A double eccentric for the Burrell will give forward and reverse. As this engine is tiny I machined the pair as one. This was machined from a piece of 1/2″ diameter mild steel. My steps were:
- Cutoff ~50mm length of 1/2″ diameter mild steel round bar
- More length than required to ensure stable holding in collet
- Machine collar step for grub screw – Warco WM240B
- Mount in square ER25 collet block
- Mount square collet block in independent 4 jaw chuck
- Centre round bar using DTi (I used a digital DTi)
- Relax one of the jaws on the 4 jaw chuck
- Rotate so this relaxed side aligns with the Dti
- Move the jaw opposite the relaxed on inwards 1mm
- Rotate the chuck by hand and check the offset both sides
- carefully tighten all jaws and recheck the measurements
- Machine the eccentric
- Now repeat steps 6 to 10 moving the part to a 1mm offset in the opposite direction.
Hopefully the video will give an idea of the workholding and DTi arrangement.
I need to carefully clean up this double eccentric for the Burrell, drill and tap for the grub screw and then I can get on to making the eccentric straps.
Crankshaft from Parts
Making the Burrell crankshaft from parts has allowed me to then have a split. The split is at the crankpin end allows me to then have a single piece connecting rod. Over 12 months ago I was concerned with the crankshaft to firebox clearance. Splitting the crankshaft means I can have a single piece conrod big-end.