Woodworker's Journal 2010-34-2, страница 58
Shop Test continued 82° Screw in a 90" Countersink 82° Screw in a 82° Countersink A fluted bit (left) has one or more straight cutting edges. Cross-hole countersinks have an angled hole through the conical tip; the trailing edge of the hole acts as the cutting edge. countersink is the correct way to go. This is especially true with hardwood, as forcing the screw tighter in an effort to properly seat the screw into the countersink puts a lot of stress on the head — try it with a brass screw, and the head will likely snap off. For hardwood, matching the countersink is always best, l or softwood, however, not nearly as much stress is created when seating an 82° screw in a 90° countersink, and the difference may be unnoticeable. The two most common countersink chamfer angles are 82° and 90°. Since most woodworking screws in North America have heads with an 82° angle, a countersink of that angle is best for a properly seated screw. Team Effort With the bits I've described so far, countersinking is a two-step process: pilot hole first, followed by the countersink. It's effective, hut time-consuming. However, a wide variety of combination countersinks do both tasks at the same time. Going by several names — countersink drills, pilot screw countersinks and piloted countersinks are just a few — these bits wrap a fluted countersink around an adjustable drill bit. (Cross-hole countersinks aren't used for these.) The original idea for these was around years ago with piloted countersinks made of a single piece of flat steel with the ends rolled to form a shank. They looked like arrowheads. There was nothing adjustable about these — you either had the right size or you didn't — and the holes and countersinks they made weren't much to look at, but they did the job quickly and in a single step. They sold by the thousands, and chances are good that anyone who's been woodworking for a few decades has a couple rattling around the bottom of a toolbox somewhere. (I did, as the photo attests.) Today's versions produce better results and are far more versatile. Styles vary, but each works basically the same way. Each countersink is really a combination tool, anil it consists of a separate drill bit centered in a through-hole in the body of the countersink and held in place by either a Shop Test continues on page 64 . Countersink Versus Counterbore Clarified Contrary to what some have suggested, a counterbore is not how the bartender referred to me at that Christmas party I went to a few months ago. Counterboring is a means of setting a fastener below the surface of the workpiece, just like countersinking. Although the terms are sometimes used interchangeably, the main difference lies in the shape of the bottom of the hole. Counterbore Countersink Countersinking creates a conical hole matching the angled shape on the underside of a flathead screw. This conical hole can be shallow, with the screw resting flush with the surface of the workpiece when driven in, or it can be made deep enough that a plug can be installed above the screw once it has been set at the bottom of the cone. Counterboring creates a flat-bottom hole, which allows the head of a screw or bolt with a flat underside to rest solidly in the counterbore, often atop a washer. While the hole above a deeply set flathead screw is usually the same size as the screw head, the hole created for a counterbore is typically larger than the head, which allows room for both the washer as well as the driving tool, such as a socket wrench. 60 April 2010 Woodworkers Jourruil |
