Lapping plate question

[BakerAndBadger]

Hi
now that my coticule is on its way I was wondering, although it is already lapped, when I have to lap it again, or if I choose to experiment with film i'll need a nice flat plate.
Ive heard people talking about glass and marble tiles but I have a load of marble window sills kicking about from when I renovated the house. Would these be ok? They are from the 1930's but I reckon they are flat. Is there a test I can do to see their flatness?
Cheers

Paul

 

[Slash McCoy]

I would be surprised if your windowsills are as flat as a recently quarried and polished marble tile or a piece of heavy glass made in the last 20 years or so. A tile is only $5 or less. I would get a tile. Be sure to get polished marble, not ceramic.

 

[Legion]

Agreed. Or find a thick piece of glass to recycle. I got mine for free.

Window sill? Not worth the risk.

 

[Wid]

I agree with the glass or marble. Although if the stone has been lapped I doubt you'll ever have to lap it again. It would take a chit ton of razors to wear a coti down enough to have to lap it.

 

[Gamma]

I test flatness with a straightedge, a ruler of sorts. Not a ruler from the corner store though - it's a cork-backed metal one from an drafting supply house.
Even so - I'm sure it's not perfectly straight in an absolute sense. But - I believe it's straight enough.
The cheap plastic ruler I have is useless for testing flatness, it came with a built in frown.
The straightedge should be longer than the tile or whatever. Just put it on on the surface you're checking with a bright light behind it.

 

[kwigibocity]

Here in the shop I use a DMT 4x10" interrupted surface plate for the daily lapping, but it is quite coarse on the finish. The stones are flat enough for any practical application thereafter, but they do look a whole lot prettier (and sell more quickly) when they've been further polished, so on occasion I'll take home a sack of bouts and run them through 400 to 2000 grit wet/dry sandpapers stuck to a ~$4 12x12" tile from Home Depot to make them terrific looking for the web page. Routinely comparing against a bright background light the stones flattened with the professional plate versus the ones flattened with a <$10 sum investment in paper and tile, I've never seen any gaps of light escaping anywhere; that cheap tile's at least as flat as the diamond plate. So unless I was making a living with it or honing lots and lots of razors for hobby, I think I'd have to go the tile/paper route.

Get a carpenter's ruler for reference and cast against your lot of marble sills, I'd not be shocked if one's dead flat - of course, this assumes you've already a reference flat ruler, which would cost more than the tile/paper solution...

 

[ladykate]

Lay a metal ruler edgewise across them. No light? It's flat enough. You can get a true-enough metal ruler at Wal-Mart for less than a buck and you can use it afterwards for measuring stuff. ;-} I regularly get them for my machine work (metal rulers are handy when working with a lathe).

If the tile are flat, they're flat. But the problem with older stuff is that there may be chips or other greebles on the surface. Very carefully examine the tile to be sure it is butt smooth. Any, even small, anomaly will be detectable with lapping film.

 

[Slash McCoy]

"Ruler flat" is probably good enough for lapping most stones with sandpaper, but for honing razors with lapping film, I think the flatness tolerance is beyond what you will eyeball with a ruler or straightedge. One of the biggest advantages of lapping film is that the honing surface can be so perfectly flat, and remain that way forever. If you compromise flatness, you may as well just hone on rocks. Flatness gets more important as you go to finer films, too. Ideally, one would use a plate specifically lapped and finished to close tolerances. In practice, polished tiles and decent quality glass of suitable thickness have been found to give acceptable results.

Checking flatness with a ruler or straightedge places a lot of trust in the straightness of that reference. How do you know that your straightedge is straight? If it did not come to you in a fitted case, did not come with a calibration certificate of some kind, and has been stored standing on end or laying unsupported, well, it isn't straight. A ruler? Forget about it. It is not straight.

Checking flatness optically is a possibility. One method that comes to mind is checking a reflection for distortion. However, simply eyeballing it won't work. The human eye deals with small amounts of distortion by ignoring it. A perfectly square and true grid should make a perfectly square and true reflection on a perfectly flat surface, but you are still back to square one... the validity of the original reference.

Given time, your windowsills could be lapped using the three stone method, to within probably .001" over the central portion of the stone. That would be 25.4u which is still not all that flat. I would guess that most polished marble tile is around that flatness or a little better. There will of course be some rounding of the areas near the edges from hand lapping due to torque on the piece. So you would end up having to cut probably the outer third from all four sides.

Best bet... get a good tile. Eyeball a reflection in the store and make sure there is no visible detectable distortion, and if there is, select another one. Good enough for razors. It's what I use and it is what many accomplished honemeisters use.

http://www.leevalley.com/US/wood/page.aspx?p=32526&cat=1,43513,51657
This is what I would probably get, knowing what I now know, if I were just starting with lapping films and did not need to go absolutely bottom dollar cheap. The price is not too extravagent, at $36. Tolerance is +/- .0001" which is 2.5u, close enough to start getting those finer films to deliver results closer to their potential. If I were honing professionally, I would go with one of these plates.

 

[hig789]

I just bought a 12"x12" granite tile from Menards for $2.98.

Lowes has 18"x18" for like $4.

 

[BakerAndBadger]

Anyone in France/Belgium bought a nice tile from somewhere? Unfortunately the marble tiles in the US are price prohibitive for me, what with the flight and lodgings etc.

 

[marcel]

I couldn't find a marble or granite tile without buying a couple of square meters at once, so instead I bought myself an old stone grill at a charity shop. It has a nice, even and polished granite tile, just the right size. About 10" x 10" and 2/3" thick.

 

[Doc226]

Anyone in France/Belgium bought a nice tile from somewhere? Unfortunately the marble tiles in the US are price prohibitive for me, what with the flight and lodgings etc.

You can always use glass, works just fine and available anywhere.

 

[toovira]

In machining, a flat reference should be at least 1" thick granite. Otherwise, the granite itself can also sag. Woodcraft had some on sale for $25. I bought one and couldn't ask for more.

 

[ouch]

I find the best surface for film is the back of a Shapton GlassStone. They're the perfect size and dead flat, and the wet film sticks to the glass nicely.

Now I have "two sided" GlassStones!

 

[danjared]

when I have to lap it again

You won't.

 

[Rschan]

You can always use glass, works just fine and available anywhere.

+1

 

[BakerAndBadger]

Right then, I'll look for some glass today then, also gonna hit some antique style shops and an old drugstore/hardware store in the hope that they have an old straight tucked away somewhere

 

[celticrusader]

I use a polished granite food prep slab, these can be purchase from most good cookware shops, then i mount this near my sink, then dip a piece of 400 git wet & dry paper in the sink filled basin, then lay this directly on top of the granite slab, now i lapped what ever stone i'm using over the paper, works perfectly well i prefer this method to using my lapping stone.

Jamie.

 

[Gamma]

Last thing I want is wet glass in my hand in the kitchen. Works or not - I see shattered glass being inevitable.
I use the back of my DMT to lap past 320x - I just wrap the sheet of w/d around it and lap under running water.