So the device is a success, though I stress that learning to use it is not something I achieved instant success at. A further test with a sharpened #38 bit and #37 test shank produced the same results. This indicates the TIN coated drills are not awful quality in respect of size they are just poorly ground. Just to be sure, I repeated this test using an expensive #40 drill with the same result. Amazing! It won't fit! This indicates that the sharpened drill has cut very close to size. The big test is to now try inserting the shank of a #40 drill in the hole. Now it cuts and cuts very nicely with chips emerging from both flutes, suggesting both lips are cutting. Next the drill was sharpened (and sharpened-but I'm getting better) and the test repeated. In its native state, that drill just would not cut. First I tried drilling a test hole in the end of a piece of drill rod. I started with one of those miserable TIN coated number drills (#41). The danger is that your results are not symmetric, meaning one lip is longer than the other, meaning that the drill cuts an oversize hole and tends to wander off axis as it drills. My eyes are about as shot as those of your average bloke halfway through his sixth decade and I certainly would not want to try sharpening anything under a #44 without better magnification. We suggest using a 20x jewelers' loupe and I'd have to agree. I used a 4x magnification head band while developing my sharpening skills (or destroying perfectly good drills if you prefer). The correct position is with the lips parallel to the "long" sides of the holder. It should be obvious that the orientation of the lips (or lips to be) of the drill when gripped in the holder is critical. In addition to positioning the drill on the axis of the holder, the collets allow the drill to be held firmly very close to the tip, thus providing the required support against deflection during honing-a vital feature as the drill size decreases. Even a couple of thou off would make it almost impossible to produce symmetrical cutting lips. The important thing about this component is that the collets are drilled concentric with the raw material, and that the hole in the rectangular holder is precisely on its axis. Smaller than that I don't care to go right now. The three collets shown should hold drills down to #60. So each collet will hold a range of drills dependant on the size of the hole in the end and the width and number of slits cut in the collet. As you can see, the collets have a tapered tip which matches a taper in the holder produced by a D-bit with a 30° included angle. This required a larger clamp screw (1BA instead of 2BA) and maintaining the 1/4" square shape over the internally threaded portion at the end of the holder. This allowed me to increase the capacity up to a #38 drill, as opposed to a #42 as drawn. I deviated a little in using 3/16" drill rod stock for the collets (his called for 9/64" drill rod). Here we see the holder, some drills, and some other collets (two of which are yet to be slit). As we'll see, this jig for small drills addresses this problem perfectly, at the expense of a lessening in precision and an increase in the degree of operator proficiency required. The problem in applying this to smaller drills is holding the drill with sufficient support that the drag of the grinding wheel does not deflect it and destroy the intended geometry. But as described in the other article, a perfectly effective tip geometry can be arrived at by grinding the drill tip using four, or even six flat, intersecting planes. The diagram here shows the effect of conical back-off in providing the required relief from cutting edge to heal. Commercial devices exist to apply this to drills in the shop, though they start to become rather inadequate as the size of the drill decreases. As explained in the earlier article, this is conventionally achieved by a method called "conical back-off". Steps for sharpening HSS tool bits.Just like lathe tools, the cutting edge of a twist drill needs relief behind the edge if it is to cut. Tool bit angles are not critical and most tools will cut material satisfactorily, just less effectively.įigure A–2. If this happens, consider starting over again from the beginning. Any discoloration on the bit indicates it may no longer be hardened. Dip the tool in coolant frequently to keep it from overheating and annealing. Next, look up the typical angles for the workpiece material, and then follow the steps in Figure A–2. What is the procedure for sharpening HSS general-purpose lathe tools?īegin by dressing the grinding wheel. Clearance and rake angles in degrees for common metals. What are typical rake and clearance (relief) angles for HSS tool bits?
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