Mirror Discharge Machining of Beveled Teeth

• 1
  How the spark gap shapes a beveled tooth flank
• 2
  What makes the flank reflective
• 3
  Which beveled teeth suit this process
• 4
  Electrode design and how it holds tolerance
• 5
  Where the process stops being the right answer
• 6
  Mirror EDM versus grinding and milling on beveled teeth

Mirror discharge machining is sinker EDM run at low energy. A shaped electrode burns a cavity into a conductive blank through thousands of controlled sparks. Each spark melts and vaporizes a tiny volume of metal, and the dielectric fluid flushes the debris away before the next pulse fires.

On a beveled tooth, the electrode carries the negative of the flank, the root fillet, and part of the adjacent gap. The spark gap sits between electrode and workpiece. For a fine finish that gap measures roughly 0.01 to 0.03 mm per side, depending on the pulse energy and the electrode undersize you chose.

That gap is not uniform in practice. Spark energy, electrode wear, and debris concentration all shift the effective gap along the tooth flank. The tip of a beveled tooth has less material behind it than the root, so it heats faster and erodes faster. Expect 0.005 to 0.015 mm of extra stock removal near the tip unless you compensate in the electrode.

This is why mirror discharge machining of beveled teeth is planned around gap compensation, not around a single cut. Rough passes remove the bulk at high energy. Semi-finish and finish passes step the energy down until the recast layer is thin and the surface reflects light.

The mirror look comes from pulse control, not from polishing. When discharge energy per pulse drops into the low-microjoule range, each crater gets smaller than the wavelength of visible light scattered across the surface. The overlapping craters average out into a surface that reads as reflective.

Finish numbers follow the energy. A true mirror pass lands around Ra 0.2–0.8 μm on hardened steel. A high-finish pass lands around Ra 0.8–1.6 μm. As-machined EDM without a mirror pass sits at Ra 1.6–3.2 μm, which looks matte gray.

The recast layer matters as much as the roughness. Low-energy passes leave a thinner white layer, often under 5 μm on a well-controlled finish pass, versus 15 to 30 μm after a rough cut. A thin recast layer reduces the risk of micro-cracking at the tooth tip.

You cannot get a mirror finish and fast metal removal from the same pass. Roughing at high energy leaves deep craters that no finish pass can fully erase. Plan two or three stepped passes and budget the extra hours.

Mirror discharge machining of beveled teeth needs a conductive workpiece. Hardened tool steel, 4140, 4340, 17-4PH stainless, and Inconel all erode predictably. Carbide and beryllium copper also work. Aluminium erodes, but it gum up the gap and usually finishes better by milling.

Hardness is not a barrier. A gear blank at 58 HRC cuts the same way as one at 30 HRC on the EDM, because the process does not rely on a cutting edge. That is the main reason this route wins on hardened bevel gears and cams that would otherwise need grinding after heat treatment.

Geometry sets the limit. A beveled tooth with a deep, narrow root and a sharp tip is hard to flush. Debris builds in the corner, and the corner erodes unevenly. If the root radius is under 0.2 mm, expect slower cutting and more electrode dressing.

Thin teeth are also a risk. A beveled tooth under 1.5 mm wide at the tip can deflect or chip when the spark energy is too high. Drop the energy and accept a slower cut, or add a support rib during machining and remove it afterward.

Copper-tungsten and graphite are the usual electrode materials for fine work. Copper-tungsten wears slowly and holds a sharp edge, which matters on a tooth flank. Graphite wears faster but machines easily into complex shapes. For mirror passes, many shops use copper-tungsten for the finish electrode.

You need to build undersize into the electrode. Subtract the spark gap from the nominal tooth profile, and add wear compensation. If the finish gap is 0.02 mm per side, the electrode is 0.04 mm smaller across the tooth thickness than the finished part.

On a 5-axis machining center, the electrode can be oriented to the flank angle instead of sinking straight down. This lets you machine a beveled tooth with a curved or helical flank in one setup, and it improves flushing because the debris has a path to escape.

GreatLight runs 16 simultaneous 5-axis machining centers, 12 four-axis mills, and 27 three-axis machines across three plants. Electrodes are cut on the same floor as the parts, so the gap compensation and the electrode profile stay in one process chain. Tolerance on the finished flank is held to ±0.005 mm.

Mirror discharge machining of beveled teeth is slow. A mirror pass removes very little metal per hour, and a full tooth set can take many times longer than milling or grinding. If the flank only needs Ra 1.6 μm, do not pay for a mirror pass.

The process also leaves a recast layer, however thin. On a highly loaded tooth in a cyclic application, that layer can be a fatigue initiation site. If the part sees high bending stress at the root, specify a stress-relief pass or consider grinding the root after EDM.

Electrode cost scales with tooth count and flank complexity. A single-tooth electrode dressed once is cheap. A full-form electrode for a 40-tooth bevel gear is a real tooling investment, and it wears out. For high volumes, grinding usually wins on cost per part.

Flushing is the hidden constraint. If you cannot get dielectric into the root and debris out, the cut stalls or arcs. Deep, narrow bevel teeth with no through-path are the hardest case, and sometimes the answer is to split the tooth or add a flushing hole.