In the field of machining process, holes are core foundational features that underpin the functional realization of parts, structural optimization, and process feasibility. Ranging from simple bolt connections and precision bearing assemblies to lightweight structural designs and fluid transmission channels, the design of holes directly determines the assembly accuracy, mechanical properties, and manufacturing costs of parts. Engineers must systematically select the type and structural parameters of holes based on the functional requirements of parts (such as positioning, transmission, and weight reduction), machining processes (such as drilling, boring, and special processing), and assembly scenarios (such as clearance fit and interference fit) to ensure that the design scheme is both reliable and economical.

 

Holes Classified by Function

Through Holes

A through hole is a hole structure that penetrates the entire thickness of a part, with both ends connected to the outside. It is one of the most widely used hole types in mechanical design.

Core Characteristics: The machining process is simple (no need to consider hole bottom treatment), chips and cutting fluid can be discharged naturally, and it is compatible with most conventional machining equipment; it has strong structural symmetry, which can reduce local stress concentration in parts.

Typical Applications: Bolt connections (e.g., fixing of machine tool beds and columns), rivet connections (e.g., splicing of steel structure frames), weight-reduction structures (e.g., lightweight through-hole arrays in aerospace parts), and bearing installations (e.g., through holes in bearing seats of motor end covers).

Machining Methods: Drilling (suitable for small and medium-diameter through holes), stamping (suitable for mass processing of thin-plate parts), milling (suitable for through holes with non-circular cross-sections), and laser cutting (suitable for high-precision through holes in thin materials).

Additional Design Notes: When through holes are used for fluid transmission (e.g., hydraulic oil circuits), chamfers (usually C1-C2) should be provided at the hole orifices to avoid local wear caused by fluid impact; if a through hole penetrates the assembly surfaces of multiple parts, the coaxiality of the holes in each part must be ensured (recommended tolerance ≤ φ0.02mm) to prevent assembly jamming.

 

Blind Holes in Machining Process

A blind hole is a hole with one end closed and the other end open. It can meet local functional requirements while maximizing the retention of the part’s material integrity and reducing structural weakening.

Core Characteristics: It can maintain the local strength of parts (e.g., blind holes for end-face positioning of shaft parts), but chips tend to accumulate at the bottom of the hole during machining, requiring additional chip evacuation structures; there is a risk of stress concentration at the hole bottom, which requires special treatment.

Typical Applications: Bearing seat holes (e.g., blind holes for tapered roller bearings in reducer casings), threaded blind holes (e.g., bolt fixing holes in engine blocks), positioning blind holes (e.g., reference positioning holes for mold cores), and deep-cavity structures (e.g., valve core installation holes in hydraulic valve blocks).

Key Design Points: A fillet (recommended R0.5-R1.5) or conical bottom (usually 120°-150°) must be reserved at the bottom of the blind hole to avoid fatigue cracking of the part caused by stress concentration at the right-angle bottom; the depth of the blind hole should not exceed 5 times its diameter (depth-to-diameter ratio ≤ 5). If a deep blind hole is required (e.g., depth-to-diameter ratio > 10), special equipment such as gun drills should be used, and chip evacuation grooves should be reserved on the hole wall.

 

Threaded Holes

A threaded hole is a functional hole with internal threads machined on its inner wall. It achieves detachable fastening through thread fitting with screws or bolts, and is widely used in scenarios requiring frequent disassembly or with limited space.

Core Characteristics: No additional fastening accessories (e.g., nuts) are needed, saving assembly space; the thread fitting accuracy directly affects connection reliability, and thread tolerances should be selected based on load levels (e.g., 6H for ordinary connections, 5H for precision connections).

Typical Applications: Threaded holes for fixing cylinder heads in engine blocks, threaded holes for guide rail pressure plates in machine tool beds, lightweight fastening holes in connecting plates, and panel fixing holes in instrument housings.

Machining Processes: For ordinary threaded holes (e.g., M3-M20), the process of “drilling pilot holes → tapping” is adopted (the diameter of the pilot hole must be calculated according to the thread specification; for example, the pilot hole diameter for M10 threads is 8.5mm); for precision threaded holes (e.g., threads for aerospace applications), a multi-step process of “drilling pilot holes → reaming → boring → tapping” is used to ensure thread accuracy; for large-diameter threaded holes (e.g., M30 and above), the rolling forming process can be adopted to improve thread strength (20%-30% higher than that of cut threads).

Additional Notes: A “tool withdrawal groove” or “empty tool section” (length ≥ 1.5 times the thread pitch) must be reserved at the bottom of the threaded hole to avoid damage to the bottom of the tap during tapping; if the threaded hole is used in high-temperature or vibration environments (e.g., automobile exhaust pipes), wear-resistant coatings (e.g., nitriding treatment) should be applied to extend the service life of the threads.

 

Bolt Holes

Bolt holes are holes specially designed for bolt insertion. They are divided into ordinary bolt holes and fitting bolt holes according to assembly accuracy requirements, and are one of the most basic hole types in mechanical connections.

Classification and Characteristics:

Ordinary bolt holes: The hole diameter is 0.5-2mm larger than the nominal diameter of the bolt (e.g., an M16 bolt is matched with a φ17-φ18 hole). A certain degree of positional deviation is allowed during assembly, making them suitable for connections without strict positioning requirements (e.g., frame splicing).

Fitting bolt holes: The clearance between the hole diameter and the nominal diameter of the bolt is ≤ 0.1mm (e.g., an M16 bolt is matched with a φ16H8 hole). High dimensional accuracy and geometric accuracy must be ensured, making them suitable for scenarios requiring strict positioning (e.g., connection between machine tool spindles and flanges).

Typical Applications: Connection holes for machine tool guide rails and sliders, connection holes for beams and columns in steel structure workshops, guide pillar fixing holes in mold templates, and connection holes for suspension brackets in automobile chassis.

Key Design Notes: When multiple bolt holes form a “hole group”, the hole pitch tolerance (recommended ±0.1mm) and coaxiality (recommended ≤ φ0.05mm) must be ensured to avoid uneven stress on the bolts; if the bolt holes are used in outdoor or humid environments, waterproof chamfers (e.g., 15°-30°) should be provided at the hole orifices to prevent corrosion caused by rainwater accumulation.

 

Pin Holes

Pin holes are high-precision holes used for installing positioning pins (e.g., cylindrical pins, taper pins) or force-transmitting pins (e.g., split pins, pin shafts). They mainly realize positioning between parts or transmission of shear force, and are usually designed in pairs or groups.

Core Characteristics: High-precision fitting is required (common tolerance grades H7/g6 or H7/h6). The dimensional accuracy, roundness, and coaxiality of the holes directly affect positioning accuracy; force-transmitting pin holes must also have high shear strength, and the surface roughness of the hole walls must be controlled (recommended Ra ≤ 0.8μm).

Typical Applications: Positioning pin holes for gears and shafts (to ensure gear transmission accuracy), reference pin holes for fixtures and workpieces (to ensure machining positioning accuracy), pin holes for connecting rods and crankshafts (to transmit shear force of reciprocating motion), and positioning pin holes for valve cores (to control valve opening and closing accuracy).

Machining Processes: The conventional process is “drilling → reaming → boring”. For boring, high-precision reamers (e.g., cemented carbide reamers) must be used to ensure the dimensional tolerance and surface quality of the holes; for long pin holes with a depth-to-diameter ratio > 8, the “floating reamer” or “honing” process should be adopted to avoid excessive taper of the holes; if the pin holes are used in high-temperature environments (e.g., turbochargers), high-temperature alloy materials should be used, and aging treatment should be performed to prevent hole deformation.

 

Holes Classified by Structural Characteristics

Cylindrical Holes

A cylindrical hole is a straight hole with a circular cross-section. It is the most basic and commonly used hole structure in mechanical design, and is suitable for the assembly requirements of most shaft and sleeve parts.

Core Advantages: The machining process is mature (achievable by drilling, reaming, boring, and honing), the dimensional accuracy is controllable (ranging from IT12 to IT5), and it is compatible with standardized tools and measuring instruments; it has a symmetrical structure and uniform stress, which can reduce local stress concentration in parts.

Limitations: When the hole diameter is too large (e.g., > φ200mm) and the part thickness is small, the effective cross-sectional area of the part will be significantly reduced. Additional measures such as increasing the hole wall thickness or setting reinforcing ribs are required to make up for the strength loss; for slender cylindrical holes (depth-to-diameter ratio > 10), “tool deflection” is likely to occur during machining, leading to excessive straightness deviation of the holes.

Typical Applications: Installation holes for bearing inner rings (e.g., bearing holes in motor rotors), fitting holes for sleeves (e.g., sleeve holes in reducer casings), bolt insertion holes (e.g., through holes in flange connections), and sensor installation holes (e.g., fixing holes for pressure sensors).

Additional Design Notes: When cylindrical holes are used for precision fitting (e.g., spindle bearing holes), the roundness (≤ 0.005mm) and cylindricity (≤ 0.01mm) of the holes must be controlled to avoid vibration caused by uneven fitting clearance between the shaft and the hole; if the hole wall needs to bear large axial forces (e.g., hydraulic cylinder holes), surface hardening treatment (e.g., quenching + grinding, hardness ≥ HRC58) should be performed on the hole wall to improve wear resistance and fatigue strength.

 

Step Holes

A step hole is composed of two or more coaxial cylindrical holes with different diameters. It realizes “assembly positioning + functional partitioning” through hole sections of different diameters, and is commonly used in scenarios requiring multi-layer fitting or countersunk structures.

Core Characteristics: It can reduce the machining depth (e.g., only machining a large-diameter hole section locally on the part), lowering machining costs; different hole sections can realize different functions respectively (e.g., the small-diameter section is used for shaft fitting, and the large-diameter section is used for bearing installation), improving the integration of the part.

Typical Applications: Step holes for bearing seats (e.g., bearing holes in motor end covers, where the small-diameter section is used for shaft fitting and the large-diameter section is used for positioning the bearing outer ring), countersunk step holes for screws (e.g., screw installation holes in housings, where the large-diameter section is used for embedding the screw head and the small-diameter section is used for thread fitting), and step holes in hydraulic valve blocks (where different hole sections are used for installing valve cores, sealing rings, and joints respectively).

Key Machining Notes: The coaxiality of each hole section must be ensured (recommended ≤ φ0.02mm) to avoid radial runout at the step; the step surface must be perpendicular to the hole axis (perpendicularity ≤ 0.01mm) to prevent poor end-face fitting during part assembly; during machining, the “large-hole-first, small-hole-later” sequence should be followed to avoid chip accumulation in the large-hole section during small-hole machining.

 

Tapered Holes

A tapered hole has a diameter that changes linearly along the axis (the taper is usually 1:50, 1:10, or Morse taper). It leverages the self-locking property and coaxial positioning characteristics of the conical surface, and is widely used in scenarios requiring high-precision positioning or quick assembly and disassembly.

Core Advantages: It has excellent coaxial positioning accuracy (conical surface fitting clearance ≤ 0.003mm), enabling automatic centering and avoiding radial deviation; the conical surface has a large contact area and uniform stress, allowing it to transmit large torques (e.g., connection between the spindle and the tool); assembly and disassembly are convenient, with no need for additional positioning accessories (e.g., positioning pins).

Limitations: Machining is difficult, requiring special tools (e.g., taper drills, taper reamers) or equipment (e.g., taper cutting function of CNC lathes); once the conical surface is worn, it is difficult to repair, and the part must be replaced entirely or the conical surface must be reground.

Typical Applications: Morse taper holes (e.g., tool installation holes in machine tool spindles, commonly using Morse tapers No. 4 and No. 5), center taper holes (e.g., center positioning holes in lathe tailstocks), taper holes for oil nozzles (e.g., oil nozzle holes in fuel injection systems, with a taper of 1:20), and positioning taper holes in precision fixtures (e.g., center taper holes in three-jaw chucks).

Additional Design Notes: The taper of the tapered hole should be selected based on the application scenario (e.g., a large taper of 1:10 is selected for taper holes transmitting torque, and a small taper of 1:50 is selected for taper holes requiring high positioning accuracy); a chamfer (e.g., C1) should be provided at the large end of the tapered hole to avoid scratching the conical surface during assembly and disassembly; if the tapered hole is used for high-pressure sealing (e.g., hydraulic joints), a sealing groove should be machined on the conical surface, and an O-ring or copper gasket should be installed to improve sealing performance.

 

Elliptical Holes and Slotted Holes

Elliptical holes and slotted holes have cross-sections that are elliptical or “rectangle + semicircle” respectively. Their core feature is “axial adjustment allowance”, making them suitable for scenarios where the assembly position of parts needs fine adjustment or where thermal expansion and contraction exist.

Core Advantages: During assembly, the part is allowed to have a positional deviation of ±0.5-5mm along the major axis direction, reducing the machining accuracy requirements for the hole group; they can compensate for thermal deformation of parts caused by temperature changes (e.g., slotted holes in engine blocks can avoid part cracking due to thermal expansion and contraction).

Limitations: Machining is complex, requiring machining processes such as milling (e.g., milling slotted holes with an end mill), wire cutting (e.g., cutting elliptical holes with slow-feeding wire EDM), or stamping (only suitable for thin plates), resulting in lower machining efficiency than cylindrical holes; stress concentration is likely to occur at the two ends of the major axis of the hole, which can be alleviated by increasing the fillet radius (recommended R ≥ 1/10 of the major axis length).

Typical Applications: Slotted holes for connecting machine frames and guide rails (allowing axial fine adjustment of the guide rails), elliptical holes for suspension brackets in automobile chassis (compensating for vibration displacement during driving), slotted holes in tube sheets of heat exchangers (compensating for thermal expansion and contraction of tubes), and slotted holes for guide pillars in molds (allowing slight displacement during mold opening and closing).

Key Design Points: The length of the major axis of a slotted hole should be determined based on adjustment requirements (usually 2-10mm larger than the bolt diameter), and the diameter of the minor axis should match the bolt diameter (clearance ≤ 0.2mm); the ratio of the major axis to the minor axis of an elliptical hole should not exceed 3:1 to avoid insufficient strength of the hole wall; if used in load-bearing scenarios (e.g., connecting parts of cranes), the hole wall should be reinforced (e.g., welding a reinforcing sleeve) to improve shear resistance.

 

Countersunk Holes

A countersunk hole is a hole structure with a chamfer, conical surface, or cylindrical groove machined at the orifice. Its core function is to “embed the fastener head” (e.g., screw, bolt) so that the head is flush with or below the part surface, avoiding protrusions that affect assembly or appearance.

Common Types:

Cylindrical countersunk holes: The orifice is a cylindrical groove, suitable for cylindrical head screws (e.g., hexagon socket cylindrical head screws). The diameter of the groove is 0.5-1mm larger than the screw head, and the depth is consistent with the thickness of the screw head.

Conical countersunk holes: The orifice is a conical surface (common cone angles are 90° or 120°), suitable for countersunk head screws (e.g., cross-recessed countersunk head screws). The conical surface must fit with the conical surface of the screw head to ensure that the head is completely embedded.

Half-countersunk holes: The orifice is a combined structure of “cylinder + cone”, suitable for half-countersunk head screws. It balances the embedding function and the head strength (30% higher strength than fully countersunk screw heads).

Typical Applications: Screw installation holes in mechanical housings (e.g., countersunk holes in instrument panels to ensure a flat surface), connection holes in mold templates (e.g., countersunk holes in stamping mold templates to avoid interference of screw heads with mold movement), and fixing holes in automobile interiors (e.g., countersunk holes in instrument panels to improve appearance quality).

Machining and Design Notes: The cone angle of a conical countersunk hole must match the cone angle of the screw head (e.g., the cone angle of national standard countersunk head screws is 90°, so a 90° conical hole should be machined), with a deviation ≤ ±2°; otherwise, the head cannot fit completely. The depth of the countersunk hole must be strictly controlled (tolerance ±0.1mm); excessive depth will weaken the part strength, while insufficient depth will cause the screw head to protrude. Special countersunk tools (e.g., countersunk drills, spot drills) should be used during machining to ensure the flatness of the countersunk surface (≤ 0.02mm).

 

Oil Holes and Cooling Holes

Oil holes and cooling holes are functional holes specially designed for fluid (lubricating oil, cooling fluid, hydraulic oil) transmission. They are usually small through holes or complex channels, and directly affect the lubrication, heat dissipation, or hydraulic transmission performance of parts.

Core Characteristics: The hole diameter is usually small (φ1-φ10mm), and the depth-to-diameter ratio of some deep holes can reach more than 50; the path of the hole may be straight, folded, or curved (e.g., intersecting oil holes in hydraulic valve blocks), and unobstructed fluid flow with no dead zones must be ensured; the hole wall must be smooth (surface roughness Ra ≤ 1.6μm) to avoid impurity accumulation and blockage of the channel.

Typical Applications:

Oil holes: Lubricating oil holes in shaft parts (e.g., oil holes in the main journal of crankshafts to provide lubricating oil for bearings), oil circuit holes in gearboxes (e.g., lubricating oil holes for gears in reducers), and lubricating oil holes in machine tool guide rails (reducing guide rail wear).

Cooling holes: Cooling water circuit holes in molds (e.g., cooling holes in injection molds to control the molding temperature of plastic parts), cooling water holes in engine blocks (removing heat from the cylinder block), and cooling holes in nozzles of laser cutting machines (preventing nozzle damage due to overheating).

Machining Processes: Conventional small straight holes are processed by drilling (e.g., high-speed steel drills) or gun drilling (for deep holes with a depth-to-diameter ratio > 20); complex folded holes are processed by “multi-axis drilling” (e.g., CNC 5-axis machines) to realize intersecting holes through multiple drilling operations; curved holes or micro-holes (φ0.1-φ1mm) are processed by laser drilling (high precision, no mechanical stress) or ultrasonic machining (suitable for hard and brittle materials such as ceramics); after hole machining, “deburring” treatment (e.g., electrochemical deburring) should be performed to avoid scratches on seals or blockage of channels by burrs at the hole orifices.

Additional Design Notes: The inlet of an oil hole should be provided with an oil nozzle installation hole (e.g., an M6×1 threaded hole), and the coaxiality between the oil nozzle and the oil hole must be ensured; the path of cooling holes should be evenly distributed (e.g., the spacing between cooling holes in molds ≤ 50mm) to avoid local overheating; “transition fillets” (R ≥ 0.5mm) should be provided at the intersections of intersecting oil holes to reduce fluid resistance and local cavitation.

 

Holes Classified by Machining Process

Drilled Holes

Drilling is an initial machining process for machining holes in parts using a drill bit. It is the most basic and economical hole-machining method, suitable for most materials (metals, plastics, wood, etc.).

Application Range: The hole diameter is usually φ0.5-φ100mm, and it is suitable for through holes or blind holes with a depth-to-diameter ratio ≤ 5; the accuracy requirement is low (dimensional tolerance IT12-IT10), and the surface roughness is Ra12.5-6.3μm. Subsequent processing (e.g., reaming, boring) is required to improve accuracy.

Common Equipment and Tools: Equipment includes bench drills (for small parts), vertical drills (for medium-sized parts), radial drills (for large parts), and CNC drilling machines (for mass machining process); tools are mainly high-speed steel drills (suitable for low-carbon steel, aluminum alloys) and cemented carbide drills (suitable for stainless steel, high-strength steel). For special materials (e.g., titanium alloys), coated drills (e.g., TiAlN-coated drills) should be used.

Machining Process Limitations and Solutions: “Excessive hole diameter” is likely to occur during drilling (caused by drill bit wear or vibration), so drill bits should be replaced regularly and cutting parameters (e.g., rotational speed, feed rate) should be controlled; “hole bottom deviation” is likely to occur during blind hole drilling, so a “center drill” should be used to machine a positioning hole first, followed by drilling with a drill bit; “hole orifice burrs” are likely to occur during thin-plate drilling, so a wooden board should be placed under the part or a machining process should be adopted.

 

Reamed Holes and Bored Holes

Both reaming and boring are machining processes. Their core purpose is to correct the dimensional deviation of drilled holes and improve surface quality, and they are key steps in machining high-precision holes (e.g., pin holes, bearing holes).

Reamed Holes

Définition: Reaming is a secondary machining process for drilled holes using a reamer to enlarge the hole diameter and correct the roundness and straightness of the hole.

Application Range: The hole diameter is usually 1-3mm larger than that of the drilled hole, the accuracy is improved to IT10-IT9, and the surface roughness is Ra6.3-3.2μm; it is commonly used as pre-processing before boring or as the final machining process for holes with low accuracy requirements (e.g., ordinary bolt holes).

Outils: Reamers have more teeth than drill bits (3-4 teeth), ensuring stable cutting and reducing vibration deviation of the hole; they are mostly made of high-speed steel or cemented carbide, suitable for parts of different materials.

Bored Holes

Définition: Boring is a machining process for holes using a boring tool to machine holes that have been drilled, reamed, or cast. It is one of the main processes for obtaining high-precision holes.

Application Range: The accuracy can reach IT7-IT6 (precision boring can reach IT5), and the surface roughness is Ra3.2-0.8μm; it is suitable for cylindrical holes and tapered holes (e.g., Morse taper holes), with a hole diameter usually φ1-φ80mm and a depth-to-diameter ratio ≤ 10.

Outils and Machining Process: Boring tools are divided into hand-held boring outils (low-speed manual operation, high accuracy) and machine-mounted boring tools (high-speed machine operation, high efficiency); the material is selected based on the part material (e.g., high-speed steel boring tools for cast iron, cemented carbide boring tools for quenched steel); cutting fluid (e.g., emulsion, cutting oil) should be used during boring to reduce tool wear and improve surface quality.

Notes: Boring tools must not be reversed, otherwise the tool edge will be chipped; the boring allowance should be controlled at 0.1-0.2mm; excessive allowance will easily cause the boring tool to overheat, while insufficient allowance will make it impossible to correct the hole deviation.

 

Precision Boring holes

Precision boring is a machining process for holes using a boring tool to machine holes that have been drilled, reamed, or cast, through tool rotation or part rotation. It is suitable for large-diameter holes (φ50mm and above), deep holes (depth-to-diameter ratio > 10), or high-precision holes (e.g., spindle holes, cylinder holes).

Core Advantages: It can machine holes of various sizes (from φ10mm to more than φ1000mm) with high accuracy (dimensional tolerance IT7-IT5, geometric accuracy ≤ 0.01mm); it can correct the coaxiality, roundness, and cylindricity of holes, and is especially suitable for machining multiple holes in box-type parts (e.g., multiple bearing holes in reducer casings, ensuring coaxiality); it can machine complex structures such as internal threads and internal grooves, improving the integration of parts.

Common Equipment and Tools: Equipment includes horizontal boring machines (for box-type parts), vertical boring machines (for large flanges), CNC boring and milling machines (for mass high-precision machining process), and deep-hole boring machines (for deep holes with a depth-to-diameter ratio > 20); tools are mainly cemented carbide boring tools (for conventional materials), cubic boron nitride (CBN) boring tools (for quenched steel, cast iron), and diamond boring tools (for non-ferrous metals such as aluminum and copper). The hole diameter can be precisely controlled by “fine-tuning boring tools”.

Typical Applications: Spindle holes in machine tools (e.g., through holes in lathe spindles, accuracy IT5), cylinder holes in engines (e.g., cylinder holes in diesel engine blocks, roundness ≤ 0.005mm), bearing holes in large gearboxes (e.g., holes in wind power gearboxes), and cylinder barrel holes in hydraulic cylinders (surface roughness Ra0.8μm).

 

Ground Holes and Honed Holes

Both grinding and honing are machining processes, used to further improve the accuracy and surface quality of holes. They are suitable for scenarios requiring extremely high accuracy (e.g., below IT5) or extremely low surface roughness (Ra ≤ 0.4μm).

Ground Holes

Définition: Hole grinding is a machining process for holes using a grinding wheel (e.g., corundum grinding wheels, silicon carbide grinding wheels). It removes a small amount of material through high-speed rotation of the grinding wheel to correct the shape deviation of the hole.

Application Range: The accuracy can reach IT6-IT5, and the surface roughness is Ra1.6-0.4μm; it is suitable for holes in quenched parts (hardness > HRC45) (e.g., holes in bearing rings), holes in high-precision sleeves (e.g., holes in hydraulic valve sleeves); it can machine holes with a depth-to-diameter ratio ≤ 15, but the efficiency is lower than that of honing.

Equipment and Processes: The equipment is internal grinding machines (for conventional hole grinding) and deep-hole grinding machines (for deep-hole grinding); during grinding, the rotational speed of the grinding wheel (usually 1000-3000r/min) and feed rate (0.001-0.005mm/r) should be controlled, and cooling and lubricating fluid (e.g., extreme-pressure emulsion) should be used to prevent grinding wheel clogging and part burning.

Honed Holes

Définition: Honing is a machining process for holes using a honing head (equipped with honing stones). The honing head performs rotational and reciprocating movements inside the hole, and a small amount of cutting is performed by the honing stones to achieve superfinishing of the hole.

Core Advantages: It has extremely high accuracy (dimensional tolerance IT5-IT4, roundness ≤ 0.002mm) and the surface roughness can reach Ra0.4-0.025μm; it can correct the cylindricity and taper of the hole, and is especially suitable for machining deep holes (depth-to-diameter ratio > 20); after honing, a uniform cross-hatch pattern is formed on the hole wall, which is conducive to the storage of lubricating oil and improves the wear resistance of parts.

Application Range: Cylinder barrel holes in hydraulic cylinders (e.g., holes in high-pressure hydraulic cylinders), spindle holes in precision machine tools (e.g., spindle holes in machining centers), and oil holes in engine crankshafts (high-precision oil circuit holes).

Machining Process Notes: The honing allowance should be controlled at 0.02-0.05mm; excessive allowance will easily cause rapid wear of the honing stones; special honing oil (e.g., mineral oil + extreme-pressure additives) should be used as the honing fluid to improve cutting efficiency and surface quality.

 

Conclusion

As the most basic and core structural feature in mechanical design, the type selection and design quality of holes directly determine the functional realization, performance reliability, and manufacturing cost of parts. From through holes, threaded holes, and pin holes (classified by function) to step holes, tapered holes, and countersunk holes (classified by structure), and to drilled holes, bored holes, and honed holes (classified by machining process), different types of holes have their own applicable scenarios. The selection must be based on the functional requirements of parts (positioning, connection, transmission), assembly requirements (accuracy, clearance), and machining capabilities (equipment, tools) for comprehensive consideration.

When designing holes, engineers must balance accuracy and strength (controlling tolerances and stress concentration), processability and economy (simplifying structures and unifying specifications), and assembly and maintenance convenience (reserving clearances and maintenance space). Through systematic optimization, the design goal of “meeting functional requirements, reliable performance, and reasonable cost” can be achieved. With the development of machining technology (e.g., laser drilling, 3D printing holes), the design of holes will also move towards a direction of “higher precision, greater complexity, and higher integration”. It is necessary to continuously update design concepts by integrating new technologies and machining process to meet the needs of high-end equipment manufacturing.