grind flat surface on rocks / granite

Hello all

Thanks for all help. Volunteering at local museum and hobby mines, benefitting from advice on things here, and hopefully starting work again as a welder imminently.

Grinding flat surface on granite samples to make visually well presented samples of geological features - eg. the "contact" between different types of rock...

Find is true a diamond-plate like for eg. sharpening plane-blades when on-site produces a beautiful find shiny smooth grind on the rock samples.

Problem - the removal rate is tiny - need a flat sample to start with if going to do that.

Advice here is glide tools over the surface and go through finer and finer grits getting a surface which is polished though not necessarily machine-flat. Comments

• this is the voice of experience
• is there really the need for the sample to be machine-flat?

Anyway, I was thinking how it might be possible to produce a flat surface.

In the steelworks labs. there was the "swing-grinder" which had a vertical spindle and you swung it back and forth over a sample in the chuck, lowering it a bit per pass to produce a flat surface to start going through the finer and finer emery grits with until you could diamond-polish it to mirror finish.

I thought of base and column of a bench-drill, clamp a collar on the pillar at height of finishing plane, and have angle-grinder on an arm you swing back and forth. Letting the grinder ride-up for light "cuts", but eventually stopping at the plane dictated by the collar locked to the pillar (column).

Anyone got a better idea / know how it should actually be done - if at all?

Regards, Rich S

---------------------------------------- I tried flattening a granite protrusion in a walkway with cold chisels, with minimal success. Apparently granite requires a single point carbide tipped chisel. An angle grinder was somewhat more useful though I didn't have a SiC masonry disk for it.

Amazon offers relatively low cost versions of the standard jewelers' facet grinder. In college we used similar machines to polish steel samples for etching and studying the grain structure.

You could try a diamond wheel on a Dremel to reduce the high spots that coarse sanding reveals. When I do that on steel I sand in one direction and file or grind in another to show the difference. After borrowing the engine for the sawmill I found that its mounting plate on my second-hand log splitter was warped from welding and I hand fitted it to the engine crankcase. Pulling sandpaper through the contact area targeted the high spots for attack.

jsw

Anyway, I was thinking how it might be possible to produce a flat surface.

In the steelworks labs. there was the "swing-grinder" which had a vertical spindle and you swung it back and forth over a sample in the chuck, lowering it a bit per pass to produce a flat surface to start going through the finer and finer emery grits with until you could diamond-polish it to mirror finish.

I thought of base and column of a bench-drill, clamp a collar on the pillar at height of finishing plane, and have angle-grinder on an arm you swing back and forth. Letting the grinder ride-up for light "cuts", but eventually stopping at the plane dictated by the collar locked to the pillar (column).

Anyone got a better idea / know how it should actually be done - if at all?

Regards, Rich S

---------------------------- I've ground surfaces quite close to shape freehand with an angle grinder by mounting them free to rotate and controlling the rotation speed vs grinding rate by the position of the grinding disk. An example is the front motorcycle tire for my sawmill which needed a wider crowned flat for the bandsaw blade. The sawmill frame helped hold the grinder steady enough to avoid high spots. I used the rear wheel as-is since the previous owner had worn it to the cords burning rubber in the street, and the crown was perfect. The blade was tracking centered after less than 15 minutes of adjustment.

When grinding the end a straightedge shows if you are grinding concave or convex. Touching the disk to both sides somewhat off the center of the work makes it rotate slowly and guides the disk to cut pretty flat despite its wear.

A variation is grinding the broken end of a drill bit or endmill square freehand by rotating it against the wheel before adding the drill point angle. I buy high quality dull or chipped endmills very cheaply second-hand and regrind them for roughing, saving the new ones for finish cuts.

Holtzapffel mentioned a cup filled with hide glue or pitch as a chuck for irregular objects, or actually high precision clock repair because the work can be precisely centered as the melted glue hardens. This describes using modern hot melt glue:

Grinding flat surface on granite samples to make visually well presented samples of geological features - eg. the "contact" between different types of rock...

---------------------------------------- I bought some masonry cutoff disks to carve firebrick and if the rain stops I could try one on the granite.

---------------------------------------- I did, it worked well. It's a black 7" SiC wheel, not diamond.

Making flat surfaces from rubbing surfaces together IS a slow process. I can't imagine doing it. The way I understand it is that it can be done to near laboratory or at least machine shop tolerances, but it takes three surfaces with swap of mates and orientation. If I were to get into it I'd look at some of Robin Renzeti's (spelling?) videos on YouTube for the basic process. He really goes down the rabbit hole on the subject I think. There is also a dentist who got into precision grinding as a side business, but I don't recall his name or if he even has a YouTube channel. Abom79 on YouTube (Adam Booth) worked with the guy on some stuff. Might have just been making precision bench stones. I don't recall. While they are not necessarily anywhere close to what you want to do, they may give you insight into how to do what you want.

That being said, the one thing you seem to catch my attention about was time and material removal rate. I am always sensitive to time, and I don't know how you could improve that except by employing mechanical means to assist in at least the rough/coarse part of the process. The problem I see with mechanizing the fine finish is you might need fairly fine mechanical bits.

That being said, the one thing you seem to catch my attention about was time and material removal rate. I am always sensitive to time, and I don't know how you could improve that except by employing mechanical means to assist in at least the rough/coarse part of the process. The problem I see with mechanizing the fine finish is you might need fairly fine mechanical bits. Bob La Londe

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Search for "gem faceting machine".

All this talk about flat rocks has pushed me right over the edge . I just ordered a 9 x 12 x 2 surface plate ... supposed to be within a tenth which is probably closer than anything I'll be doing . And just over 60 bucks delivered .

All this talk about flat rocks has pushed me right over the edge . I just ordered a 9 x 12 x 2 surface plate ... supposed to be within a tenth which is probably closer than anything I'll be doing . And just over 60 bucks delivered .

Probably everybody knows about Foundations of Mechanical Accuracy by Wayne Moore. Just mentioning it for the record.... being the first thing that came to mind after reading the thread subject.

bob prohaska