Mechanical Part Hole Machining Methods: Conventional & Special Hole Making Processes Guide

Holes in mechanical parts typically serve critical functions such as positioning, connection, power transmission, or lubrication, ensuring product quality and assembly accuracy.

The dimensional accuracy, surface roughness, and positional tolerances (such as perpendicularity and coaxiality) achievable by different machining methods vary greatly.

Learning hole-machining methods is, at its core, about developing a process-oriented mindset.

We can comprehensively consider multiple factors when meeting a hole-machining requirement.

These factors include material, precision, production volume, cost and available equipment.

This capability helps us make the most scientific and reasonable decisions.

For professionals in design, manufacturing, or quality control, this serves as an indispensable professional foundation.

Conventional Machining Methods

• Turning

Turning, also known as boring, is the process of using a lathe to enlarge existing holes in a part—such as cast, forged, or drilled holes—or to machine the inner surfaces of hollow parts.

It is a commonly used method in hole machining and can serve as both roughing and finishing operations.

The most important function of turning is to improve the dimensional accuracy and surface quality of the hole, as well as to correct its straightness or positional accuracy.

For example, while drilled holes typically have lower precision (below IT10 grade), boring can achieve a precision of IT7 to IT8 grade and a surface roughness of Ra 1.6 to 3.2 μm, or even higher.

In addition, boring ensures a high degree of coaxiality between the machined hole and the workpiece’s outer cylindrical surface.

The method for boring is essentially the same as that for turning an external circle (the workpiece rotates, and the tool moves), except that the directions of feed and retract are reversed.

The specific machining steps are as follows:

1. Tool Installation

The tool tip should be at the same height as or slightly higher than the center of the workpiece;

The shank should not protrude too far from the tool holder (generally 5–10 mm longer than the hole being machined), and the shank should be essentially parallel to the workpiece’s axis.

2. Test Cutting Method

Test cutting is required for both rough and finish turning.

The transverse feed rate should be 1/2 of the radial allowance;

After cutting approximately 2 mm in the longitudinal direction, retract the tool rapidly (while keeping the transverse feed constant), then stop the machine to measure the dimensions.

If the dimensions are not within tolerance, perform another test cut with a slight transverse feed until the requirements are met.

3. Machining Sequence for Different Holes

Through-hole turning: The feed direction (forward and retract) is opposite to that used for turning the outer diameter, and the cutting parameters should be smaller than those for turning the outer diameter.

Stepped hole turning: When turning a stepped hole with a smaller diameter, the sequence is typically to rough and finish the smaller hole first, followed by roughing and finishing the larger hole;

When turning a large stepped hole, the general procedure is to rough both the large and small holes first, followed by finishing both.

Turning Blind Holes (Flat-Bottomed Holes) requires special tool selection.

Operators must use a blind-hole turning tool whose principal rake angle is greater than 90°.

The distance from the tool tip to the outer end of the shank must be smaller than the hole radius. Otherwise, a qualified flat bottom cannot be obtained.

(1) Machining Sequences for Different Types of Holes

The key to successful hole turning lies in addressing the issues of internal turning tool rigidity and chip evacuation.

(2) Measures to Improve Internal Turning Tool Rigidity

Increasing tool rigidity: Due to the confined space inside the hole and the slender tool shank, vibration and tool deflection are likely to occur.

Therefore, the cross-sectional area of the tool shank must be maximized, and the shank’s projection length minimized as much as possible.

When finish-turning internal holes, the cutting edge must be kept sharp; otherwise, the hole is likely to become tapered.

(3) Chip Control, Cutting Parameters and Coolant Requirements

Controlling Chip Evacuation Direction: If chips cannot be evacuated smoothly, they may scratch the machined surface or cause severe tool jamming.

When precision-turning through holes, use a tool with a positive rake angle to achieve forward chip evacuation (where chips flow toward the surface to be machined);

When machining blind holes, use a tool with a negative rake angle to achieve rearward chip evacuation (where chips are evacuated from the hole opening).

Select Appropriate Cutting Parameters. Boring tools feature low rigidity of the tool shank.

They also have poor heat dissipation and chip evacuation conditions.

Therefore, boring cutting parameters including spindle speed, feed rate, and depth of cut must be set lower.

These values should be smaller than the parameters adopted for external diameter turning.

This is particularly important when turning small or deep holes.

Ensure Adequate Coolant Supply. Sufficient coolant should be supplied throughout hole machining.

Coolant delivers cooling, lubrication, cleaning and rust prevention.

It reduces thermal deformation of the workpiece and improves surface quality.

• Milling Holes

Milling holes is a machining process that uses milling to create holes in workpieces.

It removes material through the relative motion between a high-speed rotating milling cutter and the workpiece, thereby forming the hole.

Hole milling differs from conventional drilling. Its programming and machining process relies on the rotation of the milling cutter.

The cutter moves along the workpiece surface. Typical motion modes include circular interpolation and helical interpolation.

1. Milling Methods

Circular Milling (Shallow Hole Machining): This method uses a tool with helical cutting edges ground onto a cylindrical shank.

During milling, the spindle and cutter perform a circular feed motion around the Z-axis, making it suitable for machining shallow holes.

Helical interpolation milling (deep hole machining): This method uses a multi-purpose end mill, with the machining center’s X, Y, and Z axes operating in coordination.

The rotating end mill performs a helical feed motion around the Z-axis, milling a hole of the required size in a single pass.

It is suitable for machining deep holes with a depth no greater than five times the tool diameter.

2. Key Operational Points for Hole Milling

Tool Selection and Parameter Settings: End mills are the preferred choice.

The tool diameter should generally be 2–4 mm smaller than the hole diameter to facilitate chip evacuation, and the cutting edge length must exceed the hole depth to prevent tool collision.

Spindle speed, feed rate, and depth of cut per pass must be set appropriately based on the material.

Optimizing Feed and Chip Removal: When programming, be sure to enable “helical feed” to prevent tool vibration.

For deep holes or difficult-to-machine materials, use segmented cutting—such as retracting the tool every 5 mm to clear chips—or use tools with internal cooling to prevent chip buildup that could cause tool chipping or breakage.

Bore Bottom Cleaning and Precision Control: After milling, uncut protrusions often remain at the center of the bore bottom.

During finishing, select the “bottom cleaning” option or switch to a ball-nose end mill to clean the bottom.

Strict Equipment and Environmental Monitoring: In precision machining, spindle speed and feed accuracy must be strictly controlled.

When approaching the designed hole depth, switch to micro-feed to avoid over-cutting;

Simultaneously, a vacuum dust collection system or high-pressure nitrogen purging must be used to ensure no chips or dust remain inside the hole.

• Boring

Boring is a high-productivity method for the finishing of holes.

It uses specially designed, multi-edged, fixed-size cutting tools (boring bars) to machine pre-machined holes on workpieces on dedicated boring machines.

Boring bars are multi-edged tools that can sequentially complete the roughing, finishing, and polishing of a hole in a single linear stroke, eliminating the need for multiple passes and significantly improving production efficiency.

The precision of broaching primarily depends on the accuracy of the broach;

It typically achieves grades IT9 to IT7, with a surface roughness (Ra) ranging from 6.3 to 1.6 μm, ensuring the dimensional and geometric accuracy of the hole itself.

Broaches can machine not only round holes but also complex shapes such as profiled holes, spline holes, and internal keyways.

1. The Broaching Process

The broaching process primarily relies on the low-speed linear motion of the broach as the main motion.

Its core machining methods and process characteristics are as follows:

Motion and Feed: During broaching, the broach performs only low-speed linear motion, and the feed motion is achieved through the height difference (pitch) between adjacent teeth on the broach.

Broaching Methods: There are three primary methods. The first is layer-by-layer broaching, which removes material in sequential layers;

The second is block-by-block broaching, where a set of teeth removes a portion of a metal layer;

And the third is a combined method, which integrates the advantages of the first two—using block-by-block broaching for roughing and layer-by-layer broaching for finishing to balance efficiency and surface quality.

Locating Method: The workpiece is located by the hole being machined itself (with the broach’s leading edge serving as the locating element), eliminating the need for complex clamping devices.

2. Key Operational Points

Control the Number of Teeth Engaged Simultaneously. Smooth broaching and broach breakage prevention require proper control of engaged teeth.

Generally, no fewer than 3 teeth should be engaged at the same time. Fewer engaged teeth may result in annular ripples on the workpiece.

Meanwhile, the number should not exceed 6 to 8 teeth. Excessive broaching force caused by too many engaged teeth may lead to tool breakage.

Note the limitations on positioning and positional accuracy:

Since the workpiece is positioned by the hole itself during broaching, it is difficult to ensure the positional accuracy between the hole and other surfaces.

For rotary parts with coaxiality requirements, it is usually necessary to broach the hole first, then use the hole as a positioning reference to machine the other surfaces.

Limited Scope of Application: Broaches are fixed-dimension cutting tools with complex geometries and high costs;

Therefore, they are suitable only for high-volume production. Furthermore, broaching can only be used to machine through holes;

It cannot be used for stepped holes, blind holes, or thin-walled holes.

It is also unsuitable for large holes and is typically used for small-to-medium-sized parts with hole diameters ranging from Ф10 to 80 mm and hole depths not exceeding five times the hole diameter.

Preventing Bending and Damage to Broaches: After use, broaches must be hung for storage to prevent bending caused by their own weight, which can lead to chipping.

At the same time, cutting fluid and cutting speed must be selected appropriately to prevent the formation of built-up edges and the shrinkage or enlargement of the hole diameter.

Special Machining Methods

• Electric Discharge Machining (EDM) Piercing

Also known as an electric discharge drilling machine or micro-drilling machine, this is a special machining process that uses a thin copper tube or brass rod—moving vertically up and down in a continuous motion—as an electrode.

It applies pulsed spark discharges to the workpiece, generating temperatures of 8,000–12,000°C to melt and vaporize the metal on the workpiece’s surface, thereby removing material and forming holes.

This is a non-contact machining process. It is not restricted by material hardness.

It can easily process difficult-to-machine materials including stainless steel, hardened steel, cemented carbide, and titanium alloys.

In this way, it breaks through the limitations brought by material hardness.

It can fabricate holes with tiny diameters and large depths. Its maximum depth-to-diameter ratio can reach 200:1 or even exceed 250:1.

Traditional mechanical drill bits cannot achieve this performance. They tend to bend and break under such deep-hole machining conditions.

1. Electrical Discharge Machining (EDM) Piercing Process

EDM piercing employs a non-contact discharge-erosion-recovery cycle. The core process is as follows:

Electrode Structure: A hollow copper rod is used as the electrode.

A medium (typically tap water, deionized water, or gas) flows through a fine hole in the center of the copper tube, serving to cool the workpiece and remove chips under high pressure.

Discharge Erosion: A pulse power supply generates high-frequency pulse voltages.

A tiny discharge gap is kept between the electrode and the workpiece. The working fluid inside this gap will break down.

It forms a high-temperature and high-pressure electric spark channel.

This channel instantly melts or vaporizes the local material on the workpiece.

Feed and Cycle: The equipment uses a CNC system to control the adaptive feed of the electrode along a preset axis; the high-frequency, repetitive discharge process gradually forms deep holes in the workpiece.

2. Key Operating Points

Electrode Installation and Guidance Check. When installing hollow copper tube electrodes, proper clamping force must be guaranteed.

This prevents the electrode from bending or running eccentric. It also avoids burrs forming on the electrode end face.

Additionally, maintain a distance of 2 mm to 3 mm between the electrode guide and the workpiece to prevent electrode wobble, which can lead to unstable machining or electrode breakage.

Appropriate Process Parameter Selection: Select suitable discharge current, pulse width, and feed rate based on the workpiece material and hole diameter.

When machining hard materials, use low current and narrow pulses to reduce electrode wear;

Overloading parameters is strictly prohibited to prevent carbon buildup, arcing, or rough hole walls.

Work Fluid Management and Safety Precautions: The work fluid level must be 50–60 mm above the workpiece surface to prevent fires caused by a low fluid level;

At the same time, ensure high-pressure jet flow of the work fluid to promptly flush out metal debris and prevent secondary discharges from scratching the hole walls.

Electric Shock Prevention and Standard Operating Procedures: Operators must stand on an insulated rubber or wooden footplate, and must never touch the electrode with their hands during operation;

The pulse power supply must be turned off when loading or unloading workpieces or when adjusting the electrode.

• Laser Drilling

This is a non-contact machining technique. It focuses a high-power-density laser beam onto the material.

The material absorbs massive energy within an extremely short period.

This leads to material melting, vaporization, or even plasma generation. An auxiliary gas removes the material to form a hole.

It is one of the earliest laser processing technologies to achieve practical application.

Virtually all solid materials can be processed, including diamond, ruby, ceramics, and cemented carbide—ultra-hard, high-melting-point, and brittle materials that are difficult to machine using conventional methods..

It is capable of creating microscopic holes with micrometer-scale diameters and extremely high depth-to-diameter ratios (up to 200:1);

Since the process involves no mechanical cutting forces, it can drill hundreds or even thousands of holes per second.

Furthermore, there is no need to change tools; holes of different shapes and at different angles (up to 80°) can be produced simply by adjusting the program.

1. Methods of Laser Drilling

Impact Drilling (Single-Pulse/Multi-Pulse): The laser beam is focused on a single point on the workpiece, and through continuous impact from a single or multiple pulses, the material is melted, vaporized, and ejected layer by layer.

This method is fast and is commonly used for machining micro-holes or thin sheets.

Rotary Cutting (Contour Tracing Method): The laser beam and the workpiece move in relative circular motion (similar to mechanical milling), with the beam’s trajectory carving out the shape of the hole.

This method offers higher precision and better hole wall quality, and is commonly used for machining cylindrical or irregularly shaped holes.

Flying Drilling: The workpiece moves at high speed without stopping, while the laser rapidly moves to the next hole position between pulses to deliver the impact.

This method is extremely efficient and is suitable for processing large numbers of holes of the same specification that are regularly distributed (such as microholes in PCBs).

Punching Modes:Pulse punching and blast punching are widely adopted in practical cutting and hole-making processes.

Pulse punching features high peak power and low duty cycle. It creates a minimal heat-affected zone and regular hole geometry.

Blast punching uses a continuous laser beam to rapidly melt and blow away material.

This method delivers fast processing speed but generates substantial spatter.

2. Key Operational Points

Precise Matching of Process Parameters: Laser power, pulse frequency, duty cycle, and scanning speed must be set appropriately based on material type, thickness, and hole diameter.

For example, an excessively high duty cycle may cause the hole to burst, while an excessively low duty cycle may prevent the material from being penetrated; frequency affects hole uniformity and surface quality.

Focal Length and Spot Control: The focal length must be precisely adjusted so that the focal point aligns with the drilling depth.

For thick plates, stepwise drilling adjustments are typically required, gradually changing the focal length and parameters—for example, from -8 mm to -14 mm—to ensure hole wall quality and reduce drilling time.

Proper Use of Assist Gas: High-pressure coaxial gas is required, such as compressed air, nitrogen or oxygen.

It can blow away molten material and spatter in a timely manner.

This prevents slag from solidifying inside the hole and forming a recast layer. The assist gas also provides cooling effect.

Heat Affected Zone and Hole Shape Quality Control: Laser drilling is a thermal process, so care must be taken to control the heat-affected zone.

Melting and erosion are prone to occur at the junction of the hole mouth and tail; grinding and repair may be necessary when required.

Additionally, laser-drilled holes are typically slightly conical rather than perfectly cylindrical, so tolerances must be allowed for during precision assembly.

Equipment Maintenance and Safety Precautions: Optical components, such as the focusing lens, must be cleaned regularly to prevent dust from affecting beam quality; the cooling system should be inspected to prevent overheating.

Safety goggles must be worn during operation, and it is strictly prohibited to start the equipment when unattended.

• Electrochemical Machining

Electrochemical machining is a specialized machining method based on the principle of electrochemical anodic dissolution.

During the machining process, the workpiece serves as the anode and the tool as the cathode, with a minute gap—typically ranging from 0.1 to 1 mm—maintained between them.

Under the influence of a direct current power supply, the metal on the workpiece’s surface continuously dissolves to match the shape of the cathode tool, thereby replicating the tool’s shape onto the workpiece.

It can machine virtually all conductive materials and is completely unrestricted by mechanical and physical properties such as strength, hardness, and toughness.

It is particularly well-suited for machining difficult-to-machine materials such as high-temperature alloys, quenched steel, stainless steel, and cemented carbides.

Furthermore, it can machine complex cavities, surfaces, and holes in a single pass using simple linear feed motion, resulting in extremely high productivity.

Since the cathode releases only hydrogen gas and no dissolution reaction occurs, there is theoretically no material loss, and the cathode can be reused repeatedly, offering significant cost advantages in mass production.

1. Core Process Conditions and Procedures for Electrochemical Machining

Electrode Gap and Electrolyte: A minute gap must be maintained between the workpiece anode and the tool cathode.

At the same time, a high-pressure pump forces the electrolyte to flow through this gap at an extremely high velocity (typically 6–30 m/s) to remove the metal ions produced by anode dissolution and the heat generated by electrolysis, and to prevent polarization.

High Current Density: The workpiece and tool are connected to the positive and negative terminals of a DC power supply, respectively, typically at a voltage of 10–24 V.

This generates an extremely high current density within the minute gap, reaching 10–100 A/cm².

Forming Process: Due to the uneven gap, the current density is highest at the narrowest point, where the workpiece dissolves the fastest.

As the tool cathode is slowly advanced toward the workpiece, the workpiece metal is continuously dissolved by electrolysis, ultimately forming a shape on the workpiece surface that closely resembles the tool’s working surface.

2. Key Operational Points

Design and Modification of the Tool Cathode: The precision of electrochemical machining relies heavily on the accuracy of the cathode and the control of the machining gap.

Due to the diffuse erosion effect inherent in electrochemical machining, the design, fabrication, and modification of the cathode are relatively challenging and typically require iterative modifications using computer-aided systems.

Machining Gap and Flow Field Control: The machining gap must be strictly controlled within the range of 0.1–1 mm.

If the gap is too small, it can easily lead to short circuits and arcing; if the gap is too large, precision will decrease.

At the same time, it is necessary to ensure that the electrolyte flows uniformly and at high speed in the machining zone to prevent localized electrolyte depletion or product buildup.

Short-Circuit Protection: Due to the extremely small gap, short circuits are inevitable during machining.

The power supply system must have effective short-circuit protection to ensure that the current is quickly cut off in the event of sparks or a short circuit, thereby preventing burn damage to the cathode and workpiece.

Equipment Corrosion Protection and Safety Measures: Since the electrolyte is corrosive, the machine tool must possess sufficient rigidity and corrosion resistance.

Furthermore, exhaust gases such as hydrogen are generated during machining; therefore, effective ventilation must be ensured, and open flames are strictly prohibited to prevent explosions.

Unit Cost Considerations: Because cathode design and fabrication are time-consuming and the equipment is expensive, electrochemical machining is better suited for high-volume production.

For single-piece or small-batch production, the additional cost per unit would be very high.

• Ultrasonic Machining

This is a specialized machining method that utilizes the mechanical impact of ultrasonic vibrations, combined with the grinding and polishing action of an abrasive suspension, to perform micro-cutting on workpieces.

It is primarily used to machine hard and brittle materials that are difficult to cut with conventional cutting tools.

It is particularly suitable for processing hard and brittle materials such as glass, quartz, ceramics, gemstones, diamonds, and semiconductor silicon wafers—materials that are highly prone to fracturing when machined with conventional tools and cannot be processed using electrical discharge machining (EDM) or electrolytic machining due to their non-conductivity.

During the machining process, macroscopic cutting forces are extremely small, resulting in no mechanical deformation or thermal damage.

Machining accuracy can reach 0.01–0.02 mm, surface roughness can reach Ra 0.1 μm, and there is no residual stress on the hole walls.

The tool head can be fabricated into various complex cross-sectional shapes, enabling the machining of not only round holes but also square holes, polygonal holes, irregular-shaped holes, and blind holes.

1. Ultrasonic Machining Methods

Energy Conversion: An ultrasonic generator produces a high-frequency oscillating current, which is converted into high-frequency mechanical vibrations by a transducer.

The amplitude is then amplified by an amplifying rod before being transmitted to the tool head.

Abrasive Suspension Circulation: A suspension containing abrasive particles (such as silicon carbide or aluminum oxide) is continuously injected between the tool head and the workpiece surface.

Impact and Micro-Cutting: The tool head vibrates minutely at ultrasonic frequencies (with amplitudes typically ranging from 0.01 to 0.1 mm), continuously striking the abrasive particles.

The impacted abrasive particles strike the workpiece surface at extremely high speeds, causing localized micro-cracks, spalling, and fragmentation of the material, thereby achieving material removal.

Cavitation and Polishing: Under the high-frequency vibration of the tool head, the suspension undergoes cavitation (the instantaneous collapse of microscopic bubbles), generating powerful hydraulic shock waves that accelerate material removal and serve to clean and polish the bore walls.

2. Key Operational Considerations for Ultrasonic Machining

Tool Head Design and Material Selection: The tool head must possess good fatigue strength and is typically made of 45 steel or stainless steel.

Wear on the tool head directly affects the geometric accuracy of the hole, so it must be periodically adjusted or replaced.

Proper Formulation of the Abrasive Suspension: The particle size, concentration, and type of abrasive directly determine machining efficiency and surface roughness.

Coarse grinding uses coarse abrasives and high concentrations to increase material removal rate; fine grinding uses fine abrasives and low concentrations to improve surface finish.

Precise Control of Machining Pressure: The static pressure applied by the tool head to the workpiece must be moderate.

If the pressure is too low, the abrasives cannot effectively impact the workpiece, resulting in extremely low machining efficiency;

If the pressure is too high, it will cause direct contact and friction between the tool head and the workpiece, accelerating tool wear and potentially causing the workpiece to fracture.

Chip Removal and Cooling: It is essential to ensure that the slurry circulates adequately in the machining zone to promptly remove crushed particles and grinding debris, preventing machining stoppages or scratches on the bore wall caused by debris blockages.

At the same time, the slurry also serves to cool the tool head and the workpiece.

Amplitude and Frequency Matching: During machining, the ultrasonic amplitude and frequency must be appropriately adjusted based on the material properties and hole diameter to achieve optimal resonance and machining results.

• Stamping and Forming Methods

Punching is a separation process in stamping that involves using a punch press and dies to cut out various shapes or through-holes in materials such as sheet metal, tubing, leather, and plastic film.

In the punching process, the cut-out portion becomes scrap, while the remaining sheet material is the finished product.

Punching is particularly well-suited for high-volume production, as it allows for forming in a single stroke;

It can even perform multiple operations in a single pass using multi-station or progressive dies.

In architectural decoration, perforated aluminum honeycomb panels serve to absorb sound, provide sound insulation, absorb heat, provide thermal insulation, reduce weight, and enhance aesthetics;

In the packaging industry, perforations provide functions such as breathability, venting, easy-tear openings, and hook holes.

PCB manufacturing uses perforation to connect multiple circuit boards into a single panel for automated batch production, enabling easy separation later while reducing component damage caused by separation stress.

1. Punching

Conventional Sheet Metal Punching: The sheet metal is placed between the upper die (punch) and the lower die (die cavity).

The upper die moves downward into the lower die, using its cutting edge to shear the material and form a hole.

Tube Punching: This is divided into steel die punching and rubber die punching.

Steel die punching can be further categorized into vertical punching and horizontal punching.

Rubber die punching involves placing polyurethane rubber inside the tube to serve as an elastic punch, using its expansion force to punch holes in the tube wall.

CNC and Automated Punching: Using CNC turret punch presses, automatic hydraulic punching machines, or pneumatic punching machines—such as plastic bag punching machines—the position, speed, and sequence of the punch are controlled by a program to achieve high-volume, high-precision continuous punching.

Hot and Cold Punching: For thin sheets or soft metals, cold punching is typically used, as it is highly efficient and produces clean edges;

For thick steel plates or high-carbon steel, hot punching is employed, in which the metal is heated to a red-hot state (800–1000°C) before punching to reduce the force required and protect the tool.

2. Key Operational Points

Precise Positioning and Layout Design: Before punching, hole positions must be accurately marked, such as using a scribing needle or a center punch.

At the same time, the arrangement of workpieces on the sheet metal must be planned reasonably to improve material utilization and reduce costs.

Control of Punching Clearance: The clearance between the punch and die directly affects the quality of the punched parts, surface roughness, and die life.

Too little clearance accelerates wear, while too much clearance results in rough surfaces; typically, the clearance must be precisely adjusted based on the material’s thickness and properties.

Die Cutting Edge and Support Requirements: The cutting edge must be kept sharp; a dull punch can cause burrs, wire-like defects, or failure to cut through, and must be sharpened or replaced promptly.

Additionally, during punching, the material must be supported by a solid backing (such as a support plate), and the support hole must be slightly larger than the punch diameter to prevent damage to the cutting edge.

Prohibition of Dry Punching and Sequence Control: When operating CNC or hydraulic punching machines, dry punching without a tube or sheet in place is strictly prohibited, as it can severely damage the die.

Furthermore, during high-density punching, it is generally recommended to start with smaller holes or follow a specific punching sequence to prevent material deformation.

• Flanging

Flanging, also known as inner hole flanging or thread forming, is a stamping process that involves forming a vertical flange (a short, upright edge) around the inner circumference of a workpiece’s bore.

Unlike punching, flanging is a plastic deformation process; it does not remove material during processing but relies instead on localized plastic deformation of the material to achieve the desired shape.

1. Core Methods for Flanging

Flanging with a Pre-drilled Hole: This is the most common method. First, a pre-drilled hole is created in the sheet metal via punching or laser cutting.

Then, a flanging punch is used to extrude the material around the hole’s edge outward or inward, causing it to undergo tangential elongation and deformation, ultimately forming an upright side wall.

Flaring Without a Pre-drilled Hole (Piercing Flaring): Suitable for applications requiring a greater flaring height.

The end of the punch is typically designed to be tapered (e.g., 60°), allowing it to pierce the sheet directly and lift the surrounding material along with it, thereby eliminating the need for a pre-punching step.

Combined-Process Flaring: In actual production, flaring is often combined with other processes.

For example, a three-step process involving punching, bending, and flanging, or combining flanging with reverse-side punching within the same die to ensure coaxiality of multiple punch locations.

Special Auxiliary Flanging: For flanging operations with high precision requirements, rubber or polyurethane can be used as elastic punches, or a floating block ejection mechanism can be incorporated into the die to improve material flow and ejection performance.

2. Key Operational Points

Accurately Calculate the Pre-punch Hole Dimensions: Before flanging, the appropriate pre-punch hole diameter must be calculated based on the principle of constant volume and the flange height.

If the pre-punch hole is too small, it will cause excessive stretching of the material and result in cracking;

If it is too large, it will result in insufficient flange height or uneven edges.

Controlling the flanging coefficient and degree of deformation: The degree of deformation during flanging is represented by the flanging coefficient (K = pre-punch hole diameter / post-flanging mean diameter).

The smaller the K value, the greater the deformation; it is essential to ensure that K exceeds the material’s “ultimate flanging coefficient.”

If the material has poor plasticity or the flange height is too high, flanging should be performed in multiple stages or an intermediate annealing process should be added.

Pay attention to the direction of burrs on the pre-punched hole: Burrs on the edges of the pre-punched hole have a significant impact on the quality of the flanging.

It is generally recommended to perform flanging in the opposite direction of punching so that the burrs are located on the inner side of the flange;

This significantly reduces the risk of cracking on the outer side of the hole opening.

Die Structure and Ejection Design: The tip of the flanging punch is typically designed with a parabolic shape or an R-radius to facilitate a smooth transition of the material.

For downward flanging, the lower die float block must be properly designed to ensure that the punch first presses down on the float block before contacting the material, thereby preventing excessive stress that could cause the punch to fracture or make ejection difficult.

Machining of Special-Structure Holes

• Countersinking

Countersinking involves using specialized countersink drills, milling cutters, or turning tools to enlarge or flatten the end face of an existing hole in a workpiece.

Simply put, it involves creating a stepped edge or a flat base on an existing hole.

The most common application is to create a recess for countersunk screws or hex socket head cap screws, allowing the fastener head to be fully recessed into the workpiece, maintaining a flat surface, and preventing interference during assembly.

For castings or forgings with rough surfaces, countersinking the end face provides a flat bearing surface, preventing bolt breakage or workpiece deformation caused by uneven stress distribution when tightening nuts.

1. Types of Countersinking Operations

Cylindrical Countersinking: A countersink with a guide post is used to create a cylindrical step at the hole opening.

The guide post extends into a pre-drilled pilot hole, providing automatic centering and guidance to prevent tool deflection.

Tapered countersinking / chamfering: A tapered countersink drill—commonly with angles of 60°, 90°, or 120°—is used to create a tapered surface at the hole opening, primarily for use with countersunk screws.

Flattening the End Face: On uneven hole surfaces, such as those found in castings or forgings, a flat, annular support surface is created to ensure that nuts or bolt heads can sit flush against the surface.

2. Key Operating Points

Tool Guidance and Centering: When using a cylindrical countersink, ensure that the clearance between the guide pin and the pilot hole is appropriate.

Excessive clearance will cause the countersinked hole to be off-axis relative to the pilot hole, resulting in eccentricity; insufficient clearance will lead to poor chip evacuation or jamming.

Control of Cutting Parameters: During countersinking, the end cutting edges of the tool are engaged in the cutting process, resulting in a large cutting area and poor heat dissipation.

Therefore, lower cutting speeds and higher feed rates must be used to prevent severe tool wear or the formation of built-up edges.

Preventing Vibration and Tool Jamming: Since countersink drills typically have multiple cutting teeth and a large diameter, insufficient spindle rigidity or excessive tool overhang can easily cause vibration, resulting in a wavy surface at the hole opening.

When clamping the tool, minimize the tool overhang as much as possible.

Depth Control: When machining cylindrical countersinks, the countersinking depth must be precisely controlled to avoid damaging the bottom hole wall or exposing the bottom hole due to excessive depth.

When machining tapered holes, ensure that the tapered surface fully contacts the screw head and that the workpiece is not countersunk through.

• Chamfered Holes

A chamfered hole refers to a process in which a tapered or slanted surface with a specific angle—typically 45°, though 30°, 60°, and other angles are also used—is machined at the hole’s opening or bottom.

It is not usually performed as a standalone machining operation but rather as an auxiliary step following drilling, turning, or milling, or it is completed simultaneously with the hole-machining process.

A chamfered hole acts like a funnel, guiding bolts, pins, or shaft-type parts as they are inserted.

This prevents the edges of the parts from getting caught or scratching the hole walls, thereby significantly improving assembly efficiency.

1. Methods for Machining Chamfered Holes

Drilling Machine Chamfering: Use a specialized chamfering drill (a multi-edged conical cutting tool) to quickly scrape the edge of a pre-drilled hole on a drilling machine or machining center.

Lathe Chamfering: On a lathe, use an offset tool or a specialized chamfering turning tool to machine a chamfer at the hole edge or bottom while the workpiece rotates, using either manual or automatic feed.

Milling chamfering: On a milling machine or CNC machining center, a chamfering end mill (such as a 45° single- or double-edged chamfering cutter) is used to perform circular milling along the hole’s rim via program-controlled toolpaths.

Single-pass machining with a composite tool: In high-volume production, “drill-chamfer composite tools” are commonly used.

After the drill bit drills the pilot hole, the chamfering edge on the tool directly completes the chamfering of the hole opening, eliminating tool change time.

2. Key Operational Points

Precise Control of Chamfer Dimensions: These are typically designated on drawings as C1, C2, or 1×45°, among others.

During operation, the depth and width of the chamfer must be strictly controlled to avoid a chamfer that is too large—which would reduce the effective mating area at the hole opening—or one that is too small to serve its guiding and deburring functions.

Tool Selection and Angle Matching: The correct chamfering tool with the appropriate angle must be selected according to the drawing specifications.

If the chamfer is intended to accommodate a countersunk screw, the chamfer angle must strictly match the screw head angle (e.g., 90°).

Preventing Flaring or Collapsed Edges at the Hole Opening: When using a chamfering drill on a drill press, if the tool is not sharp enough or the feed rate is too fast, the edges of the hole opening are prone to being gouged out, resulting in a collapsed edge. Keep the cutting edge sharp and maintain a steady feed rate.

Note on Bottom Chamfering for Blind Holes: For blind holes (non-through holes), in addition to chamfering the hole opening, the bottom of the hole usually requires chamfering as well.

This is because the bottom center of a blind hole drilled with a twist drill will have a conical protrusion; bottom chamfering removes this protrusion, ensuring that the screw or cylindrical pin can seat fully at the bottom.

Vibration Control and Surface Quality: During chamfering, the cutting tool often makes contact with the workpiece at a single point or in a localized area along its edge, which can easily cause high-frequency vibrations.

The spindle speed should be appropriately reduced to ensure the chamfered surface is smooth and free of vibration marks.

Conclusion

Hole machining is a fundamental part of mechanical manufacturing, and different hole types, materials, precision requirements, and production volumes require different machining methods.

Conventional processes such as turning, milling, boring, and broaching each have their own advantages and application limitations, while EDM, laser drilling, electrochemical machining, and ultrasonic machining provide effective solutions for difficult-to-machine, hard, brittle, or complex materials.

Stamping, punching, and flanging are also valuable for high-volume production and sheet-metal applications.

Achieving high-quality holes requires more than simply selecting a suitable process.

Tool selection, machining parameters, positioning, chip removal, coolant or work-fluid management, dimensional control, and equipment conditions must all be considered throughout the operation.

Special-structure features such as countersinks and chamfers also require precise control to ensure proper assembly and functional performance.

Ultimately, effective hole machining depends on matching the process to the material, geometry, accuracy, production volume, cost, and available equipment.

A process-oriented approach allows designers, machinists, and quality engineers to select the most appropriate method, improve machining efficiency and hole quality, and maintain consistent manufacturing results.