A couple of set backs this last week.
My internet connection was severed (literally) by a back hoe on a JCB - my little town has embraced the need for a super sized supermarket to be built in what was formerly the town centre car-park. Utter chaos now.
That severence blew out the local server for my internet provider - and after about ten phone calls to Bombay seeking technical help (for which you have to pay for the privilege), I was told that the network was supposedly restored. It took nearly 7 days to sort!
The outer cylinder end-cover has no chucking piece and is somewhat risky to hold in the three jaw, given the "coning" of the outer edges.
SB recommends cleaning it up a bit and trying to get it to run as true as possible in the three jaw. Then centre drill and tap it 2BA and make up a similarly threaded stub mandrel from a bit of scrap bronze or gun metal bar and screw it into the "domed" side of the outer cover :-
This then provides a temporary chucking piece to machine the outer circumference true and to dimension, the inner (flat) face, bring it to thickness and machine the register on it.
The stub can then be unscrewed and the cover reversed, the stub screwed back in, and the outer (domed) face machined. The dome can be finished (bar final finish/polishing etc.) by careful twiddling of the top and cross slides.
The stub is then sawn off flush with the inner (flat) face. :-
The original plan calls for just 6 (7BA) fixings for the end covers to the cylinder block. To my mind, this does not look quite right.
SB goes a bit overboard in his build, doing 24 (!!!) 10 BA holes at each end - with my luck, a sure fire recipe for a thin tap to break off somewhere.
I opted to go for 8 (7BA) fixings.
I marked out and drilled them in the end covers using my rotary table with home made chuck adapter/receiver.
The cylinder ends were spotted through and drilled/tapped. It is worth using a small "T" piece type hand tap holder and a bush to centre the tap. Assuming all of the holes in the cylinder block are all to the same depth, count the number of turns of the tap until slight resistence is felt for tapping the first hole. Then stop!! You now know (theoretically!) how many turns each of the others should take - just a safety tip. For those of you reading this and thinking of building the same - for a depth of about 5.5mm, it takes 19 half turns! (I guess someone can work out the maths based on thread pitch etc.)
Then setback number two.
I successfully milled the three finger shaped steam ports in the valve face of the cyl. casting. I used my height gauge to mark out accurately.
Dimensions are given for milling a small "pocket" at each end face of the cylinder casting, adjacent to its valve face.
From these, three holes are drilled at each end at 30 degrees to meet the steam ports.
The first (inboard) end went reasonably well.
I was using a 1/8" Clarkson type two flute cutter (on a 1/4" shank) to mill the marked out pocket.
Stupidly, without realising how bad this is - I used my trusty set of ER32
metric collets - essentially forcing the 1/4" shank into a 6mm collet.
Without realising it, I had fractured the shank as I tightened up the collet ring at the junction of the plain part to the clarkson threaded part of the shank.
Fairly obvious to all you experienced builders - but I have learned - metric collet = metric cutter. Don't force an imperial one into a collet that's basically too small - especially a Clarkson one! I used an ER32 "5-6mm" collet - I should have used the "6-7mm" collet.
As I was milling the pocket, I realised the cutter seemed to be too deep. The fractured end was walking out of the collet. I ended up with a pocket 2mm too deep at one point. This would have repurcussions with the piston stroke.
The only way to retrieve the situation (apart from buying a replacemnt cyl. casting - cost around £40!!!) was to do a "dentist" job.
I milled out the pocket square and silver soldered a piece of scrap gun metal roughly filed to size into the enlarged pocket.
The cylinder was then set up again in the Keats angle plate, with the "good" end outermost. It was set up to turn true with a clock on the inner bore and then the inner end re-bored very carefully to remove the overspill of material and silver solder.
The three steam passages were then drilled. :-
I was a bit close to one of the tapped holes for the end cover - but all is well.
You can just see the "seam" below the pocket in the bore.
The final item was the piston rod stuffing gland - nothing appeared to be provided for this in my kit of parts (the plans said this is made from a casting!), so I turned it up from a short length of 19mm bronze bar. :-
Slide bars next.
