Welcome to
Steve's Britannia
1.1/2" scale Allchin Traction Engine
Cylinder and Motion
  1. Cylinder  
The cylinder needed quite a lot of work to complete and a start was made by stripping off the safety valve housing and removing the studs that hold the end covers in place. A mandrel to fit the main bore was made and fixed to an angle plate. The whole cylinder needed cleaning up and a start was made by skimming a few thou off the top. To get it square, the steam chest cover was clocked up, feeler gauges being used to slowly adjust the rotational position.
I'm not sure what function the threaded boss to the side has, filler plug maybe, but it was a long way out of position and this was milled away and a top-hat bush made from bronze to fit from the underside and inserted into the cylinder saddle. The cylinder mounting holes were drilled at the same time. Another job done with the mandrel mounted horizontally were the cylinder drain cock holes. Then the mandrel was mounted vertically and the ends of the cylinder given a light skim.
The safety valve rod front and back support bushes were found amongst the parts and the housings for these were machined, drilling the spindle hole 1/64" undersize. At the other end the rear support housing was machined in a similar fashion, followed by a reamer pushed through both bores for alignment. A few thou was skimmed of this face as well but the poor finish around the lower part of the steam chest will have to be filled with something on final assembly.
  2. Trunk Guide  
The trunk guide and motion bracket were soldered together but the solder had not adhered to the cast iron trunk guide. The position of the valve guide isn't that clever, either, but I will have to live with that.
After cleaning off all the solder, I finished the rest of the holes as neccessary. Lube hole first, the setting up to drill and tap the 5BA hole for the governor pulley.
Then I changed my mind and decided to bush the valve guide and get it nearer the middle. Getting the motion bracket fixed accurately to the trunk guide meant a variation of the fixture shown in Bill Hughes' book. First, a pair of end plates were machined up.
Then they were clocked out on the mill and the guide rod holes drilled and reamed. Quite a lot of extra fettling was required to get the motion bracket to the correct position. The assembly was lined up and pulled together with M6 studding.
The fixture was set up on parallels and the first four holes drilled and tapped. A more complicated setup was required for the other side because of the angle. Vee-blocks and spacers saved the day. Finally, 8BA countersunk screws hold it all together, with a dose of superglue between as well.
  3. Cylinder Attachments  
A few more parts have been finished off or cleaned up as needed and fitted to the cylinder assembly. The safety valve housing has been tidied up, holes drilled and assembled into place and the govenor frame was similarly completed, the spindle remade and oil holes drilled.
The lubricator ptatform was machined from scratch and attached, along with the shaft and arms that attach to the expansion links.
The nuts that hold the trunk guide to the cylinder were made by over-tapping 6BA nuts with a 5BA tap. Some of the nuts and bolts specified for this engine are too large, you can't get a spanner on!
  4. Valve Gland  
Another problem arose when I came to load the slide valve into position. Because I had reworked the motion bracket, the valve gland was now out of line and it was far enough adrift that I had to plug the existing hole in the steam chest and remachine the gland housing. The first job was to machine away the raised part of the housing using a lathe boring bar in the mill. The housing needed moving about 1/32" outwards and 1/64" downwards so a 7/16" end mill was sufficient to clear away the lower tapped hole but not the top one. A 7BA brass bolt was screwed into this one and cut off flush. Over on the lathe, a top-hat plug was made from 3/4" diameter Colphos and parted off. This was then silver-soldered into the steam chest. A transfer punch was made from some 7/32" dia steel and used to spot the top of the new plug through the motion bracket valve guide.
Back on the mill, the steam chest was loaded to the vice and the DRO centred on the punch mark. The 1/4" diameter housing was drilled and reamed along with the 1/8" through-hole. I used 3.2mm for that to give a touch of clearance. The two fixing screw holes were drilled and tapped and I decided to go down to 8BA for these because 7BA looks a bit chunky. Putting it all together, the valve had only a hint of stiffness in the movement which can be eased by polishing the spindle. The packing in the gland will compensate for any looseness.
And then it all went pear-shaped. I needed to bring the rim of the top hat to the same shape as the gland and started to nibble away at the raised section with a small end mill. In a moment of inattention I had a dig-in which not only damaged the remaining section of the seat but wrecked the gland as well. Resisting the urge to throw the whole damn lot out of the window, I completely milled away the seat, made a new gland and also a separate packing piece from 1/16" brass sheet. The packing piece was bolted to the steam cheat and soft-soldered into place. It has no functional purpose so soft solder is fine for this. After cleaning up, the steam chest was built up once more and this time it looked more as it should. Because the mill was still centred on the hole, the assembly was loaded back to the stop, the reamer taken down to depth again and the 3.2mm hole redrilled. With the steam chest back on the cylinder, all is now well.
  5. Governor Pulleys  
Other parts I have needed to make included the governor and countershaft pulleys. These were made from mild steel because I didn't fancy plunging the radius shape into stainless steel. The larger countershaft pulley was first and I decided to make it in two parts because of the internal protruding boss that is shown. A piece of 7/8" diameter EN1A was skimmed to 13/16", faced off and then drilled and reamed 3/8" diameter. After that, the front of the pulley was opened out to 11/16" diameter by 5/32" deep with a small boring bar.
A grooving tool was used to finish the 9/16" diameter of the smaller part behind this. I'm doing it this way round so that I can complete the wheel in a single operation. In addition to a frontstop for the tool, I am using a bedway clamp to act as a backstop so that I can just wind back and forth until I get to depth. The form tool for turning the radius was made next, using 1/4" square high speed steel. This was freehand ground with the sides relieved and front ground square, finishing with a Heath Robinson set-up on my drilling machine as this has the fastest spindle speed. The wedge is one of a pair of ball-joint splitters I made nearly fifty years ago.
The next picture shows the form quite clearly, plunged in at about 200 rpm and a firm cut applied. However, it is very easy to break the clamping lug on these small Myford-size toolposts if a dig-in occurs and I use additional clamping when doing work like this to protect the toolpost.
With the work parted off, attention turned to the core piece. This was made from 10mm diameter bar, skimmed to 3/8" diameter and a 1/8" diameter hole drilled through. The front had a little nose turned onto it 1/4" diameter by 1/32" deep using a 0.8mm tip to leave the radius, although it can't really be seen in the picture. The two parts were then silver-soldered together, although an adhesive would have done the job just as well. The drawing is not very clear due to age-fade but the small boss detail can be seen to the left.
The smaller governor pulley was made in a similar fashion although I used an 8mm slot drill as a boring bar to create the recess. The grooving tool again created the rear boss prior to stepping along and parting off. The material here is a redundant copper-plated earth rod as I couldn't find any 1/2" diameter mild steel anywhere in the workshop. This is finished apart from the cross-pin hole which I shall drill when I have made the spindle. At 3/32" dia, the slightest misalignment would be difficult to correct so I will drill both together.
  6. Mounting the Cylinder  
With the shell test completed, I have been able to start mounting the cylinder to the boiler and the first job was to mark out the position of the centre of the cylinder on the barrel. By turning the scriber upside down, I was able to zero my height gauge on the centre of the crankshaft and mark a line around the boiler at 8.3/8". To find the radial position of the cylinder, I used a piece of 3/16" dia silver steel set into the piston rod gland and lined the other end up with the centre of the crankshaft journal, checking with the crank both fore and aft. I also made a simple clamping system out of some M10 studding and a lump of wood.
After spotting the four corner holes with a hand drill, I set up the mill so that I could drill all the holes on there. Some may enjoy drilling holes by hand but I choose a machine every time. My large vee-block was mounted first, square to the table and centered under the chuck. To support the firebox end, I fabricated a flat stretcher with a 1/4" dia hole on the boiler centreline drilled through it, fixed with a couple of screws to a pair of convenient holes in the hornplates. A simple support was created from an angle plate and using a table clamp with a notch ground in the end. Centreheight was found by clamping the boiler into the vee-block and adjusting the table clamp to the 1/4" diameter rod in the temporary stretcher.
The next picture shows how the firebox end is supported while the barrel just rests in the vee-block. All that was needed now was to rotate the boiler around it's axis, wind the table to a suitable "X" position and drill the four holes.
Leaving the set-up in place, the cylinder was bolted to the boiler with 5BA screws and the rest of the holes spotted through, not forgetting the 7/32" hole for the filler hole. After drilling all the other holes, the one remaining hole was the steam feed to the underside of the cylinder and for this I chose to use a step drill although I had to be aware that I was getting close to the flue tubes in the boiler. I felt that the risk of a large jobber drill "snatching" at breakthrough was not worth taking in this situation.
To complete, all the holes were tapped freehand using a spiral-point tap which aligns itself as long as it is not forced. These taps are not cheap but they are extremely sturdy and pretty-much foolproof to use. However, considering the value in the boiler to this point, it would be penny-pinching to use anything less robust. And there we have it. My boiler which had just passed it's shell test now has an extra twenty-six holes in it!
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