Table of Contents
A router is only as good as the cutter spinning in it, and the catalog of router bits is large enough to intimidate anyone who has not learned the underlying families. Once you understand what each geometry does at the cutting edge, the wall of shapes at the store resolves into a handful of groups you can reason about: bits that cut straight, bits that cut with shear, bits that copy a shape, and bits that guide off a bearing.
This is a field guide to geometry and behavior, not a buying list. The goal is that you can pick up any bit, identify its family, predict how it will cut, and run it safely. That matters more than owning a large set, because most woodworking is done with a few dozen profiles out of the hundreds available.
The second half covers the rules that apply to every bit: shank and collet safety, feed direction, cutter speed, and the causes of burn marks. Get those right and your bits last longer, your cuts come out cleaner, and the router stops being a tool you dread.
How a Router Bit Is Built
A standard router bit has three parts. The shank is a ground steel cylinder the collet grips. The body is machined steel, usually with two or more flutes and a gullet for chip clearance. Cutting edges are carbide tips brazed to the body on most bits; many spiral bits are solid carbide.
Brazed carbide dominates the market because it balances cost, toughness, and edge life. Micrograin carbide holds a keener edge than standard grades and resists chipping better, which is why premium bits stay sharp longer in hardwood and plywood. Solid carbide spirals cost more but hold geometry and edge sharpness better, especially in small diameters where a brazed tip would be fragile.
Look at the cutting edge under good light before use. A clean, continuous edge with no nicks or glints is ready. A dulled edge reflects light along its length, and a chipped corner telegraphs ridges into every cut.
Shank Sizes and Collet Safety
Shanks come in 1/4 inch, 8 millimeter, and 1/2 inch, with 1/2 inch the standard for serious work. The bigger shank resists deflection and carries more energy into the cut instead of into vibration. A practical ceiling for 1/4-inch shank bits is about a 1/2-inch cutting diameter; above that, move to a 1/2-inch shank regardless of the package claims.
Insert the bit so at least three quarters of the shank length sits inside the collet. Then back it out about 1/16 inch so the end of the shank does not bottom out; a bottomed bit can feel tight while the collet grips poorly. Tighten with the two wrenches that came with the router, firmly but not with everything you have. Collets work by clamping, and overtightening distorts the taper and shortens its life.
Keep collets clean. A film of pitch or a metal chip prevents full contact and lets the bit shift under load. Wipe the taper and collet bore and blow out dust before every bit change. If the collet shows scoring or a shiny ring, replace it; a tired collet is a chatter and safety problem. Never stack reducers to run a large cutter from a small shank.
Straight Bits: The Workhorse Family
Straight bits cut a flat-bottomed groove, dado, or rabbet depending on how you guide them. Single-flute versions clear chips quickly and suit plastics and deep mortises; double-flute versions run smoother and leave a cleaner wall. Diameters from 1/8 inch up to 1 inch cover dados, hinge mortises, and mortises for loose tenons.
Depth control is where straight bits earn or lose their reputation. On a 1/2-inch shank bit, remove 1/4 to 3/8 inch per pass in hardwood; on a 1/4-inch shank, limit each pass to about 1/8 inch. Full-depth passes overload the router, overheat the edge, and leave burned or fuzzy walls. Multiple shallow passes cut faster overall and need no cleanup.
For dados and grooves that will show, a down-cut spiral straight bit leaves a noticeably cleaner top edge than a standard straight bit. Setting the cutter height precisely matters as much as the bit itself; this guide to depth gauges for router setups covers how to hit a target depth repeatably.
Spiral Bits: Up-Cut, Down-Cut, and Compression
Spiral bits trade the straight bit’s flat cutting geometry for a helical edge that slices rather than hammers. The shear angle reduces cutting pressure and produces smaller chips, which is why spirals dominate mortising and template work where edge quality matters.
Up-cut spirals pull chips up and out of the cut, keeping a mortise clear and reducing burning, but the upward shear can fuzz the top edge of veneered plywood. Down-cut spirals push fibers down, leaving a chip-free top edge while packing chips into the groove, where they recut, dull the bit, and scorch the walls.
Compression spirals solve both problems in one cutter: the lower portion is an up-cut and the upper is a down-cut, so both faces stay clean. They are the answer for double-sided laminate and veneered panels, and they demand decent horsepower, firm feed control, and effective chip evacuation. Pair them with good extraction; the dust collector CFM sizing guide explains what it takes to move chips fast enough at the router table.
Flush-Trim and Pattern Bits
Flush-trim and pattern bits copy a shape from one piece to another. A bearing controls the path, and the cutter removes everything that overhangs the pattern. In the basic flush-trim geometry, the bearing diameter matches the cutting diameter, so the trimmed edge lands perfectly flush with the pattern.
Pattern bits carry the bearing on the shank side of the cutter, letting you run with the template on top and the workpiece below, or the reverse depending on where the bearing sits. Top-bearing bits are the most common for template routing because the template rides above the cut where you can see and control it. Bottom-bearing bits suit applications where the pattern sits below the work, such as trimming a part against a baseplate.
Three details separate clean results from disasters. Use a shear-cutting flush trim on figured grain or veneered material. Take small bites rather than a full overhang in one pass; a 1/4-inch overhang is aggressive and 1/8 inch is comfortable. Keep the bearing in firm contact with the pattern and never let the bit drop into a gap between pattern sections. Shop-made patterns are a natural first project, and the basics of building jigs from shop scrap apply directly to template routing.
Chamfer and Round-Over Bits
Chamfer and round-over bits shape edges, and their geometry is simple. A chamfer bit cuts a flat angled face, most commonly 45 degrees, with variants at 30, 22.5, and 15 degrees. Bearing-guided chamfer bits make consistent edges on legs, aprons, and tabletops. Depth of cut controls chamfer width; a large chamfer on thick stock calls for multiple passes or a larger cutter.
Round-over bits cut a quarter-circle profile with a radius matching the bearing, in sizes from 1/8 inch to 1 inch. For a true quarter round, set the bit height so the bearing rides the edge and the cutter meets the flat face tangentially. Leaving a small step, sometimes called a quirk, turns a plain round-over into a decorative profile.
End grain is where these bits misbehave, because the cutter exits the wood and lifts fibers on the trailing corner. A light climb cut on the end grain, or a slower feed with a freshly cleaned bit, cleans it up. Small round-overs on drawer fronts and shelf edges run comfortably with a trim router.
Rabbeting, Dovetail, and Slot Cutters
Rabbeting bits are straight cutters with bearings on the shank end. Cutter height sets rabbet width and the bearing controls depth, so one bit with interchangeable bearings covers a wide range. Take two or three passes for deep rabbets and always run the bearing against a straight, solid reference.
Dovetail bits cut the angled sockets for through and sliding dovetails. Standard angles are 7 degrees for softwoods and 8 degrees for hardwoods, and the bits are nearly always 1/2-inch shank because of cutting forces. Use them only with a matching template and control depth on a test piece; a few thousandths changes the fit across a row of pins.
Slot cutters and biscuit bits cut flat-bottomed grooves for joinery hardware and biscuits. Three-wing slot cutters cut cleaner slots than straight bits because the wings balance the cut and clear chips. Match the cutter to the biscuit size and cut test slots in project material before committing to a layout.
Bearings Versus Bearingless Cutters
Bearings are small sealed ball bearings that roll along the workpiece edge or a template. They fail in a specific, dangerous way: a seized bearing stops rolling, burnishes the edge, and can let the cutter bite into the reference surface. Spin every bearing between your fingers before use. If it feels gritty, drags, or has visible play, replace it. A drop of light machine oil keeps good bearings rolling.
Bearingless cutters use a solid pilot or rub collar instead. Dovetail and many slot cutters have no bearing; you control them with a template, fence, or rub collar, and friction is part of the setup. Rub collars are sized to a template, so template dimensions account for the collar diameter rather than the cutter diameter.
Some woodworkers upgrade fixed bearings to larger aftermarket sets to gain cut depth options. That is a legitimate way to stretch a bit’s usefulness, provided the bearing seats properly and runs true.
Feed Direction and Climb Cuts
Feed direction is the rule that prevents the router from grabbing the work. With a handheld router, move along the near edge from left to right, and travel counterclockwise around the outside of a part. On a router table, the cutter turns the opposite direction from above, so feed the work from right to left across the front of the bit. The principle in both cases is the same: at the point of contact, the wood moves against the cutter’s rotation, so the edge bites and pushes the work back into your control.
A climb cut reverses that relationship, feeding the work in the same direction the cutter travels at the contact point. On a table, that means feeding from left to right. The cutter grabs the wood and pulls it through, which is dangerous with a large cutter and any significant depth of cut. Used deliberately as a finishing pass, however, a climb cut of 1/32 inch or less on a bearing-guided bit with featherboards, a starting pin, and a firm grip can eliminate tear-out on figured wood and end grain. Never climb cut freehand with a panel raiser or any large bit. The forces that make kickback violent on the table saw behave the same way at the router table; the mechanics in this breakdown of table saw kickback causes and prevention apply directly.
Speed, Chip Load, and Why Burn Marks Happen
Router speed should drop as cutter diameter rises. Bits up to 1 inch run at 18,000 to 24,000 rpm. Bits from 1 to 1-1/2 inches want roughly 14,000 to 16,000 rpm. Panel raisers near 2 inches run at 10,000 to 12,000 rpm. Check your router’s speed chart; running a large cutter at full speed is unsafe and hard on the edge.
Chip load is the thickness of material each cutting edge removes per revolution, and it should land between about 0.005 and 0.010 inch per tooth in hardwood. Feed too slowly and the edge rubs instead of slicing, which generates heat and burnishes the wood. Feed too fast and the cut gets rough and the bit deflects. A steady, moderate feed produces chips rather than dust; that is your real-time feedback.
Burn marks trace to four causes: slow feed, high spindle speed, a dull or pitch-covered bit, or dwelling in one spot. The fixes are equally direct: keep the work moving, drop the rpm, clean or replace the bit, and never pause mid-cut. The same heat mechanics show up on circular saws; this explanation of circular saw burn marks, causes, and fixes is a useful companion read.
Frequently Asked Questions
Can I use a 1/4-inch shank bit in a 1/2-inch collet?
Only with a proper reducer sleeve, and only for light cuts. Reducers add a potential slip point and should never carry a large cutter. If you find yourself needing an adapter often, buy the bits in 1/2-inch shank instead.
What is the difference between a flush-trim bit and a pattern bit?
A flush-trim bit has a bearing whose diameter matches the cutter, so it trims an edge perfectly flush with a pattern. Pattern bits carry the bearing on the shank side, letting the template sit above or below the work depending on the bearing position. Both copy shapes; the difference is where the template rides.
Why does my router burn the wood?
Slow feed, high rpm, a dull or pitchy bit, or dwelling in one spot. Clean the bit, reduce the spindle speed, and keep the work moving steadily. Chips rather than dust coming off the cut mean your feed rate is right.
What speed should I run a router bit?
Up to 1 inch in diameter, 18,000 to 24,000 rpm. From 1 to 1-1/2 inches, about 14,000 to 16,000 rpm. Large panel cutters near 2 inches, 10,000 to 12,000 rpm. Always follow the manufacturer’s chart for your specific bit.
How deep can I cut in one pass?
On a 1/2-inch shank bit, 1/4 to 3/8 inch in hardwood. On a 1/4-inch shank bit, about 1/8 inch. For pattern and flush-trim work, limit overhang removal to about 1/8 inch per pass. Multiple light passes are faster and safer than one heavy one.
How far should I insert the bit, and how tight should the collet be?
Slide the shank in so at least three quarters of it is gripped, then back it out about 1/16 inch so it does not bottom out. Tighten firmly with both wrenches until the collet stops moving; do not hang your weight on it. Re-check tightness after the first cut.