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Working with router bits

Jack Gordon - February 17, 2019 - 0 comments

Bit Diameter and Router Speed

The larger the bit diameter, the faster its rim will be moving. When using router table bits larger than 1 in. in diameter, it’s important to reduce the speed of the router, so the rim speed at the cutting edge remains within safe limits. A bit that spins too fast tends to burn the work, and the potential for kickback is greater too. Speed may be reduced either by using a variable-speed router or an after-market speed-control device. A router lift can be useful in most of those situations when working with router table bits.

The chart below suggests maximum allowable speeds for various router-bit diameters when cutting wood, MDF or plywood.

Router Bit DiameterMaximum Speed
Up to 1"22,000 - 24,000 rpm
1" to 2"18,000 - 22,000 rpm
2" to 2-1/2"12,000 - 16,000 rpm
2-1/2" to 3-1/2"8,000 - 12,000 rpm

Matched sets

Many router-table operations call for a set of matched cutters (one is the negative profile of the other). For example, in a rule-joint set, the first cutter mills a radius and the second cuts a matching roundover, making the joint that allows a drop-leaf table to fold down.

The most common use of matched sets is in the making of cope-and-stick panel doors, where each cutter assembly is composed of two to three cutters plus pilot bearings stacked on a 1/2 in. shank. The sticking assembly mills a profile on the inside edges of the stiles and rails. The coping assembly copes a matching profile on the ends of the rails.

Unless both profiles match exactly, the result will be mediocre at best. That’s why I don’t scrimp on matched sets. I buy only from first-rate suppliers who will exchange a defective set or refund my money if the profiles do not match.

Many companies offer single-cutter economy cope-and-stick sets. With these sets, the sticking cutter can be restacked on the shank in a different arrangement to make the cope cut. These cutters usually work fine, but they are not as convenient as having two separate cutters. With a matched set, changing from the stick cut to the cope cut is as easy as changing bits. With the reversible single arbor set, the operation takes much longer.

While reversible sets are fine for symmetrical shapes such as a quarter-round, they can not be used for shapes that are asymmetrical end to end, such as an ogee. A so-called ogee reversible cutter will not yield a true ogee, but rather a sort of sine-wave shape.

Guiding the Work through the Cut

The most basic way to guide a workpiece through a cut is by sliding it along a fence. That works fine for straight cuts, but what if you want to mill a curved edge or reproduce an irregular shape several times? That is where pilot bearings and guide bushings come to the rescue.

Pilot bearings

Pilot bearings allow radial guidance of the workpiece so the edge can be any shape, so long as it is of greater radius than the bearing.

A pilot bearing is a shielded ball bearing that rides over (or in some cases under) the cutter. A curved work surface can be guided by this bearing just as if it were a straight surface guided by a fence. Sometimes part of the workpiece itself rides against the pilot bearing; other times a template may be attached to the workpiece and the bearing will ride against the template. Templates may be attached above or below the workpiece, depending on the circumstances. The template may also double as a fixture to control the work.

If the bearing is guiding off the workpiece directly, always begin with a smooth edge on the workpiece, since your work will only be as true as the surface the bearing makes contact with. Most pilot bearings are of small diameter, so it does not take much irregularity in the work surface to cause a wavering profile. Edges that are to be milled should be planed or sanded smooth before routing begins.

Heavy, hogging cuts invite disaster. The quality of the cut will suffer, and the risk of the workpiece getting away from you will greatly increase. Since you can’t use feather boards with pilot-bearing work, you have to depend on your own strength to hold the work steady against the bit. The heavier the cut, the more difficult this is.
In pilot-bearing-guided work, you won’t be able to start the cut safely and smoothly unless you use a starting pin. The starting pin is positioned just to the right of the cutter, on the infeed side of the router table (see the drawing below). The workpiece is placed against the starting pin, pivoted into the cutter until the edge of the workpiece touches the pilot bearing, then slid to the left until the edge no longer makes contact with the bit. Some people like to add another pin on the outfeed side of the table so the workpiece can be pivoted smoothly off the bit without inadvertently rounding the corner. At least a starting pin is necessary for safe pilot-bearing work.

Whether or not to hold the work in a fixture is largely a function of the size of the workpiece. Large workpieces, where the operator has good leverage, may be routed without a fixture. Small pieces almost always need a fixture. A properly designed fixture always makes pilot-bearing work safer and usually delivers better, more consistent quality.

More router bits guide:

Check out how to guide Router Bits Guide Bushing Video with Matt to learn how to make your router even more useful. If you are working on a budget, I find that Yonico router bit is a sensible choice for your workshop with its reasonable price and quality.

How to clean your router bits

How to use your router bits properly

How to sharpen your router bits and Cove and Stick technique with a router table.

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