Tip #21
Choosing A Router Bit (continued)
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Carbide-Tipped Bits: the Rolls-Royce of cutters

Carbide and carbide-tipped bits were first used by industry when this extremely hard (80 on the Rockwell “C” Scale) alloy was developed in the late '50's. Widely available and popular with even the average consumer now, these bits are made by brazing extremely hard carbide cutters onto a face of high speed steel. They cost more, but they last longer.

Their ability to take harder-than-average use makes them a good investment, but you should know that carbide-tipped bits, even when brand new, are marginally less sharp than top quality high speed steel bits. This does not affect their overall performance, however, and carbide-tipped bits still outperform high speed steel bits in the long run.

When you buy them, take a good look (with a magnifying glass if you like) at the brazed joint between the carbide alloy and the steel face. Tiny, hairline fault cracks or bubbles there might indicate a poor quality tool. There's no way to forecast disappointment for certain, however, since small gaps might cause no long-range problems at all. Still, to be safe, buy a bit that at least looks well-brazed.

Not all carbide-tipped bits are created equal, either. Some are tagged simply “Carbide”, while others are called “Carbide Production Bits”. The production-type usually have a heavier deposit of carbide brazed onto the cutting edge. Since every sharpening of a bit reduces the diameter of a cutting edge a little, this extra deposit of carbide works to allow more sharpening.

If you're planning to use a router for plywood or particle board joinery or edging, by all means invest in a good carbide-tipped bit. If you're trimming high pressure plastic laminate on a countertop, it's definitely best to use a carbide-tipped trimmer with a ball-bearing pilot on the bit as well.

When these bits finally need to be touched up, alas, you really shouldn't try to do it yourself. You shouldn't, that is, unless you happen to own an abrasive diamond grinding wheel! Those rare and expensive accessories are the only sharpening tool that's hard enough to sharpen carbide well. The good news is that every small town in America seems to have at least one good professional sharpening service. If your local professional lacks the highly specialized tools to sharpen carbide-tipped tools, he probably can recommend someone who does. A qualified tool sharpener can even replace broken or chipped carbide cutting edges.

Since you should send carbide-tipped tools out, the actual sharpening and repair process of these kinds of tools remains something of a mystery for the average woodworker. How can you tell, then, if the local sharpener has done a good job? Held to the light, a well-sharpened edge should not have any shiny spots or small chips. The final test is simply to use it - you'll know from the minute it starts cutting how well it's been treated. As a matter of course, it should cut at least as well as when it was new.

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