Cutting Metals with Flame, Arc, and Plasma
Oxyacetylene Flame Cutting
The principle is simple: iron and steel heated to high temperature have a great affinity for oxygen. A cutting torch preheats the metal, then a jet of pure oxygen burns through it. The kerf resembles a saw cut when the torch is properly adjusted.
What can be flame-cut:
- Wrought iron and low-carbon steels: Cut readily
- High-carbon steels: Require preheating proportional to carbon content. Black heat is sufficient for ordinary tool steel; low red heat for some alloy tool steels.
- Stainless steel: Can be cut by the flux-injection method — a suitable flux powder is injected into the cutting oxygen stream, removing the obstructing oxides that otherwise reduce flame cutting to a slow melting process. Operating speed and procedure are practically the same as for mild steel.
- Cast iron: Practicable but more expensive. The cut contains considerable melted cast iron, indicating a partial melting operation. Speed improves by feeding a ¼-inch steel rod into the top of the cut beneath the torch tip — the rod generates slag that flows over the cut and increases temperature.
- Brass and bronze: Can be cut by interposing them between steel plates.
Maximum cutting thickness:
- Oxyacetylene: Up to 12–14 inches
- Oxyhydrogen: Up to 24 inches (longer flame penetrates deeper)
Kerf width: As narrow as 1/16 inch in light material. Up to 1/4 or 3/8 inch in heavy stock. Mechanically guided torches produce more accurate cuts in thick material because the flame doesn't wobble.
Arc Cutting
Cast iron, stainless steels, manganese steels, and nonferrous materials resist oxidation and cannot be cut easily with oxyacetylene. Arc cutting handles these materials because it doesn't depend on the chemical oxidation reaction — it melts through the material directly.
The distinction: cutting steel is a chemical action (oxygen combines with iron to form iron oxide). Cutting cast iron requires temperatures nearer to the melting point because the carbon in graphite form hinders the oxidation reaction.
Plasma Cutting
Plasma cutting uses a transferred DC arc with restricted nozzle orifice to create a high-velocity plasma jet that melts and blows away material. It is much faster than oxyacetylene for steel under ½ inch thick.
Precision plasma cutting adds a magnetic field in the cutter head to stabilize the arc via Lorentz forces. The result: a narrower kerf with no loss of cutting speed. Results are comparable to laser cutting and, with CNC control, suitable for production of small batches of blanks.
To reduce noise and fumes, mechanized plasma cutting is often performed with the workpiece submerged in water. Oxidation of cut surfaces is almost nonexistent with this method.
Files, Burs, and Precision Finishing
Understanding File Classification
Five main classes exist:
- Mill or saw files — For sharpening saws, lathe work, draw filing, brass and bronze work
- Machinists' files — For rapid metal removal where finish is secondary (mostly double-cut)
- Curved tooth files — Dual action: fast cutting in one direction, smoothing in the other
- Swiss pattern files — Made to closer tolerances, with longer tapers and finer cuts. Used by tool and die makers, model makers, and instrument finishers.
- Rasps — For soft materials (wood, leather, lead) where fast removal is needed
Coarseness grades:
- American pattern: Coarse, Bastard, Second, Smooth (4 grades)
- Swiss pattern: 00, 0, 1, 2, 3, 4, 6 (7 grades, 00 coarsest to 6 finest)
- Curved tooth: Standard, Fine, Smooth (3 grades)
Rotary Files and Burs: Speed Is Everything
Carbide burs at optimal speeds can remove stock four times faster than ordinary burs and last up to 100 times longer.
Recommended speeds (RPM) for medium-cut high-speed steel burs:
| Tool Diameter (in.) | Mild Steel | Cast Iron | Bronze | Aluminum | Magnesium |
|---|---|---|---|---|---|
| 1/8 | 4,600 | 7,000 | 15,000 | 20,000 | 30,000 |
| 1/4 | 3,450 | 5,250 | 11,250 | 15,000 | 22,500 |
| 1/2 | 2,300 | 3,500 | 7,500 | 10,000 | 15,000 |
| 3/4 | 1,900 | 2,900 | 6,200 | 8,300 | 12,400 |
| 1 | 1,600 | 2,400 | 5,150 | 6,850 | 10,300 |
Carbide bur speeds for any material: Approximately 60–70% of the aluminum speeds above. Carbide burs use a negative rake rather than radial, making them relatively brittle — keep them cutting freely to prevent crumbling of cutting edges.
Power Brush Finishing
Power brushes act as multiple-tipped cutting tools. The fill material contacts the work surface and produces a cold-working effect. Finish quality depends on wheel material, wheel speed, and application method.
Critical setup rule: The full face of the brush must contact the work. Line contact causes non-uniform wear, wire point flaring, and reduced effectiveness.
Troubleshooting power brush problems:
| Problem | Solutions |
|---|---|
| Brush works too slowly | Decrease trim length + increase fill density; increase filament diameter; increase surface speed |
| Brush works too fast | Reduce filament diameter; reduce surface speed; reduce fill density; increase trim length |
| Brush peens burr to adjacent surface | Decrease trim length + increase fill density; switch to nonmetallic brush with burring compound |
| Finer finish required | Decrease trim length + increase fill density; decrease filament diameter; try treated Tampico/cord brushes with compounds |
| Finish too smooth/lustrous | Increase trim length; reduce fill density; reduce surface speed; increase filament diameter |
| Brushing action not uniform | Use fixture to avoid irregular off-hand manipulation; increase trim length + decrease fill density |
Polishing vs. Buffing — Know the Difference
Polishing uses wheels with abrasive glued to the working surface. It removes material aggressively.
Buffing uses wheels with abrasive applied loosely (mixed with waxes or greases). It produces very fine "grainless" finishes. Buffing is less aggressive than polishing.
Polishing wheel speed: approximately 7,500 ft/min for ordinary operations. Below this, work tends to tear polishing material from the wheel. Maximum safe speed for muslin, felt, or leather wheels: 7,000 ft/min. For most purposes, 4,000 ft/min is sufficient.
Abrasive selection:
- Aluminum oxide — For high-tensile-strength metals: carbon and alloy steels, tough iron, nonferrous alloys
- Silicon carbide — For hard, brittle materials: grey iron, cemented carbide, brass, aluminum, copper
Emery grain numbers range from 10 (coarsest) to 200 (finest). Anything finer than 200 is designated as "flour" — graded CF, F, FF, FFF, FFFF (coarse flour to finest flour).
Surface Treatments — Etching, Conversion Coatings, Coloring, Anodizing, and Plating
Etching Fluids by Material
| Material | Etching Fluid |
|---|---|
| Carbon steel | Nitric acid, 1 part : Water, 4 parts (adjust ratio based on carbon content and hardness) |
| Hard steel | Nitric acid, 2 parts : Acetic acid, 1 part |
| High-speed steel, nickel, brass | Nitro-hydrochloric acid (Nitric 1 part : Hydrochloric 4 parts) |
| Bronze | Nitric acid, 100 parts : Muriatic acid, 5 parts |
| Aluminum | Alcohol 4 oz : Acetic acid 6 oz : Antimony chloride 4 oz : Water 40 oz |
For deep etching or frosted effects: 1 oz copper sulfate (blue vitriol), ¼ oz alum, ½ tsp salt, 1 gill vinegar, 20 drops nitric acid. Duration determines depth.
Conversion Coatings
Conversion coatings are thin, adherent chemical compounds produced by chemical or electrochemical treatment. They are insoluble, passive, and protective — primarily oxides, chromates, or phosphates.
Production sequence:
- Pretreatment — Mechanical surface preparation, degreasing, chemical/electrochemical cleaning
- Conversion process — Thermal, chemical, or electrochemical treatment in acid or alkaline solutions
- Post-treatment — Rinsing, drying, sealing, or dyeing
- Final protection (if needed) — Oiling, waxing, or lacquering
Passivation of Copper
The blue-green patina on copper is a natural passivated film that prevents corrosion. It can be produced artificially using: ammonium sulfate (6 lbs), copper sulfate (3 oz), ammonia (1.34 fl oz), and water (6.5 gal). Apply as fine spray to a chemically cleaned surface — allow to dry between five or six applications. Patina develops within about 6 hours.
Coloring of Copper Alloys
For alloys with ≥ 85% copper:
- Dark red: Immerse in molten potassium nitrate at 1,200–1,300°F for up to 20 seconds, hot water quench, then lacquer
- Steel black: Immerse in 180°F solution of arsenious oxide, hydrochloric acid, and water until uniform color is obtained
For alloys with < 85% copper (brass):
- Black: Tumble with copper sulfate and sodium thiosulfate solution for 15–30 minutes
- Blue-black: Immerse in 130–175°F copper carbonate/ammonium hydroxide solution for 1 minute
- Hardware green: Immerse in 160°F ferric nitrate/sodium thiosulfate solution
- Light brown: Immerse in 195–212°F potassium chlorate/nickel sulfate/copper sulfate solution
Coloring of Iron and Steel
Black oxide coatings: Applied by immersing in boiling sodium hydroxide with nitrate/nitrite mixtures. Serve as paint bases or final finishes. When impregnated with oil or wax, provide fairly good corrosion resistance at low cost.
Phosphate coatings: Three types in general use:
| Type | Color | Primary Use |
|---|---|---|
| Zinc phosphate | Light to dark gray | Paint/oil base, cold working aid, wear resistance, rustproofing |
| Iron phosphate | Dark gray | Paint base |
| Manganese phosphate | Dark gray (black with oil) | Oil base, break-in, anti-galling |
Important limitation: Stainless steels and certain alloy steels cannot be phosphated. Most cast irons and alloy steels accept coating with varying degrees of difficulty.
Anodizing Aluminum Alloys
The aluminum object is immersed as the anode in an acid electrolyte with direct current applied. The surface oxidizes, producing a greatly thickened, hard, porous film of aluminum oxide. The object is then sealed in boiling water to render the film impermeable.
Three principal processes:
| Process | Active Agent | Coating Thickness | Key Characteristics |
|---|---|---|---|
| Chromic | Chromic acid | 0.2–0.7 mil | Less brittle. Does not attack aluminum trapped in crevices. Less abrasion-resistant. Cannot be used with alloys >5% copper. |
| Sulfuric | Sulfuric acid | 0.2–0.7 mil (Type II: 0.7–1.0 mil) | More abrasion-resistant than chromic. Standard process for most applications. |
| Hard anodizing | Sulfuric acid (low temp) | Up to 2 mils | Maximum hardness and abrasion resistance. |
Before sealing, the film can be colored by impregnation with dyes or pigments. Special electrolytes can produce colored films directly in the anodizing bath.
Surface Treatments for Other Alloys
Magnesium alloys: Chemical treatments provide paint base and corrosion resistance. Chrome pickle and dichromate "dip" coatings are very thin. Anodic coatings are thicker and harder. Painting is still desirable even after treatment.
Titanium alloys: Conversion coatings improve lubricity by retaining lubricants. Applied by immersion, spraying, or brushing. A popular bath uses aqueous phosphates, fluorides, and hydrofluoric acid.
Plating Standards Overview
Industrial plating encompasses dozens of specifications. Here are the most critical:
| Coating | Key Property | Typical Thickness |
|---|---|---|
| Anodize (MIL-A-8625F) | Corrosion protection, paint base | Type I/IB: 0.00002–0.0007 in.; Type II: 0.0007–0.0010 in. |
| Black Chrome (MIL-C-14538C) | Non-reflective, heat/corrosion resistant | ~0.0002 in. |
| Black Oxide (MIL-C-13924C) | Decorative, light reflection reduction | Very thin |
| Cadmium (QQ-P-416F) | Corrosion protection | Class 1: 0.0005 in.; Class 2: 0.0003 in.; Class 3: 0.0002 in. |
| Electroless Nickel (AMS 2404C) | Hard, wear-resistant, corrosion-resistant | As specified; service up to 1,000°F |
| Hard Chromium (QQ-C-320B) | Hardness, wear resistance, erosion resistance | 0.0001–0.002 in. |
Hydrogen embrittlement warning: Steel parts with hardness exceeding 40 Rc require stress relief before plating (baking at 300–500°F for 1 hour or more) and baking after plating (375°F ± 25°F for 3 hours). This is a non-negotiable quality requirement for high-strength steel components.
The Decision Framework: Choosing the Right Process
When you stand at the process selection crossroads, use this hierarchy:
Step 1: Define the Service Condition
What forces will the finished part actually encounter? Impact? Abrasion? Corrosion? Heat? Metal-to-metal wear? The service condition eliminates 80% of wrong choices instantly.
Step 2: Match the Material to the Condition
Use the alloy comparison tables in this guide. If the part sees impact above 60,000 psi, high-chromium iron is eliminated. If it operates above 1,200°F, cobalt-base alloys have a clear advantage. If the surface must resist galling, NiCr-C is the answer.
Step 3: Validate the Process Parameters
Don't assume. Look up the cutting speeds, beam power levels, brush speeds, or plating thicknesses for your specific material and geometry. The data exists — use it.
Step 4: Plan the Secondary Operations
Every primary process creates a downstream consequence. Laser cutting creates a heat-affected zone. Hard facing may require grinding (not machining). Flame cutting produces dross. Plan for it. Budget for it. Schedule it.
Step 5: Document and Standardize
The most expensive manufacturing mistake is one you make twice. Every process selection, every parameter set, every lesson learned should feed back into your shop's process standards.
Your Next Step
Open one of your current jobs — the one that's been giving you trouble. The one with the rework, the scrap, the customer complaint.
Now ask yourself: Am I using the right process, or the familiar one?
Pull up the tables in this guide. Check the alloy specifications. Verify the cutting speeds. Compare the surface treatment options. Run the numbers on what switching processes would actually cost versus what you're spending on rework right now.
Then make one change. Test it. Measure it. And when it works — because the data says it will — make it the new standard.
What's the one manufacturing process in your shop that you've always suspected was wrong for the job? Start there.