Operating Skills of Twist Drilling for Fine Hole Processing

When processing some high-precision holes on mechanical parts, twist drills can be used to ream holes instead of reaming holes. Especially when processing non-standard precision holes, it is more convenient to operate and can adapt to various metal materials. Processing. Twist drill reaming is a kind of finishing hole processing by re-ground drills on the basis of the existing holes, and then reaming. It is called fine hole drilling for short. The accuracy is up to H7, and the surface roughness is up to Ra0.8. Generally, the accuracy of the drilled holes is low due to the operator's inability to operate. After years of practice, the author believes that it can be controlled from the following aspects. The precision of drilling fine holes will be guaranteed.

　　1. Correctly sharpen the cutting angle and improve the surface roughness:

The 　　 drill bit is similar to the reamer. If there are two main cutting edges and secondary cutting edges, the secondary cutting edges are spiral, with land and chamfers, which can increase the smoothness of cutting. As long as the cutting angle and surface roughness of the drill bit are improved and improved with reference to the requirements of the reaming edge. Basically, it has the conditions similar to the reaming blade, so it can drill holes with higher precision. Grind the second apex angle (2 2). According to the different processing materials? 2≤75°. Example: When processing cast iron, 2?2u003d75°, when processing steel, 2?2u003d50°. The length of the new cutting edge is about 3~4mm, and the connection between it and the secondary cutting edge is ground with a small fillet of 0.2~0.5mm with a whetstone, and all the sharp corners of the outer edge can be ground into a circular arc edge. Special attention: The two new cutting edges should be symmetrical, so that a combined rough and finishing cutting edge can be formed, reducing the cutting thickness and cutting deformation, improving the smoothing ability, improving the heat dissipation conditions, and improving the surface roughness of the hole. Grind out the auxiliary rear angle. On the edge near the main cutting edge, grind the secondary relief angle a01u003d6°～8°, and keep the edge width of 0.1～0.2mm, and the grinding length of 4～5mm to reduce the friction on the hole wall. , Improve the life of the drill bit. Grind the negative blade inclination. Generally, the blade inclination angle lu003d-10°～-15°, so that the chips flow to the surface to be added to avoid scratching the hole wall, which is beneficial to improve the surface roughness of the hole. The clearance angle should not be too large, generally au003d6°～-10°, so as to avoid vibration, increase stability, and improve the accuracy of the hole. The front and flank surfaces of the cutting edge are ground with oilstone to make the roughness reach Ra0.4.

　　2. Determine the reasonable cutting parameters. Among the cutting parameters, the cutting depth and feed rate have a greater impact on the accuracy of the hole. The cutting depth and cutting speed have a greater impact on the service life of the drill. For this reason, the depth of cut, cutting speed, and feed should be controlled separately. Cutting depth: It is not affected by the diameter of the machined hole, and the cutting depth of 0.5-1mm is reserved before drilling the fine hole, and the roughness is not greater than Ra6.3 to avoid large cutting volume, reduce heat, avoid impact and vibration, and eliminate Cold work hardening improves the processing quality and prolongs the service life of the drill bit. Cutting speed: When drilling cast iron, the cutting speed vu003d20m/min. When drilling steel materials, the cutting speed vu003d10m/min. Converted to the machine speed, it can be determined according to the diameter of the drilled hole and the cutting speed of the corresponding material. Nu003d1000V/(pD). Feed rate: try to use motorized feed fu003d0.1mm/r or so. However, small drills with a diameter of less than 5mm can only be fed manually due to their weak strength. valve

　　3. Cooling lubricating fluid: Because the precision and surface roughness of drilling precision holes are high, and the cutting load is small, the cooling lubricating fluid should be mainly lubricated. Use 10%-20% emulsified oil aqueous solution.

　　4. Other requirements:

　　 Choose a drilling machine with higher precision. If the radial circle runout error of the drilling machine spindle is large, the floating chuck can be used to clamp the drill bit for processing. Use newer drill bits or those whose dimensional accuracy is close to tolerance requirements. The drill bit will wear out after repeated use, which affects the accuracy of the hole diameter. The two cutting edges of the drill bit should be ground as symmetrically as possible. The axial runout difference of the two edges should be controlled within 0.05mm, so that the load of the two edges is uniform to improve cutting stability. The radial runout difference of the bit should be less than 0.03mm. Pre-drilling should prevent the generation of more chilled layers, otherwise it will increase the drilling load and wear the fine hole drill. There must be sufficient cooling and lubricating fluid during drilling.