Small Module Gear: Processing Technology And Cutting Machines

• 1
  What counts as a small module gear
• 2
  How hobbing and shaping cut a tooth
• 3
  Tolerance and surface limits that matter
• 4
  What a small module gear cutting machine does
• 5
  Choosing a process from the print
• 6
  Process comparison for small module gear teeth

Module is the pitch diameter divided by the number of teeth, in millimeters. A gear with m below 1 is usually called a small module gear. That puts tooth height in the 0.5 to 2 mm range and the whole part often under 60 mm across. At this scale the tooth is no longer a big chunk of metal you can rough out and forget. It is a thin beam that carries load, wears, and holds the timing of whatever it drives.

These parts show up in instrument gearboxes, camera and optical drives, medical handpieces, servo reducers, drone actuators and small pumps. Common materials are 303 and 304 stainless, 4140 and 1045 steel, C36000 brass, 7075 aluminium, POM and PEEK. Each one pushes the cutting process in a different direction. Brass cuts clean and holds a sharp edge. POM moves after machining unless you stress relieve it. Soft aluminium gums the cutter and builds a burr on the tooth tip.

The first decision is not which machine. It is whether the tooth is cut or formed. Cutting removes material from a solid blank with a hob, a shaper cutter or an end mill. Forming rolls, stamps or molds the tooth into shape. For one-off and low-volume work, cutting wins because no tooling cost is tied to the tooth profile. For plastic gears above a few thousand pieces, molding usually wins on unit cost.

• 1
  Cut teethHobbing, shaping or milling from a solid blank
• 2
  Form teethRolling, stamping, injection molding or die casting
• 3
  Pick cutting whenVolume is low or the profile may still change

Hobbing is the default for cylindrical gears. A hob is a worm-shaped cutter with gashes that form cutting edges. The hob and the blank rotate together as a gear pair, while the hob feeds along the face width. Each hob revolution removes a slice of the tooth gap. The tooth profile is generated by the rolling motion, not ground into the cutter, so one hob covers any tooth count with the same module and pressure angle.

For small module gear work, hobbing runs at 1,000 to 4,000 rpm on high-speed steel hobs, or higher with carbide. Feed per workpiece revolution is usually 0.5 to 2 mm. Deeper than 2 mm of tooth height on a module below 0.5 and the hob starts to rub rather than cut. Climb hobbing gives a cleaner flank, but the backlash in the hob head must be tight or the tooth will show a step.

Shaping uses a pinion-shaped cutter that reciprocates while the cutter and blank rotate in mesh. It handles internal teeth and gears close to a shoulder, which a hob cannot reach. The trade-off is speed. Shaping is slower because only one or two teeth engage per stroke, and the cutter needs sharpening often on hard material.

Gear milling cuts one tooth gap at a time with a form cutter or a ball nose in a 4-axis or 5-axis machine. It is slow for production, but it needs no special gear machine. For a prototype with an odd profile, a modified addendum, or a gear merged with a shaft in one solid model, milling is often the shortest path to a working part.

• 1
  HobbingBest all-round method for external spur and helical gears
• 2
  ShapingUse for internal teeth and gears against a shoulder
• 3
  MillingUse for prototypes and non-standard profiles

On a small module gear, the errors that matter are pitch deviation, profile deviation and runout. Total profile deviation on a cut gear typically lands in the 8 to 10 µm band for hobbed and shaped teeth, and 4 to 6 µm if the flank is ground or hard finished after heat treatment. Below that you are buying a ground gear, not a cut one, and the price step is real.

Backlash is set by center distance and tooth thickness. On a module 0.5 gear with 20 teeth, a 20 µm change in center distance moves backlash by roughly 14 µm. That is why the housing bore tolerance usually matters more than the tooth itself. If the bores are loose, no amount of gear grinding saves the mesh.

Surface finish drives noise and wear-in. A hobbed flank at Ra 1.6–3.2 μm is normal. Fine finishing gets Ra 0.8–1.6 μm, and lapped or ground flanks reach Ra 0.2–0.8 μm. For a low-speed instrument drive, Ra 1.6 μm is fine. For a high-speed reducer above 8,000 rpm, the flank finish and the lead correction decide how loud the gearbox runs.

Runout is the one that bites hardest. A gear with 30 µm of runout on a 20 mm bore will bind once per revolution no matter how good the tooth profile is. Check runout before you blame the tooth form.

• 1
  Profile deviation8–10 µm cut, 4–6 µm ground
• 2
  Backlash shiftAbout 14 µm per 20 µm of center distance change
• 3
  RunoutCheck it first when the mesh binds once per turn

A gear hobbing machine is built around a synchronized spindle pair. The work table and the hob arbor are tied together electronically or through a change-gear train so the ratio stays exact. Modern CNC hobbers use electronic gearbox coupling, which lets you set helix angle and module from the control panel instead of swapping gears. On a small module machine, the work table is small, often 100 to 200 mm, and the spindle turns fast because the cutter is thin.

Rigidity matters more than size on these machines. A module 0.3 hob is a fragile tool. Any vibration shows up as a ripple on the flank. That is why small gear hobbers use short arbors, preloaded bearings and light, fast passes instead of one heavy cut. Coolant is usually oil mist or neat oil, not flood coolant, because chips are tiny and flush poorly.

Gear shaping machines add a reciprocating ram and a cutter spindle. The stroke is short, often 10 to 30 mm, and the ram speed is high. Internal gear shaping needs a cutter smaller than the bore, so the cutter is long and thin and deflects easily. Keep the stroke just longer than the face width.

For shop work outside a dedicated gear machine, a 5-axis machining center with a rotary table can cut a gear by milling. It will not match a hobber on cycle time, but it holds position well and it can put a gear on a shaft, drill the bore, and face the hub in one setup.

• 1
  HobberSynchronized spindle pair, electronic gearbox
• 2
  ShaperReciprocating ram, needed for internal teeth
• 3
  5-axis millOdd profiles and gear-shaft parts in one setup

Start with volume. Under 50 pieces, hobbing or milling without dedicated tooling is usually the sane call. Between 50 and 5,000 pieces, hobbing with a standard hob still wins, because the hob is a stock item for common modules and pressure angles. Above that, look at whether the tooth can be rolled or molded instead.

Then look at geometry. External spur or helical gear with clear access from both sides: hob it. Internal teeth, or a gear shoulder that blocks the hob: shape it. A gear blended into a shaft with a modified profile: mill it, or hob the standard section and mill the rest. A gear with a tooth tip thinner than 0.3 mm needs a conversation before anyone quotes it, because the tip may chip during cutting.

Material decides the finishing route. Hardened steel above 45 HRC cannot be cut with a hob after heat treatment. Either cut before hardening and accept the distortion, or cut soft, harden, then grind the flanks. Brass and free-machining stainless cut clean enough that a light tumble is all the deburring needed. Plastics want sharp, polished cutters and a stress-relief pass if the tolerance is tight.

Finally, check the drawing for the data the shop actually needs: module, pressure angle, helix angle and hand, number of teeth, profile shift, backlash class, and the datum for runout. A gear print missing any of these turns into a phone call, and phone calls turn into days.

• 1
  Under 50 piecesHob or mill, no dedicated tooling
• 2
  50–5,000 piecesHob with a standard stock hob
• 3
  Hardened teethCut soft, then grind the flanks