Lap disc grit progression, Dot Discs

What do you plan to use them for - cabbing, faceting, flat lapping?

I suppose my two main questions are:

1) Is going from 120-325 reasonable? It feels like a big jump.

2) is there any risk/issue with doing 800 with a dot disc (basically "just do 600")

This is probably a really, really common question to get here now that I think about it . Sorry for that. I did do some googling before asking, but I didnt search here directly.

I only slab, cab and facet so don't have any experience polishing large bookend surfaces. I had a couple of dot discs many years ago. It's been so long that I don't remember much about using them other than I only liked them for rough grinding but as best as I remember they wore out pretty fast compared to standard laps.

As for your stated purposes of doing the backs of cabs I think that they are fine for that. And probably would work well for smaller faced specimen surfaces too. But for bookend size flat surfaces I'm not so sure. I would think that using flat lap discs of any kind for the sanding and pre-polishing size grits on bookend sized surfaces would be very time consuming and tedious. Plus could get expensive when using dot discs. I would be inclined to find another method of processing those larger flat surfaces.

That said I know I still didn't answer your question about grit successions. Back in the early to mid 1970s when man made industrial diamonds were still under patent and expensive, the coarser grits were really costly. So when cabbing using diamond instead of SC, many of us started off preforming and shaping with a less expensive 180 grit as our coarse wheel followed by a 360 grit for final shaping before sanding. Then from the 360 plated wheel to 600 sanding belt or resin disc. Then to 1200 pre-polish and depending on the material may next go to a 3000 pre-polish. Then to an oxide polish or if only using diamond then 8k to 14k to 50k polish. Today many people start out with 80 grit plated wheels and then go to a 220 grit wheel before going to sanding.

Faceting small faces follows a different succession. It used to be common that cutters would rough out the stone with a 260 grit plated lap and then jump to a 1200 plated lap for most materials. Some materials like corundum would undercut on 1200 so the next step after 260 would be reduced to 600. Then 3K on a copper or tin lap for a pre-polish. But nowadays many faceters don't want to risk sub surface damage to their material so the coarsest grit they will use for roughing is 360. Then on to 600 or 1200 or depending on the material may even skip the 1200 and go to 3k or 8k pre-polish. I know that all of this doesn't directly address your situation but I included it to illustrate how broad the gaps can be depending on materials and applications. And how narrow for others.

To reiterate, I think the dot disc will work fine for you for doing the backs of cabs and smaller polished faced specimens. But I have real doubts about them working well on larger surfaces - especially in the mid range to fine grits. For the former, choosing three of the given grit options you state are available to you, I would be inclined to try [60] 120-325-600 then to diamond belts on expando drums for anything finer. And if you find the 120-325 to be too large of a jump then you can always add a 220 into the line-up later.

The main problem with sanding and pre-polishing a large flat surface on a flat lap is that it can take a LOT of time. With a flat lap on a flat surface the cutting action is most effective nearer the outer parameters of the surface being worked while the fine scratches from the previous grit nearer the center are less affected and remain more persistently. The material being removed can quickly clog up or hinder the cutting action of finer grits (especially on plated flat laps).

When using a drum sander with the finer grits you can sand out scratches across the whole surface of rock more quickly. The finer grits (400 and finer) on a sanding drum are not so aggressive that they would too quickly cut dips or leave grooves onto a flat surface. You just have to be sure to keep the rock moving at all times to prevent that.

Most cabochon backs and smaller faced specimens can be sanded on a standard lapidary expando drum. But for bookends and large polished face specimens, in addition to reciprocating flat laps, and large iron rotary laps with SC grit, many lapidary folks who cut and polish bookends or large face polished specimens use a Bull Wheel sander/polisher with 12" diameter rubber padded drum/s and a convex polishing disc. Others use a Richardson's high speed overhead sander (dry) which has a flexible rubber disc for sanding with SC pads, and a Richardson's Buffer which has a large convex disc with sponge rubber padding and a suede covered polishing surface for polishing with oxides. I think these latter mentioned machines give faster results and are still manufactured by Covington, Highland Park, and Richardson's Rock Ranch. And others were formerly manufactured by Polaris Tool & Machine (formerly known as Rock's and prior to that Texas Rock's).

Hope this helped. I would suggest just doing a little more research. There are videos on Youtube showing how to use reciprocating (vibratory) laps, lapidary flat laps (iron rotary type), Bull Wheels and the Richardson Rock Ranch equipment.

Late response, but I wanted to let you know this was very helpful for me. Thanks for taking the time to write this.