Hand sharpening techniques for twist drills

The top angle of a twist drill is generally 118°, which can also be treated as 120°. The main tips for sharpening drills are as follows.

1. The cutting edge should be level with the grinding wheel surface

Before grinding the drill bit, the main cutting edge of the drill bit should be placed on a level surface with the grinding wheel face, that is, to ensure that the entire edge is ground when the edge touches the grinding wheel face. This is the first step in the relative position of the drill and the grinding wheel, and the position is well placed before slowly leaning towards the grinding wheel surface.

2. The drill axis should be angled at an angle of 60° to the grinding wheel surface

This angle is the sharp angle of the drill. If the angle is not correct at this point, it will directly affect the size of the top angle of the drill and the shape of the main cutting edge and the bevel of the horizontal edge. The angle is the position between the axis of the drill and the surface of the grinding wheel, just take 60°, this angle is generally more accurate. Here we should pay attention to the relative horizontal and angular positions of the drill before sharpening, both of which should be taken into account, so as not to neglect to set a good angle in order to level the cutting edge, or to set a good angle in order to neglect to level the cutting edge.

3. From the edge to the back of the grinding

After the cutting edge touches the grinding wheel, grind from the main cutting edge to the back, that is, from the cutting edge of the drill to the grinding wheel first, and then slowly grind down along the entire back face of the tool. The drill can gently touch the grinding wheel when it is cut in, and first carry out a smaller amount of sharpening, and pay attention to the uniformity of the sparks and adjust the size of the pressure on hand in time, and also pay attention to the cooling of the drill, so as not to let it over-grind and cause discolouration of the edge, and to anneal the edge. If the temperature of the cutting edge is found to be high, the drill should be cooled down in time.

4. The cutting edge of the drill should be swung up and down and the end of the drill should not be cocked

It is a standard drill grinding action that the main cutting edge swings up and down on the grinding wheel, i.e. the hand holding the front of the drill bit swings the bit up and down evenly on the grinding wheel surface. The hand holding the shank, however, must not swing and must prevent the rear shank from being cocked upwards, i.e. the end of the drill must not be cocked high above the horizontal centre line of the grinding wheel, otherwise the edge will be dulled and unable to cut. This is the most critical step, and it has a lot to do with how well the drill is ground. When the drill is almost ground, start from the edge and gently rub it towards the back corner again to make the back of the edge a little more polished.

5. Ensure that the tip of the edge is on the axis and that both sides are resharpened slowly and symmetrically

After grinding one side of the edge, grind the other side of the edge, making sure that the edge is in the middle of the drill's axis and that both edges are symmetrical. Experienced masters will check the symmetry of the drill tip against a bright light and slowly resharpen it. The back angle of the cutting edge of the drill is generally 10°~14°. If the back angle is large, the cutting edge is too thin, the vibration is strong when drilling, the hole is trilateral or pentagonal, and the chips are needle-like; if the back angle is small, the axial force is large when drilling, it is not easy to cut into, the cutting force increases, the temperature rises, the drill heats up seriously, and it is even impossible to drill. If the angle of the back angle is ground to suit, the sharp point is centered and the two edges are symmetrical, when drilling, the drill will discharge chips lightly and quickly without vibration, and the hole diameter will not be enlarged. 

6. After the two edges have been ground, for larger diameter drills, also pay attention to grinding the drill tip

After grinding the two edges of the drill, there will be a flat surface at the tip of the two edges, which will affect the positioning of the centre of the drill, so it is necessary to chamfer the angle behind the edge and grind the flat surface at the tip as small as possible. This is done by holding the drill upright, aligning it with the angle of the grinding wheel and pouring a small slot at the root behind the edge, against the tip of the edge. This is also an important point for centring the drill and cutting lightly. Note that when resharpening the tip chamfer, you must not grind to the main cutting edge, as this will make the front angle of the main cutting edge large and directly affect the drilling.

Of course, there is no certain formula for sharpening drills, so you need to gain experience in actual operation, and by comparing, observing and repeatedly testing, you will definitely sharpen the drill better.