Pressing,Peening and Glueing – a crankshaft for Befur

The theory

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 thhis would be mad!) or fabrication from journals and webs with a combination of shrink fits and loktite (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

Turning the journals with lots of suds, between centres ensures everything is really square

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 centered 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 headstock 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…

large bar ensures the blocks are square, then the dial gauge ensures they are square on the table – then you can clamp the blocks in place and remove the setting bar.
now the blocks are clamped in place the crank journal can be clamped in the blocks, and you know it’s square – a slot drill finishes the job..

The Webs

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 slighty 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. 1.5thou 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 locktited (how DO you spell locktite) joints is that these can be slid into place, ensuring that all the journals align.
  • Lastly round the crankpin ends of the journals (the other end get balance weights later).

milling the rounded ends with a rotary table – having set everything up with DRO (god’s gift of a thing)

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….

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