Popular Woodworking 2001-12 № 125, страница 5
sears Letters Why Slow-Speed Mortisers Stall in Tough Woods Don't Forget Chip Removal When Evaluating Machines I just read your article "Mortiser Slug-Fest" in the August 2001 issue (#123). It was a good article, but there is one thing I'd like to comment on. The reason slow speed mortisers tend to stall easier has little to do with horsepower or torque. If it did, then a slow-speed machine would actually be better — because all other things being equal, a 1,700 rpm motor will have twice the torque as a 3,400 rpm motor, and would therefore be more resistant to stalling. The real reason has to do with chip removal. On a mortise chisel and bit, the cutting diameter of the bit is larger than the shaft of the bit. The head of the bit tends to cut off larger chunks of wood than can easily fit between the bit and the chisel at the point where the hole in the chisel is necked down. So these chunks must be chewed up first before they can get up into the chip-removal flutes of the bit. A 1,700 rpm mortiser will only chew up and remove material half as fast as an equivalent 3,400 rpm mortiser. This is fine if the operator plunges half as fast, but as you indicate by doing time measurements in your article, we're all in a hurry and don't like to go slow. You indicated that a smaller gap between bit and chisel works better for the slow mortisers. This is for exactly the same reason. Just as when you close the gap on a hand plane to take a thinner shaving, closing the bit/chisel gap forces the bit to take a smaller bite and the slow mortis-er can keep up with the reduced amount and size of the chips more easily. There is also a second, but less significant, reason for a faster motor to work better. Even if the horsepower is equal, the mechanical energy stored in the rotating motor is four times greater on a 3,400 rpm motor than on a 1,700 rpm motor. This won't help you if you constantly plunge too fast, but it does help when once in a while the bit breaks off a large piece of wood that has to be forced up the flutes. It can offer a lot more energy momentari ly to force that one piece through before the motor slows too much and stalls. This is the same reason why an engine with a larger flywheel is harder to stall than one with a small flywheel. I still like the slower machines because they're quieter and run cooler when running idle. Once you start cutting, there isn't much difference in noise or heat, but I don't like to run them long without wood going through them because the metal-to-metal contact noise is irritating, and can really heat up the bits (I have turned some blue idling, but never while actually cutting through wood). Jim Neeb Chandler, Arizona Thinned Down Polyurethane Finish Really Works Well I was getting ready to finish some cabinets for a customer when I read your Trick of the Trade on finishing in the June 2001 (issue #122). I figured, "What have I got to lose, I can always go over it with a brush later." I diluted Minwax satin polyurethane by 25 percent with mineral spirits as you suggested, and went to work with a cotton rag. As you said, it went on smooth and fast, with no drips or runs. Why have I been cleaning brushes all these years? I was working with shop-grade birch WE WANT TO HEAR FROM YOU Popular Woodworking welcomes letters from readers with questions or comments about the magazine or woodworking in general.We try to respond to all correspondence. Published letters may be edited for length or style. All letters become the property of Popular Woodworking. How to SendYour Letter: • E-mail: Popwood@FWPubs.com • Fax:513-531-0919 • Mail carrier: Letters • Popular Woodworking • 1507 Dana Ave., Cincinnati, OH 45207 8 Popular Woodworking December 2001 |
