01 April 2011

Sharpening - Leonard Lee

This is the fourth of a number of posts about sharpening literature.

Leonard Lee (from Veritas Tools) published The complete guide to sharpening in 1995.

Flattening geometry
But first the origin of this post,  being as often a Christoper Schwarz post.  In a recent post he favors diamond based over stone-based flattening systems for flattening his waterstones and oilstones.  The problem with flattening stones being the near impossibility to keep them flat.
Before I even heard about Japanese waterstones or Belgian slate I took some interest in stargazing.  This interest is too recent to have seen the numerous workshops about telescopic mirror grinding. Today mirror grinding lost most of its interest as  it is easy to find large finished mirrors and telescopes for the price of the material, where 30 years ago it was possible to first grind a decent 6" mirror and while learning continue with a 8" or 10" .... 40"  in search of the 'big enough telescope'.
What's interesting in mirror grinding is that by randomly sliding two pieces of glass over each other it is possible to get a spherical mirror,  and then by adapting the technique achieve a parabolic, micrometrically or rather interferrometrically precise, end form, where a mirror finish does not mean finished mirror.

All this to say, the stone-based technique used to flatten waterstones is very close to the one used to grind hollow mirrors.  So it is not a surprise that flattening is rarely a success.

Flattening stones
Now Schwarz uses in his post a hammer to 'flatten' his Norton flattening stone and, looking at the technical evolution of amateur telescopic mirror grinding, that's a good idea (this is the april 1st  part of the post). Why is this a good approach? To keep prices low when grinding telescope mirrors, the second piece of mirror quality glass used as grinding tool can be replaced by a cheaper ceramic tool.  A cheap way to make this tool is to use some small 1" ceramic tiles from Home Depot, to ensure they are beveled, to lay them flat on the future mirror (the flat surface) and to embed them in dental stone or something equivalent.  Using small tiles and laying them on a flat surface, avoids any important flatness errors to be found in larger tiles.

The book at last
In his book Leonard Lee covers this flattening problem, he favors diamond plate but addressing the 'poor and parsimonious' comes up with a solution for stone-based flattening systems “These surfaces will always be flat if you are using three stones in rotation”  A good idea, but how can I implement it? The only reference to a three stones flattening system I found is documented by ERic In Nova Scotia. He uses three waterstones 1000,  4000/1000 and 8000/1000.  The flattening is done cautiously between three 1000 surfaces. As I am not in high precision sharpening (yet) it is not for me,  but it offers a good solution where you invest only in primary sharpening stones and not in stones to keep your stones flat.  Back to Leonard Lee,  he offers other flattening alternatives: sandpaper or plate glass and silicon carbide adding eventually a hard platic sheet like  Mylar to bed the particles.

Unusual in the book is a stand against quenching.  "As a general principle, quenching ... is a substitute for good technique.  ... the cracks may become invisible but they will be there." So every time someone mentions quenching,  I think now, yes but ... .  Also undercutting hollow bevels by using small <=6" grinding wheels is covered.

The complete guide to sharpening is impressive and reflects a lifelong involvement in woodworking tools and all things sharpening. It also offers by its variety a lot of coverage for real world technical problems.  For example,  I have an old wet grinder, but it is not a Tormek that offers a smooth and straight operation out of the box.  Truing of a grinder stone is covered.  In Leonard Lee's book I sometimes miss detailed enough instruction.  When sharpening a back saw,  I looked elsewhere for the final details.
As a conclusion I would say that the book will really prove its value if I stop buying any other sharpening books, it's time to be more parsimonious on the book front.