There is a lot to be said about router bits but let's focus on performance. In my view, there are 5 figures of merit to consider; finish, cutter life, efficiency, accuracy of grind and dynamic balance (rotational equilibrium).
The finish, in this instance, is the finish imparted to the work. The best of finishes are chatter free without burns or tearout, smooth and need little sanding. Most new cutters produce finishes of this caliber; some better than others.
Many variables influence the quality of the finish. Control of these variables improves cut quality and safety. To rule out these variables as the cause for a less than acceptable finish rout a very light pass (<5% of the profile) and compare the result to the cutting edge of the tool. A perfect grind (say viewed at 5x) should yield an equally good surface on the work. Also compare the cut to one that was routed under (what you think) are adverse conditions.
A new tool whose cuttings are less than desirable will not get better; regrind, return or use the tool only for the first stages of multidepth cuttings.
The second most important area of concern is probably cutter life and economics. We've all heard that the industry standard for woodworking bit material is tungsten carbide and it's supposed to last 6 - 10 times longer than high speed steel. I think the industry has come clean on this one; H.S.S. is simply no match for carbide. However, you might be surprised to learn that even at 10 times the life, carbide deteriorates rapidly.
In fact 200 - 400 feet of edge cutting is all that can be expected of the average bit and half that if the cutter is plowing (cutting on both sides and the bottom of its flutes). That's just 10 or 12 trips around a 4 x 8 sheet of ply. Or, to look at it in a different way, feeding @ 10 - 15 feet per minute, the cutter is dead in less than 45 minutes! Rout MDF or similar materials and wear lines will show up much earlier. There are strategies to extend the life of a cutter but they are unique due to the many variables involved in any one routing process. 2-stage cutting, cutting most of the profile with one bit and finishing with another, is one of the best.
The cutting efficiency is a measure of the cutter grind quality and the flute design. Again other factors apply, but a sharp cutter, well ground, is a safer cutter and requires less power.
On-shear bits (flutes not parallel to the shank) are more energy efficient than the same tool with straight flutes (parallel to the shank). Spiral ground tools are even more efficient but expect to find them only in a straight bit design and at a premium. You won't find a solid carbide spiral round-over bit for example.
For the home hobbyist who may rout only a few hours a month, these differences are of little consequence. Notwithstanding, anything that makes routing easier is to your advantage and I would recommend an on-shear tool when ever it makes economic sense.
In the production environment, flute design and efficiency are critical and determine the $peed of the run.
Just how close does the manufacturer grind the cutter to the specifications stated in the catalog? Most makers produce cutters very close (within a few thousandths) to their stated specs. As a rule, it is of little consequence, but to be sure I rarely buy a cutter and expect it to be right on spec. Rather, my strategy is to design into the routing process, a means to account for variations, especially if the cutter has been or will be reground a time or two.
Decorative cutters like roundovers and ogees don't have to be bulls eye perfection but you should expect any cutter used for joinery to perform without heroics. Dovetails, glue joints, tongue and groove sets, and cope and stickings should all be ground such that boards so routed by them fit well into one another. I have found on too many occasions they don't.
Don't be too hard on yourself if your routed joints don't fit well. Make the simplest of cuts to test for fit before routing a whole run. Technique plays a role, especially with the miter-lock but just as often joinery bits are poorly ground.
How well a cutter spins is important to cut quality and your safety. Cutters longer or greater than 1-5/8" in diameter are more subject to balance anomalies. In spite of the manufacturer's claims of dynamic balancing to more than 25,000 RPM, all too often tools vibrate at slower speeds.
Vibration is your worst router enemy. Clamps loosen, fences move, screws unwind, and chatter on the work is to be expected with vibration and resonance. Aside from avoiding big cutters, your best defense is good fixturing and the variable speed router. For your safety confine big cutter use to 2-1/2+ HP routers and begin their RPM assent at the slowest speed.At the first sign of vibration slow the motor down or shut off the tool. Resonance is so destructive that a tool that vibrates should not be used at any speed. Most router bits should be balanced to 25,000 RPM; they may be dangerous to operate at such a speed but there should be no vibration.
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Last modified: Wed Aug 31 13:20:28 PDT 2005