Thread: Lapping Plates - Glass or Granite or somthing else?

Because of what was done with it.
Lapping stones like those granite stones are meant to be used and lapped a certain way.

If you rub a small DMT over a large granite surface, different parts of the granite surface get worn differently, because the DMT will not touch every part of the big stone the same amount of time with the same amount of pressure.

The DMT may be flat to very tight margins, but used like that, it is pretty much impossible to achieve the same level of flatness on the big surface. The level of wear will be different. So if the granite plate started out with the same tolerances as the DMT, the tolerances will be much worse after the lapping operation.

You can lap a small surface with a bigger surface.
You cannot lap a big surface with a smaller surface and get a consistent overall flatness.

After reading this I am wondering about what I do. I use a large 3/4 piece of tempered glass and place wet/dry sandpaper on it to lap my hones. Did I need to check the flatness of the glass?

No. For the purpose of flattening a hone for use, you really don't need micrometer flatness.
The error in the sandpaper (which is soft as well -> variable flatness) alone is much worse than the variations in the glass.

I had a D8C that I sent back to dia-sharp because it wasn't flat. Matter of fact I sent back a 1200 at the same time. IT wasn't flat either. They agreed on the 325 and sent a new one back. They didn't agree on the 1200 and returned it with the new 325. I checked it with a Starrett straight edge. Flip your plate over and check it with a straight edge. It will tell you if it is flat within dia-sharp's tolerances. As for the granite, I agree with Bruno that it is probably no longer flat.

It is worth noting however, I talked to a machinist about granite plates and they change over time. Not only from a certain amount of wear due to use but just from atmospheric conditions IIRC. They have to be re-certified every now and again. Anyway, also like Bruno says, this business of honing razors doesn't require as much precision in flatness as some of us OCD practitioners desire.

The older members remember Howard at the perfect edge, he had a lapping plate supposedly flat to .0000000000000001 or something like that. It was around the time the Shapton GS's hit, with pond etc...I always thought (IMHO) it was a bit much. As mentioned, It's razors, not rocket science.

Seems like an awful lot of trouble to go through.

Honestly I am pretty OCD about honing razors (I know hard to believe huh )

But honestly anything more than a DMT 325 or a Shapton DGLP or something along those lines is a bit of overkill..
The stones/hones just need to be clean and flat they don't have to be perfect, most of the razors aren't anyway

I am very hesitant about using anything that releases grit to lap with also, you have to be much more conscientious about cleaning your stone's surface to prevent embedding grit... This isn't a big problem in most finishers but it is on the lower grit stuff...

Just like much of what we do it becomes a trade off as to effort-vs-gain, but also they are your hones, so the final decision is ultimately yours...