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

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)

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 now when no once has checked it after.

Brighty, the thing is that to check the flatness of the granite plate, you would
need something that is flatter than the plate itself. You used the diaflat as reference
which has a certified flattness of 0.005 whereas the granite plate should be even
flatter than that. It doesn't make sense to use something that is (supposedly) less
flat(Diaflat) to check something that should be flatter(granite plate). What you need
is a certified flat straight edge.

There is a 99.9% chance that you have reduced the flatness of the granite plate. Now from
your post I read that you were going to use abrasive powder with this plate. Well the abrasive
powder would have destroyed the flattness as well. The abrasive would have grinded away
the plate as well. Tiny amounts each time of course. But everytime you would have used it
this way would result in less flattness.

Now you don't need a certified granite plate for your purpose. It's
fun, but overkill. A 10mmthick floatglass plate is all you need. I
would advise you to buy this and also buy the thinnest floatglass
that is available in the same size as the 10mm thick floatglass.
Put the the thin plateon top of the 10mm plate and use the abrasive
powder on the the thin plate. Whenever you seethat the thin plate
has lost its flatness, discard it. There is a chance you might get the the thinfloatglass plate for free.

The 10mm floatglass will cost you about 5-10bucks. The 10mm plate s
upportsthe upper thinner plate so it won't(reduces significanly)
bend/deflect when you put pressure on it.

Not sure how much you payed for the granite plate, but don't
worry about it. It's probably still flat enough for your purpose.
If you still really want to know how flat it is, you can buy a good
certified straightedge and feel gauges. The feeler gauges are very
thin pieces of metal which you push under the ruler and the object
on which the straightedge is put on. The feeler gauges have a
certain thicknes so you can see how much certain areas are
higher or lower than they should be. A set of feeler gauges is cheap
5 bucks or so. A good certified straigtedge costs about 30-300
bucks depending on size, metal, flatness.

As for the OCD you talk about, my advice relax dude. It's all for
the fun. Forget about stones, flattness and concentrate on more
important stuff. It's just not worth it IMHO.

Sharpman