Designing compact gears isn’t just about saving space — it’s about avoiding strength failures and undercutting that make parts unmachinable. When the tooth count gets too low, common design assumptions break down, especially at the prototyping or custom machining stage.
Gears with fewer than 17 teeth using a 20° pressure angle and no profile shift will typically experience undercutting during CNC machining. This weakens the tooth root and limits load-carrying capacity. Below 12 teeth, even corrected profiles may require helical designs or advanced processing to remain functional.
Learn when low tooth counts become unmachinable, how module and pressure angle affect limits, and what to confirm with your gear vendor before finalizing specs.
Table of Contents
Why does tooth count matter in gear design and manufacturing?
Tooth count directly affects gear size, root strength, and whether the profile can be machined without undercutting. Below key thresholds, a gear may pass design software checks but fail in CNC production due to fragile geometry or tool access issues.
Reducing tooth count decreases the pitch diameter and increases curvature at the root. With a 20° pressure angle and zero profile shift, undercutting begins at 17 teeth or fewer — where the cutter path dips inside the base circle, Even if tooth geometry appears correct in CAD, it can’t always be produced cleanly with standard cutters or finishing passes.
Machining gears with low tooth counts often results in:
- Tool deflection — where the cutter bends under load, causing profile inaccuracy, chatter, or surface finish issues
- Inaccessible geometry for standard end mills or hobs
- Profile distortion due to burr formation or vibration in unsupported sections
These issues increase cost — via slower toolpaths, special setups, or scrapped parts — and become harder to fix once mating components or housings are locked in.
Tooth Count Undercut Risk (20° PA) Machining Risk DFM Guidance
≥20 None Standard machining Proceed as designed
17–19 Moderate Watch internal radii Validate profile near root
13–16 High Root thinning likely Use profile shift or redesign
≤12 Severe Often unmachinable Consider helical or compound
Design Takeaway: If your gear has fewer than 17 teeth, flag it early for DFM review. Even a 2–3 tooth increase can eliminate toolpath complications, reduce scrap risk, and keep tolerances achievable in real-world machining.
What is the minimum gear tooth count before undercutting?
Undercutting begins at 17 teeth for a 20° pressure angle spur gear with zero profile shift. Below this, the cutter path intrudes into the base circle, thinning the root and creating profiles that are difficult — or impossible — to machine cleanly.
Tooth count isn’t the only variable. Pressure angle and profile shift both affect how early undercutting occurs. Engineers can often trade a small profile shift or helical design for fewer teeth without losing tooth integrity:
Gear Type Pressure Angle Profile Shift Min Teeth Without Undercut
Spur (standard) 20° 0 17
Spur (corrected) 20° +0.5 14–15
Spur (high angle) 25° 0 13–14
Helical (15° helix) 20° (virtual) 0 10–12 (depends on overlap)
Even if CAD geometry appears clean, the base circle intrusion often shows up at the CAM or machining stage, especially when root radii or tool diameters are constrained. Thin tooth tips can’t absorb cutting loads, leading to edge breakage or poor finish.
For prototyping, undercut gears often require:
- Slower toolpaths
- Fine-pass finishing
- Custom fixtures
…which can raise machining cost by 20–40% and delay iteration cycles.
Design Takeaway: If space is tight and you’re tempted to spec 16 teeth or fewer, adjust pressure angle or shift before locking the design. Small geometric changes here prevent toolpath failures and avoid expensive manual corrections during machining.
How does low tooth count affect gear strength and reliability?
Low tooth count increases bending stress and reduces the number of teeth in contact, making gears more prone to failure under load. It also raises surface pressure and accelerates wear, especially in compact or high-torque assemblies.
As tooth count drops:
- Tooth root thickness can drop by 30–50%
- Contact ratio falls below 1.5, meaning less load sharing
- Bending stress increases non-linearly due to geometry
This weakens the gear’s resistance to both tooth breakage and pitting. For example, a 12-tooth gear with a 20° pressure angle may only engage 1.2 teeth at a time — vs 2.0+ on a 20-tooth gear — doubling localized forces.
From a DFM view, these narrow teeth often:
- Vibrate or deflect during machining
- Require soft fixturing to prevent breakage
- Accumulate heat, which affects tolerance control
Correcting strength issues late means upgrading materials, adjusting mating gears, or remaking housings — all of which increase both tooling and validation cost.
Design Takeaway: If your application involves torque, cycling, or durability, treat anything under 16 teeth as a high-risk geometry. Before moving forward, consider boosting pressure angle, adding profile shift, or switching to helical to recover strength without enlarging the part.
What are the DFM risks of specifying too few gear teeth?
Too few teeth create weak profiles, undercut roots, and tight geometries that can’t be reliably machined — leading to higher CNC cost and lower part quality. These issues don’t show up in CAD, but cause real problems during setup, finishing, and inspection.
Common DFM failures we’ve seen include:
- Tool deflection and chatter during finishing passes near thin tooth roots
- Toolpath errors when CAM software can’t finish the full profile with standard 3 mm or 2 mm cutters
- Metrology failure, especially when CMM probes can’t reach the full root or get inconsistent readings on burr-prone surfaces
👉 A customer once submitted a 13-tooth gear (m = 1, 20° PA) for CNC milling. The part passed CAD review, but the sharp root curvature caused tool deflection during finishing and failed flatness inspection — requiring a redesign to 15 teeth with profile shift.
Design Takeaway: If you’re going below 17 teeth, get DFM input early. Issues like deflection, probing errors, or CAM toolpath limits can create rework or delay — even if the drawing looks perfect.
Design has low tooth count gears?
We check for undercut + machinability • Avoid quoting delays with a fast review
What is the lowest tooth count that can be reliably machined?
12–13 teeth is the lower limit for CNC-machined gears — and only when combined with profile shift or helical geometry. Below that, involute profiles become too thin for standard tooling and deflect under load.
Tooth Count Gear Type Machining Feasibility
17+ Standard spur Fully reliable, minimal DFM checks
14–16 Spur with +shift Machinable, but verify tool access at root
12–13 Helical or shifted Marginal — needs profile correction + test cuts
≤11 Compound/compact Typically unmachinable via CNC — consider EDM/molding
📌 On a recent job, a 12-tooth gear with a 1.0 mm module required three passes with a 2 mm end mill to finish the root. Despite careful fixturing, the outer tooth tips chipped during deburring — the part was rejected due to profile loss and never entered pilot runs.
Design Takeaway: Going under 14 teeth? Plan for increased cost, higher rejection risk, and slower cycle time — or raise tooth count slightly to avoid machining instability altogether.
How do pressure angle and module affect the minimum tooth count?
Increasing pressure angle or module reduces undercut risk and improves tooth root durability — making lower-tooth gears safer to machine. These parameters directly affect tooth geometry and tool access.
Parameter Change Effect on Geometry DFM Outcome
20° → 25° PA Larger base circle More root support, fewer teeth needed
m = 1 → m = 1.5 Thicker teeth Easier to machine, better tool access
Profile shift = +0.5 Undercut eliminated Cleaner roots, fewer toolpath issues
These tweaks are often the fastest way to fix DFM issues:
- Use profile shift first — it doesn’t affect housing or center distance
- Then increase PA — easier to machine, but may increase backlash
- Only increase module if housing space allows for a larger gear
🧠 We supported a client redesigning a 16-tooth gear (20° PA, m = 1) that had undercut issues and required multiple finishing passes. By switching to 14 teeth with 25° PA and a +0.5 shift, we were able to machine the gear with standard tooling, eliminate the cleanup pass, and pass CMM checks on the first try — all while keeping the part within tolerance and budget.
Design Takeaway: If you must reduce tooth count, adjust profile shift first, then pressure angle, then module. These small spec edits preserve strength and improve machinability without redesigning the full geartrain.
How can profile shifting reduce undercutting in compact gear designs?
Profile shift eliminates undercut and strengthens the tooth root by moving the gear profile outward — without changing pitch diameter or module. It’s one of the most effective adjustments we use to make low-tooth-count gears reliably machinable.
In our production process, we apply positive profile shift (+0.3 to +0.6) when:
- Tooth count is under 17 and undercutting compromises root strength
- Toolpath simulation shows the cutter intruding into the base circle
- A compact gear must preserve center distance but avoid fragile geometry
By increasing root thickness, profile shift:
- Enables clean tool access (e.g., with 3 mm cutters)
- Reduces burr formation
- Prevents profile collapse during finishing or inspection
🛠️ In a 14-tooth spur gear project for a compact actuator, a +0.4 shift allowed us to machine the full involute in one pass, holding ±0.01 mm tolerance and avoiding additional polishing or root rework.
🧩 Suggested diagram:
Side-by-side involute curves — one standard (with undercut), one shifted (with thickened base). Show base circle, pitch circle, and cutting path.
Design Takeaway: Use profile shift as your first option to correct undercut. It preserves part size while boosting strength, reducing machining time, and eliminating costly finishing issues in production.
When should helical gears be used to avoid minimum tooth count issues?
Helical gears increase effective tooth count through axial overlap — allowing us to machine 12–13 tooth gears without undercut or root distortion. When profile shift isn’t enough, switching to helical is the next best option for compact, load-bearing designs.
In our shop, we recommend helical gears when:
- Tooth count drops to 13 or below
- Profile shift alone can’t restore root thickness
- Smooth engagement or low backlash is critical (e.g., audio, robotics, motion systems)
Helical designs:
- Distribute load across the face width
- Increase contact ratio (typically ≥2.0)
- Improve surface finish and gear noise control
- Reduce force spikes that can break thin teeth during machining
📌 For one 13-tooth helical gear used in a precision robotic drive, we held ±0.01 mm profile tolerance with a 15° helix angle. The overlap ratio prevented undercut, and the part required no special cleanup — a clean run across 200+ units.
🆚 Profile Shift vs Helical – Which to Use?
Criteria Profile Shift Helical Gear
Fixes undercut at root ✅ ✅
Changes part size ❌ ❌
Adds axial load ❌ ✅
Improves gear noise ❌ ✅
Simplifies cutter access ✅ ✅
Works for ≤13 teeth ⚠ Limited ✅
Design Takeaway: If your gear has 13 teeth or less and strength, smooth meshing, or profile accuracy are critical, it’s time to switch to helical. You’ll get more reliable machining, higher performance, and fewer DFM issues at production scale.
What should be confirmed with suppliers for low tooth count gears?
Before placing an order for gears under 17 teeth, confirm that your supplier can machine the geometry without undercut, profile distortion, or inspection failure. Don’t rely on design intent alone — validate their process capability and QA methods.
From our side, here’s what you should confirm before production:
- Have you machined gears at this tooth count, module, and material before?
- What’s your approach to eliminate undercut — profile shift, helix angle, or tool compensation?
- How is full involute accuracy maintained at the root?
- What cutting tool geometry is used — form cutter, end mill, or EDM?
- What CMM probe diameter and method are used for root inspection?
- How is burr control handled on sharp internal radii?
- What’s your dimensional rejection rate for similar low-tooth gears?
📌 In one 15-tooth gear project, we flagged the original drawing as unmachinable due to sharp root curvature and no shift. We revised it with a +0.5 profile shift and increased the pressure angle to 25°, enabling clean cutting and ±0.01 mm CMM compliance — with no changes to the housing or mating parts.
⚠️ DFM Red Flags to Catch Before You Send the Drawing:
- Tooth count < 17 with no shift or helix
- 20° pressure angle with OD ≤ 15 mm
- Root radius under 1.0 mm
- High-strength alloy with poor machinability (e.g., 17-4PH, 440C)
- Tight tolerances near base circle without clarification of inspection method
Design Takeaway: Don’t wait until CAM or inspection to discover low-tooth-count issues. Confirm supplier process limits, tolerances, and tooling strategy before finalizing your spec — especially for small, load-bearing gears.
What are the best alternatives when tooth count is too low to machine?
When gears fall below safe machining limits, alternatives like profile shift, helical geometry, or compound gearing can preserve the design intent — without sacrificing strength or tolerance. The right solution depends on what’s fixed (OD, center distance) and what’s flexible (tooth form, axial space, module).
Here’s how we evaluate fallback options:
Alternative When to Use Machining Impact
Profile shift Mild undercut, housing locked ✅ Best first fix — simple, effective
Helical gears ≤13 teeth, noise/smoothness important ✅ Raises virtual tooth count, cleaner cut
Higher module OD increase possible ✅ Thicker teeth, better tool clearance
Compound gears Center distance fixed, need high ratio ⚠ Adds parts, but solves mesh issue
EDM or molded gears Extreme miniaturization, <12 teeth ❌ Not suited for fast-turn CNC
📌 On one drive system, a client’s original spec called for a 10-tooth gear that couldn’t be cut with standard tooling. We solved it using a 14-tooth helical gear with +0.5 shift — and adjusted the mating gear to preserve the overall ratio. Result: same packaging, better strength, clean machining.
🧭 Fallback Logic for Product Developers:
If profile shift doesn’t solve it → try helical.
If axial load is an issue → raise module or use compound.
If none of those work → redesign geartrain or consider non-machined process.
Design Takeaway: Don’t force unmachinable specs through CNC. We’ll help you evaluate stronger, cleaner alternatives — using proven shifts in geometry or gear architecture — to stay within tolerance, lead time, and budget.
Conclusion
Designing compact, low-tooth-count gears comes with real machining and reliability trade-offs. We help engineers optimize specifications, avoid undercut issues, and deliver precision-machined gears that meet strength, tolerance, and cost goals. Contact us to explore manufacturing solutions tailored to your gear design requirements.
Frequently Asked Questions
Increase the mating gear’s tooth count proportionally. For example, if your 13:39 ratio becomes 15:45, the center distance grows, but the ratio stays the same. Or switch to helical to offset tooth count without growing the OD.
Usually yes — but you’ll need to account for axial force. Shaft retention (e.g., thrust bearings, shoulder stops) may need to change. We can advise if your current bearing setup can handle the added thrust load.
Ask your manufacturer:
- What probe size do they use for CMM?
- Can they inspect below the base circle?
- What’s the minimum measurable fillet radius?
Inspection is often where low-tooth gears fail — even if the geometry is cut correctly.
Yes. A 25° pressure angle strengthens the tooth root and helps avoid undercut, but it increases backlash and stiffness — which can raise gear noise in sensitive applications (e.g. audio, robotics). It’s a trade-off.
You need to define it. Profile shift isn’t automatically applied unless specified in the drawing or gear data. If you leave it out, the default will likely be zero — which may cause undercut in low-tooth designs.
Avoid root radii smaller than 1.0 mm unless you’re using custom tooling or EDM. Below that, standard end mills can’t clear the root without chatter or burrs — and even tight CMM probes can’t measure cleanly.
