Eccentric Sheave

View of Machined Sheave Showing Locating Rib

The next part to be made was the eccentric sheave.  This was a relatively straightforward turning job but there was some query about the eccentric centre in relation to the centre of the sheave.  I have been referencing the valuable little book by Andrew Smith in which he describes the manufacturing process.  When drilling the sheave for the eccentric, he says to set the centre at 11/64" (4.37mm), however the drawing shows the distance as 9/64" (3.57mm). making a difference of 1.6mm on the throw of the eccentric.  Obviously that would make quite a difference in the valve movement on such a small model.  An enquiry on the Model Engineer magazine forum returned the answer in a very short time.  It turns out that the drawing is correct at 9/64" which goes to show that it is important to double check even the best of source material.  It is well known that there are often mistakes in published drawings and text so it is o=important to check and double check.  The internet interest forums are such a valuable resource these days, no longer are we working alone and in the dark, there is always knowledgeable person willing to offer suggestions and information.

The most tricky part of the sheave was the rib which had to be machined accurately to match the groove in the strap.  Initially it was a plain cylindrical machining job. A piece of FCMS (free cutting mild) steel  with a generous chucking allowance was turned to the overall diameter of the rib.  Left hand and right hand lathe tools were then used to turn the smaller diameter to fit the main internal diameter of the strap.

The now established centre of the eccentric was marked out, centre punched.  This was set to centre using the 4 jaw independent chuck on the lathe and the boss turned to diameter.  The drilling to 7/16" (11.1mm) was done by step drilling with a succession of drills in the drilling machine.  That was quite accurate enough, no need for reaming.  To complete the eccentric a hole was cross drilled in the eccentric boss and tapped 5BA for a fixing grub screw.  Now for a bit of finishing bling with wet 'n dry.