Face gear broaching: why machine rigidity decides tooth accuracy

• 1
  How a face gear broaching machine generates teeth
• 2
  Why rigidity and stability set the accuracy ceiling
• 3
  Where broaching wins and where it does not
• 4
  Tool design, materials, and the stability loop
• 5
  Holding tooth accuracy on the shop floor
• 6
  Face gear broaching compared with CNC milling

A face gear broaching machine cuts teeth into the end face of a blank with a long, tapered tool that carries every tooth form in sequence. The spindle that holds the tool moves in a straight line. The workpiece stays clamped and does not rotate during the cut.

Each tooth of the broach is slightly taller than the one before it. As the tool travels, it removes a shallow layer per tooth and the full tooth profile appears in a single stroke. Depth of cut per tooth typically runs 0.02–0.06 mm on steel, less on titanium and Inconel.

This is different from gear milling or shaping, where a rotating cutter or a reciprocating pinion generates one tooth at a time and the table indexes between cuts. Broaching finishes all teeth before the tool exits. There is no indexing error to stack up.

The payback is speed and repeatability on high-volume parts. The cost is a dedicated tool and a machine bed heavy enough to absorb the cutting force.

• 1
  Tool moves, work does notLinear stroke eliminates rotary indexing error between teeth.
• 2
  Rise per tooth0.02–0.06 mm on steel is a common design window.
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  One stroke, full formAll teeth finish before the broach leaves the cut.

Broaching pushes a tool through metal with a force that can reach tens of kilonewtons. That force has nowhere to go but into the machine frame, the fixture, and the workpiece. If any of those three deflects, the tooth form deflects with it.

Rigidity is the static part of the problem. A frame that bends 0.01 mm under load will leave a lead error down the tooth flank. Stability is the dynamic part. The interrupted start of the cut and the sudden exit at the end of the stroke excite the structure, and any resonance near the tooth-passing frequency shows up as chatter marks on the flank.

Machine builders therefore oversize the bed, tie the columns together, and preload the slideways. A face gear broaching machine is designed considering the rigidity and stability of the machine tool, not the other way around. Tool life, tooth spacing, and surface finish all follow from that decision.

For a buyer, this means the frame mass and the slideway preload matter more than the control specification. A heavy, well-damped frame holds ±0.005 mm on tooth spacing. A light frame will not, no matter how the program is written.

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  Lead errorFrame deflection under load tilts the tooth flank.
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  ChatterResonance near the tooth-passing frequency marks the flank.
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  PreloadPreloaded slideways remove clearance before the cut starts.

Broaching wins when the tooth form is fixed, the annual volume is high, and the blank is strong enough to take the thrust without collapsing. Internal splines, face gears, and keyways on transmission parts are typical. Cycle time per part drops to seconds once the tool is set.

Broaching loses on low volume and on geometry that changes. A new tooth profile means a new broach, and broach lead time and cost do not amortize over fifty parts. For prototype or low-run face gears, a 5-axis machining center that mills the tooth form is usually the cheaper route, even at a slower cycle.

Thin blanks are a hard stop. If the rim under the face teeth is under roughly 3 mm, the clamping and cutting loads will distort it. Turn the part on a mill with light radial passes instead, or add a temporary backing ring.

Blind or interrupted faces also push you off the broaching route. The tool needs a clear exit at the end of the stroke. If a shoulder blocks the exit, the broach cannot complete the form.

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  Good fitFixed profile, high volume, rigid blank, clear tool exit.
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  Poor fitPrototype counts, thin rims, changing profiles, blocked exit.

A broach is a stack of cutting edges ground into one bar. Rise per tooth, pitch, and rake angle are set at design time and cannot be adjusted at the machine. Get the rise too high and the tool stalls or chips. Get it too low and the stroke grows long enough to flex the bar.

Tool material follows the workpiece. HSS covers 1018, 1045, and most 6061 work. Powder metallurgy HSS or carbide inserts are needed for 4140, 4340, and 17-4PH. Inconel and Ti-6Al-4V need carbide and reduced rise per tooth, with more attention to coolant delivery at the leading teeth.

Sharpness is a stability variable, not just a wear item. A dull broach raises cutting force, and higher force pushes the frame deeper into deflection. Regrind intervals on steel typically run every 3,000–8,000 parts depending on material and coolant.

This is the loop that matters: dull tool, higher force, more deflection, worse tooth form, faster wear. Break it with scheduled regrinding, not with more clamping pressure.

• 1
  HSS1018, 1045, 6061, 303 stainless.
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  PM HSS or carbide4140, 4340, 17-4PH, Ti-6Al-4V, Inconel.
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  RegrindRoughly every 3,000–8,000 parts on steel.

The fixture is half the rigidity story. A face gear blank needs full face support under the teeth and a clamp that pulls it down, not sideways. Point clamping on a thin rim is a reliable way to lose 0.02 mm of flatness before the tool touches the part.

Measure the first part completely, then measure at a fixed interval. Tooth spacing, cumulative pitch, and flank surface finish are the three numbers that move first when something drifts. Flank finish in the Ra 0.8–1.6 μm band is a normal target for a healthy cut.

Coolant type matters more on broaching than on milling because the tool is buried in the cut. High-pressure flood with a sulfur or chlorine EP additive keeps the leading teeth from welding to the chip. On stainless and titanium, check chip curl every few hundred parts.

At GreatLight we run broached and milled face features on the same floor, so we can compare the two routes on your drawing before quoting. Tolerances on our CNC side hold to ±0.005 mm, and every part ships after 100% inspection.

• 1
  Support the full facePoint clamping on a thin rim adds flatness error.
• 2
  Watch three numbersTooth spacing, cumulative pitch, flank finish.
• 3
  Flood coolantEP additive prevents chip welding on leading teeth.