Router bits: profiles, shank sizes and how to choose them

01What types of router bits are there?

Router bits divide by the shape they leave, called the profile: straight and spiral bits cut grooves, dadoes and mortises; edge-forming bits such as round-over, cove, chamfer and ogee shape an edge; trimming bits follow a template or a second surface; and joinery bits such as dovetail, rabbeting and rail-and-stile sets cut joints. Most edge-forming bits carry a ball-bearing guide that rides along the workpiece edge or a template, so the router needs no fence.

Bearing position separates two bits that look alike and are often confused. A flush trim bit has its bearing at the tip, below the cutter, so it trims the work flush to whatever lies underneath, such as a face frame, laminate or a template under the work. A pattern bit has its bearing on the shank side, so a hand-held router follows a template fastened on top. Both cut flush, but the template goes on opposite faces. Rabbeting sets go one step further and change the depth of cut by swapping bearings.

The table below lists the common profiles with an example bit and the speed its maker rates it for. The article on choosing router bits explains each profile with the joints and edges it makes, and the buyer guide to starter router bit sets compares sets for a first router.

02Which router bits fit your router?

A bit fits only a collet whose bore matches its shank diameter. Compact routers from DeWalt, Milwaukee and Makita ship with a 1/4 in collet, so they take that shank only. Mid-size and full-size US routers usually ship with both 1/4 and 1/2 in collets; the Bosch 1617 family lists capacities of 1/4 in, 3/8 in, 1/2 in, 8 mm, and the DeWalt DW618 lists 1/4, 1/2 in.

European routers add metric sizes. Bosch supplies its POF 1400 ACE with 6 mm, 8 mm, 1/4 in collets, Festool sells an 8 mm collet for its OF-series routers, and Makita lists its larger plunge routers at 12 mm or 1/2 in. For two more European pro routers only retailer listings are on file: The Power Tool Store lists the Festool OF 1400 EQ-F-Plus with 1/4 in, 8 mm, 1/2 in collets in the box, and Anglia Tool Centre lists the Trend T11EK with a 1/2 in collet, other sizes sold separately. An inch shank and a nearby metric shank are not interchangeable, which corrects one of the most repeated shortcuts in routing: a 1/4 in collet does not properly grip a metric shank of similar size, and a metric collet does not properly grip an inch shank.

Smaller shanks in a large collet need the router maker's own sleeve. Makita says small-shank bits in its larger collet must use the matching collet sleeve, and that fitting them without one can break the collet cone. The guide to router collet sizes lists collets by router, US and European, and the accessory finder shows the collets each listed router's maker states.

03Should you buy 1/4 in or 1/2 in shank bits?

Buy 1/2 in shank bits whenever the router takes them; the larger shank gives more stability and less vibration. The smaller 1/4 in shank bits are for compact routers, which have no larger collet, and for small profiles where a large shank adds nothing.

Compact routers also cap the cutter size. DeWalt says not to use bits with a diameter above 1-3/8 in in its DWP611, and Milwaukee lists a maximum bit size of 1-1/2 in for its M18 FUEL 2723 compact router. Bosch's own specification for the Bosch PR20EVS gives a maximum cutter diameter of 1-5/16 in. Large bits such as raised-panel cutters come with the larger shank and are meant for a router table, at reduced speed.

How far the shank goes in matters too. Bosch, DeWalt and Milwaukee all say to insert the bit fully and then back it out slightly so it does not bottom out in the collet, which is the opposite of the common habit of pushing it all the way home. The exact gap differs by maker, and Bosch adds that the shank must go in at least 5/8 in.

04Why do larger router bits need a lower speed setting?

Never faster than the bit's rated speed, and slower as the bit gets larger. A router bit's maximum rated speed must be at least the maximum speed marked on the router, because a bit run above its rating can break and fly apart. Router manuals give lower dial settings for larger bits, and bit makers publish lower maximum speeds as cutting diameter rises.

The figures differ by source, so read the bit's own rating first. IGM's bit safety sheet, in the table below, steps the maximum down with each diameter band, and Whiteside rates its large raised-panel bit at a maximum of 12,000 rpm. CMT describes the same pattern in its own chart, with maximum speed falling as diameter rises, and says the speed marked on the bit or its packaging governs. Compact routers run far faster than that at full speed: the DeWalt DWP611 lists 27000 rpm and the Bosch PR20EVS 35000 rpm, which is one more reason large bits do not belong in them.

The belief that a router should always run at full speed is wrong for anything but small bits. Use the speed dial, and if the router has none, keep to bits small enough for its fixed speed.

05Up-cut, down-cut or compression: which spiral bit?

Choose by which face must be clean. Up-cut spirals lift chips up and out, which suits deep cuts and mortises, but they leave the cleanest edge on the face farthest from the router and more tear-out on the near face. Down-cut spirals push chips down and leave the cleanest edge on the face nearest the router, which suits shallow grooves, dadoes and inlays, though they clear chips more slowly. Compression spirals combine both flute directions to leave clean edges on both faces of sheet material.

That corrects a common assumption that any spiral bit gives a clean top edge. If a slot in veneered plywood shows fuzz along the top, the bit is probably an up-cut, and a down-cut or compression spiral is the fix. Sheet goods raise the same clean-face question when they are cut to size first; the hub on saw blades and tooth grinds covers the blades used for that.

06Which way should you feed a router?

Against the bit's rotation. Seen from above, a hand-held router bit turns clockwise, so when routing an edge in front of you, move the router from left to right; go counterclockwise around the outside of a workpiece and clockwise around the inside. Feeding the other way, called climb cutting, can pull the router along and cause loss of control.

Feed rate and depth matter as much as direction. Feeding too slowly leaves friction burns, and a bit that makes dust instead of chips carries less heat away, so its edge dulls sooner; Freud ties early dulling to that too-fine chip. Speed can cause the same thin-chip rubbing from the other side: ToolsToday says a bit spinning too fast for the feed rubs instead of cutting and burns, and that lowering the speed while keeping the feed can help. Deep cuts belong in several passes of increasing depth rather than one heavy pass, which overloads the motor. Makita, for example, limits groove depth per pass on its RP-series routers to 15 mm with 8 mm bit; 5 mm with 20 mm bit.

07When does a router bit that fits still burn or chip?

A bit that fits the collet still burns when the feed is too slow or the bit is dull, and chips or tears when the feed direction, spiral direction or bit design is wrong for the face that shows. The table below pairs each symptom with its cause and fix, drawn from manufacturer manuals and guides.

Two failures need more than a technique change. A dull or damaged bit overloads the motor and needs extra force, which can break the bit, so manufacturers say never to use one. The load is measurable: a 2024 BioResources study, milling wood on a CNC router at 18,000 rpm with a 16 mm carbide cutter, found a blunt edge drew on average 51.6% more cutting power than a sharp one. A bit that slips or creeps in the collet points to a mismatched shank, a missing sleeve or a shank that was bottomed out before tightening. The guide to router bit burning and chipping works through each cause in order.

A router that suddenly shakes is worth stopping for, because routers are normally smooth tools. UK HSE's L140 guidance, whose 2019 second edition is still the current one, gives routers a recommended initial vibration estimate of 3 m/s², among the lowest of the woodworking tools it lists and well below its figure for jigsaws; the table reflects tools in use before 2015. Marked vibration on top of that often points back to the bit: a bent or unbalanced cutter, a small shank carrying a large profile, or a shank seated wrongly in the collet.

08Router bit guides on AccessoryFit

• Router bit profilesChoosing bits for edges and joints
• Router collet sizesInch and metric collets by router
• Router bit sets for beginnersSets compared on published specs
• Router bit burning or chippingFeed, speed and sharpness
• Accessory finderSee the collets your router's maker lists

09Questions readers ask