Triple expansion steam engine

[Eric CoxParticipant @ericcox50497]

With regards to the crank of the Triple expansion steam engine, ME 4430 p.720 fig.8, having marked out the centre line of the first crank, how do you rotate the shaft exactly 120 degrees to mark the second and third crank centre lines.

[Eric CoxParticipant @ericcox50497]

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Good morning Eric ,

The difficulty is really in getting them the same at both ends of blank – they don't nescessarily have to be 120 degrees to the nearest second of angle .

A way of doing this is to set the blank up between centres and arrange a driver and catchplate in the normal way . Using any improvised dividing arrangement on lathe spindle (based on a change wheel) you can set each of the 120 spacings . Using scribing block scribe left end then right end . Rotate 120 degrees and scribe again etc .

Need to arrange a bit of clearance at headstock end and make sure driver is always in contact with catchplate pin (rubber band) but otherwise easy to do .

A variation of the same thing uses the three jaw chuck with a freshly turned centre and the driver engaging the chuck jaws . Remove blank and reset  to each jaw position to again give 120 degrees .

To be clear on one point – blank always stays same way round relative to lathe axis – its not turned end for end .

Another way of doing it is to put the blank on a surface plate and clamp it somehow so that it can't rotate . Then use diy slips to scribe horizontal and vertical lines at precalculated distances relative to surface plate and a side touching angle plate . If one crank throw is upright the setting out is symmetric and quite easy .

Working out the thickness of slips needed can be by trig or CAD but probably a large scale drawing would be adequate .

There are ways of setting out the throws very accurately indeed using toolmakers buttons etc. but probably not needed for present purpose .

 Regards ,

Michael Williams .

 

 

Edited By MICHAEL WILLIAMS on 19/05/2012 11:57:53

[JasonBModerator @jasonb]

One other option is to clamp to place the blank on a vee block like Morgens shows and scribe a horizontal centre line on both ends with your height gauge. Rotate the work until that line corresponds to say a 60/30 set square, angle gauge etc, clamp to vee block and then scribe another line on both ends, repeat for the third line. You can next alter the height gauge to give the offsets or use a pair of dividers.

You can also do it on the mill, clamp the blank to a pair of vee blocks then stand on end, depending on size either hold the vee block in the vice or clamp to an angle plate. Indicate the centre of the blank and use co-ordinates to centre drill the holes. Flip it so the other end is pointing up and do the same again, having it in th evee blocks keeps the alignment. I've used this method several times on single throw cranks but it also works on multi throw ones. See this picture. Something similar can be done on the lathe with a vertical slide, do one end and then without taking the work off the slide turn it 180deg to do the other end so it all stays lined up.

 J

PS If using Michaels slip method the you can work out the offsets in the same way you would set out holes on a PCD and just use Zeuz tables

Edited By JasonB on 19/05/2012 12:25:35

Edited By JasonB on 19/05/2012 12:37:52

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