Twist drill is a common drilling tool with simple structure. It is very important to sharpen the drill well for workpiece processing, but it is not easy to sharpen it well. The key is to master the grinding methods and skills. If we master the grinding methods and experience for many times, we can master the grinding scale of the drill.

Procedures of Grinding Twist Drills

The top angle of twist drill is 118 ° in general, and it can also be regarded as 120 °. Grinding drill can master the following six skills, generally there is no problem.

Before grinding the drill, the main cutting edge of the drill and the grinding wheel surface should be placed on a horizontal plane, that is to say, when the cutting edge contacts the grinding wheel surface, the whole edge should be ground. This is the first step of the relative position of the drill bit and the grinding wheel. After the position is set, slowly lean to the grinding wheel surface.

This angle is the front angle of the bit. If the angle is not right, it will directly affect the size of the top angle of the bit, the shape of the main cutting edge and the cross edge angle. Here refers to the position relationship between the drill axis line and the grinding wheel surface, take 60 ° on the line, this angle is generally more accurate. Here, we should pay attention to the relative horizontal position and angular position of the bit before grinding, and both should be taken into account. Do not ignore the setting degree angle in order to level the edge, or ignore the setting edge in order to set the angle.

After the cutting edge contacts the grinding wheel, it should be grinded from the main cutting edge to the back, that is, the cutting edge of the drill first contacts the grinding wheel, and then slowly grinds down along the whole flank. When the drill bit cuts in, it can gently contact the grinding wheel, grind a small amount of edge first, observe the uniformity of sparks, adjust the pressure on the hand in time, and pay attention to the cooling of the drill bit, so as not to over grind the drill bit, causing the edge discoloration and annealing. When the temperature of the cutting edge is high, the bit should be cooled in time.

This is a standard drill grinding action. The main cutting edge should swing up and down on the grinding wheel, that is, the hand holding the front of the drill should swing the drill up and down on the grinding wheel surface evenly. But the hand holding the handle can not swing, and the back handle should be prevented from warping upward, that is, the tail of the drill should not be warped above the horizontal center line of the grinding wheel, otherwise the edge will be blunt and cannot be cut. This is the most critical step. Whether the drill is well ground or not has a lot to do with it. When the grinding is almost finished, start from the edge and gently rub the back corner to make the back of the edge smoother.

After grinding one edge, grind the other edge again. The edge must be in the middle of the drill axis, and the edges on both sides should be symmetrical. The experienced master will observe the symmetry of the drill point in front of the bright light and slowly grind it. The back angle of the cutting edge of the bit is generally 10 ° – 14 ° with large back angle, the cutting edge is too thin, the vibration is severe during drilling, the orifice is trilateral or pentagonal, and the chip is needle shaped; with small back angle, the axial force is very large during drilling, it is difficult to cut in, the cutting force increases, the temperature rises greatly, and the bit heats seriously, or even cannot drill. The back angle grinding is suitable, the front point is in the center, and the two edges are symmetrical. When drilling, the bit can discharge chips quickly, without vibration, and the hole diameter will not expand.

After the two edges are grinded, the bit tip should be grinded for the bit with larger diameter. After the two edges of the drill are grinded, there will be a plane at the tip of the two edges, which will affect the center positioning of the drill. It is necessary to reverse the angle behind the edge to make the plane of the tip as small as possible. The method is to erect the drill bit, align it with the angle of the grinding wheel, and pour a small groove at the root behind the edge to the edge tip. This is also an important point of bit centering and cutting speed. Note that when grinding the chamfer of the edge tip, never grind it to the main cutting edge, which will make the rake angle of the main cutting edge larger and directly affect the drilling.
Precautions for twist drill grinding

Cautions on grinding twist drills

Generally, the grinding wheel with grain size of 46 ~ 80 mesh and hardness of medium soft grade is suitable for grinding bit. The grinding wheel must run smoothly, and the grinding wheel with large runout must be trimmed. In order to smooth grinding the cross edge of bit, the outer corner of the grinding wheel should be polished to a smaller fillet radius. If the fillet radius is too large, the main cutting edge will be damaged when grinding the cross edge.

When the bit is cooled, the pressure exerted by the bit during grinding should not be too large. Generally, air cooling is used. If necessary, it should be dipped in water to prevent overheating annealing and reduce the hardness of the cutting part of the bit.
three The horizontal edge of standard twist drill is long, generally 0.18d (d refers to the diameter of drill bit), and the rake angle at the horizontal edge has a large negative value. Therefore, when drilling, the cutting at the horizontal edge is extrusion state, and the axial resistance is large. At the same time, if the horizontal edge is long, its centring effect and cutting stability are not good. Therefore, for the drill bit with a diameter of more than 5mm, the horizontal edge must be ground short, and the rake angle near the horizontal edge must be increased appropriately, In order to improve the cutting performance of the bit.
The grinding of the cross edge must be completed. The purpose of the cross edge grinding is to shorten the cross edge, but the cross edge cannot be too short. The too short cross edge cannot reduce the feed resistance. In the process of cutting the cross edge, the negative rake angle on both sides of the cross edge should be polished as much as possible. If the rake angle is increased properly, the cutting resistance in the cutting process can be reduced, and the whole drilling process is light.

When the bit is cooled, the pressure exerted by the bit during grinding should not be too large. Generally, air cooling is used. If necessary, it should be dipped in water to prevent overheating annealing and reduce the hardness of the cutting part of the bit.
three The horizontal edge of standard twist drill is long, generally 0.18d (d refers to the diameter of drill bit), and the rake angle at the horizontal edge has a large negative value. Therefore, when drilling, the cutting at the horizontal edge is extrusion state, and the axial resistance is large. At the same time, if the horizontal edge is long, its centring effect and cutting stability are not good. Therefore, for the drill bit with a diameter of more than 5mm, the horizontal edge must be ground short, and the rake angle near the horizontal edge must be increased appropriately, In order to improve the cutting performance of the bit.
The grinding of the cross edge must be completed. The purpose of the cross edge grinding is to shorten the cross edge, but the cross edge cannot be too short. The too short cross edge cannot reduce the feed resistance. In the process of cutting the cross edge, the negative rake angle on both sides of the cross edge should be polished as much as possible. If the rake angle is increased properly, the cutting resistance in the cutting process can be reduced, and the whole drilling process is light.

If the drill is fed manually. The vertex angle can be reduced properly in the grinding process. Because the feed pressure of the electric hand drill is insufficient, reducing the vertex angle properly can increase the positive pressure of the cutting edge on the cutting surface.

If the requirements of the hole diameter and surface roughness are not very strict, the two cutting edges can also be properly ground into incomplete symmetry. Although the hole diameter will increase on the original basis in the drilling process, it can obviously reduce the friction between the bit edge and the hole wall and reduce the cutting force. There is no strict formula for grinding the drill bit, which requires gradually accumulating processing experience in the actual operation process, constantly repeated tests, gradual comparison and observation, so that the drill bit can be grinded well.