--- title: Designing Herringbone Gears for CNC Machining: A Practical Step‑by‑Step Guide siteUrl: https://logzly.com/gearcraftinsights author: gearcraftinsights (GearCraft Insights) date: 2026-06-21T08:05:06.016793 tags: [gear, machining, herringbone] url: https://logzly.com/gearcraftinsights/designing-herringbone-gears-for-cnc-machining-a-practical-stepbystep-guide --- If you’ve ever tried to mesh two straight‑cut gears and felt the whole assembly shudder like a cheap washing machine, you know why herringbone gears matter. They cancel out the axial thrust that straight gears generate, giving you smooth power transfer without the need for thrust bearings. In today’s shop floor, CNC machines make these complex shapes possible – if you know how to design them right. Below is my tried‑and‑true workflow, straight from the bench at GearCraft Insights. ## Why Herringbone Gears Deserve Your Attention Most hobbyists start with spur or helical gears because they’re easy to draw and cut. But as soon as you push a system past a few hundred RPM or load it heavily, the axial forces become a nuisance. Herringbone gears combine two mirrored helical faces, so the thrust from one side cancels the other. The result is a quieter, longer‑lasting drive that can handle higher torque without extra bearings. That’s why many automotive differentials and high‑speed industrial gearboxes use them. If you can machine them on a CNC, you get the performance boost without the cost of a custom gear cutter. ## Step 1 – Define the Gear Basics ### 1.1 Choose the Module or Diametral Pitch The module (metric) or diametral pitch (imperial) is the size of the teeth. For most CNC work I stick with a module between 1.0 and 3.0 – big enough to machine cleanly, small enough to keep the part compact. Remember: module = pitch diameter / number of teeth. Write it down; you’ll need it for every later calculation. ### 1.2 Set the Number of Teeth Pick a tooth count that avoids under‑cutting. A rule of thumb: keep the pressure angle at 20° and make sure the number of teeth is at least 2 × (1 / sin(pressure angle)). For a 20° angle that works out to about 5.8 teeth, but in practice you never go below 12 teeth on a small gear. More teeth give smoother motion but increase machining time. ### 1.3 Decide on the Helix Angle The helix angle determines how steep the teeth wrap around the gear. Typical values are 15°–30°. Larger angles give higher load capacity but also increase axial load on each half – which the herringbone design will cancel out. I like 20° for a good balance. ## Step 2 – Sketch the Profile in CAD ### 2.1 Use a Gear‑Generating Add‑On Most of us use Fusion 360 or SolidWorks with a gear generator plug‑in. Input the module, tooth count, pressure angle, and helix angle. The add‑on will create a single helical gear slice. ### 2.2 Mirror and Align the Two Halves Copy the helical slice, flip it about the gear’s mid‑plane, and align the two halves so their teeth interlock perfectly. The key is to keep the centerline of the gear unchanged – any offset will cause uneven loading. ### 2.3 Add a Central V‑Shaped Groove (Optional) If you need a keyway or want to reduce weight, carve a small V‑groove along the center line. It also helps the CNC tool clear the middle when you use a 2‑axis roughing pass. ## Step 3 – Prepare the CNC Toolpaths ### 3.1 Choose the Right End Mill A 6‑mm or 8‑mm ball‑nose end mill works well for the curved tooth surfaces. For the outer rim, a flat end mill of similar size speeds up material removal. Keep the tool length long enough to stay stiff; flexing will ruin the tooth profile. ### 3.2 Set Up Roughing Passes Start with a high‑feed, shallow‑depth roughing pass. I usually set the step‑over to 40% of the cutter diameter and the depth per pass to 0.5 mm. This removes bulk material quickly while keeping the cutter from digging in. ### 3.3 Follow with a Finishing Pass Switch to a smaller ball‑nose (4 mm) and reduce the step‑over to 10% and depth per pass to 0.1 mm. This leaves a smooth tooth flank that meets the designed geometry. If you have a 5‑axis machine, you can tilt the cutter to follow the helix angle directly, which reduces scalloping. ### 3.4 Verify Toolpath Direction Because the two halves have opposite helix directions, you need two separate toolpaths – one for the left side, one for the right. In my CAM software I create two setups, each with the appropriate rotation sense. Forgetting this leads to a gear that looks right but won’t mesh. ## Step 4 – Simulate and Check for Interference Before you hit “Start,” run a full simulation. Look for: * **Tool collisions** – especially where the two halves meet at the center. * **Over‑cut** – the cutter might eat into the opposite half if the clearance is too tight. * **Surface finish** – watch the spindle speed and feed; too slow and you’ll get chatter, too fast and you’ll burn the steel. If anything looks off, adjust the clearance between the halves by a few hundredths of a millimeter. In my experience, a 0.05 mm gap is enough to avoid interference while keeping the gear tight. ## Step 5 – Post‑Processing and Inspection ### 5.1 Deburr the Edges A hand file or a rotary brush removes burrs from the tooth roots. Be gentle – the tooth profile is delicate. ### 5.2 Measure Critical Dimensions Use a gear tooth caliper or a micrometer to verify the pitch diameter, tooth thickness, and helix angle. For a quick check, a simple dial indicator on a rotating test jig will reveal any runout. ### 5.3 Test Fit with a Mate Gear Mount the freshly cut gear on a shaft and pair it with a matching gear. Rotate slowly; you should feel almost no axial push and a smooth, quiet meshing. If you hear a ticking or feel a wobble, double‑check the alignment and the helix angle. ## Step 6 – Iterate and Document Every CNC machine has its own quirks – spindle run‑out, tool wear, coolant flow. Keep a log of feed rates, spindle speeds, and any issues you encounter. Over time you’ll develop a “gear‑cutting recipe” that you can reuse on future projects. That’s the spirit of GearCraft Insights: share what works, learn from what doesn’t, and keep the gears turning. --- Designing herringbone gears for CNC isn’t magic; it’s a series of small, repeatable steps. Get the basics right, let the CAD software do the heavy lifting, respect the toolpath direction, and finish with a careful inspection. Follow this workflow and you’ll move from “I can’t picture a V‑shaped tooth” to “I’m machining them every week” in no time.