Lapping Plates - Glass or Granite or somthing else?

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Originally Posted by Brighty83

So what i don't understand is, I'm told i have destroyed my granite plate that wasn't flat before lapping. I have been using for the last couple of months after lapping it with no troubles.

Through out the thread, the no.1 statement is "it dosnt need to be 100% flat"

I lapped it first with my DMT to get the majority flat and then finished with my glass plate that is much larger then the granite plate, and as noted on here, the glass plate by it self is more then flat enough to lapp a hone (as mentioned previously). The granite plate was noticeably out before lapping it now its, as far as i can tell 100% flat (or 99.9% flat)

Now, this plate will go out and am i supposed to through it out or why isn't it ok to maintain it the way i have, since i seem to be able to get it flat via this method. according to the comments, this plate cant be maintained and once lapped its worthless. I really think that, by saying that, the comments are miss leading people.

I'm very OCD about everything i do and if its not 100% im not happy. I just cant see how it can be decided that its not flat and that i destroyed it when no once has checked it after.

Dose this mean i need to re lapp all my hones now?

Attachment 83582

If that's the case, i have a lot of work on my hands

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Nice Collection.

I have a 12 x 12 granite tile that cost about $2 that checks out pretty damn flat. I use loose silicon carbide grit on it. the loose grit laps a whetstone quickly. it also wears the tile. the tile has more area and I make sure to use all of the area of the 12 x 12 as equally as possible. so far it is staying plenty flat for lapping whetstones, but when it gets too far out I'll have to go splurge another $2....

I would think for lapping you want a uniform surface like glass for instance. Granite is not that. It's a rock composed mostly of Quartz, Feldspar and Mica. The grit you are using compared to mica, well the mica is softer than butter and the minerals will all wear at different rates. That's how Granite weathers. The Mica turns to mud and the Feldspar goes next and the Quartz just falls away turning to sand.

it really depends on the granite. the tile I'm using is tight black speckly stuff. the surface wears quite evenly and slowly. however, no matter what kind of granite you use the hardest components are a couple of steps harder than glass. i suppose if you are using a very loosly bound granite it might wear faster than glass, but almost all granite should outlast glass. I have used glass, which works fine. remember, we're talking about a disposable surface. after using it for a while it will go out of flat and it is much cheaper to get another surface than to reflatten a $2 tile or a piece of scrap glass. if you buy a certified reference plate ( and it turns out to actually be accurate) keep the abrasives far away from it and just use it for measuring. for lapping use whatever ' flat enough' material you can scrounge up for free or cheap - these days we are just awash in remarkably accurate durable flat materials that are great for all of your loose grit lapping needs. you don't need sub micron accuracy, and even if you had it it would be gone as soon as you lapped anything on it. just get a piece of glass or a granite tile or a granite countertop sink cutout or whatever you can drag home that will work and have at it. use it until you can detect a dish in it. if the other side is good, use that until it goes. then pitch it and get another.

as far as having a completely uniform surface - it's not necessary and even not necessarily an advantage. what does matter is that the high points all be the same height, in plane and themselves, pretty flat, about the same surface area as each other and the same hardness. as for the low areas as soon as they wear to some amount below the high points, probably something like the variance from flat of the object being lapped within the maximum distance between the high points of the lap surface plus some multiple of the diameter of the lapping grit modified somehow by the viscosity of the lapping slurry and the solubility of the feldspar or mica particle that makes up that low point... then the low points stop wearing faster than the high points. in practise granite slabs work great for lapping and in my experience wear more slowly than glass. the minute voids present due to mica and feldspar don't cause problems and may help reduce surface stiction and provide reservoirs for grit to not be pushed to the edges as fast. think of the interrupted pattern of the diamond surface on a DMT hone - it's like that. where you might run into problems with a granite surface either not performing well as a lap or failing quickly is if the granite is flaky in texture or has large or uneven patterns of softer constituents. avoid pieces like that.


one advantage of glass is that it is easy to find in convenient sized pieces and is cheap enough to use up and discard as needed. granite scraps tend to be odd sizes and shapes. both can work well, however. I think a good hard piece of granite is more ridgid than glass, but having a decently flat surface to lay the glass on takes care of that pretty well.

Spend your time more on honing than worrying about a lapping plate. Having a lapping plate 100% flat will not help you to hone any better than one that is 98.9% flat. I promise you, your face will never know the difference.