DO NOT BUILD A CRANK THIS WAY – MINE FAILED IN 20 MINUTES!!!!
See this post on the 2nd crank, for a better approach!!!!
Reading the notes for the engine a number of approaches to building the crankshaft are suggested. Forged Blanks (would need to buy a steam hammer), turn from solid (I’ve done that for a model car engine, but the waste with a 1/2 inch stroke was too much, so this would be mad!) or fabrication from journals and webs with a combination of shrink fits and Loctite (with taper pins for security.)
I opted for the latter as I have had previously good experience with heat shrinking the wheels on the loco, and metal adhesive technology has advanced a deal in the last 10 or so years.
The round bits
So the game plan is to start by preparing all the “round” bits (crank pins and journals) with shouldered ends – no great problems here, just centre drill the ends and then do all the work between centres. (Used EN24T for the journals, and had some stock provided in the casting box for the crank pins – this looks suspiciously like stainless, so I am a little concerned about the wear on this, but time will tell)…
To be really sure you also skim the headstock centre, (with it in place in the headstock) to make sure it runs straight, and check the alignment of head and tail stock centers (stick something of good dimensional quality (silver steel bar) between the centres, and then run a dial gauge along the side, to make sure there are no big errors…
This should ensure everything is nicely central and square. I checked the run-out on the shaft before starting to make sure this was OK,(I had centred the bar OK) and had run-out of about a thou or so (and this it about as good as it gets on the Harrison, as the Timken taper-roller bearings in the head-stock produce a wobble of about 1/2 a thou when running – not slack but some sun & planet effect as the bearings precess?).
No real problems, spend some time miking the journals and making notes, so you have the data for making the holes in the webs.
Then I mounted two v-blocks on the milling table, locating them by clamping a large bar into them, clocking this true, then clamping the blocks to the table and removing the setting bar. Once this was done then I could mount the crank journals in place and machine the woodruff key ways – here’s a couple of pics of this bit…
These were milled (shaped) from some over-size mild steel bar – as follows:
- First reduce stock to size (see vid),
- face one end of each piece to provide a datum,
- then clamp, drill and pin into pairs ( I learnt my lesson on the crank pins on the loco – any inaccuracy is magnified by two as the crank turns!) – the theory is if you then jig-bore them, the pairs MUST come out matched. If the two pairs are not identical then this is not a problem, the stroke on one cylinder will just be slightly longer than the other, HOWEVER if the two in a pair are not matched, then a bent crank is an absolute certainty!
- Then you can bore the holes. 1.5-thou undersize of the shrink fit journals, and 1 thou over for the loctited ones… in case you haven’t figured it out, the reason for the Loctited joints is that these can be slid into place, ensuring that all the journals align.
- Lastly round the crank-pin ends of the journals (the other end get balance weights later).
Assembling the thing
….actually this did not turn out to be the simple process I envisaged… so I think another post is in order for this bit, as I learn a bunch of new stuff in the process….