Thread: Lapping and corner wear.

Hi guys,

I'm just wondering if any of you have damaged the corners of the stone

during lapping.

I'm talking about a Norton 4k/8k and a Naniwa 12k in particular.

The surface I use for lapping is a desk which seems pretty flat to me

and 400/600 sand paper.

I give circular and/or figure 8 passes with no pressure.

I need to know if it's my technique or the surface.

Thanks a lot.

Did you mean chip the corners?
If you lap on a flat surface with nothing to bump your stone into, I think you will not damage the corners.

The edges and corners should be chamfered at all times. If you're lapping, make sure you don't lap the chamfer off - if you get close, re-do the chamfer and continue lapping. Also, the corners should never be sharp.

Cheers, Steve

Originally Posted by ssg1

The surface I use for lapping is a desk which seems pretty flat to me.

I'm bothered by the wording used here.

What do you mean? English is not my native language so I might express myself incorrectly.

Originally Posted by ssg1

What do you mean? English is not my native language so I might express myself incorrectly.

"Seems pretty flat" indicates to me, that it is estimated to be flat by an undefined method and that not much attention was paid to the flatness. If the surface on the desk is dished, the lapping would indeed be putting all of the pressure on the corners of the stone and causing the stone to be convex. How are you determining the flatness of the lapping surface?

Ok, sorry. It is a machined surface, it looks and feels flat. Thanks.

A machined desk? Hmm, pardon my skepticism, but do you have laser scanning eyeballs? You need to properly check either the desk or the resulting lapped surface if you want to be sure you have a flat surface, not a "flat'ish" surface. An easy way if you have no straightedge is to lap two items then place them lapped side together and check for gaps with a feeler gauge.

As a machinist, I will be the first to tell you that just because something is machined does NOT mean it is flat. Residual internal stresses can make workpieces go out of flat as soon as the clamps holding them are loosened. They can also slowly warp over time. Tools can also wear slightly during machining which can flex the part due to excess or changing pressure, and lastly the tool wear itself could occur in the middle of cutting a surface, slowly and gradually making it non-flat.

I personally use loose grit silicon carbide lapping with a toolmaker's cast iron lapping plate that I ground flat myself. Others use a granite scrap from countertops or granite surface plate, some even plate glass.

And the answer to your question is no. I have never damaged corners by lapping. I have seen people do so by knocking the corners into something while lapping though.

It is always best to chamfer or round off where the side meets the top of the stone before lapping to make the edge more resistant to damage.
I round all the edges and corners, even on the bottom of stones not mounted to another surface.
If a lot of lapping is necessary to get a flat surface I will check to see if the edges need touched up.
I have both of the hones you mention and their edges are somewhat delicate.
I have experienced chipped edges and they are a hassle to have to work around the dings!!

I went to a local glass manufacturer and asked them if they had any broken glass.. He gave me a big glass plate (pretty square) and charged nothing... He said the glass is flat as can be, just some worker broke the glass, so I have been using this plate with sand paper and had no big problems so far. No problems with the corners so far.. just chamfer them every here and then..
Hope this helps, Stick