Drilling, countersinking, reaming and boring
Drilling, reaming, countersinking and boring are commonly used hole processing methods in mold manufacturing, which are used for hole processing with different precision and surface quality requirements. The rational application of these processes can ensure the dimensional accuracy, shape accuracy and position accuracy of the holes on mold parts, and meet the assembly and use requirements.
Drilling is the first step in creating a hole in a solid material. It is primarily used for holes with low precision requirements (IT11-IT13 grades) or to prepare holes for subsequent processing (reaming and reaming). Drilling is typically performed using a twist drill on a drilling machine or machining center. The drill bit diameter is selected based on the desired hole diameter, and the feed rate and cutting speed are adjusted according to the material. For dowel pin and bolt holes in molds, a 0.5-1mm allowance should be left after drilling to allow for subsequent machining. For example, to drill a φ10mm dowel pin bottom hole, a φ9mm drill bit should be used, with a feed rate of 0.1-0.2mm/r and a cutting speed of 15-25m/min (for steel). The resulting hole diameter tolerance should be ≤±0.1mm, and the surface roughness should be Ra = 12.5-6.3μm. Maintaining cooling and lubrication during drilling is crucial to prevent overheating and wear of the drill bit. For high-strength materials (such as alloy tool steel), high-speed steel or carbide drill bits should be used, and the cutting speed should be reduced.
Hole reaming is the process of enlarging an existing drilled hole using a reaming drill. This process improves dimensional accuracy (IT10-IT11 grade) and surface quality (Ra = 6.3-3.2μm), while reducing cylindricity errors. Reaming drills have more teeth than twist drills (3-4 teeth), resulting in superior rigidity and smooth cutting. The machining allowance is typically 0.5-4mm. Hole reaming is often used for the final machining of medium-precision holes or as a pre-processing step before reaming. For example, a φ9mm drilled hole can be expanded to φ9.8mm in preparation for reaming a φ10H7 hole. During reaming, the feed rate should be 1.5-2 times that of drilling, and the cutting speed should be 0.5-0.7 times that of drilling to ensure machining quality. For guide pin and sleeve holes in molds, the straightness error must be maintained at ≤0.02mm/100mm after reaming to lay the foundation for subsequent reaming.
Countersinking is the process of creating a conical or cylindrical countersink with a countersink drill. It’s used to install fasteners such as bolts and nuts, ensuring the head of the fastener is flush with the part surface. Countersinking can be either conical (commonly 60°, 90°, and 120°) or cylindrical. During processing, the depth and diameter accuracy of the countersink, as well as its coaxiality with the hole, must be ensured. For example, a bolt countersink in the mold upper die holder with a diameter of 16mm and a depth of 8mm can be machined using a 16mm cylindrical countersink drill at a feed rate of 0.1-0.3mm/r and a cutting speed of 10-20m/min. The resulting countersink diameter error is ≤±0.1mm, and the coaxiality error with the bottom hole is ≤0.1mm. The cutting speed must be controlled during countersinking to avoid excessive surface roughness caused by vibration. For brittle materials such as cast iron, the feed rate can be increased appropriately. For steel parts, a cooling lubricant is required.
Reaming is the finishing process of a machined hole using a reamer. It achieves high dimensional accuracy (IT6-IT8 grades) and surface quality (Ra = 1.6-0.4μm). It is a common method for producing high-precision holes in molds, such as those for dowel pins and guide pins. Reamers are divided into hand reamers and machine reamers. Hand reamers are used for manual reaming and offer higher precision, while machine reamers are used on machine tools and offer higher efficiency. The typical reaming allowance is 0.05-0.2mm. Too much allowance can easily cause reamer wear, while too little can prevent errors from the preceding process. For example, ream a φ9.8mm hole to φ10H7 after expansion with a 0.2mm allowance using a machine reamer at a cutting speed of 3-10m/min and a feed rate of 0.2-0.5mm/r. The resulting hole diameter error is ≤±0.01mm, the roundness error is ≤0.005mm, and the surface roughness is Ra = 0.8μm. When reaming, sufficient cooling must be ensured to prevent chips from clogging the reamer chip groove and affecting the processing quality.
The combined application of drilling, reaming, countersinking, and reaming can meet hole processing requirements of varying precision. For high-precision holes (such as IT6), the “drilling → reaming → reaming” process is typically employed; for bolt holes with countersunk heads, the “drilling → reaming → countersinking” process is adopted. For example, when machining mold locating pin holes, the following steps are used: first drill with a φ8.5mm drill, then ream with a φ9.8mm reamer, and finally ream with a φ10H6 reamer to ensure hole accuracy and surface quality. During machining, attention must be paid to tool installation accuracy. The axes of the drill, reamer, and reamer must be perpendicular to the worktable, with an error of ≤0.01mm/100mm, to avoid skewed holes. For mass-produced mold parts, combined tooling or specialized fixtures can be used to improve hole processing efficiency and consistency. For example, a drill-reaming-reaming combination tool can complete all three steps in a single pass, increasing production efficiency by over 50%. With the application of CNC machining technology, the machining center can realize the automated processing of drilling, reaming, countersinking and boring, and ensure the position accuracy of each hole (coordinate error ≤ 0.01mm) through program control, meeting the hole processing needs of complex mold parts.
