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

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177 points pillars | 4 comments | 03 Sep 25 13:47 UTC | HN request time: 0.001s | source
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WillAdams ◴[03 Sep 25 16:02 UTC] No.45117323[source]▶
For folks who are not familiar w/ machine shops, the lathe is a fundamental tool in a shop, and is the only tool in a shop which can replicate itself --- there is even a book series which uses this conceit, the "Gingery Books":

https://gingerybookstore.com/

where Vol. 1 has one setting up an aluminum casting foundry in one's backyard, and Vol. 2 has one using it to make a lathe which is then used to either improve itself or make a better lathe, then one uses it to make the balance of the tools in a machine shop.

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jjk166 ◴[03 Sep 25 16:55 UTC] No.45118024[source]▶
A lathe can't actually replicate itself completely. Specifically, a lathe can only make ways smaller than its own cross slide's stroke. It would also be impossible to make a typical lathe bed on a lathe, though you theoretically could design an unconventional lathe bed that is possible to make on a lathe, even if grossly impractical.

The real starting point for machine precision is rubbing 3 granite plates together.

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mrob ◴[03 Sep 25 20:31 UTC] No.45120072[source]▶
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.

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UncleEntity ◴[03 Sep 25 20:56 UTC] No.45120333[source]▶
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?

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1. KaiserPro ◴[03 Sep 25 21:14 UTC] No.45120473[source]▶
> 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.

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2. bluGill ◴[03 Sep 25 21:46 UTC] No.45120733[source]▶
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.

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3. KaiserPro ◴[04 Sep 25 11:39 UTC] No.45126140[source]▶
> 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.

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4. bluGill ◴[04 Sep 25 12:47 UTC] No.45126647{3}[source]▶
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)