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28/05/06

After milling the slot in the manifold the underside was curved to match the valve chest. A smaller block was then made to act as the connection for the steam inlet. This also had the pipe connection facing towards the back of the cylinder. The last job was to cut a recess in the cylinder mounting plates to clear the exhaust manifold.

It was time to solder it all together which was a job I wasn't really looking forward to as I had visions of the lot ending up as a pile of scrap if it went wrong! The joint surfaces were cleaned with files and carborundum paper and all the bits assembled and carefully lined up using a couple of old G clamps to hold everything in place (I get these from pound shops so I'm not too bothered if they get damaged!)

The joints were liberally coated with flux and the whole lot silver soldered with easyflo No. 2. This was a lot easier than using the silverflo 24 as it melts at a much lower temperature and it also flows a lot better. After making sure all the joints had plenty of solder on them the job was left to cool and then dumped in the pickle bath.

The cylinder asembly just after soldering - not a pretty sight!

After pickling and a quick clean up the cylinder was given the once over. Apart from a slight gap between one of the mounting plates and the top of the valve chest everything seemed ok. Actually the mounting plates will be machined flush with the top of the valve chest anyway so it won't matter. If any leaks do show up later I'll just seal them with soft solder.

Looking better after coming out the pickle bath

 

21/06/2006

Seems a long time since the last update but I've been busy with other things such as attacking the jungle that I call a garden!

In order to finish bore the cylinder and valve chest I had recently purchased a Keats type angle plate to make the job easier. The cylinder was set up using this and the bore carefully centred with a dial gauge. This is a very fiddly job as the angle plate has to be tapped about on the faceplate to get the position right and then the mounting bolts finally tightened. A trial cut was then taken and it was immediately apparent that something was not right as when the boring tool reached the end of the bore it was only cutting on one side of the bore. Upon investigation I discovered that one side of the 'V' on the angle plate was not at right angles to the faceplate! This on a brand new piece of equipment! I finished up having to put a 15 thou shim under the rear of the cylinder to bring it true.

After this everything went smoothly and both bores were finished to size.

Finish boring the cylinder

To try and get as good a finish as possible on the bores I rigged up a crude but very effective fine feed to the lathe leadscrew. This consisted of a geared motor with a toothed pulley and an old toothed belt from a computer printer. This drove on the large handwheel on the end of the leadscrew. The motor was connected to a Hornby train speed controller which enabled the speed and direction of the motor to be adjusted to suit. It works so well that I will have to make something permanent based on the idea!

The finish on the cylinder bore was excellent so I decided not to bother lapping it as I doubt the finish would have been significantly improved. The piston valve bore was however lapped with an expanding aluminium lap and 600 grade Aloxite powder mixed with oil to make sure it was as smooth and true as possible.

Improvised fine feed!

The cylinder and valve chest end covers were simple turning jobs from ordinary brass bar but the rear covers were fitted with bushes made from the Peek material to take the piston and valve rods. The rear covers are also fitted with 'O' rings for sealing the rods and these are retained in their housing by covers, again turned from brass bar. To make the housing for the 'O' rings, the covers were bored through the correct diameter for the 'O' rings and the Peek bushes fitted in the bores. The bushes were made of such a length so as to leave the right gap for the ring at the end of the bore.

The two cylinder covers require a segment machining out of the edge to clear the valve chest and this was done using a fly cutter with the cover held on the vertical slide. The bolt holes in all the covers were drilled using a rotary table in the micro-mill.

Fly cutting the edge of the cylinder cover to clear valve chest

The boss on the rear cylinder cover required flats machining on the top and bottom to take the ends of the slidebars. These were machined by bolting the cover onto the cylinder and mounting the cylinder onto the vertical slide. The flats were then machined using a small endmill in the chuck. Beforehand I had turned up a piece of bar to act as a gauge to get the position of the flats correct. This was mounted on a length of bar pushed into the piston rod bush and the flats milled until the cutter just skimmed the edge of the bar. This ensured that both flats would be exactly the same distance from the piston rod.

Machining flats for the slidebars on rear cylinder cover

Finished end covers and 'O' ring retaining covers

Next job was the one I had been dreading and that was machining the piston valve!

A piece of the Peek material was rough machined to size, drilled through, and then fitted onto a mandrel held in the 3 Jaw. The mandrel was turned in situ to ensure that it would run perfectly truly and was turned to such a diameter that the embryo valve could be pushed on by hand but still grip tightly enough for the valve to be turned to final size.

To machine the valve heads to the correct length a similar proceedure to that used to machine the valve ports was adopted. A parting tool was used in the toolpost and a skim taken off the end of the valve. Using the leadscrew micrometer dial the tool was advanced along the valve the length of the first valve head and a groove cut into the valve. The tool was then advanced again to the start of the second valve head and another groove cut. Finally a third groove was cut to finish the valve to the correct overall length. The distances necessary were calculated to take into account the thickness of the parting tool. The material between the two middle grooves was then turned away to form the centre part of the valve bobbin and the excess length faced off.

Using a very sharp tool, the diameter of the valve head was carefully reduced until the valve would just enter the bore in the valve chest. Using a sharp 'V' shaped tool three shallow grooves were machined in the valve heads to act as oil grooves. The valve was then removed from the mandrel, coated with molybdenum disulphide grease and pushed into the valve chest bore. It was then tapped backwards and forwards in the bore until it freed up slightly. This had the effect of polishing the valve bore and left it with a mirror finish.

Finishing the outside diameter of the valve

Unfortunately, that was not the end of the process as allowance had to be made for the expansion of the valve when it got hot. Leaving it a good fit at room temperature would have meant that it would seize up solid at steam temperature. So then followed a long and tedious process of heating the cylinder in boiling water and gradually reducing the diameter of the valve by tenths of a thou until it moved freely at this temperature (approx. 100 degrees C). By leaving the original mandrel in the lathe undisturbed the valve could be put back in the lathe and re-machined a bit at a time.

The valve was now slightly loose at room temperature but will still seal well enough until the temperature of the cylinder reaches working temperature. I think I will eventually have to reduce the diameter of the valve still further so that it has the correct fit at say 150 degrees C but I'll leave that until the outside cylinders are finished and the chassis can be run under steam.

The last job on the valve was to turn up a brass sleeve to hold the valve head. This has a threaded nut at one end so that the fit in the valve head can be adjusted. The fit needs to be such that the valve cannot move endways on the sleeve but can still 'float' up and down slightly so that the valve can centralise itself in the valve bore. The sleeve itself is threaded onto the stainless valve spindle to allow for easy valve setting when the time comes. A brass nut locks it in position on the spindle. The traditional way of valve adjustment seems to by using shims in between the end of the valve and a collar on the valve spindle but that seems way too fiddly to me. Being able to screw the valve up and down the spindle seems far easier!

Finished valve with sleeve and valve spindle

The cylinder components were then assembled to see what the finished article looked like and I must admit I'm very pleased with the result. The piston rod in the photo below is just for show as I haven't made the piston yet!

Cylinder components temporarily assembled

21/07/06

Another big gap since the last update! This time I've been doing some machining for my brother whose bought a part built Rob Roy to finish off . I think he's got fed up with going to rallies etc. with me and not having a loco of his own !

Next job on Helen is to finish the inside motion and then I can make a start on the outside cylinders.

The piston was tackled first and is a simple turning job from some aluminium bronze bar that I happened to have. The blank was turned slightly oversize in diameter to allow for finishing after fitting to the 3/16" stainless piston rod. The groove for the O ring was cut using a parting tool but left slightly shallow, again to allow for finishing after fitting to the rod. The blank was then drilled and tapped for the piston rod and parted off to the right length. Incidently, I find aluminium bronze dreadful stuff to drill although it turns OK. Drilling has to be taken very steadily to avoid overheating and the drill grabbing in the hole.

The piston rod was held in a collet to ensure it ran truly and threaded at one end to take the piston blank. The thread was made slightly large so that the piston is a very tight fit when screwed on. After screwing the piston onto the rod (still held in the collet) the outside diameter was carefully turned down until it was an easy fit in the cylinder bore but with no discernable sideways play. I've read conflicting advice on how good the piston fit should be when using O rings. Some say that the piston should be as close a fit as possible, others say that the piston should be about a thou undersize - take your pick! Finally the groove for the O ring was finished to the correct depth and the end of the rod turned flush with the end of the piston. Finish turning the piston after fitting to the rod ensures that everything is concentric and the piston will run truly in the cylinder. To make sure that the piston cannot work loose in service the thread on the end of the rod was punched with a centre punch to make sure that it couldn't come undone.

Piston, rod and O ring

The crosshead is a fairly simple design and machined from 3/8" x 3/4" mild steel bar. First operation was to machine the slots at the top and bottom to take the slidebars. Care was taken to ensure that the slots were exactly parallel and the same distance apart as the flats on the rear cylinder cover boss. The blank was then very carefully centred in the 4 jaw chuck and the hole for the piston rod drilled and reamed. The reamer was not put through all the way so that the rod would be a fairly tight fit in the crosshead. On final assembly the crosshead will be held onto the rod by a taper pin. I prefer this method of attachment to the often used method of screwing the crosshead onto the piston rod as I think it is easier to ensure everything lines up properly.

The piston rod boss was then turned with a long pointed tool and a scallop taken out of the connecting rod end just to make it look a bit better. The blank was then mounted on the vertical slide and the slot to take the little end of the connecting rod drilled and milled out. The holes for the small end pin were drilled with the same setup. The pin is just a length of 3/16" silver steel turned down to 1/8" dia at one end and threaded 5BA to take a securing nut. Finally a couple of slots were cut on each side of the crosshead using a slitting saw, again just for appearance. After a trial assembly to check fits etc. the crosshead will be case hardened to reduce wear.

Crosshead and pin

The two slidebars are simply 1/4" wide strips cut from 1/8" gauge plate. A bevel was filed on the inside ends nearest the axle to give a bit more clearance for the connecting rod when at its highest and lowest point. The cylinder ends of the bars are fastened to the rear cylinder cover by a single 6BA bolt. The other ends will be secured to the motion plate by a small length of angle. I won't harden the slidebars as gauge plate wears quite well without this.

Slidebars

The inside connecting rod is also a fairly simple design and was machined from 1/4" steel plate in similar fashion to the outside coupling rods. The end cap is secured by two 5BA studs.The big end bush is turned from aluminium bronze with the bore drilled and reamed after fitting to the rod. A brass oil cup was turned from brass rod and pressed into a hole in the top of the big end. The small end is a simple plain bush pressed in and reamed in situ. The sides of the rod were left plain rather than fluting them to save a bit of work.

Inside connecting rod

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