Every stones is different.It has a different hardness,density,refractive index,crystal system,and then there are physical properties related to the individual stone even in the same group or crystal system. This means a different approach is needed to fit the properties of that particular stone to the cutting,pre-polish and final polish laps. As an example a copper 3000 pre-polish lap from a half hard plate or round bar section will cause facet peel with tourmaline, zircon,peridot and demantoid garnet just to name a few.Facet peel is where the facets appears half polished and half ground.It can be a difference in hardness or a crystal direction phenomena and eliminating this means abandoning the pre-polish early and doing alot more final polishing.This means that the copper is too hard and a softer metal pre-polish lap is needed for maximum pre-polish results before proceeding to a final polish lap.So by using a zinc to pre-polish the result is immediate. Why? because you have reduced the hardness of the lap and the imbedability of the diamond particle.A softer lap will allow the diamond to deeper impregnate the lap surface giving a softer finish eliminating or reducing the facet peel effect. This is the same for a 1200 nickel plated lap compared to a bronze sintered lap.While the nickel will improve over time,nickel is much harder than bronze so the finish for the same grit will be different,not too mention quality differences between manufacturers. A sintered diamond copper lap will give a greatly different finish compared to a bronze sintered lap of the same grit.By getting the best final cut before pre-polish you have lessoned the time to finish following processes, saving allot of time.The lap set pictured above is a 1200 sintered copper fine grinding rim lap with a zinc cast pre-polish lap with a special tin alloy final polishing centre lap. Another lap in a lap in a lap.
This configuration gives me a final grind that is both aggressive and very silky smooth. I often dont need a course grind from rough on smaller stones.I then move the stone to a 3000 pre-polish on the zinc. You may notice that I have knurled the lap using a single blade medium hand held tool.This makes thousands of indentations at right angles to the facet flow direction acting as both a counter measure and feed source for the diamond paste I use.Finally I go to the special alloy I use to make a cant miss final 100,000 polish. This I also knurl.
Do not knurl copper or sintered diamond laps.
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