Stone Compatibility Is Critical for Polishing Success - RA Stones

Most stones have a narrow range of finishes that they can produce, but a resin adhesive stone can do just about everything.

Did you know that a resin adhesive (RA) stone could be the fastest stone of all and by fastest I mean having the greatest range of finishes that can be produced?

Most stones have a narrow range of finishes they can produce, but an RA stone can do just about everything - it all depends on how much the polisher knows about how to use the stone and the steel on which he/she will use it.

An example is 420 stainless steel heat treated to 54 Rockwell then flat stoned with a 220-grit aluminum oxide stone using a heavy pressure to cut through the 20 RMAX EDM, then a lighter pressure using the honing fluid to polish up the scratches to a 400-grit finish.

Because the steel is 54 Rockwell, all of the scratches are actually finer than normal. The veteran polisher knows that because the steel is harder than normal, the scratches would come in finer than normal.

Next, a 600-grit RA stone is used with enough downward pressure to practically break the flat stone into pieces. The grit - now pouring out of the dissolving stone - is mixing with the mineral spirits and creating a honing fluid, which is super-lapping the hardened steel.

Apprentice polishers use the stone to polish; veterans use the honing fluid the stone generates as it breaks down to polish the steel instead. The RA stone - under the proper pressure - dissolves and the resulting honing fluid changes the finish from a fine 400-grit to a semi-mirror, 900-finish - almost instantly. The steel actually shines with a semi-mirror gloss after only a stone finish! A tiny bit of brushing with a stiff, nylon cup brush and nine-micron diamond, and in seconds only a diamond polish remains that is ready for hard buffing with three-micron diamond for a Liquid Black optic finish.

What happened here? How did things happen so fast? It's because this polisher took advantage of the steel's hardness and the tremendous range of the RA stone to accelerate the polishing process. The steel is hard, so it takes a scratch finer than it would if it were soft.

A good 220-grit stone, which dissolves at the proper rate of speed under the proper downward pressure guarantees a smooth, satin finish instead of a rough bunch of jagged zigzags - which come from stones that do not dissolve fast enough to keep their pores constantly clean. By going over the 220-finish with a 220-stone at a lighter downward pressure while using a slower, gentler profiler stroke and criss-crossing using the naturally occurring honing fluid as a polish, the polisher in just minutes can turn a 220-finish into a fine 400-finish.

The RA 600 stone - which produces excellent results over the greatest range of scratches out of any stone on the market - under heavy pressure on hardened steels produces an immediate 900-grit finish with mirror-like qualities. This polisher has gone from EDM to a fine 400-grit on a four square-inch surface flat stoning in about 20 minutes and from a 400-grit finish to a reflecting semi-mirror 900-finish in about five minutes more. Nine-micron diamond takes about two minutes or less and a hard buff with three-micron diamond takes two more minutes.

Smart polishing is all about taking advantage of the strengths and weaknesses of the steel. For example, taking four square inches of 54 Rockwell stainless steel from a 20 RMAX EDM to a Liquid Black optic in about 30 minutes without pit holes and with all of the contours perfectly intact. Smart polishing is about taking advantage of the way one type of 220-grit stone will dissolve to produce an exact scratch with this steel, while another type 220-grit won't. It's about having total control over the finish you want from the stone you are using. It's knowing that the RA stone will be able to instantly pick up where the 220-grit stone left off.

Remember all of that black fluid you keep wiping off and throwing away when you stone? Stop doing that. That black gritty fluid is actually the best stuff you have going for you - it's actually a honing fluid of consistent granularity, which is what creates satiny soft scratches of a uniform, consistent quality.

Stop throwing away your best tool and start using it as a polish. Don't use your stone as a stone; use it as a "massager" for the honing fluid as if it were an expensive polish. Then polish the very top surface finish of the steel with that stone, but stop grinding your stone into the steel. That's only for the initial rough stoning when you're first fighting the tool marks.

Once that step is completed you press down only hard enough to take out the existing scratches, then lighten up to massage in the honing fluid "polish." Then switch to the next finer grit stone pressing down only hard enough to take out the ever finer scratches. Once again, lighten up on your downward pressure and use the honing fluid to "polish" the now finer surface with your massage stone.

Stone, then polish, with the honing fluid, switch up to the next finer grit stone, then stone and polish. This process of pressing down only hard enough to get out the increasingly finer scratches and then lightening up to use the honing fluid as a 220-grit polish, 320-grit polish, 600-grit polish will never change. The only thing that will change is how fast you can finish a job because now polishing steel has suddenly become very easy and your final diamond finishes have dramatically improved.

Related Content

Throwback Thursday: Best Practices for Surface Finishing

Surface finishing is as critical an aspect of building a mold as the design, so a review of some best practices is always a good idea.