Here I’ll discuss the basic design, geometry and materials of straight and spiral bits used for trimming, shaping, and making dadoes and mortises. Adding a pilot bearing or two simply takes these basic designs to a new dimension.
Basically, a rotary cutter fitted within the collet of a table-mounted or hand-held router, straight router bits come in two formats: straight fluted and spiral. Today’s straight fluted bits generally have one or two tungsten carbide inserts brazed onto a supportive steel body. Straight spiral bits are available in either solid high speed steel (HSS) or solid tungsten carbide. Straight fluted bits are more common than straight spiral bits.
HSS is formed from molten molybdenum, tungsten and chromium steel. Beginning as a liquid, it has a fine molecular structure capable of achieving and maintaining a very keen edge.
Tungsten carbide, on the other hand, is a chemical compound of tungsten and carbon particles compressed into “blank” shapes via a process called sintering. Twice as stiff and double the density of steel, tungsten carbide is less susceptible to normal wear and abrasion, offering a much longer useful life than HSS, but is comparatively brittle and unable to develop as keen an edge.
While the cutting edge is the “business side” of a router bit, there’s a lot more behind it. On closer inspection, the side edges of a straight carbide-tipped bit have a single “cutting angle” of +/- 30°. Spiral router bits, actually two flute machinists’ endmill cutters with a “standard” 30° helix designed to mill aluminum or brass, have a primary cutter angle of 3° to 5°, supported by a secondary angle of a further 10° to 15°. These clearances behind the cutting edge reduce the potential for friction and heat to develop as the severed wood fibres (a.k.a. “swarf”) make their way via the flutes to exit the bit as it repeats the process thousands of times per minute.
On the end of most router bits is also some form of edge with a relief or clearance angle. Behind it, the shape is dependent on whether the bit is designed to be simply “end cutting” or “bottom cleaning”. The ends of carbide-tipped bits have a relief angle of +/- 15°. The ends of spiral bits, on the other hand, have a “primary cutter angle” of 7° to 10° supported by a secondary cutter (clearance) angle of a further 5° to 15°.
All straight bits, standard or spiral, will have a “dish angle” of 1° to 2° within the end, ensuring the bit contacts the workpiece primarily at its outermost tips with the centre in relief.
A “pilot” or “guide bearing” allows router bits to smoothly follow a pattern or template so a shape or profile can be duplicated perfectly, in a controlled manner, as often as required. The less common option, the pilot is “plain”, simply a polished area of the bit’s shaft either above, below or on either side of the cutting edge. A “panel pilot” bit is a good example of this. More on these bits later. But in most cases, a ball bearing “guide” is fitted, reducing friction as the bit follows the pattern, thus extending each duty cycle.
Continuing on my “straight bit” theme, bearing placement defines the bit’s intended use. Having the bearing at the end creates a “flush trimming bit”; having the bearing at the shank end, above the cutting edge, creates a “template” or “pattern bit”; and if there are bearings both above and below, the cutter produces a “trimming/template” or “combination bit”. In use, this variety allows us to select the best bit for each operation, assuring the fibres of the wood are being cut in the direction of the grain for the coolest, finest result while minimizing effort on our part. Available generally in 1/4″ and 1/2″ shanks, size the bit to the project, though a 1/2″ shank is less prone to chatter. All can be used with either table-mounted or hand-held routers.
Shop-made patterns or templates can be crafted to best favour the grain direction of the individual part you’re going to shape, secured to the piece being worked either above or below the part, or clamped to a jig using toggle hold-downs. Then, simply select the right bit to follow the profile.
Guide bearings also feature on “profile bits,” shaping a host of edges (coves, bullnoses, ogees) and joinery profiles. None of these bits are straight, so I won’t cover them in detail here.
Pilots/bearings are manufactured exactly flush with the tip of the cutting edge when the bit is first machined. Each time the bit is sharpened, its outer diameter (o.d.) becomes a few thousandths of an inch smaller, trimming less material during use, thus producing a shape slightly larger. In most cases this isn’t an issue. Also, it’s strongly recommended you remove ball bearing guides yourself before sending bits off for sharpening.
Router bit shanks, the part gripped within a router’s collet, are generally 1/2″ and 1/4″ in diameter. Most routers have interchangeable collets to allow the use of either, smaller handheld 1/4″ “trim” routers being the exception. Given the choice, 1/2″ bits are the best investment, offering the largest cutting-edge surface, heat dissipating mass and reduced vibration.
It’s hard to tell the quality of a bit by simply looking at it, but there are signs. Tungsten carbide comes in different grades and densities and appears much the same, regardless. On all straight fluted bits, you can look for the size and thickness of the insert, how uniformly it’s brazed to the body and the keenness of its edge. You can also tell a lot about a bit by its body mass. A larger mass will dampen vibration and dissipate heat better, plus often allow for the inclusion of an anti-kickback profile in the maker’s design to reduce the potential for the bit to take too deep a cut resulting in chatter, a catch or kickback. A quality bit will have an appropriate cost but will be well balanced and a solid investment that you can sharpen again and again. Spiral bits come as solid HSS and solid tungsten carbide, with carbide bits more costly up front but offering comparable value long term if you take care of your tooling.
Most tooling retailers offer a precision sharpening service and the cost is quite reasonable vs buying a new bit, as long as the bit is simply dulled, not damaged. For straight fluted bits, including angled and/or profiled styles, sharpening is applied inside the cutting edge within the flute, reducing the outside diameter merely a few ten thousandths of an inch each time. If the bit’s end requires sharpening, that may be applied outside the cutting edge along the primary and secondary bevel. For spiral bits, sharpening is again done inside the cutting edge within the flute and the end may be sharpened outside, as described above, if required.
If a straight carbide-tipped bit is damaged because it hit a nail in use or was accidentally dropped, a sharpener can remove and replace the damaged insert before resharpening the bit to return its balance. For a solid spiral bit, often the damage is near the end and that area can be cut off, and a fresh edge, gash, primary and secondary bevel can be shaped and sharpened, affecting only the bit’s length.
Typically, router bits are delivered sharpened with 600-800 diamond wheels for a fine cutting edge.
The enemies of router bits are the same as for any cutting tool: abrasion; impact; heat; and lack of cleanliness.
Making sure your wood is grit-free will minimize abrasion. Brush, scrape and vacuum the surface of materials you think may not be clean, and avoid using cheap particleboard, which is prone to having grit imbedded in the particles during manufacturing.
Impact is pretty obvious. Avoid dropping router bits, keep them protected from contacting each other by not storing them loose in drawers or tool bags, and inspect wood for elusive nails, screws, stones (odd but true), etc. before routing. Many bits are supplied with stands or holders to keep them separated on a shelf, or simply keep them in the display box they came with.
Allied with this is making sure the bit is firmly, fully seated within the router’s collet. Make sure the collet and the tapered bore it sits within are each quite clean and mate well — no galling or damaged threads. Insert the bit fully, but not totally, and tighten the collet retainer firmly so it draws the bit in to seat fully. A bit that’s loose or in a damaged or dirty collet rotating at several thousand RPM can really mess up your day, not to mention all it contacts, if it comes free.
The usual sources of heat are overworking a bit, using a bit that’s dull or forcing a bit that’s bound with a film of resin or glue. It’s always better to take several light passes, allowing the bit to do the work and the swarf to exit behind the cut rather than bind up within it. Making sure your bits are sharp rewards you with having to put less effort into the cut, minimizes edge-damaging heat and produces a smoother, burn-free result. Pitch and resins build up on bits, affecting the relief angle behind the cut and masking the face of the cutting edge within the flute, making the cutter effectively dull. Simply cleaning the bit using a citrus-based cutter-cleaning product is easy. These types of products are often sold where you buy your bits. Apply using an old toothbrush or a small square of abrasive-free, ultrafine (grey) or fine (maroon) “Scotchbrite” or equivalent microfibre hand pad. You’ll add untold life to your bits. It’s important to never use oven cleaner to clean your bits or saw blades. Its harsh chemicals harm the brazing that bonds the carbide inserts to the steel body and will damage or remove the anti-stick coating on the tool.
Manufacturers have also developed proprietary non-stick coatings for their bits, reducing pitch and sawdust buildup, which in turn reduces friction and heat.
Feed speed, router RPM (variable speed routers rule!) and depth of cut are a balancing act we need to perform so the router bit can deliver peak performance and not get overworked. All need to be tailored to the material being cut for a clean cut and effective swarf extraction. Sensitivity to how much material you’re removing and whether you’re routing resinous softwood, dense hardwoods, oily exotics, plywood or melamine-coated particleboard is important.
Router bits come in a huge range of styles but it’s the presentation angle of the cutting edge that defines their ability to produce a good result. Realizing this, we can decide which cutter is best suited to any given material and for the finish we expect to produce. There are three basic cutting-edge styles: straight; slightly angled; and spiral.
Most straight bits today are tipped with a brazed-in insert of tungsten carbide that slices or scoops material as it cuts. Most are “end cutting” and capable of shallow plunge cuts while some offer “bottom cleaning” by having a cutting edge in some form extending across the full diameter of the bit’s end.
Bits with slightly angled cutting edges are designed to be “finishing” bits. Because they shear as well as slice wood fibres, they produce a smoother finish. This design is typical on bits that shape final profiles, such as face edges, coves, bevels and ogees. Most often, these have bearing “pilots” for referencing a pattern as they cut.
Spiral bits, those that feature a helix running around the bit from tip to shank, are the ultimate for clean cutting and plunge routing. The continuous outer cutting edge along the bit’s helix shears wood fibres exceptionally well, while generous flutes extract and direct the severed fibres as the cut is made so the edge is kept clean, friction is reduced and the bit remains comparably cool. Most spiral bits are “end cutting” and “bottom cleaning”, ideal for machining mortises, tenons, dadoes, rabbets, etc. They are available in both solid HSS for keenness and solid tungsten carbide for durability.
Spiral bits come in three styles: “up-cutting”; “down-cutting”; and “compression”. These feature up-cutting edges at the tip end and down-cutting edges at the shank end, overlapping in the bit’s central body.
Accepting that routers rotate in a clockwise or left-toward-right direction, hold a spiral up-cut bit in one hand and a down-cut bit in the other, and examine the direction of the flutes of each. The up-cut bit is designed to draw swarf from the tip upward toward the bit’s shank. On the other hand (pun intended), the down-cut bit extracts swarf from the bit’s shank end downward to exit at the tip end. An up and down cut or “compression” bit draws swarf from both ends towards the center of the bit.
Because of its extremely good ability to extract swarf behind the cut, an up-cut bit is excellent for deeper mortises, when you want to extract a lot of fibre in a very controlled manner. However, it also draws wood fibres surrounding the cut upward, leaving a slightly frayed edge. In most cases, this isn’t an issue as the mortise will be covered once the joint is assembled. This fraying effect can be minimized by taking a very shallow first cut to sever the surface fibres only.
A down-cut bit is well suited for forming dadoes and shallow mortises when you want to preserve a crisp edge surrounding the cut; perfect for crisp dadoes in veneered sheet goods. Its cutting action presses the wood fibres downward, away from the router, so it cuts very cleanly. In this case, the cut should be advanced slowly and with shallower depths of cuts so the swarf has a chance to be cut, ground and drawn away as dust rather than chips.
A “compression” bit or “up and down cut spiral” bit is well suited for applications where both the top and bottom surfaces of wood or veneered sheet goods need to be left crisp, as it will draw cut fibres upward from below and downward from above, compressing the swarf to exit the cut from the center of the board face or panel thickness.
A panel pilot bit is an aggressive cutter that does a smooth job of routing shapes within sheet goods like a countertop for a sink basin or house sheathing for windows or doors. The bit comprises a pointed tip; a smooth, plain pilot area; a single flute cutting area; and above that, the smooth shank. When chucked in a plunge router set so the bit’s pilot area can follow a pattern, the pointed tip is plunged just inside the edge of the area to be removed, moved in a clockwise direction until the smooth pilot or shank contacts the guide surface, then slowly advanced within the cutting area as the pilot follows the guide/framing. A router with adequate grips and horsepower is recommended for this uniquely capable bit.
I’m guessing that now you’ll never look at a straight router bit the same way again.