Hi Guys - thanks for looking in and your comments - cryptic or otherwise
Neil - I've made all the cranks so far using high tensile En24T leaving it in it's as machined state. It is a tough but fairly easy material to machine and produces a good, hard wearing, finish on the bearing surfaces. These engines are never intended to be used in flying mode though I'm sure they would stand up to that requirement if desired - the need for further heat treat is not felt necessary given their intended display use though.
The cranks were set back between centres for finish turning the shaft diameters then held in a collet for drilling the intake hole. Finally set back in the fixture the crank-pins were brought to diameter. The fine tolerance on the bearing areas was achieved by stoning with a pair of fine grade thin flat India stones.
With those ops completed a simple fixture was made to hold the shafts for drilling the inlet
Set at 20 degrees the hole was 'drilled' though first using an FC3 cutter
Then the fixture set on its side to machine the inlet timing area
The webs could then be milled to profile
And finally set back between centres to screw cut the shafts. Nothing special - HSS ground tool and the tool pulled out at the end of the run ie no run out groove for maximum strength.
Incidentally - I don't think I've seen this tip mentioned before on here but if no chaser is available to clean a thread the flanks can be polished to a degree by using the end grain of a piece of wood charged with carborundum paste. Pushed hard into the work the wood will run along the thread until the end.
This 'still' gives a bit better idea
The three finished shafts ready to fit
I thought it best to finish off the steel parts - rounding off the characteristic 'Oliver' ball ended compression screw - the GFS tool, hardened but not tempered, cuts really well. Note this is En1a - not En8 which I had called for on the drawing!
Removing the parting off pip by rotating the tool
Finished comp screws - all screw cut to a tight-ish fit in their respective heads
That's state of play as of today - still raining and more forecast so more 'shid' time
AS - regarding the cutter -
This was made from silver steel. The blank was turned first relieving the inner diameter to give a slotting saw type relief. The teeth (5) were cut on the mill/dividing head then shaped/sharpened by filing. Heat treated and quenched in oil (not water) but not tempered. This gives maximum hardness and the oil quench offsets any tendency to crack in use.
Once heat treated the teeth are sharpened using a diamond file.
Hope that's all of interest
That's it for tonight - Tug