The top angle of the twist drill is usually 118 °， It can also be used as 120 ° Look at it. The grinding bit mainly master the following skills:

## the edge shall be leveled with the grinding wheel surface

Before grinding the bit, the main cutting edge of the bit and the grinding wheel surface shall be placed on a horizontal plane, that is, when the edge contact the grinding wheel surface, the whole edge shall be ground. This is the first step in the position of the bit relative to the grinding wheel. Set the position and then slowly lean on the grinding wheel surface.

## the axis of the drill bit shall be inclined out from the grinding wheel surface by 60 ° Angle of

This angle is the front angle of the bit. The wrong angle will directly affect the size of the top angle of the bit, the shape of the main cutting edge and the angle of the horizontal edge. This refers to the position relationship between the bit axis and the grinding wheel surface, taking 60 ° OK, this angle is generally quite accurate. Here, the relative horizontal position and angle position of the drill edge before grinding should be paid attention to. Both should be considered. Do not neglect the setting angle for the purpose of flattening the edge, or ignore the blade edge for the sake of setting the angle.

## grind back from the edge

After the edge contacts the grinding wheel, it is necessary to grind from the back of the main cutting edge, that is, the edge of the drill bit first contacts the grinding wheel, and then slowly grind down along the whole rear blade surface. When the drill bit cuts in, it can touch the grinding wheel gently, and conduct a small amount of edge grinding first, observe the uniformity of sparks, adjust the pressure on the hand in time, and pay attention to the cooling of the bit, so as not to make it wear too much fire, causing the edge to change color, and then anneal it to the edge. When the temperature of the edge is high, the drill bit shall be cooled in time.

## the edge of the drill bit shall be swinging up and down, and the tail of the bit shall not be warped

This is a standard bit grinding action. The main cutting edge should swing up and down on the grinding wheel, that is, the hand holding the front of the bit should swing the bit up and down evenly on the grinding wheel surface. The hand at the grip can not swing, and the back handle should not be warped upward, that is, the tail of the drill bit cannot be higher than the horizontal center line of the grinding wheel, otherwise the edge will be blunt and cannot be cut. This is the most critical step, the bit is well and bad, and it has a great relationship. When grinding is similar, start with the edge, and gently rub the back corner to make the back of the blade more smooth.

## ensure that the blade tip is opposite to the axis, and the two sides are symmetrically and slowly repaired

After one edge is well polished, then the other edge must be worn. It is necessary to ensure that the edge is in the middle of the drill axis and the edges on both sides should be symmetrical. Experienced masters will look at the symmetry of the drill point in the light and slowly repair and grind. The back angle of the cutting edge of the drill bit is generally 10 °~ fourteen °， The back angle is large, the cutting edge is too thin, and the vibration is strong during drilling. The hole is in the shape of three or five sides, and the chip is needle shaped; The back angle is small, the axial force is very large during drilling, it is not easy to cut in, the cutting force increases, the temperature rises greatly, the bit is hot, and even cannot be drilled. The back angle grinding is suitable. The front point is centered and the two edges are symmetrical. When drilling, the bits are light and fast in chip discharge, without vibration and the hole diameter will not expand.

## after grinding the two edges, pay attention to the bit tip of the bit with larger diameter

After the two edges of the drill bit are well ground, there will be a plane at the tip of the two edges, which will affect the center positioning of the bit. It is necessary to reverse the angle behind the edge to grind the plane of the tip as small as possible. The method is to erect the drill bit, align with the angle of the grinding wheel, and pour a small groove towards the edge tip at the root behind the edge. This is also an important point for the bit to be centered and cut lightly. Note that when grinding the chamfer of the edge tip, it is not allowed to grind on the main cutting edge, which will make the front angle of the main cutting edge larger and directly affect the drilling.

Of course, there is no certain setting of grinding bit, and experience needs to be accumulated in practical operation. Through comparison, observation and repeated test, it is sure to grind the bit better.