Pin this on Pinterest

How to use a flex shaft: Part 4 of 7

Abrasives and grinders
Abrasives and Grinders

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

If the drill is the tip of the flex-shaft iceberg, accessories are everything below the waterline. I see them as rotary analogues for many processes in my studio: Sandpapers become disks, files are rotarized into burs and wheels, and buffs are miniaturized. Shaping, smoothing, and finishing follow the standard progression from coarse to fine. It’s worth remembering that you can stop at any point to make a variety of finishes.

Common abrasives

An abrasive’s aggressiveness is governed by grit hardness — what materials it will cut — and the size of the grit. The grit number loosely corresponds to how many grit particles can be packed into one square inch; so 80 is very coarse, 800 is very fine. Grit hardness is measured on the Mohs scale (there is no Larry and Curly) which ranges from talc (1), easily scratched with a fingernail, to diamond (10), the hardest thing we know of.

Why care about abrasive hardness? Well, unless you want to turn that faceted topaz into a cabochon, keep abrasives like aluminum oxide grinders or wet/dry sand-papers away from it! Knowing what a given abrasive will cut helps to avert accidents. (See “Common Abrasives,” above)

TIP: There is a reason that spent sandpaper looks yellow or gray — that’s gold or silver clogging the paper. Save worn-out strips and turn them in with your scrap.

Protect yourself

As abrasive accessories wear down, they get smaller. That lost material goes somewhere. It’s in the air, on the bench, and — if you don’t take precautions — in your eyes, nose, mouth, and lungs. Always wear a mask and eye protection, and check out the manufacturer’s advice for recommended speeds.


For really removing material — smoothing sprue stumps from castings, trimming a protruding edge, or cleaning up a solder spill — grinders present a versatile arsenal of tools. Most grinders are a cake of solid abrasive, most commonly aluminum oxide or silicon carbide.

Separating disks
Separating disk

Separating disks 

My “desert island” accessory, these wafer-thin disks mount on a screw mandrel and cut straight grooves with their edge. Man, are they useful. Separating disks come in diameters of 3⁄4 in. (19 mm), 7⁄8 in. (22 mm), and 1 in. (25.5 mm), various thicknesses, and come in either reddish-brown aluminum oxide or harder, dark gray silicon carbide, either of which will cut anything but diamond. 

Use the edge, or even the side of the disk, to cut and grind. Standard disks will abrade on either side, but there are also “safe-sided” disks, with one or both sides smoothed and nonabrasive. I prefer the standard silicon carbide, double-sided, 7⁄8-in. (22 mm)-diameter, about.022-in. (0.6 mm)-thick disks, which leave a slot (kerf) like a 2/0 blade. If you stack several disks on your mandrel, you can cut a wider slot. 

To prolong the life of your disks, run them into a cake of bur lube. Save smaller, worn disks to use in tight places. Fragile and more expensive, 7⁄8-in. (22 mm)-diameter, 0.008-in. (0.2 mm)-thick ultra-thin disks are great when you want a thinner slot, more like a 4/0 blade (these are great for cutting jump rings). Always wear eye protection; disks can shatter. 

Some applications:

  • Make grooves and slots.
  • Clean up the inside angle of a T seam.
  • Cut a groove or slot in the middle of a sheet or tube (plunge cut). Try doing that with a saw!
  • Use the side of a standard (not safe-sided) disk like a lapidary flat lap.
  • Cut sprues from castings.
  • Make a variety of surface textures by cross hatching, etc.
Mizzy and busch wheels
Mizzy/Busch wheel
Mizzy & Busch wheels

The Mizzy is the classic small grinding wheel; Busch and Mizzy brands are pretty much the same. Originally designed for dental labs, these wheels wear fairly quickly, which is actually an advantage; because there’s always a fresh cutting face, they don’t clog much. They come in a variety of diameters and thicknesses and — you guessed it! — they mount on a screw mandrel. These wheels leave a finish much like an extra-coarse file. 

Gray wheels are silicon carbide and white are aluminum oxide, so both will grind most things. These wheels are often used to give a “brushed” linear-line-type finish, which can be interesting but a bit crude if not applied consistently. Smaller-diameter wheels can get into tighter spaces but still can’t get into corners.
Mounted grinder
Mounted grinder
Mounted grinders

Mounted abrasives are shaped masses of abrasive material permanently bonded to a mandrel. Mandrel shafts vary in diameter. There’s no screw to get in the way, so they’re agile and can get into spaces that screw-mandrel wheels can’t. Most suppliers offer an abundance of mounted abrasives for the flex shaft in a multitude of grit materials and shapes, including tapered cylinders, cones, inverted cones, tiny wheels, balls, points, and bullets. 

As with a file, select the profile that best approximates the shape you’re working on. As is often the case with flex-shaft abrasives, color-coding varies between manufacturers and suppliers with no consistent indicators for abrasive type or fineness (although aluminum oxide is often reddish-brown and silicon carbide is frequently black or dark gray). I stick to medium and fine grits. 

Diamond wheels

Diamond wheels are steel disks coated with diamonds. They’re great for stones, ceramic, glass, metal, etc. Good diamond wheels aren’t cheap and are best saved for when another tool just won’t do.
Sanding and shaping
While the flex shaft excels at some things, it’s not so great at others. It may not be the best choice for larger areas, where you need a flat, uniform finish, or inside corners where that scrunched-up bit of sandpaper works better. But for detail work, complex surfaces, and even small flat areas, the flex shaft can do magic.
PSA disks
PSA (Pressure Sensitive Adhesive) disks
PSA (Pressure Sensitive Adhesive) disks

These disks are basically peel-and-stick circles of aluminum oxide, silicon carbide, or even diamond-coated film that you press onto a special mandrel that ends in a 3⁄4–1-in. (19–25.5 mm) rubber disk. Grits range from 180 to 1200. Since there’s no hole or screw, the entire area of the disk is available to use. It’s easy to peel away and discard spent disks. I don’t use these much, because the grit is always face up, which isn’t how I usually work. I also find the rubber a bit stiff; it doesn’t conform well. But they can be handy. Applied in consistent, overlapping arcs, PSAs can give a nice finish.
Moores snap on disk
Moore’s snap-on disks

Moore’s snap-on disks 

The abrasive on Moore’s disks — sand, garnet, emery, aluminum oxide, or silicon carbide — is bonded to one side of a paper or plastic disk. In the center of the disk is a little brass grommet with a square hole that snaps over the corresponding nubbin of a special 3⁄32-in. (2.4 mm) mandrel. Snap on, snap off! Diameters range from 1⁄2 to 7⁄8 in. (13 to 22 mm).

Most people use snap-ons with the grit side up; I wasn’t crazy about them until I realized I could use them grit-side down. It’s the flat side that cuts (not the edge), and I use pressure so that the disk lies flat against the piece and the sanding surface covers as much metal as possible. They are now my go-to rotary sander. 

Paper disks don’t tolerate flexing, unlike plastic disks; make them bend and you’ll end up with piles of little brass grommets. My favorites are the more flexible 7⁄8-in. (22 mm) plastic-backed Adalox (aluminum oxide) coarse (150) and medium (220) disks.

Drums & cartridges

Small sleeves of aluminum oxide or silicon carbide cloth, in a range of dimensions and grits, slide onto the rubber drum of a special mandrel. Tighten the screw and the rubber expands to grip the sleeve. These can be handy for cleaning up the inside of rings and bezels — any place that a cylinder will work. Slower speeds are best. 

Sanding cartridge rolls

Not high on my list. Cartridges are layers of sandpaper that you screw onto an unusual mandrel and peel away as they wear. I find them to be lumpy and unwieldy, and a sticky mess if they get wet.

3M bristle disks
3M bristle disks
3M bristle disks 

I am becoming a convert to these little abrasive wheels. They are basically Cubitron grit (an abrasive manufactured by 3M, similar to aluminum oxide) embedded in a plastic disk edged in bristles or fingers. For some jobs, they can augment — even replace — sandpaper, and they can more easily access tight spaces, like corners or the juncture between prongs. Because these are conforming wheels (the little fingers follow the contour of the metal) they excel at evenly removing oxides and firescale from complex and detailed surfaces, such as reticulation, or smoothing concave or undulating forms. 

Color-coded disks range from 36 to 400 grit. Brown disks (the most aggressive) are handy for removing the stubborn oxide from forged steel. I use the white and brown disks for a nice matte finish. 

Used singly, the disks are prone to lose bristles; stack three to six disks on your mandrel to present a broader working surface. Apply light pressure — the bristle tips are doing the work. Those tips should point against the direction of handpiece spin, so don’t use reverse, unless you reverse the disks (and change mandrels). 

Catalogs are full of abrasive accessories, with more grinding onto the scene every year, so keep investigating. As always, I am writing from my experience and from my point of view; individual results may vary.This especially holds true for rubberized abrasives, which we’ll look at next time!
A bit about mandrels

Whereas drill bits are held directly in the handpiece, most abrasive and finishing accessories don’t come with their own shank, so they need to be mounted on mandrels. The various shapes of mandrels reflect the variety of attachments and accessories for the flex shaft; mandrel shanks vary in diameter from 1⁄4 in. (6.5 mm mm) to 3⁄32 in. (2.4 mm). All fit in the adjustable #30 handpiece, while its quick-release cousins will be limited to one shank size (most likely 3⁄32 in./2.4 mm).

You can extend the reach of an abrasive disk or grinding wheel by adjusting how much of the mandrel sticks out of the handpiece; however, remember that, like drills, the longer the mandrel is, the more likely it is to bend or wobble.
Screw Mandrel
Screw Mandrel

Screw mandrels

The workhorse of the flex-shaft mandrels, and the most versatile, is the screw mandrel. A tiny screw is threaded through a hole in the center of a disk, wheel, buff, etc. and then into the end of the mandrel shank to hold everything together. The screws can be long or short to accommodate the width of the accessory.

There are different styles of screw mandrels: slim, heavy shouldered, etc. I like a plain mandrel with a 3⁄32-in. (2.4 mm)shank and a 1⁄16-in. (1.5 mm) screw. Screw mandrels have standard, right-hand threads and are used in the forward mode; however, left-hand threaded mandrels, which won’t unscrew when you’re using a wheel in reverse, are also available. (Un-like a drill, abrasive particles cut in any direction.) Oh, and the little washer under the screw? It’s there to grip thinner disks. Whether it’s above or below the disk makes no difference. Screw mandrels are usually sold in packages of six or ten. 

There are also other mandrels, each designed to hold a specific accessory. Animal, vegetable, or mineral, if I can hold it on a mandrel, I’ll put it in my flex shaft. Which brings me to:

Split mandrel
Split (slot) mandrel
Split (slot) mandrels

Love ‘em. The shank of the mandrel steps up to a large or medium cylinder or a long, tapered cone that is split down the middle. Rip a piece of sandpaper (any grit) to about 2 in. (51 mm) long and as wide as the slot is deep. Slide that paper into the slot with the grit facing you (I always get this part backward) and the paper extending to the left, like a little waving pennant. With the motor at slow speed, hold the spinning sandpaper against your bench pin. The sandpaper will wrap around the mandrel with the grit outward, becoming a little sanding cylinder. (Hint: If the smooth paper is on the outside, you have it backward.)

This works great for sanding the inside of rings or for smaller jobs that require a wide, flat sanding surface. When the paper gets worn or clogged, just rip off that end and keep sanding. The beautiful thing is that you can change grits quickly without changing mandrels. For a satin finish, flatten one end of a strip of scrubby pad, or even steel wool, and shove it in the slot in the mandrel. Use slower speeds.
FIND MORE: metal

Want to leave a comment?

Only registered members of are allowed to leave comments. Registration is FREE and only takes a couple minutes.

Login or Register now.
Get awesome news, tips, & free stuff!