Flattening ceramic whetstones on plate glass?

[rsser]

Some say this can't be done.

I disagree. This is my experience with a strip of glass on MDF:

1. 3 episodes with a Shapton #120 using #36 SiC grains. No problem with stone flatness and my impression was that the glass got slightly flatter. Didn't measure it with a feeler gauge, just eyeballed with a straight edge.

2. 6 faces of Sigma Power Select IIs or Sigma Power: #240, #400, #1000, using #90 SiC grains and for finer stones some of the slurry left behind from the previous. Did some skewing and some circling of the stone on the strip. Result: the #1000 is slightly higher in the centre on the long axis and the strip is dished by 1.5 thou across.

At a guess the flatness of the glass would be better with a square piece and more circular movement of the stone. Will try that next.

Someone wrote that lapping properly understood involves an abrasive medium used between a flat surface and the surface to be flattened. So the abrasive grains must move around. They do, visibly, and it works.

[snafuspyramid]

Some say this can't be done.

Who?

It most certainly can be done. Use a decent sized piece of glass and use a figure-8.

Although I have a diamond plate I still use this method to flatten courser stones (120, 400 and sometimes 1000).

[Robson Valley]

When the time comes to work in the wood, does +/- 0.001 really matter?Honestly, does that show in a table top? I have no idea.Where I live, there are sand deposits all over the place = you pick the grit size that you want and we will go shovel up 50kg.There's a company here that cuts stone counter-tops with a diamond rope bandsaw with 8' (240cm) wheels.Big saw and big slabs. If they cut fractured rock, off to the junk pile for people like me.I use the small 30 x 30cm bits for carving bases, maybe 2-3cm thick. I can drill a bolt hole in any of it in less than 20 minutes. I use about 120 grit sand to flatten one stone slab against another.The diamond rope doesn't run true and leaves 2-4cm ridges that I can grind down whith out too much trouble.Figure 8, up/down, left/right, spin a few times.No, it isn't optically flat. I rue the day when I'm told it needs to be. Sorry for them.

[rsser]

When the time comes to work in the wood, does +/- 0.001 really matter?

Not then but beforehand. If you want a honed edge you need a polished tool back. To get that running up through the grits each abrasive surface must be flat. A cumulative error of a thou over 4 to 5 stones means the last stone or two won't work the whole section of the back you want polished. Or if you have variable flatness you find missed sections of the back have to be corrected two stages down the process which is a PITA.

[rsser]

Anyway, after c. 9 flattenings with loose grains that strip of glass is dead and has been replaced by a 30cm square of 6mm float glass on a flat carrier.

On that a sheet of PSA 100 micron (c. #150 P rated) 3M micro-finishing abrasive was laid.

It flattened 3 ceramic whetstones (Sigma PS II or PS; #3000, #1000, #240), both sides of each, before running out of puff.

A new sheet was laid, and exhausted, doing both sides of a PS II #240.

The glass was $25. Can't recall the cost of the 3M films. If anyone's interested I can trawl through the receipts. This stuff came from RioGrande in the US.

The work took about 45 mins. Not onerous with 8 faces of quality stones prepared for the next task.