Flattening a Hard Silicon Carbide Honing Stone

[Stewie Simpson]

A few weeks ago I mentioned I ordered some 280grt Silicon Carbide Powder to flatten my hard Silicon Carbide Stones; the powder arrived a week ago. I also upgraded my old sheet of float glass to a 12mm (2x laminated); 1250mmL x 300mmD. http://www.glassforeurope.com/en/pro...s-of-glass.php

The stone chosen had a noticeable hollow down its full length; the surface of the glass was 1st sprayed with water, before applying 5 grams of powder within a localized area; its important to bear in mind that these Silicon Carbide Powder's itself break down in size the longer you work it; as a general rule of thumb- when you can no longer hear the sound of the grit working the surface of the stone its an indicator you need to add some additional fresh powder; over a 30 min period; 2 grams of additional powder was added every 10 min. To achieve a totally flat surface over the Silicon Carbide Stone took me 30 min of work; longer than I initially anticipated; most likely that's an indicator that the commensurate grit of 280 is not coarse enough. I have ordered 400g of the 150grt for the next trial.

Random spraying of water over the surface of the Silicon Carbide slurry is a requirement to prevent the effects of air drying. Not difficult to identify, as the drying slurry will start to impede the free movement of the stone across the glass surface.

I should make mention that I previously trialled a Course DMT Diamond Stone to flatten these hard Silicon Carbide Stone's; the diamond grit was non existent after about 10 min of work. imo ; its the bonding agent that adheres the diamond grit to the top surface of the plate that cannot withstand the hardness of these Silicon Carbide Stone's.

Stewie;

[Kees Heiden]

I have some 80 grit carborundum and used it for various flattening purposes. 280 grit seems very fine to me!

[Stewie Simpson]

Crystolon" is a trademark of Norton Abrasives for man-made stones of Silicon Carbide that are vitreously bonded. In other words, they are heated to such a high temperature that the grains fuse together. Other companies can make similar stones but cannot call them "Crystolon". Silicon Carbide has a mohs hardness of 9.5.

[Tom M King]

I haven't flattened anything that hard, but do use 80 grit on the lower grit Sigma stones. http://www.willbell.com/atmsupplies/atm_supplies.htm

[Patrick Chase]

Like others I prefer coarser grits for this sort of work, though it depends on the grit of the stone being flattened. I use #36 SiC for *really* coarse stones (Sigma Power 120 for example, which is an SiC waterstone with a remarkably hard binder), and #90 SiC for most others.

I don't flatten polishing stones on loose SiC, though if I did I'd probably use #220 or #400 powder.

I also highly recommend plastic laminating sheets. They speed the process up a *lot*.

[Steve Voigt]

Like others I prefer coarser grits for this sort of work, though it depends on the grit of the stone being flattened. I use #36 SiC for *really* coarse stones (Sigma Power 120 for example, which is an SiC waterstone with a remarkably hard binder, and #90 SiC for most others.

I don't flatten polishing stones on loose SiC, though if I did I'd probably use #220 or #400 powder.

I also highly recommend plastic laminating sheets. They speed the process up a *lot*.

Patrick,

Where did you find 36 grit SiC? I've never found coarser than the 60-80 loose stuff on Ebay.
And, ditto on the laminating sheets, they do help.

[Mel Fulks]

I'm guessing the 36 grit is mainly a masonry thing.

[Stewie Simpson]

The 12mm rubber seals serve the role of containing the flow of the slurry.

[Patrick Chase]

Patrick,

Where did you find 36 grit SiC? I've never found coarser than the 60-80 loose stuff on Ebay.
And, ditto on the laminating sheets, they do help.

Stewie includes some with the Sigma Power 120. It's also available in bulk on Amazon.

[Stanley Covington]

Patrick,

Where did you find 36 grit SiC? I've never found coarser than the 60-80 loose stuff on Ebay.
And, ditto on the laminating sheets, they do help.

Try the sandpaper belts for industrial floor sanders. I have bought these in the past at Home Depot's that rent such equipment. Expensive, but effective. You can cut it with a hacksaw to make smaller pieces. Also useful for rough truing of metal plane soles. Too aggressive for fine tuning.

Stan

[Warren Mickley]

I have used Carborundum stones since 1962. Carborundum was the first company to make Silicon carbide stones and at one time I liked the quality of their stones a lot better than Norton's Crystolon. I have worn through a few stones but am still using old stock. Norton and Carborundum are now both owned by Saint Gobain.

For flattening, I use SiC powder like Stewie; it is probably around 100 grit. I put a piece of old glass on a large scrap board and work on the floor, like Japanese sharpening. It is kind of messy to do on a bench and I would not want so much loose grit around my finer stones. Any old piece of glass is fine because we are using a coarse grit and we are not grinding telescope lenses.

[Tom M King]

I kept a large stack of float glass window panes that we replaced on a job, use one, and toss it when done. Just like when I ground telescope mirrors when I was a teenager, water is sprayed on the glass, the grit sprinkled all over the working area. Rather than deal with cleaning it up, the pane is just tossed when the job is done.

When we were grinding mirrors, it was much harder to make an optical flat than a concave mirror. The mirror was made concave by letting it go off the edge of the tool (matching size glass disk as the mirror blank). The extra time the center stayed on the tool caused more wear in the center of the blank. You would take some strokes. Rotate the mirror blank one way a little bit, take a step around in the opposite direction, and keep it up like that for a LONG time. Knowing how we did that, I try to keep what I'm flattening on top of the glass "tool" all the time.

I didn't really look around, but the SC grit I bought from that telescope making place I linked to seems to be very good quality for 7 bucks a pound. I don't know if I'll ever use up a pound.

[Patrick Chase]

On a semi-related note...

I also use loose SiC to flatten and condition (read: rough up) the metal plates that I use with diamond pastes. Plates used with coarse pastes in particular eventually go out of flat and become smooth, and both conditions need to be corrected at some point. When working those I use white mineral oil as the lapping fluid, with grits anywhere from #65 to #400 depending on what grade of diamond paste I use with the plate. I've used Kerosene in the past, but after a nasty bout of contact dermatitis (enough so that I landed on Prednisone) I compromised a bit on viscosity and switched to a food-grade mineral oil. I use lapping film, and swap the film between plates of different grits - in addition to speeding things up the film also protects against cross-contamination in this case.

I use loose SiC for this for a few reasons:

1. It's able to remove embedded diamonds from the plates and thereby get to the underlying metal substrate, presumably by "eroding" the metal around the diamonds (probably similar to how it attacks diamond plates now that I think about it). Sandpaper (AlOx or SiC) doesn't do so well at this - plates that have been used with coarser pastes just dull the sandpaper without any material being removed from the plate.

2. When moved in a random-ish pattern it leaves a fairly rough surface that seems to capture the diamond grits fairly well.

3. It's fast and gets very good/flat results

[Steve Voigt]

Stewie includes some with the Sigma Power 120. It's also available in bulk on Amazon.

Thanks for the link!

[Patrick Chase]

Thanks for the link!

I should probably have noted that I don't think the Panadyne stuff is the most tightly graded abrasive in the world, particularly in the finer grits. I use it for flattening and think it's just fine for that use, since the biggest particles get broken down pretty quickly anyway, but for more critical work I'd stick with a more reputable source like Tom's optical supply house.