Part 11 - by Ramon Wilson
Part four - by Ramon Wilson

While on the surface the prop drivers these seemingly innocuous little parts which look like basic little turnings, they do need to be machined with care if the propeller is to run in a true plane to the crankshaft. There are several means of attaching these to the crankshaft from a push fit onto serrations, holes with a drive flat in them and varying means of tapers. Described here is the separate, tapered split collet and matching driver, where the force of the prop nut tightens the collet to the shaft and one which provides a degree of self release. The 'challenge', if there is one, is to get the taper in the rear of the driver true to that front face which has to be knurled to provide a grip on the prop as well as, importantly, a matching taper on the collet.

The following method was adopted on the first Eta build and proved successful. Used again on the Tigres and now these latest engines it has proved to be quite straightforward and gives an accurate part.

Began with roughing the OD and front face and drilling a 6mm hole then parting off the blank.

Held by the faced end in soft jaws the rear face was faced, the bore opened up to 8mm by first taking a lick out by boring to true the hole and the taper turned - all ops using the same boring tool (this one's ground up from a discarded slot drill).
Popped back into the soft jaws the recess to define the knurled area was cut - not critical, the soft jaws providing more than sufficient concentricity ....
....ready for knurling
First off is to turn a mandrel. To maintain a matching taper means not moving the topslide so this is cut with the lathe in reverse and using a boring tool - it's tapped 4 BA. This was already turned for a previous build but a skim is taken to true it up.
The prop driver is held to the mandrel using a thick 'washer' which is stepped to just fit inside the bore. The knurl is not captive on the holder - it just slides onto a dowel and rotates freely and is prevented from moving by the washer. The knurled area is skimmed before knurling to bring this true to the taper.

The lathe is run slowly - around 150 rpm and the knurl pushed into the driver using moderate pressure. It does need lots of coolant, I don't have a system set up so use a Squeezy bottle to flood it.  It's better to back off and clean the knurl - it will soon 'pick up' again if brought up slowly - as the chips formed can soon spoil the effect if allowed to be crushed into the grooves.

Ready for finishing off ....
.... but first the collets need making before that topslide setting is broken. Made from brass - could be free cutting steel but ali is a definite ‘no no’ as in use it could 'pick up' on the driver and prove a devil to remove. The collet is reamed to suit the shaft (6.35mm) and the taper turned as for the mandrel by running in reverse and using a boring tool
A driver is positioned on the collet and a parting tool set to the back face.
With the blank removed the tool is advanced 0.25mm to define the rearmost face then withdrawn 0.5mm and plunged to create a step at the base of the collet. (Sometimes when these collets are split they close tight onto the shaft. This step allows something flat to act as a lever to prise it away from the step on the crankshaft enough to be able to grip it.
With the collets all finished the mandrel is set back up, given another skim then the top slide can finally be moved to finish turn the OD true to the taper.
Ready to fit.


See part one here  part two  part three  part four  part five  part six  part seven  part eight 

part nine  part 10  part 11  part 12  part 13  part 14  part 15 part 16