Metal Cutting Processes: Mechanical, Thermal and Arc Methods: The Technical Guide

The Technical Guide

A Machine Shop Foreman, a Blown Die, and the Lesson That Changed Everything

the practitioner had been running the fabrication floor at a mid-size aerospace subcontractor for eleven years. He knew his way around a lathe. He could set up a press brake blindfolded. But the morning a rush order for 4,000 stainless steel brackets landed on his desk—with tolerances tighter than anything his team had attempted—he realized something uncomfortable.

He didn't actually understand metal cutting.

Not the deep, process-level understanding that separates a technician from an engineer. Not the kind that lets you choose between flame, plasma, laser, EDM, or mechanical shearing based on physics rather than habit. Not the kind that saves you from scrapping an entire production run because you picked the wrong clearance on a blanking die.

the practitioner's education started that week, and what he learned over the next several months is exactly what you are about to absorb in this guide.

This is the complete reference to metal cutting processes—from the oldest oxidizing flame torch to precision plasma arc systems accurate enough to replace laser cutting in certain applications. Every process. Every variable. Every data table you will ever need to make the right call on a shop floor, in a design office, or during a bid review.

No shortcuts. No summaries. Just the engineering.

The Landscape: Why "Metal Cutting" Is Not One Process—It Is an Entire Discipline

Before the practitioner could fix his production problem, he had to understand the taxonomy. Metal cutting is not a single operation. It is a family of fundamentally different physical processes, each governed by its own physics, each optimized for different materials, thicknesses, tolerances, and production volumes.

Here is the territory you need to map:

Each process has a sweet spot. Each has failure modes. And as the practitioner discovered, choosing the wrong one does not just cost money—it costs time, reputation, and sometimes safety.

Punches, Dies, and Press Work: Where Mechanical Cutting Begins

Understanding Clearance: The Foundation of Every Die Operation

Clearance is the space between the punch and the die on one side. It is not the total difference between punch and die diameters—though some shops mistakenly define it that way. The correct definition:

Cutting clearance = the space between the punch and die on each side Die clearance = the angular clearance below the cutting edge that allows parts to fall through

For round dies: Clearance = Die Radius − Punch Radius

Why this matters: Using the wrong definition on an unsymmetrical or angular die leads directly to dimensional errors. Always specify clearance as the space on one side.

Clearance Values in the supplied reference general rules for clearance selection

Critical insight: Soft, ductile metals actually require more clearance than hard metals—the opposite of what most inexperienced operators assume. the practitioner's team had been using excessive clearance on the stainless brackets, thinking the harder material needed more room. It did not.

Where Clearance Is Applied

This is the question that determines whether your blanks or your holes are the correct dimension:

• Blanking to a given size: The die is made to the required dimension. The punch is made smaller (clearance is subtracted from the punch).
• Perforating (punching holes): The punch is made to the required hole diameter. The die is made larger (clearance is added to the die).

Get this backwards, and every part is out of tolerance.

Angular Clearance for Dies

Below the cutting edge of a die, angular clearance allows the blank to fall freely:

Two methods exist for applying this clearance. For soft, thin metals (e.g., thin brass), the clearance extends from the cutting edge all the way to the top face of the die. For harder materials (hard brass, steel), a short section of approximately ⅛ inch below the cutting edge is left nearly straight, with the angular clearance beginning below that. This second method dramatically extends die life—grinding the die face does not significantly enlarge the hole.

The Effect of Clearance on Punching Pressure

Clearance directly affects the force required:

Test data (¾-inch holes in 5/16-inch mild steel plate):

Less clearance = more force = more punch wear = higher risk of breakage. But too much clearance produces ragged edges. The engineering is always a trade-off.

Calculating Punching Force

For circular holes:

$$F = \frac{\pi \times D \times S \times T}{2000}$$

Where:

• $F$ = Force in tons
• $D$ = Hole diameter (inches)
• $S$ = Shearing strength (lbf/in²)
• $T$ = Stock thickness (inches)

Approximate formula for steel:

$$F = D \times T \times 80$$

For non-circular holes:

Replace $D$ with one-third of the perimeter of the hole.

Approximate tensile strengths for punching calculations:

Worked example: Punching a 2-inch diameter hole through ¼-inch steel:

$$F = 2 \times 0.25 \times 80 = 40 \text{ tons}$$

Worked example: Punching a 1-inch square hole in ¼-inch steel: Perimeter = 4 inches. One-third of perimeter = 1⅓ inches.

$$F = 1.333 \times 0.25 \times 80 = 26.67 \text{ tons}$$

For brass: Replace the factor 80 with 65.

Lubricants for Press Work

the practitioner learned that lubrication was not optional—it was a precision variable:

Target film thickness: Approximately 0.0001 inch, achievable with about 1 pint of fluid covering 500 sq. ft of material.

Drawing Cylindrical Shells: Depth and Diameter Reductions

The depth to which metal can be drawn in one operation depends on material quality, thickness, die geometry, and the degree of wall thinning.

General rules:

• First draw depth should never exceed the shell diameter
• Alternative rule: First draw depth = one-third of the blank diameter
• For sheet steel up to ¼ inch thick: First shell diameter ≈ 60% of blank diameter

Successive diameter reductions (single-action presses, with annealing between draws):

Double-action presses (with internal bushing support) allow increased reductions: 30, 24, 18, 15, and 12% respectively. These figures also apply to brass in single-action presses.

Blank Diameter Formula

For cylindrical shells:

$$D = \sqrt{d^2 + 4dh}$$

Where:

• $D$ = Blank diameter
• $d$ = Shell outside diameter
• $h$ = Shell depth (add trimming allowance)

For shells of known weight:

$$D = 1.1284 \sqrt{\frac{W}{w \times t}}$$

Where:

• $W$ = Weight of shell
• $w$ = Weight of metal per cubic inch
• $t$ = Shell wall thickness

Fine Blanking: When Standard Press Work Is Not Precise Enough

How Fine Blanking Works

Fine blanking uses special presses and tooling with three separate, distinct movements:

1. Clamping the work material
2. Performing the blanking operation
3. Ejecting the finished part

The process requires 1.5 to 2.5 times the force of conventional stamping. Cutting clearances between punch and die are held to an extraordinarily tight 0.0001 to 0.0003 inches.

The V-Projection: The Secret Weapon

The critical innovation in fine blanking is the V-projection—a sharp, 90° V-section ridge that follows the outline of the workpiece. These projections are built into the stripper plate (and for thicker materials, also into the die plate).

What the V-projection does:

• Bites into the material surface before blanking begins
• Prevents sideways movement of the blank
• Squeezes material toward the cutting edges, reducing edge rounding
• Creates flow that produces perpendicular, fracture-free edges

V-Projection Dimensions (Stripper Plate Only):

V-Projections on Both Stripper and Die Plate (thicker materials):

Fine Blanking Force Calculations

Cutting force:

$$F_{cut} = 0.9 \times L \times T \times UTS$$

Where:

• $L$ = Length of cut (inches)
• $T$ = Material thickness (inches)
• $UTS$ = Ultimate tensile strength (lbf/in²)

Clamping force (V-projection pressure):

$$F_{clamp} = L_v \times h \times UTS \times f$$

Where:

• $L_v$ = Length of V-projection (inches)
• $h$ = V-projection height (inches)
• $f$ = Empirical factor (2.4 to 4.4 for tensile strengths of 28,000–113,000 lbf/in²)

Rule of thumb: Clamping pressure ≈ 30% of cutting force.

Fine Blanking Capabilities

• Edge surface finish: Down to 80 µin. Ra
• Edge perpendicularity: Within 0.004 in. on 0.2-in. thick material
• Minimum hole-to-thickness ratio: 0.7:1 (vs. 1:1 for conventional tooling)
• Additional operations in same tooling: Countersinking, coining, bending up to 60°
• Tooling materials: 12% chromium steel (standard), high-speed steel or tungsten carbide (long runs)
• Strip width requirement: 2–3 times material thickness plus part width (transverse to feed)

Steel Rule Dies: The Cost-Effective Cutting Alternative

A Different Kind of Problem

Meanwhile, a sister division of the practitioner's company faced its own challenge. They needed to cut 500 aluminum brackets—not the precision aerospace parts the practitioner was sweating over, but simple flat shapes for a prototype housing. A conventional blanking die would cost thousands of units of value and take weeks to manufacture. The production quantity did not justify it.

The answer was steel rule dies—patented by the practitioner in 1879, and still one of the most cost-effective cutting methods available.

What Steel Rule Dies Are

Steel rule dies use cutting edges made from hardened steel strips—similar in proportion to graduated measuring rules—held on edge in a die block. They operate on the same principle as a cookie cutter: a sharp edge pressed through the workpiece against a counter-surface.

Cost advantage: Steel rule dies typically cost 25 to 35% of the cost of conventional blanking dies, and can be produced in far less time.

Construction and Materials

Die blocks can be made from:

• Plaster, hot lead, or type metal (poured to shape)
• Epoxy resin
• ¾-inch thick, five- or seven-ply maple or birch wood (most common for light work)
• Lignostone densified wood (for heavier forces, metal sheet blanking)
• Metal

Steel rule specifications:

Common choices: 0.138 or 0.166-inch thickness (10 and 12 points in printers' measure). Thinner rules for intricate outlines and short runs of ~50 pieces.

Cutting Edge Profiles

Different edge profiles suit different materials and operations:

• Profile A (sharp V): Used initially for shearing in the punch, later modified to flat
• Profile B (90° flat): Working edge for blanking and piercing metal sheet
• Profile C: For thin soft materials pressed against a flat surface
• Profile D: For thicker soft materials

Heat Treatment of Steel Rules

Carburizing process for intricate dies: Automatic cycle furnace → carburize in liquid compound at 1500°F → quench in oil → temper at 550°F → furnace cool.

Making Steel Rule Dies

1. Begin with a drawing of the required shape
2. Mark cutting lines on the die block (or paste/draw on Mylar sheet)
3. Drill a starter hole off the cutting line
4. Jig saw or circular saw slots under manual control
5. Recommended saw speed for Lignostone: 300 strokes/min (2-inch stroke)
6. Trial-fit rule thickness in scrap die block before cutting
7. Bend rules to contour using purpose-built bending machines
8. Join complex shapes by welding or brazing mitered edges
9. Install rules, load in press, shear the counter-pad to match
10. Grind sharp edges flat to produce ~1/64 inch land for working edges

Electrical Discharge Machining (EDM): Cutting Without Contact

The Two Types of EDM

Sinker (Plunge) EDM:

• Resembles a vertical milling machine
• Electrode is a shaped replica of the cavity to be formed
• Electrode attached to a servo-controlled vertical slide
• Gap between electrode and workpiece: 0.0005 to 0.030 inches
• Used for mold cavities, die sinking, complex 3D shapes

Wire EDM:

• Resembles a bandsaw with brass wire replacing the blade
• Wire is unwound from one reel, passed through the workpiece, wound onto another
• Wire is used only once (constantly renewed)
• CNC controlled for complex profiles
• Used for stamping die profiles, precision shapes, through-cuts

The EDM Process: Spark in the supplied reference physics of each spark cycle

1. Voltage builds between electrode and workpiece across the dielectric fluid gap
2. Dielectric breaks down — fluid transforms into a plasma of hydrogen, carbon, and oxides
3. Plasma creates a conductive pathway of ionized particles
4. Current flows — heats and vaporizes a tiny area of the workpiece
5. Striking voltage reached — voltage drops, plasma loses energy, spark extinguishes
6. Voltage rises with increasing resistance
7. Control cuts power — plasma implodes, creating a low-pressure pulse
8. Dielectric fluid rushes in — flushes away debris, cools the impinged area
9. Cycle repeats — typically lasting a few microseconds (µs)

A typical cycle: 100 µs total. Current on for 40 µs, off for 60 µs. This gives a duty cycle of 40%.

The Duty Cycle: Where Speed and Finish Are Controlled

$$\text{Duty Cycle (\%)} = \frac{\text{On Time}}{\text{Total Cycle Time}} \times 100$$

Key relationship: Longer on time → more material removed per cycle → rougher finish → deeper heat-affected zone. Shorter on time → better finish → more electrode wear.

Metal Removal Rates

Typical roughing operation (electrode positive, high-carbon steel):

Effect of Electrical Control Adjustments:

Electrode Materials

The choice of electrode material determines speed, finish quality, and wear rate.

Graphite:

• Most commonly used electrode material
• Easy to machine conventionally
• High resistance to heat and wear at lower frequencies
• Premium grades cost up to 5× the least expensive, 3× the cost of copper
• Fine-grain, high-density graphites provide superior wear characteristics

Copper (with 5% tellurium):

• Most commonly used metal alloy electrode
• Better surface finish in fine-finishing operations
• Can be discharge-dressed in the EDM under CNC
• Smoother surfaces, slower wear rates

Copper-tungsten:

• Superior wear resistance compared to pure copper
• Used for finishing operations and precision work

Electrode/Workpiece Combinations and Corner Wear

Workpiece Material Characteristics

Machine Settings: The Rule of Thumb

Power selection for graphite and copper electrodes:

$$\text{Current (A)} = 50 \text{ to } 65 \text{ amps/in}^2 \text{ of electrode engagement}$$

Example: A ½ inch square electrode → 0.5 × 0.5 × 50 = 12.5 amps

Voltage across the arc gap: Ideally ~35 volts. Should be as low as possible to maintain stability. Gap voltage should never drop below 35–40 volts.

The Recast Layer: EDM's Hidden Defect

Every EDM operation on steel produces a recast layer—a thin, hard, brittle surface created by the interaction of spark heat, oil dielectric, and rapid quenching:

• Oil breaks down into hydrocarbons during sparking
• Carbon atoms are drawn into the molten metal surface
• Rapid quenching in oil creates a white, carbon-enriched, martensitic layer
• This layer covers the heat-affected zone (HAZ) beneath

Wire EDM difference: Uses deionized water as dielectric. Carbon is extracted from the recast layer rather than added. Copper atoms from the wire may migrate into the surface, slightly softening it. HAZ depth can be held below 1 micron (0.00004 in.) with proper on/off time adjustment.

Flushing: The Most Underrated Variable

Flushing is critical to EDM success and is the most common source of problems:

• Pressure flushing: Fluid pumped through holes in the electrode or workpiece
• Vacuum (suction) flushing: Used when straight, accurate side walls are required
• Side flushing: External nozzle directs fluid into the tank

Dangers:

• Excessive flushing pressure can displace the electrode or workpiece
• Many low-pressure flushing holes are preferable to few high-pressure holes
• Trapped gases from sparking may explode, causing fire or equipment damage
• Pressure-relief valves are strongly recommended

Wire EDM Specifics

Wire materials:

• Yellow brass: Copper 63%, Zinc 37%, tensile strength 50,000–145,000 lbf/in²
• Diameter range: 0.002 to 0.012 inches
• Zinc-coated brass: Faster cutting, reduced breakage (zinc vaporizes at 906°C while brass core melts at 930°C)
• Steel core with brass/copper coating: For added strength

Wire precision requirements:

• Diameter tolerance: ±0.00004 in. (drawn), ±0.00006 in. (plated)
• Surfaces must be smooth, free from nicks, scratches, and cracks

EDM Drilling (for wire start holes):

• 0.04-in. diameter hole through 4-in. steel: ~3 minutes
• Practical minimum hole diameter: 0.012 in.
• Practical maximum: ~0.12 in. (standard), larger in carbide
• Example: 0.2-in. hole through 2.9-in. thick carbide in 49 minutes

Machining Graphite Electrodes

Graphite electrodes require special handling:

• Tool materials: Tungsten carbide or polycrystalline diamond (must be kept sharp)
• Graphite does not shear—it fractures and produces fine dust
• Respiratory hazard: Efficient exhaust system required, especially for copper-infiltrated graphite

Recommended cutting speeds:

Turning parameters:

• Positive rake angles, nose radii of 1/64 to 1/32 in.
• Depth of cut: 0.015–0.020 in. (produces better finish than lighter cuts)
• Feed rates: 0.005 in./rev (roughing), 0.001–0.003 in./rev (finishing)

Bandsawing: Standard carbon steel blades at 2,100–3,100 surface ft/min

Milling: Rigid machines, short tool extensions, firm clamping. Feed/tooth for two-flute end mills: 0.003–0.005 in. (roughing), 0.001–0.003 in. (finishing)

Drilling: High-spiral tungsten carbide drills for production. Use pecking cycles. Feed rates: 0.0015–0.002 in./rev (up to 1/32 in. drill), 0.002–0.005 in./rev (larger drills)

Surface grinding: Medium (60) grade, silicon-carbide wheel, vitreous bond. Wheel speed: 5,300–6,000 surface ft/min. Roughing: 0.005–0.010 in./pass. Finishing: 0.001–0.003 in./pass. Achievable finish: 18–32 µin. Ra.

Flame Cutting: The Original Thermal Process

When Brute Force Meets Chemistry

Every metal cutting process on the practitioner's floor was precise, controlled, and clean. But when the maintenance crew needed to cut a 14-inch-thick steel base plate for an equipment relocation, none of those processes could touch it. The answer was as old as the acetylene torch itself: flame cutting.

The Physics: Oxidation, Not Melting

Flame cutting is fundamentally a chemical process, not just a thermal one. When iron or steel is heated to a high temperature, it develops an intense affinity for oxygen. The cutting torch exploits this:

1. Preheating flame raises the metal to white-hot temperature
2. Jet of pure oxygen contacts the heated metal
3. Rapid oxidation occurs—the metal literally burns
4. Kerf is left behind, resembling a saw cut when properly executed

The cutting torch consists of:

• A heating jet (oxyacetylene, oxyhydrogen, or other fuel gas + oxygen)
• An auxiliary jet of pure oxygen for the cutting action
• Some torches have multiple preheating flame ports surrounding the central oxygen port

Metals That Can Be Cut

Cutting Stainless Steel with Flux Injection

The elements that give stainless steels their desirable properties produce obstructing oxides during conventional oxy-fuel cutting. The flux-injection method solves this:

• A suitable flux powder is injected into the cutting oxygen stream before it enters the torch
• A vibrator-type dispenser with rheostat control regulates flux flow rate
• Works with both machine and hand-controlled torches
• Operating procedure and speed are practically the same as for mild steel

Cutting Cast Iron

Cast iron cutting is a partial melting operation, not pure oxidation:

• Carbon in graphite form hinders the oxidation reaction
• Larger preheating flame and greater oxygen consumption than steel
• Temperature must approach the melting point (vs. bright red heat for steel)
• Speed improvement technique: Feed a ¼-inch diameter steel rod into the top of the cut beneath the torch tip—provides additional slag that increases cast iron temperature

Maximum Cutting Thickness

Important: Mechanically guided torches produce better results on thick material than hand-guided torches. Hand guidance causes wobble, which widens and degrades the kerf.

Kerf width ranges:

• Light material: ~1/16 inch
• Heavy stock: ¼ to ⅜ inch

Cutting Steel Castings: The Blowhole Problem

When flame-cutting steel castings, blowholes present a specific hazard:

• If the flame penetrates a blowhole, molten oxide splashes into the cavity
• The flame is diverted, and excessive sparking indicates the problem
• Corrective action: Immediately move the torch back along the cut, angle it to strike metal beneath the blowhole, burn away the material beyond the cavity, then resume

Mechanically Guided Torches

For production work and precision outlines:

• Pantograph mechanisms: Trace outlines from patterns or drawings
• Straight-line designs: For linear cuts
• Circular cutting attachments: Dedicated designs for round cuts
• Numerical control: Modern systems use CNC for complex profiles

Arc Cutting of Metals

When Oxidation Fails

The cutting of steel with an oxyacetylene torch is a chemical action—oxygen combines with iron to form iron oxide. But not all metals oxidize easily. Cast iron, stainless steels, manganese steels, and non-ferrous materials resist this oxidation, making flame cutting difficult or impractical.

Arc cutting addresses this limitation. Instead of relying on chemical oxidation, arc cutting uses the intense heat of an electric arc to melt through the material. The process works on:

• Cast iron (where graphite carbon hinders oxidation)
• Stainless steels
• Manganese steels
• Non-ferrous materials

Key difference: In steel, the cutting action starts at bright red heat. In cast iron, the temperature must approach the melting point for sufficient reaction. Arc cutting bypasses this limitation by providing temperatures far above the melting point.

Plasma Arc Cutting: Precision at the Speed of Lightning

How Plasma Cutting Works

1. An electric arc is struck between the electrode and the workpiece
2. Gas flowing through the torch is superheated into plasma
3. A restricting nozzle orifice accelerates the plasma to extremely high velocity
4. The concentrated energy melts through the material
5. The high-velocity gas jet ejects the molten metal from the kerf

Plasma vs. Oxy-Fuel: Speed Comparison

Plasma cutting is much faster than oxygen/fuel torch cutting on steel less than ½ inch thick. However, it produces kerfs with some variation in width and bevel angle, affecting part precision. Some molten metal may recast on cut edges and be difficult to remove.