Drawing 8 - Cylinders & Accessories
1. 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 chuck.
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 driving axle.
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...