How to Machine Toothed Pulleys on 3 Axis CNC

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  Key takeaways
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  What Fits 3 Axis CNC Before You Cut
• 3
  Preparing the Blank and the Datum
• 4
  Fixture and Indexing Setup
• 5
  Cutting the Tooth Profile
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  Inspection and Pitch Verification
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  Step by Step: Machining a Toothed Pulley on 3 Axis CNC
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  3 Axis vs 4 Axis vs 5 Axis for Toothed Pulleys

A 3-axis mill moves in X, Y and Z and nothing else. To cut a toothed pulley you must turn the tooth profile into a series of flats or a cut that runs along the part axis, then index the part by one pitch and repeat. On a plain vise that means re-clamping. On a rotary table the index is numeric and repeatable. Everything below assumes you have one of those two options.

The typical 3 axis cnc part here is a timing pulley between 12 and 40 teeth, a face width under 25 mm, and a pitch from 2 mm to 5 mm. The teeth run parallel to the bore, so a profile cutter or a small end mill can drop straight through the tooth space in one Z pass. Outside diameter usually lands between 15 mm and 120 mm.

When the job stops fitting: fewer than 12 teeth, because the tooth space gets narrower than a Ø3 mm cutter; a face wider than 40 mm on a fine pitch, because cutter deflection grows with depth of cut and the index count rises; or a crown or helical tooth form, which needs coordinated rotation and cannot be indexed in flat steps.

If your part falls in the second group, the honest answer is a 4-axis or 5-axis machine. We run 16 simultaneous 5-axis centers and 12 four-axis mills, so we can move the job rather than fight the setup. A 3-axis quote on a crowned pulley is a quote for a part that will not hold pitch.

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  Good fit12–40 teeth, pitch 2–5 mm, face under 25 mm
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  Borderline40–60 teeth, face 25–40 mm, pitch under 2 mm
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  Wrong fitCrowned or helical teeth, 8 teeth or fewer

Turn the blank first. The bore, the two faces and the outside diameter should come off a lathe or a mill-turn center before the pulley ever touches the 3-axis table. The bore is your only reliable datum for indexing, so it needs to be round within 0.01 mm and finished to Ra 1.6–3.2 μm. A reamed or bored hole beats a drilled one every time.

Leave 0.3–0.5 mm of radial stock on the outside diameter for the tooth passes. More stock means more cutter load at full tooth depth. Less stock risks the profile cutter rubbing instead of cutting on the first pass, which work-hardens stainless and burns aluminium.

Materials behave differently here. Aluminium 6061-T6, 7075 and 2024 cut clean with sharp two-flute cutters and air blast. Stainless 303 and 304 need slower surface speed and constant coolant or the flank tears. POM and PA machine easily but move with temperature, so let the blank sit at shop temperature for a few hours after turning.

Check the blank runout on the rotary table with a dial indicator before you cut anything. If the outside diameter runs out more than 0.02 mm, the tooth depth will vary around the part and belt tracking will suffer. Correct the setup, not the program.

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  Bore toleranceRound within 0.01 mm, Ra 1.6–3.2 μm
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  Radial stock0.3–0.5 mm on the outside diameter
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  Runout checkUnder 0.02 mm TIR before the first tooth pass

The standard setup is a Ø400 mm rotary table with a three-jaw chuck or a dedicated expanding mandrel. An expanding mandrel that grips the bore is better than a chuck because it holds the pulley concentric to the index axis and lets you reach both faces. Make one mandrel per bore size if you are running a family of pulleys.

Clamp the part against a shoulder so the axial location repeats. If the pulley walks along the mandrel between indexes, the tooth lands shift in Z and the belt sees a step at every index joint. A shoulder plus a cap screw is enough for most parts under Ø120 mm.

For low tooth counts on small pulleys, a collet block with a square or hex body is a workable alternative. Index by rotating the block one facet at a time. This only works when the tooth count is a multiple of the block facets, and it sacrifices the fine index control of a rotary table.

Verify the index before cutting metal. Program one pass, cut air, and measure the angular step against the model. On a rotary table, a 20-tooth pulley needs an 18° step with backlash taken out in the same direction every time.

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  MandrelExpanding type, one per bore size, concentric to the index axis
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  Axial stopShoulder plus cap screw to repeat Z location
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  Index checkCut air first; 20 teeth = 18° per step

Two strategies cover most work. A form cutter matched to the belt pitch cuts the full tooth space in one plunge and is fast and repeatable. A small end mill roughs the space and then profiles the flank in two or three passes, which costs more cycle time but lets you hold a specific flank angle and finish.

For the end mill route, use a cutter no wider than 60% of the tooth space width at the root. Run aluminium at 180–250 m/min surface speed and 0.05–0.12 mm per tooth feed. Stainless wants 60–90 m/min and a heavier chip load, not a lighter one, or the edge rubs. Take the roughing pass to 0.3 mm off the final depth, then finish with 0.1 mm radial steps.

Depth of cut per pass should stay under 0.25 × cutter diameter. On a 3 mm cutter that is 0.75 mm. Going deeper is where most broken cutters and scrapped pulleys come from, and the deflection shows up as a tapered tooth space that no amount of deburring fixes.

Aim for Ra 0.8–1.6 μm on the flanks. We hold Ra 0.2–0.8 μm when a belt manufacturer calls for it. If the flank finish is worse than Ra 1.6 μm, the belt wears the tool marks and pitch diameter effectively grows as the tooth tips round over.

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  Form cutterOne plunge per tooth space, fast, needs the exact pitch
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  End millRough plus two finish passes, more control on flank angle
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  Depth limitUnder 0.25 × cutter diameter per pass

Outside diameter is the easiest and least useful check. A pulley can sit on nominal OD and still miss pitch, because the tooth space is what the belt actually engages. Measure the space: roll a pin gauge or a pair of gauge pins into the tooth space and compare the span against the model.

A belt wrap check is the practical field test. Wrap a length of the correct pitch belt around the pulley, hold light tension, and look for gaps at the tips or interference at the roots. A pulley that passes this check will run quietly at speed. One that fails will not, no matter what the drawing says.

For production quantities, run the first part through 100% inspection and keep the record. We inspect every part before shipment and can supply raw material check, in-process monitoring and final inspection reports on request. Tolerance is ±0.005 mm when the drawing calls for it, though most timing pulleys run comfortably at ±0.02 mm on pitch diameter.

Watch for two failure patterns. Pitch drift usually means the index has accumulated error, often backlash taken up in different directions. A tapered tooth space usually means cutter deflection, and the fix is a shallower pass or a shorter cutter, not a program change.

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  Pin gaugeRoll pins into the tooth space and compare the span
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  Belt wrapCorrect pitch belt, light tension, look for tip gaps
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  Pitch driftCheck index backlash direction on every step