Lapping, Polishing and Grinding


I have doubt that these terms are used uniformly - but they have specific meanings.

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 lapping.

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 process
with loose abrasive grains dispersed in a paste, which is guided on the lapping
tool with nondirectional paths"

Lapping Plate

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.

Lapping thoughts:

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 well.)..

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 want).

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 80-160mm[3-6"].

From lens making it is best to make a lap almost the size of the plate.

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 mechanical 'dodge'."

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 (Tyman)

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 (using Tripoli)

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 optics

"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.