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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 E1- Frames
  1. Buffer Beams  
Usually, one machines the frames first but I have started with the buffer beams. Whilst passing the local blacksmith's the other day, I stopped by and purchased a couple of feet of 40 x 40 x 6mm black angle for the princely sum of 2.00 and he even cut two pieces just overlong for me as well as handing me the offcut. So the first job was to get them faced accurately to length in the mill. I used a 16mm end mill for this and an angle plate to act as a backstop. They were mounted back-to-back in the mill, raised on parallels, and skimmed with a homemade flycutter. However, the overhang from the vice caused the material to "sing" so I added a pair of "G" clamps to the ends to dampen the oscillations. Then the finished faces were clamped together and the second faces machined.
The pair were left clamped together, upended in the vice and the bottom edges milled to size with a 16mm boring bar mounted in the collet. Once again, the faces were reversed and the other edges milled to finished depth. Because the height differs from the depth, I marked the cutaway on the outer faces to prevent errors, then mounted the front beam face up in the vice, setting the top edge as Y0 and the centre of the beam as X0 on the DRO. I also modified all the dimensions on the drawing to reflect distances from my data points. Not all holes are shown on the drawing, however, and a hunt through found more details on a completely different drawing.
The 3/16" dia hole for the front drawhook was drilled first, followed by the holes for the buffers. However, I'm not happy with Don Young's method of fixing the buffer stocks and have made some changes.The drawing shows a separate boss screwed into the beam and the stock fitted to the boss but this means getting the rotational alignment just right. I will cover the detail a little later but, for now, I have reamed a 10mm dia hole at the central position, drilled two clearance holes and tapped two others for fixing the stocks. The rest of the holes in both beams were finished off, including the slot in the rear beam, and attention turned to the shaping at the lower corners of each beam. My largest end mill is a 20mm diameter and this was used to rough out the shape.
To get the radius correct really requires a 1" dia cutter and, as luck would have it, one of the 20mm dia boring bars I have cuts a circle a touch under the inch. I calculated the "X" and "Y" co-ordinates, positioned the tool and plunged downwards to finish the internal shape.The buffer support angles have been made from the same material as the buffer beams, cleaning up all the outer faces in a similar manner to the beams themselves. First, though, I had to skim the inside of the buffer beams to get flat faces and one of my seldom-used cutting tools was used to perform the task. This can cut quite close into a corner.
I wanted the four support angles to all be the same size and they were clamped together, on edge, in the vice. I didn't lock this up too tight because I didn't want to deform the angle and force them out of true. I already had a 16mm end mill in the collet so this was used to clean them up. I also wanted them exactly 1.250" in height and a quick and easy method was used to set the height of the final pass; touch on the Hoffman roller, zero the "Z" axis DRO, remove roller.
I wasn't confident about setting up acurately in three planes simultaneously so I calculated the position of the holes for fixing to the front beam then added ten thou to my numbers, hoping to leave five thou per side to skim off the side faces. However, I managed to get my knickers in a twist with the first pair and, after dry-assembling, ended up with rather more to come off than I bargained for. That should be reading about 4.135". I marked the two side faces 53 with a felt tip, readjusted my numbers and drilled the other pair. These came out as expected at 4.137" so those faces were marked with a 6. The angles were then returned to the vice and the faces skimmed by the marked amount. On reassembly, these now sit in the correct position.A couple of thou over but I'm happy with that. The angles for the front beam are a little thinner than designed but they are still plenty strong enough and won't affect anything else. I need to rivet these together before I continue with the side holes and the machining to the top of the beams but I am going to phosphate these parts, as well as the rivets, first.
Once the parts had been phosphated, I temporarily assembled the support angles to the beams with countersunk iron rivets and scribed a line on each of them with the height gauge to match the dimension given on the main frames drawing. These were then loaded to the mill vice and the frame-fixing holes drilled and tapped 4BA. The fixed jaw of the vice controlled the distance from the inside of the buffer beam face and the scribed line the height from the top of the frames. I find that I can see the position accurately to a couple of thou like this so don't bother with a wobbler.
That left a couple of jobs to do on the buffer beams, one of them forming the 1/8" wide slot that locates the frames. These were made using a 1/8" slot drill taking 25 thou depth of cut and winding till the DRO read 0.250" on the "Y" axis. To set the "X" positions, I just touch the cutter on one end of the work, zero the DRO, touch the cutter on the other end and hit the 1/2 button with the "X" axis. Then the two cutter centrelines are plus and minus 2.125". The top surfaces show a cutaway to the sides but I don't know what they're for yet, no doubt all will come to light further down the drawing chain. I touched the cutter on, moved along a further 90 thou and wound across until I reached the inner edge of the front face. Left-hand side one way up and right-hand side the other.
At this point, I could have filed the square shoulders shown on the drawing but got lazy and set up a cutter instead. I could have formed the complete cutaway like this but felt the risk of a dig-in too great at this stage of the job. The parts were riveted together and the front faces lightly skimmed to lose the rivet heads, followed by another dip in the phosphate tub. There is some work to do on the top of the front beam and the rivet detail to add to the rear beam but this can all be done later. They've gone even darker with the extra soak.
  2. Frame Stretchers  
There are four frame stretchers shown, two the same and two different. Three are fabricated and the fourth, although being drawn as bent metal, was milled from solid. I started by making the support angles for the first three, cleaning up some 12mm black bar to finish at 3/8" square, making them in pairs and bringing the finished faces together ready for the next cut. Next, I milled out the insides, again in pairs and roughing out with a 12mm dia end mill and a finishing pass with a 1/2" dia end mill. This leaves the sides 1/8" thick which is enough to stop them popping out of the vice but I wouldn't try this method with anything thinner.
The plates were sawn from a piece of 3mm steel sheet and the widths milled to size but leaving the length up about thirty thou. These are the parts for the first three stretchers although I still need to cut the angles in half for the lower pair. To ensure all the stretchers were the same length, I set up a backstop on the mill vice, then milled each of the plates to 4.125".
The end of the plates was set to "X0" on the DRO. We all have our favourite way of using a wobbler; mine is to run the wobbler at about 300-400 rpm, light up a piece of white paper and slowly watch the gap disappear as it's wound closer to the work. With my deteriorating eyesight, the contrast works well for me and I can easily see the moment when there is no flicker of white remaining. The DRO gets 50 thou added or subtracted to compensate for the width of the wobbler head. With the back of the vice set to "Y0", the holes were drilled using a 2.4mm PCB drill for 3/32" iron rivets. The plate was flipped over left-to-right but not back-to-front and the other end drilled at the same settings.
The angles were drilled in similar fashion to the bogie bolster in a previous post and the angles riveted to the plates. I moved the holes ten thou nearer to the edges to allow me to face the ends of each stretcher once they were riveted together. I haven't bothered making these look pretty, they are inside the frames and out of sight. The stretchers were loaded back into the mill with the backstop adjusted to leave about 1/16" poking out the right-hand end. This requires a sharp cutter, because of the width of the cut, and the bigger the better for rigidity. The stretchers are handed so each was marked with felt-tip pen to reduce the chance of errors, then drilled and tapped 6BA, using the top and centreline as the zero references. The top of each stretcher was always loaded to the inside of the vice and the top ("X0") found each time with a point in the chuck. The "Y" axis stayed the same for all of them.
The remaining stretcher was made from 20 x 12 black bar, milled all round to finish at 3/4" x 3/8" and the ends dressed to 4.125". The centre was cleared away with a boring bar in the mill, working back and forth at twenty five thou DOC each time and leaving end shoulders of about 3/16". To finish, the stretcher was held on end and the three holes also drilled and tapped 6BA. This should be a little more rigid than what was drawn but, at the moment, I'm a little concerned that the chassis is not going to be stiff enough. If there is room, I may try and incorporate a longitudinal stretcher somewhere on the model. At present, there is nothing to stop the frames assuming a banana-shape and the frame plates, as supplied, have a slight bow to them but both in the same direction. I might try and put a slight set in the opposite direction of one of the plates to compensate. Anyway, with the stretchers finished, I can now drill the frames and get the basic chassis assembled.
  3. Main Frames  
This is my first experience with laser-cut parts and I didn't know what to expect regarding quality or price. The frames have arrived and I'm very impressed with them. Accuracy is spot-on and the cut edges are very crisp and clean but not razor-sharp. I was also pleasantly suprised with the cost. They are advertised as "spotted" and I was a little concerned that there would be largish dimples where each hole should be but it's just a small cross at each point similar in depth to an etching. I won't be using these, all holes will be positioned using the DRO on the mill, but they would be a godsend for someone without one.
There is not much to say about the mainframes. They all have to marked out and drilled, whether by hand, with a DRO or laser-etched / laser-cut. I'm using laser-cut frames from MEL and DRO positioning to drill the holes. However, the etched position of the holes is a good reminder to do the right number of holes in the right places. I set mine up as a pair and used a Hoffman roller in the horn guides as a means of pinning them in line. The size of the cutout is extremely precise. I'm not drilling all of the holes at present, just the ones for the buffer beams, the various stretchers and the main horns. Once I have a lot more photos of the full-size loco, I will see what I need to drill to replicate the full-size features. I've levelled the frames against tee-slot packers and have clamped them onto a strip of 9mm MDF. On most things, I would use PCB drills but I'm using a centre drill and HSS drills on these. The material is quite tough and HSS drills are more forgiving than solid carbide.
Each group of holes was referenced from their own datum point to suit what was going there. Buffer beam holes were referenced from the top and respective end, stretchers from the top and their centre-line, horns from the base and centre of the first horn guide. The second horn was positioned with respect to the first one, winding along 6.7/8" from the centre of the first and resetting the "X0" before drilling the second set. The beams and stretchers have been assembled with a few screws to get the basic chassis together. Now that the really mundane work is done, it's time to start on some of the more interesting parts.
  4. Main Horns  
There are castings available for the main horns but I'm fabricating as much as possible and four billets were cut from some 13mm bronze plate. The first job was to clean up all the edges and bring the height and width of each to finish size. Then they were laid flat in the vice and the faces skimmed to clean up. I'm leaving these oversize at present because I wanted to recover the four inner sections for another job.
To remove the centre section, I have stitch-drilled a series of 2.9mm dia holes on a 3mm pitch but left the two holes at the bottom spotted but undrilled at present. This section can be sawn and chiselled out later but is being retained for strength at the moment. The centre and bottom of the horns have been chosen as the reference points and will remain so throughout all the following operations. My top corners were at X41.5mm and +/- Y10.5mm. The frame location faces were next, roughing out all four to 175 thou deep and leaving 15 thou on each side and the top. Once again, twenty-five thou DOC and full width going round anti-clockwise. I needed two passes across the top because my cutter was only 12mm dia. With all four done, I replaced the end mill with a new one and made a final pass round each in turn, taking ten thou more from the base and bringing the top and sides to size.
The parallels were replaced with a taller set, the work flipped over and cutaways machined to leave the bracing webs on the back of the horns. For this, I used a 4mm dia solid carbide end mill at 1200 rpm, cutting full depth. The cutter was set to the top of the packer, lifted 0.156" and locked tight. The tool path for each pocket was worked out and the waste removed with a perimeter cut 10 thou inboard all round, the island nibbled away and a final pass round at finish size. The other side used the same numbers, just negative on the "Y" axis. The end pockets were formed in a similar fashion but with sharp corners left at the top. It was a slow job that required concentration to avoid working to the wrong set of numbers. I nearly did once but caught myself just in time. In the photo, the two lower ones have had the corners dressed with a rotary burr although the feature doesn't stand out very well.
The endmill was replaced with a 2.5mm PCB drill and all the holes drilled. I am using M3 countersunk socket screws rather than the specified 6BA. The dimensions I had entered onto the frame drawing were transferred to the horns drawing, adding 0.125" to the verticals because the horns sit 1/8" below the bottom of the frames. The horns were held firmly in the vice and the parallels removed from beneath before drilling. This is why I've kept the middle intact for the time being. After deburring the holes, they were tapped freehand in my small tapping fixture - all seventy of them. I wasn't sorry to see that operation completed.
Six of the supporting ribs are machined at an angle and the two outer top ones are at about fifteen degrees. It was easy to just rest them on an angled packer and mill these by eye. The centre rib is left square for an oil hole. I tried the same arrangement for the side ribs but they weren't loading as nicely as before, the result of milling away some of the clamping face at the top. Instead, I set up a small vice an an adjustable angle table and milled away the four upper ribs, the lower ones left untouched. I had a bit of a dig-in on the first one with the work lifting in the vice so added a top clamp for safety.
Now that I no longer needed the support of the centre piece, it was cut through with a hacksaw and then attacked with a small chisel. Holding on edge in the vice, the four horn stay holes were drilled. Care had to be taken here because the drill would have broken out into a half-and-half void, risking drill breakage. Using the vertical DRO, I drilled to precisely 170 thou, leaving a blind hole. They were tapped 6BA freehand, held in the bench vice. To complete this stage of the work, the horns were upended in the vice and the oil hole drilled. I used a pilot drill of 2.4 mm dia to a depth of 3/16", then followed through with a 1.9mm drill. I don't know what the oiling arrangements are at this stage but that will allow me to set a piece of 3/32" dia copper tube into the hole. Finally, a filing button was clamped to each in turn and the corners linished away. I will mill the internal horn faces once these are assembled into the frames.
  5. Horn Keeps  
A rummage around in the scrap box unearthed some embryonic horn keeps, bits of 1/2" x 1/2" x 1/8" mild steel angle. The holes in one side of the angle will be milled away and not be a problem. The outside faces were linished clean and didn't require any machining, there will be very little to see by the time these are completed. The wider leg was milled to size first and, because I particularly wanted this edge square to the side, I used a packer to clamp the angle against the back jaw of the vice. They were all finished to the width of the horns, 0.406". Next, they were reloaded and the other edge milled to 0.250" high. Then the inside faces were cleaned up with a single pass of a 12mm dia cutter. Removing the cutaway was next and an end-stop was set up to get them all in the same place. After setting the centre of the component at "X0", one cut through the centre plus one either side leaving ten thou on each of the inner edges saw them roughed out. Then they were returned to the vice and the width of the cutaway completed to size, a nice fit to the base of the horns. The advantage of doing all these operations in this order was that, once the depth was set, the cutter didn't get moved until completion.
The final job on these was to drill the holes and, once set down on the packer, this was removed and the six holes centre-drilled and drilled to their respective sizes. I re-dimensioned my drawing so that it more accurately reflects the important sizes. I've also made a few of the related items including the spring pins and retaining plates, items which don't require any description. These have been phosphated along with the horn keeps.
  6. Next Item...