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Steve's Britannia
5" gauge Adams O2 - Calbourne
Drawing E1(a). Drawing E1(b). Drawing E2(a). Drawing E2(b).
Drawing E3. Drawing E4. Drawing B1. Drawing B2.
Drawings S1 and S2 Boiler Assembly Assembly and Alignment Errors and Ommissions
Drawing E2- Gylinders and Valve Gear
  1. Cylinders  
I've made the cylinders using cast iron billets and, I must say, it's lovely stuff to machine. However, I could have saved myself a bit of time if I'd ordered them to be cut a bit shorter. They had been cut very square and I ended up facing off 5/16" of material. First, I cleaned up two sides to finish at 2.1/8" then finished the other two faces to 1.11/16". I faced them square and to length on the lathe using a 4-jaw chuck, then returned to the mill to reduce the bottom of each to leave the 1/4" thick bolting flange and something similar on the top.
The top incorporates a boss for the exhaust pipe to fit to and this was left as a square section for now. With bronze it would probably be easier to silver-solder the boss in separately later. None of this can be seen once asssembled, anyway. The centre-line of the cylinder bore was found next and a No.2 centre drill used to mark the spot. On the lathe, each billet was clocked true to the centre, drilled with a 1" dia dril, then bored out to thirty thou shy of 1.375". The plan was to use a between-centres boring bar to finish but the parallelism was so good that I just carried on to finished size. One of the advantages of a big, rigid boring bar: I wouldn't try this with anything of smaller cross-section.
They came up a couple of thou under the 1.3/8" diameter but, after honing, I shall make pistons and rings to suit. Back to the mill and the drain cock holes were drilled and tapped followed by the exhaust steam flange mounting holes. Steam ports were milled next using a 1/8" diameter slot drill. I can't follow the circular form at the bottom - that requires using a cylinder lining sleeve - but a later picture shows the lowered cuts at each end of the ports. The undercut of the bolting flanges was next and I was able to use one of the fancy cutters from my odd-ball collection. This makes space for the nuts to fit on the fixing studs. A bit like a standard dovetail cutter but with a much sturdier stem and flatted teeth.
Next up were the fixing holes which were drilled 2.9mm diameter with a PCB drill. The holes are not equi-distant so care needed to be taken here. The picture also shows (just) the step-downs at the end of each steamway and how the dovetail has to end short of the exhaust flange boss. The cylinders were turned on end, the bore clocked for centre and the holes for the end-cap bolts drilled. The Zeus book doesn't have a 16-hole solution as is required here so the numbers were worked out using the Sine and Cosine of 11.1/4 degrees and 33.3/4 degrees and a radius of 13/16". Those four numbers were the only dimensions needed, just plus or minus respectively. The two spaces at the top leave room for the steam ports to be drilled.
The steam ports were drilled next followed by drilling and tapping the drain cock ports and drilling the angled hole to get it exiting at 3/32" from the ends. Loading onto a 30 degree angle gauge appeared to be the required tilt. The exhaust port was machined next and I used my tilting table to support the work at 10 degrees from the horizontal. The dimple I made earlier was centered and a hole drilled through at 3mm, followed by a 3/16" end mill and a 1/4" dia end mill, stopping soon after breaking through.
After that, I changed over to the tilt and swivel vice and machined the external shape of the exhaust port, the four cutouts for the steam ports and any other odds and ends that needed finishing. I could have done the exhaust port in this vice but I wanted to be able to see where the bottom of the end mill was. All the endcap holes were tapped freehand with a 6BA spiral point tap, likewise the exhaust port holes, albeit 8BA. I would like to have smoothed the steam passages using burrs in the dremel but the port slots are bang on size with very crisp edges and it's not worth risking damaging these. I've done a little light filing but that is all. Nothing more can be done with these until the steam chest is made so that's one of the next jobs on the agenda.
  2. Steam Chest  
The draughtsman suggests making the steam chest from a 3/4" thick block of gunmetal and brazing the upper steam chamber and it's connection point on the top. I've chosen to make it differently, using a couple of pieces of the 1/2" bronze plate that I have a fair bit of. Starting point, as always, was to square up the two billets to the maximum dimensions, then remove the waste from the centre of each. Once again, I've used a 4mm carbide slot drill to track round in a rectangle using plus and minus co-ordinates on the DRO. I also pre-drill four corner holes 4mm diameter so that the slot drill doesn't need to plunge-cut unneccessarily. I prefer this method to stitch-drilling because the resulting billets are perfectly square and ready to be used on another job.
This was followed by facing both sides was my large flycutter. With the billets cleaned up all round, the upper steam chamber was milled out next to a depth of 3/16". The fixing holes were drilled next so that I could position the steam slots, then realised that I didn't need to make them as per drawing and just milled them 5/16" wide by 3/16" deep. To assist with location, I also drilled a pair of 3/32" dia dowel holes at two corners.
I painted the insides of the fixing holes with correction fluid to stop the solder running away and then the two billets were given a light rub over some fine emery cloth. The faces were fluxed, a ring of 1mm dia silver solder shaped to fit all round and rest in between, the upper one placed on top and a weight above that to help gravity do it's thing. Unfortunately, I forgot to take a picture at this point. It took a few minutes to get up to temperature because of the mass but, eventually, the top section settled down on the lower part and heat was removed. The photo taken afterwards shows the two corner dowels in position. I've definitely had a good melt because solder has wicked up the dowels even though they weren't fluxed because they will be milled away. The next couple of ops were about getting the outside shape. First was to mill all round the upper steam chamber.
This was followed by facing the inboard end to length and forming the valve spindle ports. I'm using 4mm dia stainless for the spindles and have also changed the gland-fixing screws to 8BA rather than 7BA. 5/32" x 5/16" "O" rings will be used as packing. The suplus material was then cleared away from the face, ready for shaping the dual spindle boss.
Following some sound advice from Nigel, I also drilled and tapped a couple of 4BA holes at top of the front face to assist with valve-setting. A couple of cap screws will seal them on completion. Final job was to create the steam inlet port and I'm making a change here. The drawing shows a threaded tube soldered to the top of the steam chamber but I don't want to do any further soldering to this and am making a screwed flange fitting instead. A 5/16" dia was drilled through which was spot-faced with a 7/16" dia slot drill to take another "O" ring. Four 10BA holes were drilled and tapped, taking care not to break into the steam chamber.
The fitting for the top was turned from 3/4" dia Colphos and threaded 1/2" x 40 because I have a matching tap and die but not a 1/2" x 32 as drawn. I've also formed a small undercut at the root because, otherwise, it will be a bit tight to get a spanner on. After parting off with a 1/8" thick flange, it was screwed into a mandrel (actually, my thread gauge) and loaded into a chuck on the mill for the spanner flats to be machined, then back to the lathe for the other end to be turned.
After seeing Nigel's drawing, it seemed sensible to have the steam pipe vertical but the cylinders are inclined by about six degrees so I set the tilting table to compensate. The flange was milled to leave 1/16" on the thinnest section, then the centre of the hole found and four 1.9mm holes drilled at +/- 0.218" for 10BA mounting screws. And to finish off, here is a picture of the steam chest mounted between the cylinders with a couple of temporary studs and nuts. And this one shows the slide bars fitted to the top of the cylinder covers, making quite a nice, rigid assembly.
  3. Cylinder End Caps  
These have been made from three sawn sections of bronze plate and it took quite some time to turn squares into circular discs. The 4-jaw chuck was used to get one end turned and faced true before changing to soft jaws in the 3-jaw chuck. One of the billets was sawn through to make two thinner discs for the front covers, then all four were faced turned and the spigot for the cylinder bore turned. The two front caps were reversed and faced to finish length, the two back covers were faced to length and the boss for the spindle gland turned. A 3/32" wide undercut was formed at the of the boss, the piston-rod hole drilled and the reccess for the gland bored to size.
The lathe chuck was unbolted and mounted on the mill to allow the fixing holes to be drilled 2.9mm diameter on the same PCD as the cylinders. At this stage, they are symmetrical but the rear covers are handed on subsequent operations. The flats on the edges of the covers were milled next, simply hanging on two pins in the vice.
At this point, the rear covers were bolted to their respective cylinders and set up for milling the shape of the boss. I made a few modifications to my rotary table to allow for better clamping and for a central pull-down bolt. The two fences are there to stop the work rotating around the single clamping screw. With the DRO zeroed on the centre of rotation, a 12mm end mill was used to cut down one side to "Y0", the table wound through 180 degress then continuing with the "Y" axis to clear the work. Multiple passes of twenty-five thou were made until reaching 3/4" across the flats. I was keeping track of the sizes after each pass and made a note of the "X" reading on the DRO when I reached exactly 1.1/8" across the flats and this number was used as the finish pass of the top of the boss, bringing it to exactly 9/16" above the centre-line, the same as the crossheads. Before machining this face, however, the side face of each cylinder was clocked exactly square to the table.
With the vice back on the table, the gland mounting holes were drilled and tapped 8BA. Finally, the two holes in the top of the boss were drilled and tapped 6BA to allow for fixing the slidebars.
  4. Pistons and Rings  
I've changed the design of these slightly because I prefer to use cast iron rings on cast iron pistons rather than an "O" rings or graphited string. The pistons were completely machined in a single set-up to ensure concentricity. The O/D was turned about three thou below nominal, the ring grooves were placed at 3/32" from each end and were made a few thou wider than 3/32". The spindle hole was drilled, bored for concentricity then reamed 1/4" for half it's depth, the balance tapped 1/4" x 40 tpi. Four rings were made by turning the O/D to the same size as the cylinder bore and the inside dimension bored to a few thou more than the diameter at the bottom of the groove. They were parted off a smidgeon under 3/32" wide to allow a couple of thou endfloat.
The rings were split by giving them (carefully) a sharp tap in the vice, then a needle file used to dress the break. They were sprung onto a piece of 3/16" wide material held in the vice and heated from the bottom until they turned bright red, at which point they dropped off the bar onto a thermal mat and were left to cool.
I needed a means of getting the assembled pistons into the cylinders and modified a scrap axlebox from the Britannia project. The axlebox was loaded to the 4-jaw chuck and roughly clocked out, then the compound slide was set over to about 1.5 degrees and the a tapered bore created. The piston was used as a gauge, boring until the piston just passed through the back of the bore. The rings were sprung onto the pistons and then pushed through the guide and into the cylinders. One of the pistons was very stiff and a check showed that the ring groove was a bit too shallow so it was clocked up in chuck and a touch more removed from the bottom of the groove, On reassembly, it was much better.
Holding them up to the light, I could see that the rings were not perfectly circular and were touching at three points in the bores. I took the rings off and gave them a further heat treatment, hoping to get them to reshape a little. First, I set them part-way down the loading block, resting them on a piston to get them square and under tension. Then I blasted them up to as hot as I could get them, almost orange in colour, keeping the heat full-on for a few minutes. Then I dropped a thermal block on top and left them to cool slowly for half-an-hour. The result was an improvement because, although there is a very small amount of light around the rings, it's much less than before. If anyone wants real accuracy, I can't recommend my method for obvious reasons. However, I rebuilt the complete drive, retimed the valves and set it running on air. It runs a heck of a lot smoother and also I can get it much slower.
  5. Motion Plate  
The drawing for the motion plate took a little while to decipher and it's a fabrication that could be made as a simple plate because it's buried within the frames and won't be that visible once the loco is complete. However, I thought it might be interesting to try and make it look a bit closer to the prototype. Starting point was a lump of plate sawn from some 3" steel angle and a couple of offcuts of flat black steel. After squaring the plate all round, it was skimmed to 7/32" thick with a flycutter although it will be thinned later to nearer 1/8" thick. The height and length of the plate were finished to size for the top and side plates to be silver-soldered to.
The three cutouts were made next, drilling the corners of the con rod openings with a 4mm drill and a pair of 1/4" diameter holes where the valve spindle guide bracket sits. The centre of the spindle guide was used as the "X0, Y0" reference point. The six fixing screw holes were also drilled but after this picture was taken. The spindle hole was formed by opening the two holes out with a 12mm end mill followed by a 14mm end mill, clearing the centre at the same time. To remove the con rod openings, a 4mm end mill was wound round clockwise between the four holes taking a 50 thou depth of cut each time.
After that, I lifted the plate higher in the vice and, with the same cutter, milled away thirty five thou from the surface to leave a raised section around the openings, then flipping the plate over and doing the same on the other side. I swapped over to a 1/8" dia cutter and milled a pair of channels to locate the slidebar brackets into. The plate was stood upright and a 3/32" deep cut taken out of the top for the same purpose. To take the corners off, I set them against a small angleplate and used an angle gauge to rest the workpiece on. A pair of holes were drilled and tapped 9BA in each of the edges so that I could screw the outer plates on before silver-soldering. One supplier, I don't know who, sent me a pack of 9BA hexagon bolts instead of 10BA so I've been using them up on jobs like this.
The slidebar brackets were fabricated from offcuts of 10swg M.S. sheet and the two cross-pieces soldered to the arms with 40% high temperature solder to ensure they didn't fall to bits at the next heat-up. Some 1/4" square was milled down to 3/16" thick and the corners filed to create an oval cross-section. These and the two side plates were bolted on with flux between them. The top one holds the slidebar brackets in place. The corner pieces were mades next from 1/8" thick material and with the matching angle milled on the ends. These had to be made individually to get a snug fit.
A simple fixture was made to hold the corners in place and a bar was clamped to the slidebar brackets to keep them square. Lengths of 55% silver solder were placed along the outer edges and horseshoes placed around the slidebar brackets, then everything was well-fluxed. The spring clamps in the picture were replaced with miniature "G" cramps before soldering as I've found that the springs in these clamps lose their temper at the elevated temperatures and the clamps fall off. I rested the fixture on a couple of blocks on the hearth and applied most of the heat to the underside of the plate. Once cooled and pickled, I then set about clearing the excess material from the corner pieces. Conveniently, a 16mm end mill fitted nicely between the slidebar brackets and the side plates and allowed me to form a nice radius.
Apart from drilling some holes to suit the slidebars and cutting off the corners of the brackets, the rest of the work was handwork, filing and linishing the various forms. I've held off drilling the frame fixing holes for the time being as I want to see how it all comes together further down the line. This is how the plate looks now and this is a photo of the motion plate that I took when I was allowed to clamber in the pit at Havenstreet over which Calbourne was stood.
  6. Valve Guide  
I've made the valve guide from a couple of offcuts of bronze silver-soldered together. After milling all round to get a clean block at the maximum dimensions, I centred it up on the DRO then roughed out the back of the flange that bolts to the motion plate. This was followed by drilling and reaming the two 5/16" diameter spindle holes, then using a 7/16" dia end mill to clear out the recess. I also started to centre the six mounting holes, then realised that I couldn't do the four corner ones until the circular shape had been made. That's what the two spots on the flange are.
The other side of the guide is not symmetrical because of the oil pocket so the block was roughed out in a different manner, mounting end-on into the vice and using a tipped boring bar to remove the waste. It's better this way, anyway, because I dont need to use the side of a long cutter. Once the basic shape of the valve guide had been machined, I milled out the two pockets on the top of the guide, one an oil pocket and the other an access pocket for the top fixing screw. I drilled four corner holes for each pocket then used a 1/8" diameter end mill to clear the waste.
The cylindrical shape of the guide was formed on a rotary table. I made a mounting pin with an M6 tapped hole over which one side of the valve guide was mounted. A 1/4" dia long-series end mill was used to make a series of plunge cuts every three or four degrees of rotation before making a final pass at full depth and winding back round to the starting point. The other side was done in similar fashion, then the valve guide upended and the flange milled to shape. Finally, the valve guide was reloaded to a vice and the six mounting holes drilled 6BA clear.
  7. Valve Spindles  
These have an offset portion that needs to keep it's orientation and the easiest way seemed to be to make them from square material. I started by machining some 3/8" square mild steel, turning and polishing the main body of the spindle to 0.312" diameter and leaving a section at the end for the bit that joins to the valve stem. This actually turned out to be a waste of time and was turned away later.
Then I made an offset sleeve to carry the spindle for the next turning operation. After clocking true in the mill chuck, the 5/8" diameter billet was offset by the required 1/16" in the "Y" axis and a 5/16" diameter hole drilled and reamed right through. Before removing from the chuck, a 3/8" diameter endmill was used to make a pocket for the end of the valve spindle to locate in. Three slots were milled part-way through the sleeve using a slitting saw, the final slot being cut right through in the bench vice with a hacksaw. I chose to use a vee-block for this operation and the engineer's square clamped to the bed is just to help align the block on each cut.
Each spindle was loaded, in turn, into the sleeve with the square shank pulled tight into the pocket then turned to 5/32" diameter, finishing off with a radius tool to profile the shoulder. I also put an M4 thread onto the front on the spindles to take a threaded boss. The spindles needed to be kept square whilst finishing the head end so, for this, I used a collet in a square block. These things tend to rotate the collet slightly at final tightening so I clamped the work to a 1-2-3 block to keep it aligned.
From there, it was quite straightforward to mill out the slot, relieve the sides and drill and ream the cross-hole. Finally, a pair of 7/16" long bosses were made from 1/4" diameter mild steel, tapped M4 and screwed tightly onto the end of the spindles with the help of some Loctite. An hour later, they were loaded back to the sleeve and a 4mm endmill used to relieve the ends to 7/32" deep. These are the finished articles.
  8. Slidebars  
The slidebars were made from 1/4" thick gauge plate. The hardest part about this job was hacksawing two pieces 13/16"" wide by 4" long from a piece of 2" wide material. They were milled all round to bring them to 3/4" x 4.1/4", then reduced to 1/2" wide for most of the length. They were then held on the milled edges and the end clearance made with a 5/16" end mill, reducing the waist to 7/16". Continuing to hold the bars in this manner, the four holes were drilled and tapped to suit. Then they were tipped to 15 degrees in the vice and the motion plate ends milled away.
  9. Crossheads and Covers  
When I made the Britannia crossheads, they were a fabrication of two sections so I thought I would have a go at making these ones from a solid lump. The only difficult part looked to be the piston rod boss so worth a try. The starting point was a couple of billets sawn from some 1.1/4" square mild steel bar. These were faced both sides in the 4-jaw chuck, as this was the easiest way of getting the faces square, leaving the finished width of 13/16". Then it was over to the mill and create the slidebar channel. For this, I roughed them both out first with a 12mm end mill then put them back and finished with a brand-new 1/2" dia one.
Using a snug-fitting packing piece, they were loaded sideways and the main body section reduced to 1/2" wide. Holes next, small-end pin first and piston rod second. These are both 1/4" reamed and the tapered part of the crosshole will be added later.
Then the slot, roughed out with a 6mm end mill and finished using a new long-series 1/4" diameter slot drill. After milling in one direction, I tipped the part up and milled in from the other direction so that the bottom of slot was square both ways. Up on end and the front section was cleared away.
Now the back of the crosshead was shaped with a 1/4" slot drill, followed by the cutaway at the front.
And finally, for this session on the mill, drill and tap the six top-plate fixing holes. The drawing calls for 8BA but I'm using up a dozen more of the 9BA screws that I have. Over on the lathe, a 1/4" dia mandrel was made with an M5 cap screw as the clamp.
Because of the square form of the boss, it was expecting too much from the form tool to machine the boss on it's own. The front was turned to 1/2" dia for a short way and a 5/8" hole saw without the pilot drill used to rough out the shape. Then the form tool was used to reduce the diameter to 7/16" and form the radius at the root. This picture shows how the tool has to fit into the gap without hitting the underside of the slidebar guide.
The only thing left to do after that was the large radius at the lower section of the body. The work was mounted on a 1/4" diameter spotting drill and rolled around the vice taking back-and-forth cuts to remove most of the metal. I think this well-established technique is what David Scott was referring to the other day in answer to another members' request. The composite picture demonstrates how the cutter remains at a fixed height and creates many facets which can easily be linished to the finished shape. These are the finished for the time being, the only job left to do being the coned seat for the small-end gudgeon pin. I need to find my missing cone cutter, it's in a safe place somewhere....
The crosshead cover plates were made from 1" x 1/4" mild steel bar and the first part was to mill the shape of the oil reservoir. I've left the base plate a bit thicker that shown on the drawing, finishing at about seventy-five thou. Then they were placed back in the vice and the oil reservoir formed by using a 7/32" slot drill to form the hole and a 1/4" x 40 tpi tap run down afterwards. Finally, the six fixing holes were drilled and the covers fitted to the crossheads.
  10. Slide Valves and Buckles  
I have made the valves and valve buckles from bronze and the valve spindles from stainless steel. After cleaning up all round, I formed the pocket on the underside of the valves with a small end mill. Once most of the material was cleared away, I crept up carefully to finished width, just a couple of thou per pass, because I wanted the size as accurate as possible. The length of the port is not important but the width is accurate to within a thou. The buckle was made next and this needed a little planning because the spindle is not central to the workpiece, being offset by 1/16". The blank was machined to 1/4" thick and a centre drill used to mark the spindle position. The work was then loaded to the independent 4-jaw on the lathe, trued up and the valve spindle boss machined to 1/4" diameter by 3/16" long. I then drilled and tapped it M4
Back on the mill, the material was cleared away from the middle using a 1/8" diameter end mill to get reasonably sharp corners before filing square. Then the thickness of the buckle was reduced to 3/16", all the material coming off one side. The parts were paired up and numbered so that I could get a good fit between the buckle and the valve. The outside of the valves were roughed out all round, then the longest dimension finished with about five thou clearance. Floating up and down doesn't matter and it will help to keep the valve free in the buckle.
Next, the longer sides were milled to be a very precise but not sticky fit to the buckle. Having no play in this direction should help to keep the timing accurate. I decided to mill some channels on the backs of the valves to try and overcome any Casimir-effect that might happen between the two valves. Angled at thirty degrees, they are 2mm wide and 2mm apart. I made a crosshatch on one of them but decided not to do that on the second, settling for a single direction only. Putting the valves back-to-back, they slide over each other without snagging so that appears to have worked.
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