Finished machining the wagons' 4 eccentrics. Oh what a struggle!
Today's epic was cutting their keyways.
Lacking any form of slotting tool, I milled them with the eccentric flat on parallels, in the corner between two small angle-plates. I aligned the sheave, trapped in its strap by aluminium-foil packing, by tiny centre-drill spots I'd already made.
I considered using a vice, briefly, but my milling-vice has so much slop in it I don't trust it. However, this meant using clamps, and those commercial clamping-sets are so thick and clumsy they are frustratingly awkward to manipulate, and severely limit access for close-in work on small parts. So allowing spindle room meant barely-adequate clamping area on the disc, despite rather unorthodox clamp assembling.
Indeed a sudden, harsh vibration indicated the first eccentric was slipping round in its strap. Luckily this was on the first depth iteration of 0.05" so it has damaged only the entry edge of the keyway. Re-set it all, finished that and completed the other three without further ado. but taking only 0.03" deep cuts.
Yes I know the keyways are arched, but that does not matter as a grub-screw will lock onto the key.
@
Grub-screws…. Yesterday's epic. I realised if give the strap an oil-hole near the rod mounting, I could select their diameters to use them also for guiding the tapped hole in the sheave below. I marked the sheave and strap with tiny centre-drill dots for alignment, and set the assembly on one of the angle-plates, with an adjustable bevel to set the appropriate angle.
Drill clearance down to just below the circlip groove on the sheave's equator, taking care not to snatch the drill on the groove sides: gives tap guidance by what will be the oil-way.
Drill tapping-size down to the bore.
Enlarge the outer hole to 6mm, so giving adequate initial tapping depth below the floor of what will be the oiling-point.
Take drill-bit from chuck, lower the knee to comfortable working height for a short-++++ like me, tap the hole, M3.
Three eccentrics done, then … Crack! "Bother" or similar, and despair. Dismantling showed the tap had broken in the circlip groove, with bits jammed irrecoverably in both components. I could reverse the sheave and drill its opposite shoulder, but I was faced with having to make a new sheave.
Luckily, I remembered what looked like a spare sheave, on the dining-room window-sill. Well, where else? (There is about as much engineering stuff indoors as in the workshop.) Phew! It was a spare sheave! And, the milling-machine's basic set-up for drilling and tapping was also that for boring the shaft hole, so still in place – though I took the precaution of verifying its co-ordinates..
Finally came indoors at about 11pm, knackered but now with four eccentrics all back up to the same stage.
While at it, today I enlarged the grub-screw holes to M4. Still to make the circlips fit properly, or make new ones. Or buy! I have discovered you can buy similar, and quite cheaply, for such applications. They resemble piston-rings but of opposite proportions, and without the usual circlip lugs. The existing, mild-steel and parted from bar stock, are slightly too thick.