tapped freehand using a 4BA
spiral point tap (except the draincock holes, of course) and I am relieved
that all went well. To break a tap at the last moment would be disappointing
to say the least.
||Before I fit the valve sleeves, I needed to hone the cylinder
bores because access may have been a bit limited later. The hone kit came
from Cromwell Tools and did the job perfectly. If you've never used a hone
before, the trick is to start AND stop with the stones in the bore (never
pull the hone out whilst under power or you will probably destroy the hone),
and run at high speed for twenty seconds or so, stop, clean both the bore
and the stones, another squirt of light oil ( I used 3-in-1) and repeat.
Follow this routine until you're happy with the finish. As regards size,
you will probably need to work for half an hour or so to remove about three
thou in cast iron. It's all about surface finish, not removal of material,
get your size nearly correct before honing.
|2. Cylinder Backplates
| I cut the cylinder backplates out a while
ago using some 10 swg mild steel plate, offcuts of the frames material,
and had marked out and drilled them. I have now countersunk the holes and
screwed a few of the 4BA screws into place but one or two of the holes are
not as accurate as I would have liked. These were made before I had co-ordinate
drilling facilities whereas the cylinders were drilled using the DRO. I've
opened the lower row of holes out an extra four thou and the screws are
now pulling in, but I may remake these plates as there is not a lot of work
involved. One advantage would be that I can get the dowel holes that I have
added to the cylinder in the correct place.
|3. Valve Sleeves
||I am making most of the parts for the cylinders
from cast iron and I have now made the piston valve sleeves from some old
sash weights. The material is not of the best quality and I got black as
a coalman whilst machining it but the end product is quite acceptable. Unlike
the majority of components in this build, both the bore and the O/D need
to be very accurate and perfectly concentric because of how they are assembled.
This would usually lend itself to making from bar stock, machining both
the O/D and the bore to size and parting off to length as a single setup.
However, my lathe is not the most accurate machine in the world and that
is why I chose to do the job this way instead. The first job was to cut
up and rough out the four billets, although they were faced to finished
length at this stage.
||Next I modified one of my between-centres boring bars
to get down to this smaller diameter and also made up some brackets to hold
one of my machine vices on the home-made vertical slide. Although a lot
of time has been spent in preparing this, the vice arrangement will come
in useful again in the future, I am sure. All four sleeves were finished
in multiple passes and, therefore, marked with a felt-tip pen to ensure
they went back in the same place each time. To machine the outside, a mandrel
was made from some spare material and the sleeves loaded in turn and brought
to final diameter in multiple passes, the final cut being about six thou.
They were then reloaded to machine the slightly reduced diameter of the
external part of the sleeve, and for the steam port groove in each one.
||The last operation in the lathe was to load to the 4-jaw
and clock the bore true prior to opening out the locating spigot (0.950"
dia) for the end covers. Finally, it was over to the mill and the steam
port holes all put in. The inlet holes are just a series of eight drilled
holes of 3/16" diameter and the outlet hole a 9/32" x 9/16" slot. These
will now be cleaned up, pressed into the cylinders making sure the outlet
ports line up with the cast-in holes and the hone ran through each to get
a perfect central join. The piston valves will then be made to suit whatever
the final size turns out to be.
|| I decided to take the cylinders to a local garage and
make use of their 30 ton press to push the four valve liners into place.
The liners protrude from the main casting by 0.712" at each end and a ring
of this thickness and with a biggish bore was made to rest the cylinder
on and act as a depth stop. A packer with spigot was also made to place
inside the liner to protect the liner from damage. The first one was aligned
with the cast-in exhaust ports of the cylinder and pressed in to just past
the finished depth. The assembly was then upended and placed on the depth-stop
ring the second one likewise aligned and pressed home until it touched the
other liner. A bit of extra pressure then moved both to the correct depth.
This was repeated for the other cylinder and the whole job took no more
than ten minutes.
||Now I needed to mount them to their backplates and get
them located on the frames for spotting through. The cylinders on Britannia
are inclined and it is not very easy to use co-ordinate drilling to mark
the holes in the frames. The other problem is that, because the cylinders
are castings, the centreline of the cylinders won't neccessarily be where
I think they should be. On the frames, I had previously marked out the position
of where the exact centre of the cylinder should be and scribed a line from
a given dimension at the front of the frames to the axle centrline of the
|All I needed to to do was find the exact centre
of each cylinder and transfer this to the cylinder backplate and I could
then line everything up, clamp together and drill through. First I set up
an angle plate on the mill, accurately squared off, and mounted the first
cylinder and backplate assembly. Using a wobbler, I found the exact centre
of the bore then swapped the wobbler for a scribing point and, without moving
the "Y" axis, dragged it across the edge of the backplate. The assembly
was then upended and the procedure repeated so that I had a scribed line
at each end of the backplate.
||The backplates were then removed and a line scribed on
the plates between each of the two marks. Using a digital caliper, the half-way
point was marked, followed by centre-punching and drilling 3/32". Back with
the frames, I also drilled the previously marked cylinder centre point with
the same 3/32" drill. The plate was then laid on the frame. a 3/32" rivet
dropped through the holes and the marks on the end of the plates aligned
with the scribed line on the frames. The whole lot was then clamped together
and all the holes drilled through with a 4BA clearance drill. Clamping the
two frames together and doing as a single item wouldn't work because the
other cylinder alignment would probably be slightly different so he other
frame was machined separately and finished the same way. I also scribed
the position of the steam exhaust hole on the frames and these, along with
a few other holes such as the saddle mounting holes will also be completed
while I have the frames disassembled.
|4. Rear Valve Guides
||The rear valve guides are usually machined from gunmetal
castings but I decided to make mine from a couple of billets of cast iron
which are 40mm dia Meehanite and 75mm long. The drawing is not dimensioned
around the rib area and I have had to scale from the drawing to get the
sizes somewhere near. The first part of the job was to turn the O/D and
the front boss that fits in the valve sleeve. The hole for the valve stem
was also drilled and then bored with a tiny boring bar to ensure concentricity.
It chattered like a cage full of monkeys but finishing with a 7/32" reamer
cleaned it up nicely, These were then loaded to soft jaws to machine the
outer form of the guide rails and face to length.
||Next it was over to the mill to make the final shape of
the ribs. This entailed multiple operations, starting with hacksawing a
large lump from the front. The 4-jaw chuck was mounted sideways using an
angle plate and the centreline clocked and set on the DRO with all y-axis
dimensions were referenced from this. A 12mm endmill was used to get the
majority of the front-end material removed and a rummage in the "specials"
box came up with this cutter to produce the angled form using multiple cuts
and sneaking up on the shape until reasonably smooth.
||Now that there was some working room, the holes for mounting
the guide rails were next, drilling for 8BA. These will be tapped by hand
later. Because of restricted access up near the chuck, removing the waste
from the centre section meant making a holder to extend the reach of a 6mm
end mill. The holder is a piece of 5/8" dia bar reamed 6mm, then cross-drilled
and tapped M4 for a clamping grub screw. There was a small amount of chatter
but easily dressed out with sanding drums in the dremmel.
||The outside form was also machined with the same tool.
Most of these cuts for both operations were plunge cuts, moving over about
forty thou per cut and finishing with a ten thou facing cut to lose the
plunge lines. That is as much as can be done for now. They need to go back
in the lathe to machine the pocket for the valve gland and they will need
some relieving under the guide rails to clear the slidebar casting but I
will leave this until assembly time.
|5. Valve Guide Glands
||Tthe valve glands have been made from bronze bar and this
was just straightforward turning and parting off followed by loading to
the 4-jaw on the mill and drilling the 8BA clearance holes. I then marked
out the shape of the flange and the edges were hacksawed off and linished
smooth. I have also now put the tapped holes and the pocket in the valve
guide and this needed a couple of extended tools made to reach. The drill
is jammed into a short length of brass tube but not soldered, remaining
tight during the drilling of the four holes. I didn't have any tube suitable
for the 8BA tap so I drilled a hole in the end of 1/8" key steel and banged
that onto the end of the tap, which saved having to file some flats for
the tap wrench.
||The pocket was created using a 10mm long-series slot drill
since I didn't have a 3/8" one and the gland turned to suit. These were
then assembled but without the PTFE washer and it became apparent that I
had an alignment issue. It wasn't much but enough to stop a 7/32" turned
rod from sliding through, even when I loosened or turned the glands through
one-eighty degrees. I must have messed up on my clocking up somewhere. I
decided that the best thing to do was to increase the size through the gland
and valve guide to the next standard size. 7/32" is approximately 5.5mm
so I decided to lift that to 6mm, the advantage being that 6mm dia stainless
rod is easily available as a stock size
||The soft jaws were returned to the lathe and re-skimmed
- the chuck had been off in the interim - and the guides and glands were
assembled and marked as pairs, complete with orientation, just in case I
have to disassemble them again. A special boring bar had to be made but
this will come in useful for other things in the future so worth the investment
in time. The home-made boring bar is a piece of silver steel which I have
hardened and tempered. The assemblies were then loaded to the soft jaws
and a few passes made taking three or four thou at a time until I could
get the shank of a 5.7mm drill in the hole. This was enough to true them
up and then the 6mm reamer was pushed through.
|6. Next item...