There is a little more to using a router bit than slipping it into the router collet and cinching the collet nut down. You’ve got to match the bit to the cut and the router. You’ve got to balance the bit speed with the feed rate. If you already have a router bit set, learn how to sharpen your router bits and how to clean your bits properly here.
Here are some tips for using your bits to their best advantage.
What or which router bit to use?
Always use the bit with the shortest cutting edge that will do the job. The longer the bit, the greater the chance of its breaking. This is because excessive length amplifies vibration and deflection, which manufacturers cite as the leading causes of tool breakage. If you are cutting a 1/8 inch deep dado, use your dado bit with the 5/8 inch long cutting edges, rather than your straight bit with 1 1/2 inch long cutting edges. Consider reading this small guide to select the best router bit on the market.
Always use the bit with largest diameter shank you can.
Each time you fit a bearing pilot bit in your router, give the bearing a flick to ensure that it spins freely that its rim is smooth and clean. A frozen bearing is the prime cause of tracks and scorch marks on the edge of a workpiece, but perhaps surprisingly, it isn’t the only cause.
The purpose of the pilot is to guide the cut and to control its width. In the days when HSS was what all bits were made from, pilots were turned steel pins. You needed a light touch to avoid burn marks left by a pilot spinning at the same speed as the cutter. Nowadays, you need a heavier hand to avoid those burn marks. The bearing is supposed to roll along the workpiece edge at the feed rate, while the bit spins inside it at the router’s speed. But if you don’t put enough pressure on that bearing, it can spin along with the bit and really stink up the edge, if you know what I mean. (On the other hand, press that bearing too hard against a softwood, and you crush the wood fibres, leaving a different sort of track along the edge.)
Dirt or grit stuck on the bearing’s rim can cause a wavy or choppy cut. The dirty pilot acts as a kind of cam, lifting the cutting edge infinitesimally away from the work each the dirt speck hits the guiding edge. (The same unsatisfactory finish can be achieved by running a clean pilot along with a dirty, chipped work edge, by the way.)
That integral pilot on the HSS bit has on big advantage over the typical pilot bearing – its small diameter. That allows you to cut deeper into a corner than a 1/2 inch or 3/8 inch ball-bearing pilot.
On the whole, though, the integral pilot is inflexible. Its size is what it is. But a bearing can be changed, thus altering the width of the cut to yield different profiles from the same bit.
Always use the correct collet for your router, and avoid using sleeves or bushing to make a 1/4 inch shank bit fit in a 1/2 inch collet. These reducers add to vibration and runout, and they generally don’t hold the bit as well as collet alone. (A number of router manufacturers don’t make separate 1/4 collets for their 1/2 inch collet machines and if you’ve got one of them, using bushing is unavoidable.)
Using a collet that is in good condition is essential. A worn and scored or out of round collet doesn’t hold the bit tightly, which increases run out and vibration. Don’t assume that a new collet is perfectly round or even the correct diameter. Check the shank each time you remove a bit from the collet. Dark marks or grooves in it usually indicate slippage. Either you didn’t tighten collet nut sufficiently or the collet is worn and should be replaced.
Always insert the shank as far into the collet as it will go, then back it out slightly (approximately 1/16 inch). The reason for doing this is to ensure that the collet alone is securing the bit and that the shank is centred in the collet. If you bottom the bit and leave it bottomed when you tighten the collet nut, the bit can be off-centre by a thousandth or two. That is enough to cause pretty severe vibration.
How deep the bit shank must be inserted in the collet is not something all bit manufacturers agree on. Most urge you to avoid cheating the bit out of the collet to extend its reach. It is a tempting idea. You need to cut just a little deeper, so you the bit out of the collet an extra 1/4 inch or an extra 1/2 inch. The maker of Byrom bits says the minimum insertion is twice the shank diameter. As a practical matter, you don’t always have a lot more length than that on 1/2 inch shank cutters. But having a hefty cutter on a 1/4 inch shank inserted only 1/2 inch into the collet seem excessively venturesome to me. Bear in mind that Byrom also recommends the 1/2-inch shank for any bit over 1/2 inch in diameter.
Rockler has a very good video to demonstrate these techniques.
Update on 28th of Dec: Part 2 of this article, how to use router bits properly is up.