Tooling and Toolmaking Process Handbook: Flushing Methods

Flushing Methods

Flushing away particles is vital to successful EDM operations. Methods include:

Warning: Excessively high flushing pressures can displace the electrode, the workpiece, or both, causing inaccuracy in the finished product. Pressure-relief valves in the system are recommended.

Electrode Materials

Graphite is the dominant electrode material, providing superior metal removal rates because of its resistance to thermal damage.

Infiltrated graphite (copper particles in graphite matrix) provides a trade-off: lower arcing and greater wear with slower metal removal, but better machinability of the electrode.

Polarity and Its Effects

Newer generators can provide less than 1% wear with either copper or graphite electrodes during roughing operations using positive polarity and elevated on times.

Machine Settings: Rules of Thumb

Power selection for graphite and copper electrodes: 50–65 amps per square inch of electrode engagement.

Example: An electrode that is ½ inch square:

$$\text{Current} = 0.5 \times 0.5 \times 50 = 12.5 \text{ amps}$$

Ideal gap voltage: About 35 volts, but should be as small as possible to maintain process stability.

Spark Frequency and Surface Finish

Higher frequencies are used for finishing operations and for work on cemented carbide, titanium, and copper alloys.

The Duty Cycle: Optimizing On/Off Times

A typical EDM cycle might last 100 µs, with current on for 40 µs and off for 60 µs. The effects of varying these parameters are dramatic:

Optimization technique: If overcut, wear, and finish are satisfactory, slowly decrease the off time in increments of 1–5 µs until machining becomes erratic, then return to the previous stable setting. Do not allow gap voltage to drop below 35–40 volts.

Workpiece Material Properties for EDM

Matching recommendation: The melting points and specific gravities of the electrode material and the workpiece should preferably be similar. Aluminum, brass, and copper workpieces should be processed with metallic electrodes of low melting points. Carbon and stainless steel should be processed with graphite electrodes.

The Recast Layer: Understanding EDM's Metallurgical Signature

One drawback of the EDM process on steel is the recast layer — created wherever sparking occurs. The oil-based dielectric turns the EDM operation into a random heat-treatment process:

1. Metal surface is heated to very high temperature
2. Then quenched in oil
3. Heat breaks down the oil into hydrocarbons, tars, and resins
4. Molten metal draws out carbon atoms and traps them in the resolidified metal
5. This forms a very thin, hard, brittle surface that covers the heat-affected zone

The recast layer has a white appearance and consists of material that has been melted, enriched with carbon, and drawn back to the surface or retained by surface tension.

Machining Graphite Electrodes

Graphite is extremely abrasive. Here are the recommended practices:

Cutting speeds by tool material:

Turning recommendations:

• Positive rake angles
• Nose radii: 1/64 to 1/32 inch
• Depths of cut: 0.015–0.020 inch (better finish than light 0.005-inch cuts — graphite chips rather than flowing)
• Feed rates: 0.005 in./rev for roughing; 0.001–0.003 in./rev for finishing
• Cut-off tool angle: 20°

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

Health and safety: Graphite dust can cause respiratory problems and allergic reactions (especially copper-infiltrated graphite). An efficient exhaust system is essential. Air velocities of at least 500 ft/min for flushing and 2,000 ft/min in collector ducts are recommended.

Wire EDM: Precision Profile Cutting

Wire EDM machines are numerically controlled and use a fine brass or copper wire as the electrode, continuously wound from one reel through the workpiece to another reel.

Wire specifications:

• Material: Yellow brass (63% copper, 37% zinc), or brass alloyed with aluminum/titanium
• Tensile strength: 50,000–160,000 lbf/in²
• Diameter: 0.002 to 0.012 inch
• Diameter tolerance: ±0.00004 in. (drawn), ±0.00006 in. (plated)
• Polarity: Wire negative (wire used only once, so wear is irrelevant)

Zinc-coated wires give faster cutting and reduced wire breakage — zinc boils off at 419°C while the brass core (melting at 930°C) continues delivering current.

Key capabilities:

• Gap control within 0.1 micron (0.000004 inch) of programmed position
• Heat-affected zone can be held below 1 micron (0.00004 inch) with proper settings
• Carbon is extracted from the recast layer (opposite of sinker EDM in oil)
• Wire-cut surfaces may be slightly softer than parent metal (copper migration from wire)

Drilling holes for wire EDM:

• A hole must be provided in the workpiece before apertures can be cut
• EDM "drills" a 0.04-inch hole through 4-inch steel in about 3 minutes (brass or copper tubing electrode)
• Practical minimum hole: 0.012 inch (limited by overcut and electrode rigidity)
• Practical maximum hole: ~0.12 inch (larger sizes require too much material removal)
• EDM can drill large holes in tungsten carbide (e.g., 0.2-inch hole through 2.9-inch thick carbide in 49 minutes)

Electrode Discharge Dressing

Copper electrodes have a unique advantage: they can be discharge-dressed in the EDM itself, usually under CNC control. The worn electrode is engaged with a premachined dressing block (copper-tungsten or carbide), renewing the original shape with sharp, burr-free edges.

Laser Processing: Cutting, Welding, Drilling, and Heat Treatment

By the third year of his contract, the practitioner's operation had grown to include laser processing — and with it came an entirely new dimension of toolmaking capability.

Understanding Laser Fundamentals

The word LASER stands for Light Amplification in the supplied reference of Radiation. A laser produces optical-frequency radiation in intense, controllable quantities that cause localized effects on material surfaces with low part distortion.

The four basic components:

1. Amplifying medium (solid crystal or gas mixture)
2. Excitation source (flashlamp or electrical discharge)
3. Optical resonator (mirrors)
4. Output transmission device

Industrial Laser Types

Key relationship: The CO₂ laser beam (10.6 µm) produces a focused spot ten times larger than the Nd:YAG beam (1.06 µm) at the same focal length.

Cutting Metal with Lasers

The fundamental relationship: Process depth is proportional to power and inversely proportional to speed. Doubling power doubles penetration depth.

• Maximum cuttable thickness: 25 mm (1 inch) for steel alloys
• Most economical range: Up to 12.5 mm (0.49 inch)

Oxygen-assisted cutting of ferrous alloys: A jet of oxygen concentric with the laser beam is directed against the heated surface. The molten puddle's heat causes oxygen to combine with the metal, and the gas pressure ejects molten metal from the kerf.

Kerf Widths in Laser Cutting

Surface Roughness in Laser Cutting (with Oxygen Assist)

Heat-Affected Zones in Laser-Cut Mild Steel

Pulsed CO₂ laser cutting reduces HAZ values significantly — beneficial for end-use applications requiring minimal metallurgical change.

CO₂ Laser Cutting Rates for Nonmetals

Laser Welding

Two types of laser welding exist:

Conduction welding: Relies on thermal diffusivity to conduct heat into the joint. Used for spot welding and partial penetration seam welding.

Deep penetration (keyhole) welding: Beam energy creates a hole through the metal thickness. Vapor pressure holds molten metal against the hole wall. As the hole moves, molten metal flows around and solidifies behind the beam. Maximum practical penetration: approximately 25 mm (2 inches).

The key advantage of laser welding: Low total heat input. With the beam moving faster than thermal conduction speed, significant heat flow occurs only perpendicular to the direction of motion. This produces minimum thermal distortion.

Joint tolerances for laser welding:

• Corner, tee, and lap joints: Gaps not more than 25% of the thickness of the thinnest section
• Butt and edge joints: Gaps not more than 10%

Laser Drilling

Three methods produce holes of increasing quality:

Laser Heat Treatment

The defocused CO₂ laser beam at room temperature has 90% or more of its power reflected by metal surfaces (93% for steels). Surface preparation is essential:

• Surface roughening creates tiny craters that trap beam energy
• Black enamel paint coating absorbs energy then vaporizes, leaving a clean surface

Materials suitable for laser heat treatment:

Typical hardening rate: 130 cm²/min (20 in²/min) for a 1-mm (0.039-in) case depth in 4140 steel.

Laser Cladding

Laser cladding applies a hard metal coating to a softer alloy using a shaped or defocused beam to heat preplaced or gravity-fed powdered alloys.

The key technical advantage: Controlled minimal dilution.

The result is a dense, homogeneous, nonporous clad layer that is metallurgically bonded to the substrate — in contrast to the mechanically bonded, more porous layers produced by other methods.

Laser Marking

Extrusion: Tooling for Continuous Cross-Sections

The Basic Process

Extrusion squeezes a solid slug of metal from a closed container through a die, producing long, straight semifinished products. During extrusion, compressive and shear forces (but no tensile forces) are developed in the stock, allowing heavy deformation without fracturing.

Extrusion Methods

Hot Extrusion Temperature Ranges

Pressures range from 5,000 to over 100,000 psi, requiring careful lubrication and protection of chamber, ram, and die.

Minimum Extrusion Cross-Sections

Cold Extrusion Applications

Cold extrusion produces parts with high mechanical properties, narrow tolerances, good surface finish, and fast extrusion speeds. Examples include collapsible tubes, aluminum cans, fire extinguisher cases, shock absorber cylinders, automotive pistons, and gear blanks.

Powder Metallurgy: Tooling for the Impossible

The Process

Powder metallurgy compresses and sinters powdered metals (brass, bronze, aluminum, iron) into finished parts using accurately formed dies and punches in hydraulic or mechanical presses. The "green" compressed pieces are sintered in atmosphere-controlled furnaces at high temperatures, bonding the powder into a solid mass.

What Powder Metallurgy Can Produce

• Controlled porosity: 5 to 50 per cent in the final product
• Self-lubricating bearings: Porous bronze and iron bearings impregnated with oil
• Filters: For liquids and gases
• Dense products: Refractory metal wire and sheet, cemented carbide tools, electrical contacts
• Complex shapes: Gears and intricate forms

Parts Only Powder Metallurgy Can Produce

• Irregular curves, eccentrics, radial projections, or recesses
• Irregular holes, keyways, flat sides, splines, or square holes
• Tapered holes and counter-bores
• Axial projections (up to one-quarter the length of the part)
• Slots, grooves, blind holes, and recesses of varied depths

Tooling Design for Powder Metallurgy

Dimensional tolerances: 0.0002 inch with super-finished surfaces.

Critical design rules:

• Use corner radii, fillets, and bevels — avoid sharp corners
• Feather edges, threads, and re-entrant angles are usually impracticable
• Allowances must be made for dimensional changes due to growth after pressing and shrinkage or growth during sintering

Powder Metallurgy Tolerances

The difference in achievable tolerances is due to the elasticity of the powder and spring of the press.

Precision Investment Casting: Tooling for Extreme Accuracy

When to Use Investment Casting

Investment casting is applicable when:

• Metals are too hard to machine or otherwise fabricate
• It is the only practical method of producing a part
• It is more economical than any other method of obtaining required quality
• Exterior or interior contours of intricate form must be produced on surfaces that could not be machined readily, if at all

The Process

1. An expendable pattern (usually wax or injection-molded plastics) is created
2. Several patterns are joined together or to wax bars forming runner channels
3. The assembly is coated with investment material (ceramic slurry)
4. The investment is hardened, and the wax is melted out
5. Molten metal is poured into the resulting mold cavity
6. After solidification, the investment is removed
7. Finishing may include removal of risers, sand blasting, and minimal machining

Casting Milling Cutters in the supplied reference

A practical demonstration of investment casting's capability: high-speed steel milling cutters of various forms and sizes have been produced by this method. The only machining required is removal of risers, sand blasting, and grinding the cutting edges. The bore is used as cast. Tests show the life of these cutters compares favorably with conventionally manufactured high-speed steel cutters.

Shrinkage Allowances

Investment castings require greater shrinkage allowances than die castings due to:

• Pattern shrinkage (wax or plastic)
• Investment shrinkage
• Metal solidification shrinkage

These allowances must be carefully calculated for each material and geometry combination.

Flame Spraying: Rebuilding and Protecting Tools

The Process

Flame spraying deposits metals, alloys, ceramics, and cermets onto metallic or other surfaces. Applications include:

• Building up worn or undersize parts
• Providing wear-resisting or corrosion-resisting surfaces
• Correcting defective castings

Equipment and Operation

Wire is fed automatically through the spray gun nozzle. A combustible gas (usually acetylene), oxygen, and compressed air serve to:

1. Melt the wire
2. Atomize the molten metal
3. Propel the particles against the surface to be coated

Your Next Step

Pick one process from this guide that applies to a current project or challenge. Go deeper into that section. Run the calculations. Compare the specifications against what you are currently doing.

The gap between what you find and what you currently practice is your immediate opportunity for improvement.

Whether that means recalculating your die clearances, investing in fine blanking capability, adding EDM to your shop, or optimizing your laser parameters — the data is here. The formulas are proven. The principles are timeless.

The only variable left is action.

What tooling challenge are you currently facing? Which section of this guide addresses it most directly? Start there — and build from that foundation outward.