Manufacturing Process Selection from Raw Material to Finished Part: Punches, Dies, and Press Work

The Complete Machining Operations Guide Every Engineer Needs on Their Workbench

an illustrative engineering practitioner once stared at a rejected batch of 4,000 stamped brackets and realized his team had been using the wrong die clearance for three production runs straight. The scrap bin was overflowing. The client was calling every hour. And the root cause was embarrassingly simple: nobody on the floor truly understood the relationship between material thickness, clearance percentage, and fracture quality.

the practitioner's story isn't unusual. It's the story of every shop that treats manufacturing knowledge as tribal wisdom instead of engineered certainty.

This guide exists so you never become the practitioner. Every process. Every parameter. Every critical table and formula you need — from punches and dies to laser cladding — organized so you can find it in seconds and apply it for decades.

Punches, Dies, and Press Work: Where Raw Sheet Becomes Precision Parts

Understanding Clearance — The Foundation of Every Stamping Operation

If you take away one concept from press work, make it this: clearance between the punch and die controls everything — fracture quality, tool life, required press force, and dimensional accuracy.

What "clearance" actually means: The space between the punch and the die on one side only. For round dies, clearance equals die radius minus punch radius. Some shops define clearance as the total difference between punch and die diameters, but this creates confusion with irregular shapes. Always specify clearance as the gap on one side.

Where to apply it:

• Blanking to a given size → The die is made to the required size, the punch is made smaller (clearance is subtracted from punch diameter)
• Perforating holes of a given size → The punch is made to the required diameter, the die is made larger (clearance is added to die diameter)

General clearance guidelines:

Key insight: Soft, ductile metals actually require more clearance than hard metals — the opposite of what many shops assume.

Angular clearance for dies ranges from 1° to 2° for production dies requiring large quantities of blanks. Dies intended for short runs may use 4° to 5° for faster fabrication. Two methods exist for providing clearance: extending the clearance to the top face (used for very soft, thin metals like soft brass), or leaving a short straight section of about 1/8 inch below the cutting edge before the angular relief begins (used for harder materials like steel, allowing thousands of blanks to be cut with minimal size variation after regrinding).

Pressure Required for Punching

The force needed to punch or blank depends on the material's shear strength, the perimeter of the cut, and the stock thickness.

$$P = L \times t \times S$$

Where:

• P = punching pressure (force units)
• L = length of cut (perimeter of the shape)
• t = stock thickness
• S = shear strength of the material

Effect of clearance on pressure: Clearance directly impacts the force required. In one documented test, punching 3/4-inch holes in 5/16-inch mild steel plate required approximately 32,000 pounds of force at 10% clearance, 33,000 pounds at 4.5% clearance, and 34,500 pounds at 2.75% clearance. Tighter clearance = more force.

Speeds and Pressures for Press Operation

For punching and shearing ordinary metals not over 1/4 inch thick, press speeds typically range between 50 and 200 strokes per minute, with 100 strokes per minute being a solid average. For stock over 1/4 inch thick, geared presses operating at 25 to 75 strokes per minute are commonly used.

Lubricants for Press Work

Your lubricant selection should match the operation severity:

Lubricant thickness should be approximately 0.0001 inch — roughly 1 pint of fluid to cover 500 square feet of material. For sheet thicker than 1/8 inch and for stainless steel, use high-pressure lubricants containing sulfur and chlorine compounds.

Drawing Cylindrical Shells

The fundamental rule: The depth of the first draw should never exceed the diameter of the shell.

Another practical guideline: the depth given on the first draw should equal approximately one-third the diameter of the blank. Ordinarily, sheet steel of any thickness up to 1/4 inch can be drawn so that the diameter of the first shell equals about 60% of the blank diameter.

Diameter reductions for successive draws (with annealing between operations):

In double-action presses with internal bushing support, reductions can increase to 30%, 24%, 18%, 15%, and 12% respectively.

Ironing limits: Wall thinning (ironing) per draw ranges between 0.002 and 0.004 inch per side, and should not exceed 0.001 inch on the final draw if a good finish is required.

Blank diameter formula for cylindrical shells:

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

Where D = blank diameter, d = shell diameter, and h = shell depth. For shells with a rounded bottom corner, subtract the corner radius from the table value.

When significant wall thinning occurs:

$$M = \frac{h \times t}{T}$$

Where M = mean height (used in place of actual height), h = shell height, t = final wall thickness, and T = original stock thickness.

Drawing Rectangular Shapes

Corner radius is critical for rectangular draws. Maximum achievable depth correlates directly with corner radius:

Annealing Drawn Shells

When drawing steel, iron, brass, or copper, anneal between at least every two draws because the drawing process work-hardens the metal. Tin plate or stock that cannot be annealed without damaging the finish must be drawn to size in one or two operations. Aluminum can be drawn deeper with less annealing than other commercial metals — if annealing is necessary, heat in a muffle furnace and never exceed 700°F.

For brass shells: making the depth of the first draw equal to the shell diameter is generally possible. Heating brass to just below a dull red (visible in a dark room) enables drawing of otherwise impossible shapes with square corners.

Bending Sheet Metal

The critical calculation for bending is the length of straight stock required for each bend. Three formulas, based on material type:

For 90° bends in soft brass and soft copper:

$$L = (0.55 \times T) + (1.57 \times R)$$

For 90° bends in half-hard copper/brass, soft steel, and aluminum:

$$L = (0.64 \times T) + (1.57 \times R)$$

For 90° bends in bronze, hard copper, cold-rolled steel, and spring steel:

$$L = (0.71 \times T) + (1.57 \times R)$$

Where L = length of straight stock for the bend, T = material thickness, and R = inside bend radius.

For bends at angles other than 90°: Calculate L using the appropriate formula above, then multiply by (angle of bend ÷ 90). The "angle of bend" is the actual angle through which the material bends — not necessarily the angle shown on the drawing.

Fine Blanking

Fine blanking uses special presses and tooling to produce flat components with exceptional dimensional accuracy — edges perpendicular to the surfaces within 0.004 inch on 0.2-inch thick stock, with surface finishes down to 80 µin. Ra.

Key differences from conventional blanking:

• Forces are 1.5 to 2.5 times those of conventional stamping
• Cutting clearances are held between 0.0001 and 0.0003 inch
• Sharp V-projections (90° cross-section) on the stripper plate bite into the material before blanking, preventing sideways movement and causing material to flow toward cutting edges
• Three separate press movements required: clamping, blanking, and ejection

V-Projection Dimensions (Stripper Plate Only):

For material thicker than 0.157 inch, V-projections are placed on both the stripper plate and the die plate to enhance material flow.

Steel Rule Dies

Steel rule dies (or knife dies) use cutting edges made from steel strips similar to graduated measuring rules. They cost 25 to 35% of conventional blanking dies and can be produced in much less time.

Die block materials:

• Light work: 3/4-inch thick, five- or seven-ply maple or birch wood
• Metal sheet operations: Lignostone densified wood (35 plies of compressed lignite wood bonded with phenolformaldehyde resin) or metal
• Other options: Plaster, hot lead, type metal, or epoxy resin

Heat treatment of steel rules by application:

Carburizing treatment: liquid compound at 1500°F, quench in oil, followed by tempering at 550°F and furnace cooling.

Electrical Discharge Machining (EDM): Shaping the Unmachineable

How EDM Works

EDM removes metal by generating electric sparks between an electrode and a workpiece submerged in dielectric fluid. The spark creates a plasma channel (ionized particles at extreme temperatures), which melts and vaporizes tiny amounts of workpiece material. Each spark creates a small crater. Thousands of sparks per second, controlled with microsecond precision, shape the workpiece to the desired geometry.

The two main types:

• Sinker (Plunge) EDM — Uses a shaped electrode (usually copper or graphite) as a "positive replica" of the cavity to be created. Resembles a vertical milling machine with the electrode on the vertical slide.
• Wire EDM — Uses a fine brass or copper wire (0.002 to 0.012 inch diameter) as the electrode, moving through the workpiece like a bandsaw blade. Numerically controlled for complex profiles.

EDG (Electrical Discharge Grinding) uses a graphite wheel as an electrode, mainly for producing complex profiles on polycrystalline diamond cutting tools and carbide tooling.

The EDM Process in Detail

1. Voltage applied across the gap between electrode and workpiece
2. Dielectric fluid ionizes in the smallest gap area, forming a plasma channel
3. Spark fires through the plasma, heating and vaporizing workpiece material
4. Voltage drops as the plasma loses energy and the spark cannot be sustained
5. Plasma implodes, creating a low-pressure pulse that draws in dielectric fluid
6. Dielectric flushes away metallic debris and cools the work area
7. Cycle repeats — typically lasting a few microseconds (µs)

Flushing: The Unsung Hero of EDM

Flushing away particles is vital to successful EDM. Poor flushing leads to DC arcing, surface pitting, electrode displacement, and inaccurate cavities.

Flushing methods:

• Pressure flushing — Fluid pumped through holes in the electrode or workpiece. Use many low-pressure holes rather than a few high-pressure ones.
• Vacuum flushing — Fluid sucked through the gap. Best for straight, accurate sidewalls.
• Side flushing — External nozzle directs fluid movement in the surrounding tank.

Critical warning: Excessively high flushing pressures can displace the electrode, the workpiece, or both. Pressure-relief valves are recommended. Trapped gases from sparking may explode — proper ventilation and safety measures are essential.

Polarity and Its Effects

Spark Frequency and Duty Cycle

Spark frequency = number of on/off cycles per second.

• Low frequency → larger spark gaps, rapid metal removal, rough finish, reduced electrode wear
• High frequency → small gaps, finer finish, increased electrode wear; required for cemented carbides, titanium, and copper alloys

Duty cycle = on time as a percentage of total cycle time.

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

A typical cycle might last 100 µs with 40 µs on and 60 µs off, yielding a 40% duty cycle.

Effect of Electrical Control Adjustments on EDM Operations:

Optimization tip: To increase machining speed without changing overcut, wear, or finish — slowly decrease off time in increments of 1 to 5 µs until machining becomes erratic, then return to the previous stable setting. Gap voltage should not drop below 35 to 40 volts.

Machine Settings Rule of Thumb

For vertical sinker machines using graphite or copper electrodes:

$$\text{Amperage} = 50 \text{ to } 65 \text{ amps} \times \text{electrode engagement area (in}^2\text{)}$$

Example: A 1/2 inch square electrode → 0.5 × 0.5 × 50 = 12.5 amps

Lower amperage for thin electrodes, sharp details, or very large jobs (to avoid overheating and recast layer issues). Ideal gap voltage is approximately 35 volts.

Electrode Materials

Graphite is the dominant electrode material due to its superior metal-removal rate and thermal damage resistance. Instead of melting, graphite sublimates (solid → gas) at 3,350°C (6,062°F).

Fine-grain, high-density graphites provide the best wear characteristics, finish quality, and detail reproduction — but cost up to five times more than the least expensive grades and about three times more than copper.

Infiltrated graphites (copper particles in a graphite matrix) offer a trade-off between lower arcing, greater wear, and slower removal rates.

Electrode and Workpiece Material Combinations

Characteristics of Common Workpiece Materials for EDM

Material-electrode pairing rule: Workpieces with high melting points (carbon steel, stainless steel) should be processed with graphite electrodes. Workpieces with lower melting points (aluminum, brass, copper) should be processed with metallic electrodes (copper or copper-tungsten). Sintered materials (tungsten carbide, cobalt-based PM) require high frequencies with very short on times to prevent excessive heat buildup.

The Recast Layer Problem

Every EDM surface on steel develops a recast layer — an extremely thin, hard, brittle surface created when the oil dielectric causes a random heat-treatment effect. The surface is heated to extreme temperature, quenched in oil, and the molten metal draws carbon atoms from the broken-down oil, trapping them in the resolidified surface.

Wire EDM differs: Deionized water extracts carbon from the recast layer rather than adding to it. Copper from the wire migrates into the surface, actually softening it slightly. With proper adjustment, the heat-affected zone (HAZ) can be held below 1 micron (0.00004 inch).

Machining Graphite Electrodes

Graphite is extremely abrasive — carbide tools are mandatory. The material fractures and generates fine powder rather than flowing chips.

Recommended cutting speeds:

Turning: Positive rake angles, nose radii of 1/64 to 1/32 inch. Depths of cut of 0.015 to 0.020 inch produce better finish than light cuts (0.005 inch) because graphite chips away rather than flowing. Feed rates: 0.005 in/rev rough, 0.001 to 0.003 in/rev finish.

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

Drilling: High-spiral tungsten carbide drills for production work; diamond-tipped drills last longest. Use pecking cycles to clear dust. Feed rates: 0.0015 to 0.002 in/rev for drills up to 1/32 inch; 0.001 to 0.003 in/rev for 1/32 to 1/8 inch; 0.002 to 0.005 in/rev for larger drills.

Surface grinding: Medium (60) grade, medium-open structure, vitreous-bond, green-grit, silicon-carbide wheel at 5,300 to 6,000 surface ft/min. Roughing: 0.005 to 0.010 in/pass. Finishing: 0.001 to 0.003 in/pass. Normal finish range: 18 to 32 µin. Ra.

Health warning: Graphite dust causes respiratory problems and allergic reactions, especially with copper-infiltrated grades. An efficient exhaust system with air velocities of at least 500 ft/min for flushing and 2,000 ft/min in collector ducts is essential.

Wire EDM Specifics

Wire is typically yellow brass (63% copper, 37% zinc) with tensile strength of 50,000 to 145,000 lbf/in², available in 0.002 to 0.012 inch diameter. Zinc-coated brass wires cut faster with reduced breakage due to zinc's low vaporization temperature (906°C) — the zinc boils off while the brass core (melting at 930°C) continues delivering current.

Drilling holes for wire EDM: A 0.04-inch diameter hole can be "drilled" through 4-inch thick steel in approximately 3 minutes using brass or copper tubing as the electrode. Practical limits: minimum 0.012 inch (due to overcut and rigidity), maximum approximately 0.12 inch (due to excessive material removal requirements).

Wire precision: Diameter tolerance ±0.00004 inch for drawn wire, ±0.00006 inch for plated wire. Power source maintains the arc gap within 0.1 micron (0.000004 inch) of the programmed position.

Iron and Steel Castings: From Molten Metal to Finished Form

The Four Fundamental Cast Irons

Cast irons contain more than 2% carbon and 1 to 3% silicon. The shape and distribution of free graphite, and the matrix surrounding it, determine all mechanical properties.

Gray Cast Iron — The workhorse of the foundry. Contains 1.7 to 4.5% carbon and 1 to 3% silicon. Excess carbon exists as graphite flakes, producing the gray fracture surface that gives it its name. Excellent machinability, high damping capacity, easily cast in complex shapes. Used for machine tools, automotive cylinder blocks, pipe and fittings.

ANSI/ASTM A48-76 classes range from 20A (20,000 psi minimum tensile) through 60C, where the prefix number indicates minimum tensile strength in thousands of psi. Lower classes (20–35) offer excellent machinability and high damping; higher classes (40–60) are more difficult to machine but offer greater strength.

White Cast Iron — Nearly all carbon is in the combined (cementite) form. Silvery-white fracture. Very hard and brittle; ductility is practically zero. Compressive strength usually exceeds 200,000 psi (compared to 65,000 to 160,000 psi for gray iron). Mostly used as a precursor for malleable iron production, plus applications demanding maximum wear resistance.

Chilled Cast Iron — Gray iron castings with localized white iron surfaces, produced by using metal chills in the mold for rapid cooling. Combines the machinability of gray iron with wear-resistant surfaces where needed.

Malleable Iron — Produced by annealing white iron castings. The graphitization process creates temper carbon (graphite in compact, rounded aggregates). Offers strength, ductility, machinability, and shock resistance. The process starts with a hard, brittle white iron casting from pig iron and scrap, then transforms it through controlled annealing.

Ductile (Nodular) Cast Iron — Created by adding magnesium or cerium to the molten iron, causing graphite to form as spheroids instead of flakes. This shape change dramatically improves ductility and impact resistance compared to gray iron.

Alloy Cast Iron — Contains alloying elements (nickel, chromium, molybdenum, copper, manganese) in amounts sufficient to change physical properties. Machinable alloy cast irons can achieve tensile strengths up to 70,000 psi or higher. Used for automotive cylinders, pistons, piston rings, brake drums, crushing/grinding machinery, and high-temperature applications.

Casting Methods

Green-sand molding — The most common method. Sand mixed with a binder is packed around a pattern. The term "green-sand" means the binder is not cured by heat or chemical reactions. The pattern is made in two halves attached to a flat plate with channels for metal entry.

Shell molding — Uses a resin binder to create a 1/4 to 3/8 inch thick shell of sand/resin mixture that adheres to a heated pattern plate. Produces higher accuracy than green-sand.

Vacuum (V-process) molding — Uses no binders. Dry, unbonded sand is held against a thin plastics film on the pattern by vacuum. The film melts and evaporates when molten metal is poured. Castings emerge cleanly; sand needs only cooling before reuse.

Permanent mold (gravity die) casting — Mainly for nonferrous metals. Iron or steel molds (or graphite) with water or air cooling. Cavity surfaces coated with heat-resistant material. The alloy must be sufficiently ductile to accommodate cooling restrictions without fracturing (hot short alloys cannot be used).

Low-pressure casting — Similar to permanent mold but uses 6–10 psi gas/air pressure to force metal up a hollow refractory tube (stalk) into the die. Metal entering the die is free from surface oxides and impurities.

Pattern Materials and Shrinkage Allowances

Pattern woods: White pine (most common — easy to work, takes glue and varnish); mahogany (close grain, resistant to atmospheric changes); cherry; maple and birch (for turned parts). Wood must be well-seasoned — kiln-dried or kept 1 to 2 years. Always use heartwood, not sapwood.

Shrinkage allowances per foot of pattern dimension:

Critical note: These are approximations. Shrinkage depends on casting shape and mold resistance. A straight round steel bar required approximately 9/32 inch/ft, but the same bar with large knobs on each end needed only 3/16 inch/ft, and with large flanges, only 7/64 inch/ft. Always consult your foundry.

Obtaining casting weight from pattern weight: Multiply the pattern weight by the appropriate factor. A white-pine pattern weighing 4 pounds for a solid cast-iron part yields approximately 4 × 16 = 64 pounds of casting.

Die Casting

Die casting uses permanent metal molds (dies) under pressure to produce high volumes of dimensionally accurate parts. Key alloy families:

• Aluminum-base alloys — Lightweight, good strength, widely used
• Zinc-base alloys — Excellent dimensional stability, complex shapes
• Copper-base alloys — Good strength and corrosion resistance
• Magnesium-base alloys — Lightest structural metal
• Tin-base and lead-base alloys — Lower melting points, specialized applications

Design considerations: Uniform wall thicknesses between 0.040 and 0.375 inch. Gradual transitions from thick to thin sections. Molten metal should not pass through thin sections to fill thick portions. Fillets on all internal corners to avoid stress concentrations.

Precision Investment Casting

Investment casting achieves casting accuracy unmatched by most other methods, forming intricate contours with surfaces that would be impossible to machine conventionally.

General procedure:

1. Create a master mold (usually carbon steel or soft alloy)
2. Inject expendable pattern material (wax or plastics) into the master mold
3. Assemble patterns into clusters ("trees") with wax runners
4. Coat the tree with refractory investment material
5. Remove wax by heating (creating the cavity)
6. Pour molten metal into the mold
7. Break away the investment after solidification

Dimensional capabilities:

• General tolerance: ±0.005 inch
• Specified dimensions: as low as ±0.002 inch
• Minimum wall thickness: 0.020 inch (high castability alloys) to 0.040 inch (low castability)
• Weight range: fractional ounce to 75+ pounds (practical limit typically 10 to 15 pounds)
• Length: ordinarily under 12 to 15 inches, but much longer parts possible

Machining allowances: 0.010 inch on small parts to 0.040 inch on large parts. For many parts, machining is eliminated entirely or reduced to a minimum.

Extrusion of Metals: Pushing Through to Complex Profiles

The Basic Process

Extrusion forces a billet through a die opening to create long pieces of uniform (or complex) cross-section. The stress conditions allow working of materials that would crack under other primary metalworking processes.

Two fundamental types:

Cold Extrusion — Uses cold or slightly warm billets. Not limited to soft metals — aluminum, copper, zirconium, titanium, molybdenum, beryllium, vanadium, niobium, and steel can all be cold extruded. Advantages include no oxidation, high mechanical properties from cold working, narrow tolerances, and good surface finish with optimum lubrication. Common products: collapsible tubes, aluminum cans, fire extinguisher cases, shock absorber cylinders, automotive pistons, gear blanks.

Hot Extrusion — Performed in horizontal hydraulic presses rated from 250 to 12,000 tons.

Temperature ranges for hot extrusion:

Pressures range from 5,000 to over 100,000 psi. At lower temperatures, oil and graphite mixtures suffice for lubrication; at higher temperatures, glass powder becomes a molten lubricant.

Minimum extrudable cross-sections: