Lapping, Polishing and Grinding
I have doubt that these terms are used uniformly - but they have
Lapping - like polishing uses rolling grit and sliding grit - and
embedded grit. - two surfaces are rubbed together with an abrasive
between them - for forming
Polishing - uses one or two of the following - rolling grit and
sliding grit - probably melts the surface. Much finer surface than
honing - Honing produces a precision surface on a metal workpiece by
scrubbing an abrasive grinding stone or wheel against it along a
controlled path. Honing is primarily used to improve the geometric
form of a surface, but can also improve the surface finish.
But - contradicted by
"According to DIN 8589 part 15, lapping is defined as a cutting
with loose abrasive grains dispersed in a paste, which is guided on
tool with nondirectional paths"
Also called the 'tool' is often of a softer material that the
cutting grit can embed in. Lens makers use a layer of pitch to
hold polishing grit - can be tweaked to cut more in certain areas.
The groves are NOT about the swarf - it is for the abrasive - to
keep the lap from cutting to much at the outer edge of the lap. The
abrasive builds up at edges - where it rolls in and cuts. The groves
even out where rolling actions happens - with grooves, the few loose
grits can be anywhere. In lens making - if you polish without
grooves you get a rolled edge. (I've not seen this written up
A lot of the info on grinding and lapping is proprietary, but you
will find that telescope makers have written about a lot of this -
and they work to 1/20 wave - 34nm..
Lens makers use a pitch lap - it can be cold pressed to conform to a
good surface plate - and they cut SQUARE profile groves - not V
grooves (V grooves would tend to encourage rolling - not what they
A very cheap DIY spherometer can measure in the um range - thinking
of making one like this guys:
I would make the gauge holder a flexture (don't want to hold an
indicator with a screw) and it really needs a fine screw to adjust
the range. The legs need to be adjustable - a radius from
From lens making it is best to make a lap almost the size of the
Surface Plates - oldest 3-plate method
Granite is not ideal - it can take on water, can be too soft vs not
stiff enough.. I would like to see synthetic quartz plates - or huge
ceramic plates.. that were cheap<grin>. Granite is cheap.
Cast-iron isn't bad - faster at adjusting to 20but can rust.
History of flatness
James Nasmyth 1808 - 189- (he worked on early steam engines and
development of the steam hammer.) Nasmyth worked under Henry
Maudslay (1771-1831) Early lathe when he was young (The first
guy to standardize screw threads) .. He talks of Maudsley
using 3 plate method 1800's. But says "I believe, a very old
Joseph Whitworth 1803 - 1887 (1825-33?) perfected a scraping
technique for making true metal plane surfaces.
Huygens - lens still exists - measured to 1.5 wave over its 300mm
diameter - probably polished on a flat metal plate.
Egyptian - Tanis - dated 150AD (British Museum) except for the
tarnish. They are about 50mm diameter and about 90mm in. focus and
would magnify three diameters. They have been
ground and are not merely cast. The flat surface has been ground
against another flat surface with a rotary motion as at the present.
".. by Mr. E. ]. Forsdyke, in Crete, of two crystal magnifying
lenses that date back at least as early as 1200 B.c., and probably
1600 B.c., as most of the small objects from the tombs
where they were found are of that date."(From Tyman) - not flat..
Spectacles - 1282?
"Dr Greeff attributes, however, to Prof. Hirth of Columbia
University, New York, the statement that the Chinese had mirrors
both concave and convex, of bronze, in the first century B.C." From
Pere Cherubin d'Orleans book - Telescope and microscope lenses
(1671) describes lens making using metal tools of iron and
brass. First mention of the use of pitch.
Hooke - 1667 - describes making microscope objectives - one side
polished flat - first on a stone - then on a smooth metal plate
Newton's reflecting telescope 1668 - two mirrors - one flat - used
pitch for polishing - metal mirrors.
I think the 3-plate method goes way back.. before 1600's - Lens
making in the Netherlands was quite advanced - Huygens used flat
metal plates to grind optical flats.. 1683..
My hunch is flour Miller's noticed the three plate method
first. I will have to look at the history of old grain Mills.
First use of pitch to make optics?
della Porta in 1558 talking about using pitch (colophon) to make
"How Spectacles are made."
We see that Spectacles were very necessary for the operations
already spoken of. Or else Lenticular Crystals, and without these no
wonders can be done. It ~emains now to teach you how Spectacles and
Looking-glasses are made, that every man may provide them for his
use. In Germany there are made glass balls, whose diameter is a foot
long, or thereabouts. The ball is marked with the Emril- stone
round, and is so cut into many small circles. They are brought to
vemce. Here with a handle of wood are they glued on, by Colophonia
melted. And if you will make Convex Spectacles, you must have a
hollow iron dish, that is a portion of a great sphere. As you will
have your Spectacles more or less Convex, and the dish must be
perfectly polished. But if we seek for Concave Spectacles, let there
be an Iron ball, like to those we shoot with Gunpowder from the
great Brass cannon. The supersicies whereof is two, or three foot
about. Upon the dish, or ball there is strewn white sand, that comes
from Vincentia, commonly called Saldame, and with water it is
forcibly rubbed between our hands. And that so long until the
supersicies of that circle shall receive the from of the dish.
Namely, a Convex supersicies of it exactly. When that is done, head
the handle at a soft fire, and take off the Spectacle from it and
join the other side of it to the frame handle with Colophonia, and
work as you did before, that on both sides it may receive a Concave
or Convex superficies. Then rubbing it over again with powder of T
ripolis, that it may be exactly polished. When it is perfectly
polished, you shall make it perspicuous thus. They fasten a woolen
cloth upon wood. And upon this they sprinkle water of Depart and
powder of T ripolis. And by rubbing it diligently, you shall see it
take a perfect glass. Thus are your great Lenticulars, and
Spectacles made at Venice.