Compared to the machining of the outer surface, the hole machining conditions are much worse, and the machining of the holes is more difficult than machining the outer circle. This is because:
1) The size of the tool used for hole machining is limited by the size of the hole to be machined, the rigidity is poor, and bending deformation and vibration are easily generated;
2) When machining holes with a sizing tool, the size of the hole machining often depends directly on the corresponding size of the tool. The manufacturing error and wear of the tool will directly affect the machining accuracy of the hole;
3) When machining the hole, the cutting area is inside the workpiece, the chip removal and heat dissipation conditions are poor, and the machining accuracy and surface quality are difficult to control.
1. Drilling and bearizing
1). Drilling
Drilling is the first step in machining a hole in a solid material. The diameter of the hole is generally less than 80 mm. There are two ways to drill holes: one is the rotation of the drill bit; the other is the rotation of the workpiece. The error caused by the above two drilling methods is different. In the drilling mode of the drill bit rotation, the center line of the machined hole may be skewed due to the asymmetry of the cutting edge and insufficient rigidity of the drill bit. Not straight, but the aperture is basically the same; in the drilling mode of the workpiece rotation, the opposite direction, the drill lead will cause the aperture to change, and the hole centerline is still straight.
Commonly used drilling knives are: twist drills, center drills, deep hole drills, etc. The most commonly used ones are twist drills, the diameter of which is .
Due to the structural constraints, the bending stiffness and torsional stiffness of the drill bit are low, and the centering property is not good. The precision of the drilling process is low, generally only IT13~IT11; the surface roughness is also large, Ra is generally 50. ~12.5μm; but the metal removal rate of the drill hole is large, and the cutting efficiency is high. Drilling is mainly used for processing holes with low quality requirements, such as bolt holes, thread bottom holes, oil holes, etc. Holes with high machining accuracy and surface quality requirements should be achieved by reaming, reaming, boring or grinding in subsequent processing.
2). Bearizing
bearizing is to further drill the hole that has been drilled, cast or forged with a bearizing drill to enlarge the hole diameter and improve the processing quality of the hole. The hole expanding process can be used as a pre-machining before finishing the hole or as a pre-machining hole. Final processing of holes that are not required. The reamer is similar to a twist drill, but with a large number of teeth and no chisel.
Compared with drilling, bearizing has the following characteristics: (1) The number of bearizing drills is large (3~8 teeth), the guiding property is good, and the cutting is relatively stable; (2) The bearizing drill has no chisel edge and the cutting conditions are good; (3) The machining allowance is small, the chip pocket can be made shallower, the core can be made thicker, and the strength and rigidity of the cutter body are better. The precision of the hole expanding process is generally IT11~IT10, and the surface roughness Ra is 12.5~6.3. bearizing is often used to machine holes with a smaller diameter. When drilling a hole with a large diameter (D ≥ 30mm), it is often pre-drilled with a small drill (0.5 to 0.7 times the diameter of the hole), and then the hole is reamed with a correspondingly reamer. Processing quality and production efficiency.
In addition to the cylindrical hole, the bearizing can also be used to process various countersunk holes and flat end faces with various special-shaped bearizing drills (also known as boring drills). The front end of the boring drill often has a guide post that is oriented with the machined hole.
2.Reaming
Reaming is one of the finishing methods of the hole and is widely used in production. For smaller holes, reaming is a more economical and practical method than internal grinding and fine grinding.
1). Reamer
Reamers are generally divided into two types: hand reamer and machine reamer. The reamer of the hand is a straight shank, the working part is long, and the guiding effect is good. The hand reamer has two structures of integral type and outer diameter adjustable type. The machine reamer has two structures with a handle and a sleeve. The reamer can not only machine circular holes, but also taper reamer to machine taper holes.
2). Reaming process and its application
The reaming allowance has a great influence on the quality of the reaming hole. The margin is too large, the load of the reamer is large, the cutting edge is quickly blunt, and it is difficult to obtain a smooth surface. The dimensional tolerance is not easy to guarantee; the margin is too small. It is impossible to remove the knife marks left by the process, and naturally there is no effect of improving the quality of the hole processing. Generally, the coarse hinge allowance is 0.35~0.15mm, and the fine hinge is 01.5~0.05mm.
To avoid built-up edge, the reaming is usually machined at a lower cutting speed (v <8m/min when machining steel and cast iron with high speed steel reamer). The value of the feed amount is related to the hole diameter to be processed. The larger the hole diameter is, the larger the feed value is. The feed rate of steel and cast iron for high speed steel reamer is usually 0.3~1mm/r.
Reaming must be carried out with appropriate cutting fluid for cooling, lubrication and cleaning to prevent build-up of the built-up edge and timely removal of chips. Compared with the grinding hole and the boring hole, the reaming hole has high productivity, and it is easy to ensure the accuracy of the hole; however, the reaming hole cannot correct the position error of the hole axis, and the positional accuracy of the hole should be ensured by the previous process. Reaming holes are not suitable for machining stepped holes and blind holes.
The reaming dimensional accuracy is generally IT9 to IT7, and the surface roughness Ra is generally 3.2 to 0.8. For medium-sized, high-precision holes (such as IT7-level precision holes), the drill-expand-hing process is a typical machining solution commonly used in production.
