Building the most accurate DIY CNC lathe in the world [video]

Does anybody actually use the three plate method with granite? It was originally done with cast iron, and I thought cast iron was still the standard material. The plates are covered with dye and rubbed together to find the high points, which are then scraped off, instead of being removed by the rubbing directly.

Granite is a common material for modern surface plates (and a good one because it doesn't rust and doesn't raise burrs if it's chipped), but I believe these are still made using cast iron reference plates.

From what I understand the three plate method is when you are going from 0 to flat as the errors are averaged out.

Doing the "covered with dye and rubbed together to find the high points, which are then scraped off" thing is only if you already have a flat reference surface as you wouldn't have a way to know if the thing you're trying to make flat is really flat.

The real question is how do you get the first flat reference surface when all you have are a few somewhat flat things?

Also for those who aren't familiar, there's also "hand scraping" for flatness which is more common and used for things like refreshing the "ways" (ie precision linear bearings) on a lathe or other precision machine tool.

This is done like the "dye / rub / scrape" method described above, which I believe is still used as it's superior to grinding for these applications.

See video below for the process: https://www.youtube.com/watch?v=T7w84CrBEE8

> The real question is how do you get the first flat reference surface when all you have are a few somewhat flat things?

my understanding is the threeplate method allows you to build the reference plate in the first place.

Hand scraping is done, but for ultimate flat you need to lap three plates not hand scrape. Hand scrapping is not as flat - but the average is close enough and the imperfections are needed anyway for oil so hand scraping is used for bearing surfaces.

Right. You start with any two plates and make them flat with respect to each other. One will be convex and one concave of course, then you take one of those and your third plate and make those two flat with each other, the switch out again using the third plate and the one not swaped out. Keep repeating until all three are flat with respect with each other - only true flat will have all three flat and the repititions keep getting closer.

of course if you have a known flat surface you can save effort by making the new plate flate to the known flat.

New Scientist published a reminiscence of somebody in the relatively modern era doing the 3 plate dance. I wish I could find it online. They said it was tedious work.

Maudsleys 3 plates are in the London science museum along with Whitworths screw, and some of Marc Brunels stuff. Same room as the meccano differential analyser and the harmonic calculator for tide charts and Babbage bits.

Edit: found it - https://archive.is/iyCzB

It's a straight edge, not a bed, but this is a fun watch: https://www.youtube.com/watch?v=pq47yXFmj24

Much the same process, in one less dimension. And it looks time consuming but I bet immensely satisfying.

> of course if you have a known flat surface you can save effort by making the new plate flate to the known flat.

All the effort!

one of the good things about granite flat references, is that they last for ages, so you can get them reasonably cheaply second hand, if you can find a second hand machine shop specialist.

Even if your second hand plate is no longer flat enough it is still close enough so there is very little effort needed to bring it back to flat. (machine shops often check them on a regular basis and bring them back to flat as needed)