The CNC Hobbing Machine Explains Gear Cutting

• 1
  How a CNC hobbing machine generates a gear tooth
• 2
  Axis configurations on a CNC hobbing machine
• 3
  Hob selection, swivel angle, and cut depth
• 4
  What tolerances and finishes a CNC hobbing machine holds
• 5
  When hobbing is the wrong choice
• 6
  Hobbing compared with other gear cutting methods

A hob is a cylindrical cutting tool with gashes and relieved teeth that look like a worm. It is not a form tool. The hob and the gear blank rotate together at a ratio set by the tooth count and the hob's number of starts. As the hob turns, its teeth sweep through the blank and remove material in a series of overlapping cuts. Each hob revolution advances the cut by one tooth space. The result is an involute profile generated by the motion itself, not by the tool shape.

That distinction matters. Because the profile comes from the relative motion, one hob can cut any tooth count within its module range. A shop does not need a dedicated cutter for every gear size. A single hob handles a 17-tooth pinion and a 120-tooth wheel in the same setup, as long as the module and pressure angle match. This is why hobbing is the default process for external spur and helical gears.

The cut is continuous. There is no indexing pause between teeth, unlike gear shaping or form milling. The hob stays in contact with the blank through the whole cycle, which keeps the cutting load steady and the cycle time short. For a module 2 spur gear with 40 teeth in 1045 steel, a roughing pass and a finishing pass are typically enough.

• 1
  Generated profileTooth shape comes from hob and blank motion, not cutter form.
• 2
  One hob, many tooth countsAny tooth count within the module and pressure angle range.
• 3
  Continuous cutNo indexing pause, so the cutting load stays even.

A basic three-axis hobbing machine moves the hob radially, the table axially, and the hob swivels to match the helix angle. That covers spur gears and straight helical gears. It is the workhorse layout for most job shops. Setup is straightforward: mount the blank, set the hob swivel, set the radial depth, and let the cycle run.

A four-axis machine adds a tangential slide. The hob can shift along its own axis between cycles, which spreads wear across more of the hob's length. Tool life climbs. The trade-off is a longer setup and a more expensive machine, so it pays off mainly on high-volume runs where hob cost matters.

A five-axis hobbing machine adds a second rotary axis, usually a tilting hob head or a tilting work table. This allows crown and taper on the tooth flank, and it lets the machine cut worm gears and other profiles that a fixed-swivel setup cannot reach. GreatLight runs 16 simultaneous 5-axis machining centers alongside its hobbing capacity, so a hobbed gear blank can move straight into secondary milling or drilling without a second setup.

The axis count should follow the part, not the other way round. A straight spur gear in low volume does not need five axes. A crowned helical gear for a gearbox does.

• 1
  Three axesRadial, axial, and hob swivel. Spur and straight helical gears.
• 2
  Four axesAdds tangential shift. Longer hob life on high-volume runs.
• 3
  Five axesAdds a tilting axis for crown, taper, and worm gears.

Hob selection starts with module and pressure angle. A module 2 hob cuts module 2 gears, and a 20° pressure angle hob cuts 20° gears. Mixing them produces a tooth that looks close but will not mesh correctly. The hob's number of starts also matters: a single-start hob gives the smoothest finish and is the normal choice for finishing, while a multi-start hob removes material faster in roughing.

The swivel angle is set to the gear's helix angle plus or minus the hob's lead angle, depending on hand. Get this wrong and the tooth flank will not be straight. For a helical gear with a 15° helix, the hob head is typically set near 15° plus the hob lead angle. The machine's control handles the exact value once the gear data is entered.

Cut depth is set from the whole depth of the tooth. For a standard full-depth tooth, whole depth is roughly 2.25 times the module. A module 2 gear therefore needs about 4.5 mm of radial infeed. Roughing takes most of it in one or two passes. Finishing removes the last few tenths of a millimeter at a lower feed to hit the surface finish target.

Coolant matters more than most people expect. Hobbing generates a long, stringy chip that carries heat away from the cut. Flood coolant keeps the hob and the blank at a stable temperature, which holds size across a run. Dry hobbing is possible with coated carbide hobs and high-speed spindles, but it demands rigid machines and tight chip control.

• 1
  Match module and pressure angleA module 2 hob will not cut a module 1.75 gear correctly.
• 2
  Set swivel from helix angleWrong angle means a flank that is not straight.
• 3
  Whole depth ≈ 2.25 × moduleModule 2 gear needs about 4.5 mm radial infeed.
• 4
  Flood coolant for size controlStable temperature holds dimensions across a run.

Hobbing is a roughing-to-semi-finishing process by nature. The generated profile is accurate, but the surface left by the hob is not a ground finish. On a well-set machine, tooth profile and lead errors land in the 0.01–0.03 mm range for a mid-size gear. That is fine for many power transmission parts. It is not fine for a high-speed gearbox input pinion that needs a ground flank.

At GreatLight, general machining tolerance is ±0.005 mm and surface finish runs from Ra 0.2–0.8 μm on fine work to Ra 0.8–1.6 μm on standard work. A hobbed tooth flank typically sits at Ra 1.6–3.2 μm as cut. If the drawing calls for Ra 0.8 μm or better on the flank, plan for a grinding or honing step after hobbing. Do not assume the hob alone will reach it.

Material choice shifts the numbers. Aluminium 6061 and 7075 cut cleanly and hold a good finish off the hob. Stainless 303 and 304 work-harden, so a light finishing pass with a sharp hob is important. Steel 1045 and 4140 hob well in the normalized or annealed state; hardened steel above roughly 35 HRC is usually ground instead. Titanium and Inconel need slower speeds, more coolant, and a realistic tolerance budget.

Inspection is where the process proves itself. Gear measurement covers profile, lead, pitch, and runout. GreatLight inspects 100% of parts before shipment and provides reports on request, with raw material checks, in-process monitoring, and final inspection as standard.

• 1
  As-cut flank finishUsually Ra 1.6–3.2 μm. Grinding is needed below that.
• 2
  Profile and lead errorAbout 0.01–0.03 mm on a well-set machine.
• 3
  Material mattersAluminium hobs clean; stainless work-hardens; hard steel is ground.

Hobbing cannot cut internal gears. The hob has to approach the blank from the outside and sweep through the tooth space. An internal ring gear needs shaping or broaching instead. This is a hard limit, not a setup problem.

Hobbing also struggles when the gear is close to a shoulder or a flange. The hob needs clearance to run past the tooth end and exit the cut. If a gear sits against a large diameter shoulder, the hob will hit it before the teeth are complete. A relief groove or a larger clearance gap solves it, but that is a design change, not a machining fix.

Very large gears run into machine capacity. The workpiece has to fit the table and clear the hob head. GreatLight's largest travel is 4,000 × 400 × 150 mm, with a Ø400 mm rotary table. A gear beyond that envelope needs a different route or a larger machine.

Low-volume, one-off gears are often cheaper to mill. A three-axis mill with a form cutter or a ball nose can cut a small spur gear in a few minutes without a hob setup. Hobbing wins when the tooth count is high, the volume is real, or the profile needs to be a true involute. For a single prototype gear, the setup time can outweigh the cycle time saved.

• 1
  No internal gearsThe hob approaches from outside and cannot cut a ring gear.
• 2
  Needs exit clearanceA shoulder next to the teeth blocks the hob.
• 3
  Size limitPart must fit the table and clear the hob head.
• 4
  One-off gears may be milledSetup time can beat hobbing on a single part.