How to use a flex shaft: Part 5 of 7

Each Sunday for seven weeks, we will be posting a different part of our series "How to use a flex shaft"

• Part 1: Flex shaft fundamentals
• Part 2: Handpieces and foot controls
• Part 3: Drilling
• Part 4: Abrasives and grinders
• Part 5: Finishing and polishing
• Part 6: Burs
• Part 7: Odds and ends

What is gray, green, white, brown, blue, or black; has a square edge, a knife edge, is pointy, or cylindrical; is hard, flexible, yielding enough to be shaped; can smooth, blend or contour metal, ceramic, plastic and glass; can polish and shine and is available from pretty much any supplier?

Rubberized abrasives are exactly what their name suggests: abrasives bound in a solid cake of rubber or plastic. You are probably more familiar with the brand names, as the most common — Cratex — has become something of a default label for this type of attachment. There’s more than one maker, however, and tons of options in grits, shapes, and sizes.

Most wheels are sold loose and are used with a screw mandrel, although some come permanently mounted. For some jobs, rubberized abrasives can be used with bobbing, tripoli, or rouge compounds for buffing and polishing. As always, however, large surface areas are better served by larger buffing and polishing wheels charged with these compounds.

There are several factors to consider when selecting a rubberized abrasive attachment. With silicon carbide being the most common grit (Mohs hardness 9.5), and a consistent array of sizes and shapes among brands, the main differences between rubberized abrasives are the nature of the binder, how fast it wears, and its relative hardness or flexibility.

Rubber or silicone 

When it comes to the binder, there are two main options: rubber and silicone (not to be confused with silicon carbide, which is the grit). Some say that silicone is a softer binder than rubber, but the main difference is shelf life: Like a pencil eraser, rubber grows dry and brittle over time; silicone doesn’t.

Shapes

Makers include the aforementioned Cratex, Dedeco, AdvantEdge, Pacific, and the dental companies Eve, Edenta, and Shofu. Unfortunately, there is no consistent color-coding across brands; one’s blue is another’s black. Choose these accessories as you would a file: Select the one that approximates the shape that you are working on. Wheels, cylinders, and points all have their applications, and are available from most manufacturers.

Mandrels

If you buy loose wheels, you’ll need to mount them on a mandrel to use them in the flex shaft. Aside from screw and specialty mandrels, there is the point, spiral, or threaded taper mandrel, which is like the tapered spindle on a buffing machine. Cylinders, bullets, and points, all of which have a hole in the base, are twisted or screwed onto the mandrel. Some people mount wheels on these tapered mandrels as well, but I don’t like the protruding end of the spindle and prefer a screw mandrel.

Fine tuning: Dressing 

I consider the fresh-from-the-factory shapes of attachments to be pre-forms — close, but in need of fine-tuning. In the shops where I’ve worked, the bottom inch or so on the flat side of any ring file was invariably concave and shiny, the teeth polished off long ago from shaping rubber wheels. (Most of these abrasives are harder than steel or even sapphire, and they can damage most stones.) 

It seems a shame to wound a nice file, so I dress my rubber wheels and points by spinning them against a large silicon carbide separating disk, which I manipulate to give the abrasive accessory the shape I need. Square-edge wheels can be sharpened into a knife-edge or rounded, and points can be made pointier. I often bevel the back edge of a wheel so that it trails off toward the underside; this lets me get in close to the piece without hitting the screw on the mandrel. There are also small diamond or carborundum dressing stones. 

SAFETY NOTE: Wear a mask! Abrasive dust is hazardous!

Cratex

Hard synthetic rubber, silicon carbide grit

“Cratex” is often used to mean any rubber wheel. The company has been around for a long time, and despite the Cambrian Explosion of brands and species in recent years, Cratex is still a popular choice. The rubber (not silicone) is fairly hard and, once dressed, holds its edge. I pretty much use only the gray/green extra-fine 7⁄8 x 1⁄8-in. (22 x 3 mm) square-edge wheels (which I shape) and points. Cratex seems to come from the factory with a glaze; spin the wheel against a dressing stone or separating disk to expose a fresh surface in order to ensure the wheel performs well. I have found old Cratex wheels to be brittle and not of much use. In brief, Cratex wheels:

• Come in square-edge and knife-edge wheels, cylinders, and bullets (points)
• Are available in coarse, medium, fine, and extra fine grits
• Should be mounted on screw mandrels (for wheels) or on spiral mandrels (for cylinders or points)
• Will cut most stones (including sapphire), steels, precious metals, ceramic, wood, glass, and plastic

AdvantEdge

Various grits and binders

According to my research and the friendly folks at Rio Grande, this broad family of rubberized abrasives is their baby, available only through them. AdvantEdge products come in the usual shapes, either loose or mounted, the latter including small wheels and points similar to Shofu and other dental manufacturers (see below). The basic abrasive is silicon car-bide, but there are also pumice wheels. 

The big variable within this family is the binder: rubber, silicone, or “rubberized,” a term that is unclear. In any case, these wheels, disks, and points are worth checking out. I primarily use the 22 x 3 mm (7⁄8 x 1⁄8 in.) black (medium), silicone, square-edge silicon-carbide-grit wheels for shaping and smoothing, prior to using tripoli or creating matte surfaces. They conform nicely to round or curved surfaces. The harder rubber version is useful when a crisp edge is required.

Shofu

Silicone rubber, silcon carbide grit

When I was a sparsely feathered fledgling metalsmith, a young dental-technology student named Morty (who was intent on dating my sister) gave me a small kit of crown and bridge polishers (it’s still available from some suppliers, such as Otto Frei). At that time Shofu abrasives were available pretty much only from dental suppliers, used in the lab — and the mouth — to polish metal crowns. I fell in love (so did Morty). Here’s the scoop on Shofu:

• Relatively hard silicone binder with silicon carbide grit
• Come mounted on 3⁄32 in. (2.4 mm) mandrel or loose (fit screw and point mandrels)
• Cylinders, bullets, square-, and knife-edge wheels, thin and ultra-flexible “floppy” wheels
• Will cut like Cratex but much finer
• Coarsest to finest: Brownie, Greenie, Supergreenie

Brownies are brown, but Greenies and Supergreenies are actually an identical blue color; the latter is distinguished by a yellow stripe on the mandrel. Shofus aren’t cheap, so use them for special occasions rather than general cleanup. My go-to’s are mounted brownie points (bullets) and mounted 5⁄8-in. (16 mm) knife-edge wheels, and less often square-edge wheels. 

The knife-edge is great for cleaning up the corner of a T-seam or even a decorative saw cut for a super-thin, polished groove (even though the rubber is hard and can hold an edge, knife-edge wheels, like any rubber wheel, will need to be dressed or sharpened as you go). Use a light pressure and a medium speed. You can go straight from a Brownie to rouge. The Greenies and Supergreenies are great for delivering a precise polish in places where you only want a detail to shine, like a tiny appliqué, or a gold inlay on a molar. Try doing that with a rouge wheel. Thanks, Morty!

Assorted types

Skinny polishing rods/pins/points/sticks

Many rubberized abrasives are available in skinny 1-in. (25.5 mm)-long rod-like points (no hole). They are held in a special mandrel that’s similar to a pin vise or a small drill adaptor. These points are wonderful for getting into tight places and can be sharpened into really fine points. Be sure you’ve set the rod securely; the longer the rod extends from the mandrel, the more likely it is to wobble loose or bend under pressure.

Pumice wheels

These accessories aren’t very aggressive — and that’s the point. Pumice is a relatively soft 6 on the Mohs scale, so it won’t hurt most stones. As a loose powder, it provides a nice low-luster finish. When bound in silcone or rubber wheels, it’s ideal for cleaning up bezels and prongs in which a stone is set. 

Cratex, AdvantEdge, Dedeco, and Silipum all make pumice accessories. Cratex makes the classic pumice “Bright-boy” wheels in natural rubber, which can dry out and harden; blue is finer, and white is coarser. Silipum and AdvantEdge wheels use silicone as the binder for their pumice wheels; the silicone remains flexible and the wheels hold up well. I like these. The light gray is less aggressive, the green more. 

Pumice accessories are available in 1⁄2–1-in. (13–25.5 mm) square, rounded, and knife-edge wheels. All can be shaped and dressed.

NOTE: Even though pumice won’t hurt most stones, heat, which is generated by excessive speed and pressure, can. Be safe when working around stones. Test the back of the stone first if it’s possible to do so.

Inside-ring polishing cylinders

This shape comes in a variety of brands, grits, and binders. The cylinders are about 5⁄8 x 1 in. (16 x 25.5 mm) with a hole in one end that is threaded onto a strange-looking mandrel. This is a nice tool for the inside of rings or any curved surface. I have had great results “lapping” the outside of a flat band with these.

Assorted brands

Edenta: A precise system of progressive polishers. The coarsest are rubber based. The finer polishers are silcone based and can be used on semiprecious stones.

Titan: Hard rubber and silcon carbide. Great for platinum and harder materials like stainless. Too hard and stiff for my taste.

Eve: Mounted silicone polishers (most likely silicon carbide), similar to Shofu. Worth a look. 

Pacific: I really like these silicone wheels, because they cut quickly and are a nice blend between hard and flexible. They are similar to AdvantEdge. I use the medium, grey/green wheels (silicon carbide) and the pumice. Search the Web for availability.

The overwhelming and ever-increasing diversity of brands and types make the prospect of deciding “Which do I choose?” a tad initimidating. The bottom line on all of these accessories is experimentation. Order a selection and get scientific. Mark each one — you can write the serial number and supplier on the side of rubber wheels with a fine-point permanent marker — take an afternoon and try each one, and record your conclusions: “Too stiff, too fine, falls apart, glazes,” etc. You’ll know in a minute which works best for you.

Buffing and polishing with the flex shaft is like using an agile and diminutive buffing machine. Along with variable speed and small size, you can move both the work and the buffing wheel, which gives you access to tight spots. You’ve got a range of choices for finishing accessories.

Muslin buffs as well as hard and soft felt wheels and cones come in a variety of sizes and mount on a screw or threaded-point mandrel. Use the same logic as you would at the buffing machine: Keep your different buffing compounds separate, use wide, soft muslin wheels for broad areas, and use hard felt for details and high spots.

Bristle brushes get a lot of use in my studio, most often for bobbing compound, tripoli, and rouge. They are great for accessing small or complex areas. As always, keep your compounds separate. Available in (way too) stiff, medium, and soft bristles, both natural and synthetic, and mounted or unmounted. My brush of choice is the 3⁄4-in. (19 mm), medium bristle, mounted. I use them at slower speeds (faster for rouge), charging them frequently. High speeds tend to leave a glaze of baked compound on the metal. I sometimes dip a new wheel in water and then pumice for a soft sheen. Beware the splatter!

Permanently mounted steel or brass bristles that burnish as they spin are wonderful in any situation that would benefit from a satiny finish; they’re especially awesome for bringing a reticulated surface to life. They’re available as 3⁄4–1-in. (19–25.5 mm) wheels and end brushes. Although conventional practice calls for using brass on yellow metals and steel on silvery, I use a 3⁄4-in. (19 mm) medium-soft steel wheel at slow speeds for everything. Dipping the brush or the work in a jar lid filled with soapy water can improve the finish a bit, but it is messy — and never put a spinning wheel in the water! Wheels with crimped wires are stiffest. Occasionally using them in reverse keeps them from matting in one direction. 

There is a variety of scrubby-type mini-fiber wheels that produce a satin finish. Light gray (coarse), gray (medium), dark gray (fine), and brown/purple (fine) indicates the aggressiveness. I often make my own by cutting a little disk from a kitchen scrubbing pad and poking a hole in the center. They don’t have to be perfect. (Sounds like heresy, but I stamp out mine with my disk punches!) Use a screw mandrel with these.