Thread: Hone lapping on a glass plate with silicon carbide powder - pictures

I hate to be the one to break bad news but your surface for using the glass should be flat when using any glass below 3/8" thick. Truth be known 1/4" glass can flex and when you apply pressure when used for lapping the glass will flex to match the uneven surface below. The surface below the glass is very uneven the glass will break or crack (run). For the sake of trying to keep the glass from flexing even in slightest form one should use a thickness of 3/8" or greater and the weight should help reduce the glass from sliding around.

I used to be a glazier and used various thicknesses and trust me, when you load a plate about 110" onto a vaccum support table you can actually watch the glass flex. It does, it can and it will even on uneven tables. So, saying 1/4" plate is flat may be a true statement but if the surface below it is not... the glass will flex as you apply pressure when lapping. Just a friendly heads up for all you flatness junkies out there.

A couple of points:

1)The floor tile is lapped with a diamond tool. It is not as smooth as glass, but is probably about as flat.It will wear smooth right away.

2) When you lap, you randomize the orientation for precisely this reason. Keep the stone moving, vary the stroke, change the direction, etc... It all averages out and the end product depends much more on the smoothness of the plate than the flatness. I ground a telescope mirror this way using a tool made with porcelain bathroom floor tiles and the resulting accuracy was much better than anything a razor would ever need. The upshot is that you don't need a perfectly flat lapping surface if you do this, since it wears with the stone.

All I can say is that it seemed to work quite well.

Originally Posted by Logistics

... the glass will flex as you apply pressure when lapping. Just a friendly heads up for all you flatness junkies out there.

What would you recommend to keep the glass flat? I also know that another option is to use cast iron plate.

Being lapped with a diamond tool does not mean it is flat. Take a
precision straight edge and check it.

As someone who has lapped quite a few plane soles, I disagree on point
2. If the tile has tiny inaccuracies, that is fine, but if it is out of flat by
like a few thou, then use another flatter tile or just get float glass which
is(almost always) very flat.

Originally Posted by Traskrom

What would you recommend to keep the glass flat? I also know that another option is to use cast iron plate.

Don't worry about this. Just get a thick piece of glass and use
light pressure whilst flattening.

You could also buy a small granite surface plate. Stick thin self
adhesive foil on it and sprinkle your loose abrasive powder on it
and flatten your stones.

I did check it at work on a granite layout table with a dial indicator in our machine shop. It was within a few 10,000ths of an inch. Just how good do you think you needs it to be?

I think the surface you grind on matters less than you think. Both surfaces wear (the tool and the stone you are lapping). After a few strokes, they will 'fit' perfectly. With glass, the initial flatness is not particularly relevant since it will not be perfectly flat after a few strokes, but will be slightly dished. What matters more is how you vary and randomize the strokes. I recommended the granite tile simply because I was getting $20-30 quotes for a piece of 3/8 float glass vs. $4 for a tile.

Originally Posted by invention13

I did check it at work on a granite layout table with a dial indicator in our machine shop. It was within a few 10,000ths of an inch. Just how good do you think you needs it to be?

I think the surface you grind on matters less than you think. Both surfaces wear (the tool and the stone you are lapping). After a few strokes, they will 'fit' perfectly. With glass, the initial flatness is not particularly relevant since it will not be perfectly flat after a few strokes, but will be slightly dished. What matters more is how you vary and randomize the strokes. I recommended the granite tile simply because I was getting $20-30 quotes for a piece of 3/8 float glass vs. $4 for a tile.

What you write is true, but that is why I recommend using very thin self
adhesive foil on the glass plate. This will reduce the rolling of the grit and
allows you to true your stones without losing flatness of the glass plate.