Flat stones

[FulmenParticipant @fulmen]

This project went quicker than anticipated. I figured I should have a set of flat stones in the shop, so I cut out 3 octagonal stones from a cheap two-sided sharpening stone on the water jet (sorry, no pics). I thought I would spend the Christmas holiday lapping them together, but I ended up finishing both sides in an evening.

This video describes the lapping process: https://www.youtube.com/watch?v=6emh90BexpE

And of course a 3D printed box to round off everything

[FulmenParticipant @fulmen]

And don’t worry, you can use a hacksaw if you don’t have a water jet. The teeth will of course full in seconds, but it wil still cut. Diamond hones are great for roughing in very uneven surfaces, I also used a coarse diamond wheel dresser to refine the sides.

[Wade BeattyParticipant @wadebeatty78296]

Nice work Fulman

[Nigel Graham 2Participant @nigelgraham2]

Fine work!

 

I guessed before we reached there that the video demonstrates what I would call a “reciprocity” technicque.

This was used to make the first surface-plates, in fact; by hand-scraping and/or mutual lapping trios of them.

 

I wonder if too, this was used for the early astronomical telescopes and mirrors. That quote part-way though does talk of “speculae”, the fancy name for mirrors.

In one of his books Helmholtz describes grinding concave surfaces by abrading and polishing the surface of a circular blank.

It is clamped round its rim down to rings of very accurately (or precisely?) set screw-jacks. This raises the centre so when that convexity has been ground down flat, on release the glass springs back to leave the centre low. Obviously the underside will have been ground flat in the first place.

Note for anyone intending to experiment: use new glass. Glass becomes more brittle with age, I am informed by those in the windows trade.

 

I knew the basic principle from a very different field, or rather, lake – testing nominally-identical hydrophones.

If of sufficiently “reciprocal” characteristics they will act as “projectors” (underwater loudspeakers) as well as hydrophones (microphones).

The three we can call A, B and C are made to “ping” at each other in water, using the same set of amplifiers, signal-generators and analysers. The results come out of a load of maths described in the text-books, though a computer did that for us; but in essence the technique is still the same, A to B, A to C, B to C.

And reverse if wished as with mutually-lapping those stones, although the soggy analogue is not necessary if one of the transducer trio has been calibrated independently, by an external laboratory, to give the necessary confidence test of the whole system.

 

 

 

[FulmenParticipant @fulmen]

The method is usually linked to Joseph Whitworth, but I can’t believe that nobody figured this out before him. I mean, they’d been grinding lenses for centuries at that point. But I also see a lot of people conflating the Whitworth method of scraping with this lapping technique. And while the logic is the same the execution is somewhat different since lapping alters both surfaces at once.

[BazyleParticipant @bazyle]

The amateur method of making a concave mirror is sneakingly cunning. A round blank is inverted over a round lap of the same size, both flat at the start,  and moved in a straight line across it, rotated a little and repeat for ages. A little thought will show that the middle of the blank spend more of its time over the lap while the edges are over air some of the time. Hence the middle gets more grinding than the edge and it ends up dished.

[FatgadgiParticipant @fatgadgi]

Well done Fulham – funny, I’ve been thinking of making a pair of flat stones as well.  Didn’t think about using the three flat surfaces method for them.  Does it close the pores and blunt the grit as per the ones used on machine ways?

I read somewhere that a flat diamond lap would do a good job as well.  Thin ones are pretty cheap, so I was thinking about bonding them to a steel plate.  Anyone tried this?

I do have a surface grinder, but not a diamond wheel for it, which are not cheap!

Cheers Will

[Pete RimmerParticipant @peterimmer30576]

I use a 8″ diameter cbn wheel I think it was £40 when I bought it. It wears down pretty quickly doing stones but that’s compared to normal grinding and I’m still on my first one.

[FulmenParticipant @fulmen]

@Will: I can’t judge grain shape or anything, but it certainly feels reasonably sharp with dry lapping. Wet had more of a dulling effect. They will of course quickly clog from stoning metal dry, but a quick lap against another stone cleans that up. That suggests that the best way to keep them is to use all three and just keep them lapped against each other with random use and cleaning.

I should stress that my stones are fairly soft. I don’t have have any data on it, it’s just a two-grit no-brand aluminium oxide sharpening stone. But a hack saw will cut it even when the teeth are worn completely away, just to illustrate how soft the binder is. It might not work the same with harder stones.

[FatgadgiParticipant @fatgadgi]

Mmm, torn between 3 stones and trying to grind flat.

Something to ponder over whilst stuffing too much turkey down my throat 😊

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