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How to use router bits properly


Kamille - December 25, 2018 - 0 comments

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, cove and Stick with a router table and how to clean your bits properly here. But if you want to learn how to use a router bit then you’ve come to the right place.

Here are some tips for using your bits to their best advantage.


What or which router bit to use?

Bit selection

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.


Pilots

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.

The collet

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.

Feed rate

The rate at which the router is fed along the work (or the work is fed across a router table) is very important to the overall quality of the cut to the longevity of the bit. You should feel a constant, even pressure when the work meets the cutter. Feed rate ultimately depends on the type of material being cut, the amount of material being removed and the type of bit being used.

The most common feed rate mistake is excessive restraint. And feeding too slowly is a quick way to ruin a bit. Letting the bit “dwell” in the cut will lead to a burned cut, caused by the bit heating up, which in turn reduces the bit’s life immensely. Remember, heat can ruin a sharp tool. So keep the router (or the work) moving.

If you are concerned about bogging the router down, make several light passes to complete the cut instead of trying to hog away too much material in one pass. This is especially true if you are using large-diameter bit. This will reduce the stress on the bit and will generally be a safer practice.

Router Speed

The speed at which the bit turns can be important. The typical router runs at somewhere between 20,000 and 24,000 rpm, depending upon the brand and model. Router bits are designed to cut at this operating speed. If you were to run the typical router bit at a reduced speed, say 10,000 to 12,000 rpm, you might be surprised at how poor a job it does. The finish of the cut probably will be rough and choppy.

This is true of most bits. But as the diameter of the bit increase, the router’s high operating speed becomes a problem. The cutter is too darn big to be revolving at 22,000 rpm. While there’s little dispute that large-diameter bits — for example, 3 to 3 1/2-inch-diameter panel raisers should be spun at about 12,000 to 14,000 rpm, opinions vary as to the appropriate speeds for other sizes of bits.

Once bit source recommends operating any bit larger in diameter than 1/2 at no more than 17,000 rpm and cutting that speed to no more than 14,000 rpm when the diameter exceeds 1 inch. More commonly, you’re advised to slow the bit when its diameter hit 2 inches.

What’s working here is confusion between safe operating speed and an appropriate balance between bit speed and feed rate. We slow down big bits simply because they’re unsafe at “full router speed”. But we often slow down midsized bits so a workable balance can be struck between bit rpm and feed rate. As I mentioned before, a feed rate that is too slow is common. The bit moves too slowly through the cut, the heat builds up, the wood scorches.

The prevalence of plunging operations fuels this problem. Say you have a short slot to cut. Each time you want to plunge the bit deeper, you tend to pause, allowing the spinning bit to dwell in the cut. You can just smell the wood scorching. The cut is short, and each change in direction brings another slowing of the feed rate, another pause. One way to moderate the problem is to slow down the bit’s revs — even when the cutter is relatively small in diameter.

Check out this video to see u “how to make a shaker style cabinet door using tongue and groove router bits AKA rail and stile router bits”. This is my favourite video when it comes to getting started with routers, router bits and router tables for woodworking.

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