Twist Drill - 2: Geometry
|WARNING: Machine tools present a safety hazard. Improper operation can result in severe injury. These topics are for non-laboratory study only and are not to be used in conjunction with the operation of any tool or machine described herein. Never use a machine tool without the supervision of a qualified instructor.|
|For a drill to bore true and on location it is absolutely imperative that the margin and the point be ground concentric and be on center. The lips must not only be ground on center but the angles must be consistent on both sides.
Margins are the outside tip of the flutes and are always ground the drill diameter. The land is the portion relieved from the margin. Without this relief there would be a great deal of friction created during the drilling process. Margins hold the drill in check during the drilling process.
|As the lips are being formed in the sharpening process to the desired angle at the very tip the "chisel point," "spiral point" or a "split-point" is formed. The point is the heart of the drill and is the first to start penetration into the material, if the point is ground off angle or center it will cause the drill to start wobbling possibly moving the drill off the desired location and a deformed hole.|
|General purpose drills normally have the chisel point. The big disadvantage here is that this point does not penetrate well at the start and has a tendency to wobble. To eliminate this wobble a center drill (above) is used to start the hole or a drill bushing to guide the drill. However, efficient production methods can eliminate the need for these measures.|
|Although drills with spiral points or split points (right) are more expensive they eliminate the need to use a center drill or a drill-positioning tool. Historically they also have longer tool life than drills with chisel point. Of the two points the split-point drill is the most efficient and also the most costly to manufacture. However machine hours are also costly the efficiency of the split-point maybe the greater savings in the long run.|
|Regrinding of drills at one time was an economical necessity as the cost of drills was comparatively expensive. Nowadays the cost of new drills are so low and the accuracy of grind so perfect that smaller drills are simply thrown away and replaced. Lager drills are more expensive and are usually reground. R.S.|
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