Router Bit Dimension Lingo

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16.000 Woodworking Plans by Ted McGrath

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fe In wood-working with a router, there arc three essential components to success. One is the router itself. Another is the woodworker—you. The last is the bit. A sawy woodworker using good bits can do a lot with a lousy router. But the best woodworker with the greatest router will be stymied by a dull, poorly balanced bit.

So the bit is easily the most important part of the whole routing operation. The bit is doing the cutting. The router only powers the bit.

The thing about woodworking with a router is that the more bits you have, the more different jobs you can do with the basic machine. That's not really true of other woodworking machines.

The table saw—vital as it is to the woodworker—is limited to sawing. dadoing and grooving with a dado set, and maybe forming moldings with a molding cutter. I've got a combination blade in my table saw, and I can't ravcmhcr the last time 1 changed it. I could buy a rip blade, a crosscut blade, or a plywood blade, and they'd each produce a somewhat better finish. But each is really doing the same job as the others. Buying more blades doesn't make it a more versatile machine.

Okay, the drill press. The machine is useless without a collection of bits, but beyond boring holes, what does it do? Mortising with a hollow-chisel mortising attachment, an accessory chat's time-consuming to install and remove. As with the table saw. buying different kinds of bits—wist -drill bits, brad-point bits. Forstner bits, hole saws—refines its ability to bore holes, but more bits don't expand its repertoire.

But the router's capabilities expand as you add bits. 1 recently saw a videotape showing a woodworker making a little tabletop cabinet using a router almost exclusively. He used the router to rip and crosscut the stock, to plane and joint it, to cut all the joinery. It's posturing, yes, but it's also demonstrating a versatility that no other woodshop power tool has. And without the bits, the router wouldn't have any use whatsoever.

YOUR NEED TO KNOW

What do you need to know about bits?

• What's available

• Something about buying them

You should know that there are zillions of them out there, made in the United States and Gmada, in Italy and Israel, in Asia. They're sold in hardware stores and building centers throughout the land. Tool dealers sell them. You can buy them through mail-order catalogs.

The lion's share of bits made and sold these days are carbide-tipped . because they work the best in the widest range of routing circumstances. Carbide is an extremely hard material, close to the hardness of diamonds. It's relatively insensitive to heat, so it won't lose its temper when the pressure to perform mounts. The main weakness of carbide is brittleness: It chips easily. It's also pretty expensive.

Hence the carbide-tipped bit. The shank and body of most bits are machined from steel, then slips of carbide—the cutting edges—are brazed to the bit.

The cost and quality of router bits range widely. A particular profile made by one manufacturer may cost two or three times that made by

BIT BODY FREE OF RUST AND PITTING

CUTTING EDGE FREE OF GRINDING MARKS

TIDY JOB OF BRAZING THICK CARBIDE TIP

CUTTING EDGE FREE OF GRINDING MARKS

TIDY JOB OF BRAZING THICK CARBIDE TIP

BIT BODY FREE OF RUST AND PITTING

another. The cost difference may stem from where the bits are made (the United States versus the Far Last, for example) and how they're marketed. It may stem from how they're made. So when purchasing a bit. look for visible signs of quality: the thickness of the carbide, how evenly it is brazed to the bit body, and the smoothness of the cutting edge. These aren't necessarily the most important aspects of bit quality, but they are things you can see. If they're poor, the likelihood is that the invisible aspects will also be poor.

The carbide's thickness suggests the ultimate life span of the bit. A skinny strip of carbide will disappear after a couple of sharpenings. (Whether or not you ever will have it

The cheapest hits around are made of steel that's stamped into bit shape, rolled to configuration, then hardened. The bits aren't meant for long-term routing or a depth of cut greater than Vh inch. Fred has a small selection that he uses when he wants to create a special profile for a small, one-of-a-kind job. The steel is soft enough to be shaped with a file or grind wheel. Fred picks the bit with the profile closest to what he wants, files or grinds and sharpens it, then makes the cut. The hits keep getting smaller and smaller.

The cheapest hits around are made of steel that's stamped into bit shape, rolled to configuration, then hardened. The bits aren't meant for long-term routing or a depth of cut greater than Vh inch. Fred has a small selection that he uses when he wants to create a special profile for a small, one-of-a-kind job. The steel is soft enough to be shaped with a file or grind wheel. Fred picks the bit with the profile closest to what he wants, files or grinds and sharpens it, then makes the cut. The hits keep getting smaller and smaller.

resharpened is immaterial.) A fat carbide tip indicates the maker expects it to last a good many years, even if you are a pro and will have the bit resharpened a half-dozen times.

A corollary to thickness is sup-pon. The carbide tip should be supported along its full length. It's britdc. remember, and if it isn't fully supponed. and especially if it's not too thick, the carbide may break. That could be dangerous! The steel behind the tip shouldn't be visibly pitted.

The quality of the brazing job is an indication of the skill of the workers who made the bit. If it looks sloppy, it probably was not made well. That's not a good sign.

Finally, check the quality of the edge ground onto the carbide. You may want serrations on a knife edge, but a router bit doesn't cut wood the way a knife slices bread or cheese. If you can see grinding marks, the edge has been ground only roughly. If the bit's cutting edges look under magnification like a serrated knife, pass it by. That bit will leave a serrated finish. Better bits will appear and feel smooth.

Of the quality aspects you can 't sec. the most critical are the roundness of the shank and the overall balance of the bit. You can't tell about the bit's balance until you use it. Then you'll know! If the bit's shank isn't perfectly rounded, or if the bit isn't perfectly balanced, it will vibrate. When you cut with it. it'll chatter. Vibration is hard on the router and the bit and the cut.

Somewhat less important than bit balance, though still critically important, is the quality of the carbide that makes up the tip. The carbide used for router bits usually is graded on a four-level hardness scale:

CI to C4. To relate this to a more universal hardness scale—the Rockwell c scale (abbreviated Rc)—CI carbide is approximately Rc89. while C4 carbide Ls Rc94. To put this in perspective, figure that a good woodworking chisel's edge will be between Rc58 and Rc62, while a diamond is RclOO. A few manufacturers specify in their catalogs the grade of carbide they use. This can be a little disingenuous, since even the hardest carbide can be poorly made, with hidden voids and weak spots. But knowing that the bit you're about to buy is made of C3 or C4 carbide is useful. CI carbide is the softest and most brittle. Soft carbide dulls more quickly, and more of it must be ground away to renew the edge, so it simply won't last as long. In the absence of specifics, you have to rely on a manufacturer's reputation.

BIT LINGO

To stan. it's useful to know the lingo. Here arc some definitions and discussion, starting with the names of any bit's components.

Solid bit: A bit machined out of a single piece of tool steel. In some cases, a ball-bearing pilot is screwed to it. Also called one-piece bits, solid bits usually arc machined to closer toleranccs than assembled bits.

Assembled hit: A bit made up of several pieces. The arbor usually accommodates interchangeable cutlers and pilots. Slot cutters typically are assembled bits.

Pilot: A bit's built-in steering mechanism. A noncutting portion of the bit, it is cither a turned steel pin that's an integral part of the bit or a ball bearing attached to the bit. The pilot rubs on the edge of the workpicce, limiting the cut and guid-

Carbide-Free Bits

Yes, Johnny, there arc bits that don't have any carbide in them. Used to be the only kind you could get. High-speed steel (HSS), the>' called em. We don't use them much anymore, but they're still out there. You sec them in hardware stores and at building centers all across the country, though they're seldom advertised in woodworking periodicals.

You don'tsee HSS bits much anymore because they simply aren't as durable and maintenance-free as the carbide and carbide-tipped tools. The one advantage HSS bits have is cost: A HSS bit is generally about one-quarter the cost of a carbide-tipped bit of the same profile. I would say "comparable carbide-tipped bit." but the fact is, the two bits just don't compare

HSS is relatively easy to machine, so bits made from it are inexpensive. It can be honed razor-sharp, much sharper than carbide, in fact, but it dulls quickly. Though good for routing softwood, it dulls pretty quickly if used more than occasionally on hardwood. And routing man-made materials like plywood and panicleboard can bum out a HSS bit in a half-dozen passes.

A couple of manufacturers coat the cutting edges of their HSS bits with a gold-colored titanium alloy to make them more durable. Titanium-coated HSS bits stay sharp longer than the uncoated variety, but they still won't hold up long in hardwoods. Consequently. you've got to hone the cutting edges routinely.

Some will argue that a HSS bit is a reasonable choice in two situations: when you're working with softwoods, or when you need a profile that'll get limited use. Neither situation says "Buy high-speed steel" to me.

The softwood scenario overlooks the destructive properties of the pitch and resins that are so abundant in the most common softwood, pine. Heat is a natural pan of cutting wood with a bit spinning at 22,000 rpm, but HSS bears up to heat less well than carbide.

Tho of the shortcomings of HSS hits arc visible here. One is the delicate cutting edge. The dark blobs at the cutting edge of the rabbeting bit and the overall darkness of the straight hit are result of routing plywood. The glue used in composite materials and plywood is much more abrasive than wood and quickly damages a HSS cutting edge, as you can see. The second shortcoming is the integral pilot. It rotates at the same speed as the bit. heating up quickly. At bat, it merely burnishes the stock's edge, and at worst, it burns it.

The hotter the bit gets, the faster it loses its edge sharpness. The duller it gets, the hotter it gets. Accumulations of pitch and resins add substantially to heat buildup.

Now look at the second scenario. Only the most commonplace profiles are made in HSS: round-overs, coves, an ogee or two, maybe a Roman ogee. In each of these categories, you'll find a wider variety of sizes in the carbide-tipped form than in the HSS form. And you will find a broader array of categories in carbide-tipped forms: classical coves, cove-and-bcads. edge-beading, flutes, full- and half-radius hull-nose, multiform moldingcutters, and many others. Want that bit with a '/¿-inch shank? You are unlikely to find it in HSS.

Neverthless, the cost difference can make your head swim. A HSS bit may cost only a quarter of what its manufacturer charges for a carbide-tipped bit of the same profile. Go ahead, think-about it. Five bucks for \ ISS versus 20 buck-, for carbide-tipped. Or S8 versus $30. Or $ 10 versus $35. The carbide-tipped bit has to last three to four times longer than a HSS bit to eliminate these price disparities. The claim most frequently bandied about is that

Tho of the shortcomings of HSS hits arc visible here. One is the delicate cutting edge. The dark blobs at the cutting edge of the rabbeting bit and the overall darkness of the straight hit are result of routing plywood. The glue used in composite materials and plywood is much more abrasive than wood and quickly damages a HSS cutting edge, as you can see. The second shortcoming is the integral pilot. It rotates at the same speed as the bit. heating up quickly. At bat, it merely burnishes the stock's edge, and at worst, it burns it.

a carbide-tipped bit will last 20 times longer between sharpenings than a HSS bit.

See the value of the carbide-tipped bit? Three or four times the cost, hut 20 times the life span. Good economics.

Here's how I look at it: If the opportunity to sharpen cutting tools is what draws you to woodworking, buy HSS and sharpen away Me? I stick strictly with carbide-tipped tooling. If any of my bits ever do need sharpening— and so far they have not—I'll take them to an experienced tradesman and have him do it.

ing the path of the bit. The cut width of a piloted bit is half the difference between the bit's diameter and the pilot's diameter.

Arbor: The shaft of an assembled bit. The typical arbor consists of a shank, which fits into the router's collet, and a spindle, threaded at the end. The cuttcr fits onto the spindle and is secured by a nut.

Dust shield: A washerlikc disk, sometimes called a slingcr. that fits between the pilot bearing and the cutter. It deflects chips and dust coming off the cutter, helping to prolong the life of the bearing. Not ever>r bearing-piloted bit has a dust shield.

Interchangeable cutter: A steel body with carbide cutting edges brazed to it that fits onto an arbor, thus forming a bit. In some interchangeable-cutter bit systems, a given cutter will fit both a Winch-shank arbor and a '/>-inch-shank arbor. Often, a cutter can be used with or without a pilot, depending upon the arbor used. (See "Interchangeable Cutter Systems" on page 24 for more information.)

Shank: The pan of the bit insened into the router collet. The majority of router bits have either 'A-inch or '/¿-inch shanks. Some manufacturers make bits with Vo inch shanks.

Bits are either solid or assembled. The straight bit (left) is the quintessential solid bit, hut the cove bit next to it is also a solid hit, even though the pilot bearing can be removed. The assembled bit is an alternative, made up of an arbor and a removable cutter. The slot cutter (right) is the most common assembled bit.

TUE BIT PlrtEMSIOMS YOU MELD TO KN0*1

High-speed steel (HSS): The most common tool steel used to make woodworking cutters.

Carbide: An alloy formed of powdered tungsten-carbon panicles fused together with a cobalt binder under extreme heat and pressure. It is one of the hardest synthetic substances. Though it is harder than steel and more heat-resistant than steel, it is also brittle and breaks much more easily than steel.

Carbide tip: A cutting edge, made from carbide, that is brazed to the router bit's steel body. This type of bit construction puts carbide where it does the most good.

Solid-carbide bit: A bit made completely of carbide. The size or design of some bits—spirals and small-diameter straights—precludes their being constructed of steel with carbide rips. Because carbide is so costly, these bits arc expensive. And because carbide is so brittle, these bits are seldom guaranteed against breakage.

Titanium nitride: A surface coating applied to both HSS and carbide-tipped bits by some manufacturers as an extra-cost option. Applied to HSS. the gold-colored coating increases the surface hardness and supposedly provides "extra lubricity" to the bits. Applied to carbide-tipped bits, the coating is said to allow a sharper edge to be ground on the carbide.

Tejlon: A surface coating applied to the bit body by at least one manufacturer, CMT The slippery substance has been used on some saw blades for years, but it is best known as the operative coating on nonstick cooking pans. On a router bit, the Teflon is supposed to help the bit shed pitch and other residues and—arguably—to aid in chip clearance.

When you look at a bit catalog, you see a variety of dimensions listed. Some are helpful, some not. Some pertinent (and irrelevant) measures are explained below, to help you know what you're about to buy.

Cutting diameter: The largest diameter the bit cuts, which isn't always the bit's largest diameter (a '/»-inch straight with a '/2-inch shank, for example). This dimension is most pertinent when used with straight bits and groove-forming bits. With some plunge-cutting profiles, it's useful to know both small and large cutting diameters. With an edge-forming bit, you must know the outside diameter of the pilot to calculate the cutting width from the cutting diameter.

Cutting length: The length of the cutting edge. This is usually a measurement parallel to the bit axis, telling you how deep into the wood the bit can cut at full extension. But sometimes it refers to the longest dimension of the cutting edge. If the edge is at an angle to the bit axis, knowing how long it is doesn't give you much of a clue as to how deep it cuts.

Qirbide height: The vortical height of the carbide tip, a measurement that parallels the bit axis. This usually indicates how deep the bit will cut. With some profiles, an additional measurement—the minor carbide height—is provided so you know how deep a fillet or step the bit is able to cut.

Shank diameter: The largest diameter of the shank. The inside diameter of the router's collet must match this dimension.

Overall length: The total length of a router bit, from top to bottom.

Radius: Half the diameter. In bit lingo, this usually indicates the size of an arc in the profile formed by the bit. A ye-inch-radius round-over bit, for example, cuts a quarter-round profile with a radius of Ye inch. A cove-and-bead bit may have the same radius for both cove and bead. If the radii of cove and bead are different, they'll usually be labeled Ri and R->.

Bexel angle: The angle formed between the cutting edge and a line perpendicular to the shank axis. This is most pertinent in chamfer and V-groove bits.

The cutting properties of a bit depend upon a lot of angles. Hook angles, radial clearance angles, penetration angles. Even the cutting edges involve angles. And what the tool engineer designing a bit is angling for Ls a felicitous compromise. Incorporating a high hook angle so the bit cuts fast, for example, compromises the durability of the cutting edge. Adjusting this angle to strengthen the bit, on the other hand, compromises the cutting speed.

No bit catalog mentions this arcana of the tool engineer's work in more than general terms. You accept the choices he or she has made, without knowing what alternatives were considered. The catalog will razzle you with some tech talk, in the hope that you'll be dazzled enough to buy. But you won't get a choice of hook angles or radial clcarances.

Will knowing what these terms refer to be of any practical value to you? Probably not. But you should know about flute designs, and about when to use what design.

Hook (or rake) angle: The degree to which the cutting edge "leans into the cut." It is the angle (or "hook") between the tip of the cut-ting edge and a straight line passing through, and perpendicular to, the bit axis. The hook angle affects feed

The distinction between straight flutes and shear flutes is subtle hut important. The cutting edges of the straight-flute bits (the bit on the left in each pair) hit the wood straight on, in a chopping action that requires brute /tower. The cutting edges of the shear-flute hits (on the right in each pair) slice into the wood, an action that yields a finer finish yet demands less power. While we tend to think of straight versus shear in terms of straight bits, good-quality profile cutters often have shear flutes, as you can see. (The straight bit is a downshear; the rabbet and cove bits arc upshears.)

The distinction between straight flutes and shear flutes is subtle hut important. The cutting edges of the straight-flute bits (the bit on the left in each pair) hit the wood straight on, in a chopping action that requires brute /tower. The cutting edges of the shear-flute hits (on the right in each pair) slice into the wood, an action that yields a finer finish yet demands less power. While we tend to think of straight versus shear in terms of straight bits, good-quality profile cutters often have shear flutes, as you can see. (The straight bit is a downshear; the rabbet and cove bits arc upshears.)

rate and bit control. High hook angles are easier to feed fast and give better chip clearance, but low ones are stronger.

Clearance (or relief): The space between the surface of the cut and the bit body. The greater the clearance, the faster the bit can be fed. Reduce clearance and you reduce the rate at which the bit can be fed. This is the principle behind the so-called safety or anti-kickback bit design. (See "Bit Drawer" on page 29.)

Penetration clearance: The angle formed between the cutting tool edge and a line perpendicular to die shank axis. This angle allows gradual penetration of the bit into the workpiecc.

Web diameter: The thickness of the bits ground steel body.

Flute: The opening in front of the bit's cutting edge. It provides clearance for the wood chips. Bits may have one or more flutes, and they may be straight, angular, or spiral. Flutes are also referred to as chip pockets cr gullets.

Single-flute bit: A bit with a single cutting edge. A single-flute bit allows a higher feed rate but makes a cut with a rougher finish. It is always a straight bit, never a profile cutter, and seldom exceeds Y* inch in diameter.

Double-flute bit: A bit with two cutting edges, 180 degrees apart. Because it cuts twice in each revolution, a double-flute bit can't be fed as fast as a single-flute bit But those two cuts yield a smoother finish.

Triple-flute bit: A bit with three cutting edges, spaced 120 degrees apan. Used primarily on laminate trimming bits, three cutting edges provide an extremely smooth finish on plastic laminates, which tend to chip easily.

Straight flute: A cutting edge parallel to the bit axis. This is THE basic bit geometry.

Shear flute: A cutting edge that is slightly angled in relation to the bit axis. Much like hand planing at a slight angle, shear-angled bits slice the wood rather than chop at it. The results mimic hand planing at an angle, too. Because of the angle, it takes less effort (you can get by with less horsepower), yet it yields a better finish. More and more manufacturers arc designing bits other than straights with shear flutes.

Spiral flute: This cutting edge resembles a twist-drill bit. It takes the shearing action a step or two further, combining it with chip augcring. The spiral-flute bit is usually recommended for mortising and other deep-cutting work, where chip removal is a problem. The quality of the cut is improved, but the cutting action is slowed. Because of the twist-

The double flute configuration (top left) is the most common by far. All profile cutters—ogees, round-overs, coves, chamfers, and the like—have two flutes, and so do most straight hits. .Some straight hits, usually small-diameter ones, have hut a single flute (center), while others, called stagger-tooth hits, have two flutes hut only half-height cutting edges. The latter style (right) shows the compromises designed into it much more baldly than do the other hits. Each cutting edge extends only half the length of the full cut, so the bit cuts fast and clears chips, just like a single-flute bit. But two cutting edges give a better finish than a single cutting edge (even if they are only half-length). In addition, the Paso Rohles hit shown has one upcut edge and one downcut edge, a compromise intended to yield fuzz-free edges on both surfaces of through cuts.

Ml&U CLEMUNCC« FfcST FUD PossitLt

LOW CLCfcRKUCC; SLOW FEED

Ml&U CLEMUNCC« FfcST FUD PossitLt

LOW CLCfcRKUCC; SLOW FEED

BIT GEOMETRY

rhe spiral bit takes the shear action lo its logical extreme. The down cut spiral (left)—its cutting edge descends in a counterclockwise twist—shears the wood fibers downward, giving the cut a cleaner edge, but it doesn't clear chips from the cut rery well unless it is a through cut. The upcul spiral (ccntcr) —its cutting edge descends from the tip in a clockwise twist—will lift chips from the cut, but also the wood fibers from the edge of the cut. Both these bits are solid carbide. On the right is a tarhide-tipped (upcut) spiral. Its spiral is less aggressive, but its steel body is considerably stronger than a solid carbide one.

ing edge, spirals are either solid HSS or solid carbide.

Duwncui. A shear 01 spiral flute that slices downward as the bit rotates. The downward slicing action leaves a clean, generally fuzz-free edge on the cut. One disadvantage to it is that it tends to drive the router and the workpicccapart, so a secure grip or the use of hcld-downs is important. Another disadvantage is that it doesn't clear chips from the cut quickly. On a through cut. however, it moves the chips away from the router (and onto the floor). You can identify a downcut shear or spiral by looking down on the bit's tip. If the flutes descend in a counterclockwise spiral, the bit is a downcut one.

Upcut: A shear or spiral flute that slices upward as the bit rotates. The upcut bit is favored for deep grooves and mortising, where the upward spiral helps clear chips from the cut. It also augers the bit into the workpiece. thus pulling work and router together. A disadvantage is that the upward slice tends to lift the wood fibers at the edge of the cut. You can identify an upcut shear or spiral by looking down on the bit's rip. If the flutes descend in a clockwise spiral, the bit is an upcut one.

Stagger tooth bit: A double flute bit on which the cutting edges do not extend the complete fcngth of the flute. Instead, the overall cutting length is split between the two flutes. Intended for cutting dense or abrasive man-made materials aid panel goods, these bits combine the speed and chip clearance of single-flute bits with a better-quality finish. The finish is improved because the design balances the bit's weight and the cutting forces to reduce deflection and thus vibration. The bit movement is less erratic; its cut thus is smoother.

JUST STARTING?

So many different bits! There are so many bits to choose from that whittling the possibilities down to the affordable and versatile few—the ones you need to stan a router bit collection—can be overwhelming.

I stancd by buying a "set" of high-speed steel bits at the same time I bought my first router. I remember peeling the waxy coating off several of the new bits and trying them one at a time. And I remember musing about what I'd ever do with one or two of them. But pretty quickly the four that got the most use were replaced with carbide-tipped bits: '/¡-inch and V*-inch straight bits, a Winch round-over bit. and a Mnnch rabbeting bit. Later the Roman ogee bit was replaced. Then 1 branched out. (I still have the old HSS set. and a couple of the bits still have the waxy coating on them.)

You have to ask yourself, "What

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