First, accurately grasp the "cross out"
Correct marking is the key to correct drilling. Therefore, before drilling, you should be familiar with the mode requirements. Benchmarks for machining workpieces; The flatness of general datum is ≤0.04mm, and the verticality of adjacent datum is ≤ 0.04 mm ... According to the requirements of drilling position and size, draw the cross center line of the hole position with a height ruler, which requires clear and accurate lines. The thinner the line, the higher the accuracy. Because the scribing always has a certain width, the accuracy of scribing can generally reach 0.25 ~ 0.5 mm, so it should be checked with vernier caliper after scribing. If students don't do enough inspection after marking and drill holes with wrongly marked workpieces, the position accuracy of holes can't be guaranteed at all. Therefore, it is necessary to educate students to develop the good habit of checking after drawing lines.
Second, master the "grinding gap" of the drill.
Bit is the fundamental guarantee of drilling quality and accuracy. Choosing a reasonable bit diameter, being familiar with the geometric dimensions of the bit and mastering the manual grinding method of the bit are the basis for students to master the drilling skills.
1. Standard twist drill grinding
To grind an ideal drill with a standard twist drill, it is necessary to correctly understand and judge various geometric parameters on the drill, such as vertex angle, cross-blade bevel angle, rake angle and rake angle. There are many operating essentials for sharpening a knife. Teachers should repeatedly explain and demonstrate when coaching sharpening. On the basis of students' perceptual knowledge and rational knowledge, practice sharpening the main cutting edge, and repeatedly train the visual judgment of specific angles such as120, 60,10 ~14 and 55. According to the shortcomings of the standard twist drill, the drill bit must be sharpened, and the requirements of the sharpened cutting edges are as follows: the top angle of the two main cutting edges is 2 =118 2; The length and height of the two cutting edges are symmetrical; Back angle α =10 ~14; The oblique angle ψ of the cross blade is 50 ~ 55, and the cross sharpening is short. These parameters need to be skillfully imprinted in students' minds to accurately judge the various angles of the bit after grinding.
2. Grinding of standard group drills
The standard group drilling integrates a variety of bit grinding methods, which has the advantages of good centering, fast cutting, high drilling accuracy and low axial force and torque. This kind of drill not only grinds the chisel edge, but also grinds a symmetrical crescent-shaped arc groove on the main cutting edge to form a new chisel edge, which increases the rake angle of the chisel edge and improves the cutting conditions. A turning point is formed at the junction of straight edge and round edge to realize chip separation. When drilling, the straight edge will cut off the strip-shaped chips with large pitch, and the round edge will cut off the curled chips. Chip separation reduces the pulling torque between chips, so group drilling greatly improves drilling efficiency.
3. Grinding of other groups of drills
In order to improve the drilling efficiency and the drilling accuracy requirements of different processing materials, it is necessary to grind the drill bit into different shapes, which is other drilling groups. For the drill used to drill thin plates, we need to grind the outer edges of the cross edge and the two main cutting edges into three tips, and ensure that the tips of the outer edges of the two main cutting edges are about 0.5mm lower than the center drill tip. This kind of drill is suitable for drilling thin plates and can achieve high accuracy. To process holes made of other materials, it is only necessary to grind the drill bit to the specified requirements, and the accuracy requirements can also be achieved.
Third, seriously study drilling "punching".
Before drilling, the cross center line of the hole position should be drawn according to the drawing requirements, and the lines should be clear and accurate (the thinner the lines, the higher the accuracy), and the inspection circle or inspection frame should be drawn as the inspection line of drilling, which is convenient for checking and correcting the drilling position. Then hit the center punch carefully. According to the use requirements of the sample punch, punch a little bit in the center first, and then observe carefully in the vertical and horizontal direction of the center line of the cross to see if the center punch is symmetrical at the intersection of the cross. Finally, punch the center punch straight, round and deep to center the drill accurately. Practical training is very important to improve students' training ability. Students must be educated to be careful and not to worry, which is an important link to ensure the accurate drilling position. If the center hole is punched correctly, the center position of drilling can be correct, and drilling can be successful at one time; If there is any missing drilling, corrective and remedial measures should be taken in time, and drilling can only be carried out after the inspection hole is confirmed to be accurate at the marked position.
In addition, the author also summarized a drilling method without proofing: after marking the alignment line on the workpiece, directly clamp it on the drill chuck of the drilling machine with a tap below M6 or a thimble with high coaxiality, first start the drilling machine and observe whether the center is eccentric. Then stop and aim at the center of the cross to be drilled, then turn on the drilling machine and press it down gently and forcefully. At this time, you will see a very accurate round hole in the center of the cross, and then replace it with a small drill or a central drill, pre-drill, and then use a fine reaming drill to enlarge the hole, which not only ensures the accuracy of the hole, but also ensures the position of the hole.
Fourthly, choose the right bit to ensure "drilling"
When drilling, it should also be done step by step. Don't rush for success. Don't use the last bit to drill holes at one time. First, drill with a drill bit of about 3mm or drill with A2.5 as the center. At this time, the workpiece does not need to be clamped, and it is placed on the workbench of the drilling machine by hand, so that the workpiece is in a state similar to floating clamping. The drill bit can be automatically centered, and the drilling position is more accurate. Then, the workpiece is clamped and reamed to the required size. Rig installation should be smooth, spindle speed should be selected reasonably, and different speeds should be selected according to the diameter of drill bit. Under the condition of not affecting drilling, the exposed part of the drill bit should be as short as possible to reduce the bit jitter and improve the centering accuracy of drilling. If the small hole is not careful, it should be corrected when clamping and reaming, because the disadvantage of twist drill is poor cutting performance at the drill center. If the small hole is not drilled first, it will be difficult to correct the hole deviation because of the unfavorable cutting in the drilling center. However, it is easier to correct it by drilling small holes first, not cutting the center, but cutting the edge.
Five, the rational use of cutting oil to improve the "accuracy"
The selection and use of cutting oil plays an irreplaceable role in bit life and hole machining accuracy, and can help to remove chips. According to relevant research, active sulfur composition has great influence on the roughness of machined surface, and increasing active sulfur composition can improve the roughness of machined surface. To sum up, the cutting oil for deep hole machining should choose active vulcanized chlorinated oil with low viscosity. Because the drill bit is seriously worn when using water-soluble cutting oil, this cutting oil is not used for deep hole machining of steel. Emulsified cutting oil is used for deep hole machining of aluminum alloy and cast iron with good cutting performance.
Reaming technology. Reaming is a process aimed at improving drilling accuracy and surface roughness. When reaming, the blade is used for continuous cutting and polishing. As for the cutting function and calendering function of reamer, it will be influenced by the clamping mode, driving mode, cutting edge shape, cutting conditions and pre-hole state of cutter and workpiece. The requirements of reaming for cutting oil are good surface roughness, high hole accuracy and long reamer life.
Six, to ensure that the hole after processing "repair"
When drilling, although the accuracy of the hole itself is ensured in the operation, there are errors in marking, machine tools, drill bits and so on. Moreover, there are inevitably errors in the position and size of the workpiece, so the drilling accuracy sometimes deviates. In view of this situation, the author suggests that the following methods should be used to repair the hole to improve the dimensional accuracy and geometric accuracy of the hole.
1. Edge filing method
This method is suitable for situations where the accuracy of hole spacing is high and the shape of workpiece can be machined or has a margin.
As shown in figure 1, it is required to process a hexagon with a hole in the center, and the distance to all sides of the hexagon is required to be equal. It is easier to ensure the distance from the center of the hole to each side by drilling and reaming the hole first and then machining the file surface. The processing method is as follows: firstly, mark and punch holes according to the drawing requirements, then drill the bottom hole of 3mm, chamfer the hole with a 7.8mm drill bit, and finally ream the hole with an 8H7 reamer. Then file around. When measuring, insert an 8mm standard test rod into the hole, and measure the distance from the cylinder to each surface with a dial indicator, while ensuring the dimensional and angular accuracy.
2. File hole method
This method is suitable for the case that the shape has been processed or the shape can not be processed again.
As shown in Figure 2, the shape of the workpiece has been processed. To ensure the hole spacing on the pattern, you can draw the center line of the hole and punch a hole in the center, and then drill the bottom hole with a 5mm drill. At this point, first measure the distance from the hole to the edge. If it exceeds the requirements of the pattern, the hole will be filed flat.
As shown in Figure 3, if the distances between the hole center and the datum measured after drilling the bottom hole with a 5mm drill are 14.8mm and 15. 1mm, that is, the hole spacing in the length direction is 0.2mm smaller and the hole spacing in the width direction is 0. 1mm larger. At this time, just file the right side of the hole to 0.4mm, that is, file the transverse diameter of the hole to 5.4mm；; Then file off the bottom of the hole by 0.2 mm, that is, file the longitudinal diameter of the hole to 5.4 mm.
As shown in Figure 4, reaming holes with a 6mm drill bit, and checking the hole spacing. If it meets the requirements, it can be reamed with 7.8mm, and finally reamed with 8H7 reamer. Generally, after filing twice, the hole spacing requirements of the pattern can be accurately guaranteed.
(Author: Changxing Technician College, Huzhou City, Zhejiang Province)