Valve Gear For the Stuart Beam Engine

Eccentric and Strap - Part 2 Eccentric Sheave.

The sheave is quite a simple turning operation initially.  The blank is turned in the first place to the diameter of the groove in the strap.  The blank is then turned to the smaller diameter on both sides leaving the ridge to locate in the groove of the strap.  Unfortunately I did not take many pictures of these operations so I will explain as best as I can.  The finished sheave (except for final polishing etc.) is shown here. 

 

 I turned the wider section, with allowance for the eccentric boss, first and then, using a parting tool, turned the narrower section on the other side of the ridge while the sheave was still attached to its parent material. 

  At this stage I took the part, still attached to the parent material, inked it with a broad tipped Sharpie permanent marker and marked the centre of the end diameter.  Then, using a pair of dividers I struck off the centre of the eccentric from that centre and centre punched.  A word of caution here, I was using the Anthony Smith Stuart Beam Engine book, often quoted as the 'bible' for this engine.  In the drawings (from the Stuart originals) the eccentric centre is shown as being offset from the sheave centre by 9/64" (3.57mm), however the text said that the dimension should be 11/64" (4.37mm).  I decided to ask the model engineers forum members which would be correct and got the answer - 9/64" - almost by return.  What would we do without the internet now it is so ubiquitous?

Then it was time to turn the eccentric and drill the 7/16" (11.11mm) hole.  I set the job up in the 4 independent jaw chuck by setting the eccentric centre mark approximately on centre using a dead centre in the tailstock.  I achieved final accuracy by the old method of trapping a bar between the centre of the work and the tailstock dead centre and using a clock on the bar near the work was able to set the work with some accuracy.  I have no picture but perhaps a drawing will make the process clear.  I will put a later post up when I do the process again.