Interesting to see everyone's thinking!
I'll put you out of your "misery", but some more photos first. Here's most of the bits I've made so far put together:
The large holes in the top plate are for magnets; the small holes either side of the central spigot (if that's the right word) are for drive dogs. The plain holes in the flat plate next to the cap screws and smaller threaded holes are for bits of steel as the aluminium in that area would wear too much.
Another view:
This photo perhaps gives a better idea:
The motor runs at about 3600 rpm; the gearbox slows it down to about 400 rpm. Those discs are cheap (about £5.50 each) 152 mm diameter diamond coated steel discs (available in grits 80 to 3000), that get magnetised onto the top plate and driven by the little drive dogs (5 mm diameter steel pins in the holes I mentioned earlier in this post). It's based loosely on an idea from Stefan Gotteswinter, although using bigger discs than his and the whole thing has been designed from scratch by me
The plate with the curved side on the top is held in place with four cap screws and has four grub screws that bear on the steel inserts and, between the cap screws and the grub screws, the height and angle of the plate can be laboriously tweaked to get it level with the top of the diamond disc. The table (bottom of the last picture) will eventually be attached on the side (in one of two positions) to give a reference for grinding (slowly) at angles down to 20° (@novocaine: your assessment of the taper angle was spot on!)
The aim is that I can use it for reshaping second-hand plane blades and chisels to get an accurately square end on them. Running slowly will mean it takes a lot longer than a bench grinder, but the blade shouldn't get as hot. I hate putting plane blades and chisels on the bench grinder and I much prefer the idea of something that goes slowly and has a flat grinding surface. The groove in the table will be for a fence that helps keep everything lined up as it's plunged into the grinding disc.
The flat plate on the right of the photo, once set at the right height, is there to support a tool when you're flattening the back.
When all complete it should look something like this (although I haven't designed the bit at the bottom of this picture yet, so the whole thing is just floating above the brown-coloured power supply block at the moment):
I used it to sharpen a penknife to 3000 grit last night and it worked remarkably well - a penknife doesn't need the table, so that's about as much as I can do until the table is attached. Having played with it a bit, I think it'll be most often used in the orientation shown in the first test photo above: with the disc horizontal at the top rather than vertical on the side like in the CAD model.
In theory I should also be able to use it with my
Harold Hall grinding rest for accurate metalworky grindy things, although I'll probably stick to a bench grinder for anything that involves high-speed steel.
