Gear undercut isn’t just a design calculation—it’s a manufacturing reality that can compromise tooth strength and create costly failures in precision assemblies. When designing gears for tight-tolerance applications like aerospace actuators or medical device housings, understanding undercut thresholds becomes critical to both performance and cost control.
Gear undercut happens during the manufacturing process when the cutting tool (hob, shaper, or planer) cuts deeper than the interfering point and removes part of the involute tooth profile near the root. This occurs specifically when gears have fewer than 17 teeth for standard 20° pressure angle gears, as the cutting tool’s geometry physically interferes with and removes material from the dedendum area.
Learn what causes gear undercut, how to calculate minimum tooth counts, and ways to eliminate undercut without sacrificing performance—plus machining warning signs.
Table of Contents
What design conditions lead to gear undercut?
Three design conditions trigger gear undercut risk: fewer than 17 teeth, standard 20° pressure angle, and standard addendum height (ha = 1.0).* When combined, the cutting tool removes involute tooth material, increasing machining cost 40-60%.
Undercut severity scales with tooth count. At 16 teeth, expect minor profile removal affecting mainly cost. At 12 teeth, material removal reaches 0.3-0.5mm depth, reducing tooth strength 25-35%. Smaller modules worsen this effect proportionally.
The root cause is geometric interference during manufacturing. Standard cutting tools cannot generate the full involute profile below 17 teeth without removing root material—this requires either profile shift correction or accepting weakened teeth.
ISO 6336 confirms undercut reduces bending strength by removing material at peak stress locations. However, many applications accept 10-20% strength reduction for significantly smaller assemblies or higher ratios.
Design Takeaway: Accept undercut if loads are well below capacity, size trumps cost, or volume under 100 units. Otherwise, start with 18+ teeth and optimize module/pressure angle for size targets with standard manufacturing.
What pressure angle prevents gear undercut?
25° pressure angle allows gears as small as 11 teeth, while 14.5° requires 32+ teeth compared to standard 20° (17 teeth minimum). ISO 6336 validates pressure angles “from 15° to 25°” for load capacity calculations, making your choice critical for both size and manufacturing feasibility.
Pressure Angle Min Teeth Noise vs 20° Manufacturing Cost Best Applications
14.5° 32 teeth 3-5 dB quieter +30-50% tooling Medical, precision instruments
20° 17 teeth Baseline Standard General manufacturing
25° 11 teeth Noisier operation +30-50% tooling Compact, high-load designs
Decision Matrix for Pressure Angle Selection:
- Choose 14.5° if: Noise is <60 dB requirement, you can accept 38% larger gear diameter, low-speed applications
- Choose 20° if: Standard manufacturing, balanced performance, AGMA Q8-Q9 quality achievable with regular CNC
- Choose 25° if: Space constraint <50% of standard design, can accept higher bearing loads, noise >70 dB acceptable
AGMA 2101-D04 compliance requires “ISO A7/AGMA Q8 for 90% of applications” with standard 20° tooling readily available across CNC shops, while custom angles add 2-4 weeks lead time.
Design Takeaway: Use 20° unless specific performance requirements drive the choice. For noise-critical products requiring <60 dB, use 14.5° despite size penalty. For ultra-compact designs, 25° enables smallest pinions but verify bearing capacity first.
How many teeth minimum prevents gear undercut?
17 teeth minimum for 20° gears theoretically, but “gears with 16 teeth or less can be usable if their strength and contact ratio still meet design requirements” and “adverse effects can be neglected” for light loads. The key is understanding your actual load requirements versus theoretical limits.
Tooth Count Risk Assessment Load Limit Guidance Testing Protocol
18+ teeth Zero undercut Any load level Standard production
15-17 teeth Minor undercut ≤10% of bending capacity Prototype validation required
12-14 teeth Moderate undercut ≤25% of surface capacity Accelerated life testing
<12 teeth Severe undercut Risk assessment critical Extensive validation
Load-Based Decision Framework: Engineering analysis shows “maximum allowable torque due to surface failure is only 2.83 Nm” for 20-tooth gears vs “46 Nm” for bending strength. For positioning mechanisms, instrumentation, or intermittent duty cycles operating below 25% of theoretical capacity, moderate undercut often acceptable.
Manufacturing Reality Check: AGMA inspection standards require “CMM capabilities, climate-controlled environment” for precision grades. Standard CNC achieves ±0.005mm accuracy with ISO A7/AGMA Q8 quality but undercut gears need enhanced inspection protocols.
Design Takeaway: Start with 18+ teeth for reliable production. If space forces <17 teeth, quantify your actual loads against gear capacity – many low-duty applications successfully use 15-16 teeth after prototype validation. Always test rather than assume failure.
Design might cause gear undercut?
We check module, pressure angle & tooth count • Get expert feedback before quoting
What problems does gear undercut cause?
Gear undercut creates three distinct problems: progressive noise increase, characteristic root wear patterns, and uneven power transmission. Unlike random gear failures, undercut issues are predictable and identifiable through specific symptoms.
Problem Identification Guide:
- Noise signature: Whining or humming that worsens over time, not present at startup
- Wear pattern: Horizontal grey-black line along tooth root – unique to undercut
- Performance change: Increasing backlash, vibration during direction reversals
Quick Problem Assessment: If your product shows noise increase >3dB within first 100 operating hours, undercut is likely affecting performance. Uneven load sharing creates this progressive degradation pattern.
Immediate Decision Criteria:
- Acceptable: Noise stays <60dB, no visible root scoring after 500 cycles
- Monitor closely: 3-5dB noise increase, minor root wear visible
- Action required: >5dB increase or deep horizontal scoring
Manufacturing cost impact: Profile shift corrections add 25-40% to gear cost but eliminate these progressive failure modes. Standard CNC achieves ±0.005mm without premium processes for most undercut prevention.
Design Takeaway: Undercut problems are gradual and recognizable – not catastrophic failures. If prototypes show stable noise/wear after 500 hours testing, undercut acceptable for your application. Budget 3-4 weeks additional development time if corrections are needed.
When does gear undercut affect strength and performance?
Undercut becomes critical when operating torque exceeds 30% of gear’s theoretical capacity or when precision degrades below acceptable limits. Contact ratio below 1.2 creates dangerous load concentrations requiring immediate design changes.
Immediate Load Assessment Formula: Critical Threshold = (Your Peak Torque / Gear Pitch Diameter) × Safety Factor
- Safe zone: <0.3 × theoretical capacity
- Test zone: 0.3-0.6 × capacity – validate with accelerated testing
- Danger zone: >0.6 × capacity – redesign with more teeth
Performance Impact Timeline: Minor undercut effects “can be neglected” below 25% capacity, but precision applications need tighter limits. Expect 10-15% precision loss at 40% capacity loading.
Go/No-Go Decision Matrix:
- Production approved: Peak torque <30% capacity, positioning accuracy maintained after 1000 cycles
- Extended testing required: 30-50% capacity utilization
- Design revision mandatory: >50% capacity or precision loss >5% after 100 cycles
Manufacturing Timeline Impact: Profile shift corrections require 2-3 additional weeks for tooling but prevent field failures. Maintaining “contact ratio of 1 or greater” ensures reliable long-term performance.
Design Takeaway: Calculate your torque-to-capacity ratio immediately. Below 30%, proceed with confidence. Above 50%, add 3 weeks to development timeline for undercut corrections. Between 30-50%, build test units and validate performance over extended duty cycles.
How to fix gear undercut in existing designs?
Three ways to fix undercut: add more teeth (easiest), use profile shift (keeps size small), or switch to 25° pressure angle (most compact). Most product developers just need to know which option fits their constraints.
Simple Decision Guide:
- Space available? Add 2-3 teeth – cheapest and fastest solution
- Size critical? Apply profile shift – adds 25% to cost but keeps gear small
- Ultra-compact needed? Switch to 25° pressure angle – enables smallest gears
Timeline Reality: Adding teeth: 2-3 weeks standard tooling Profile shift: 3-4 weeks for specialized setup Pressure angle change: 4-6 weeks for custom tooling
Cost Impact: Profile shift increases manufacturing cost 25-40% but may be worth it if your assembly space is severely constrained. Adding teeth is almost always cheaper if you can accept the size increase.
Design Takeaway: Try adding 2-3 teeth first – it’s the simplest fix. Only use profile shift or pressure angle changes when space absolutely requires it. Most undercut problems disappear with 18+ teeth.
What does gear undercut look like on machined parts?
Undercut looks like a small notch or groove where the tooth meets the root circle. You can spot it with basic visual inspection – it interrupts the smooth curve from tooth flank to root.
What to Look For:
- Normal tooth: Smooth transition from flank to root
- Minor undercut: Slight interruption in the curve, barely visible
- Problem undercut: Clear notch or groove, rough surface texture
Quick Check Method: Run your fingernail along the tooth root. Normal gears feel smooth, undercut gears have a noticeable depression or rough spot. Under 10x magnification, undercut appears as an obvious geometric interruption.
When It Matters:
- Ignore: Barely visible, smooth to touch
- Monitor: Visible notch but still smooth operation
- Fix: Deep groove, rough texture, or performance issues
Design Takeaway: Most undercut is visually obvious once you know what to look for. If your prototype gears look and feel smooth in the root area, undercut probably isn’t your problem. Focus on testing performance rather than measuring tiny geometric details.
How do experienced gear vendors spot undercut risk in a drawing?
Good gear shops immediately check if you have enough teeth for your pressure angle, then suggest fixes during quoting. They’ve seen these problems before and know the quick solutions.
Red Flags Vendors Spot:
- Less than 17 teeth with 20° pressure angle
- Missing profile shift notation when needed
- Unrealistic tolerance callouts for small gears
Vendor Quality Indicators:
- Good: Points out undercut risk during quote review
- Better: Suggests specific solutions with cost/timeline impact
- Best: Provides design alternatives and explains trade-offs
Quick Vendor Test: Send your gear drawing to 2-3 shops. Good vendors will mention undercut risk within 24 hours if it exists. Vendors who just quote without technical feedback probably lack gear engineering expertise.
Evaluation Questions: “Do you see any manufacturability issues with this gear design?” “What would you suggest if we needed to reduce the size further?”
Design Takeaway: Use vendor feedback as a reality check. Experienced gear shops catch undercut problems during quoting and suggest practical solutions. If multiple vendors raise the same concerns, listen to their recommendations.
Conclusion
Gear undercut becomes critical above 25% load capacity or in precision applications requiring consistent performance. While 17+ teeth theoretically prevent undercut, many products successfully use 15-16 teeth after proper testing validates acceptable performance for their specific loads and duty cycles.
Contact us to explore manufacturing solutions tailored to your gear requirements.
Frequently Asked Questions
For <500 units annually, usually not unless testing shows actual problems. The cost premium rarely justifies itself for replaceable low-volume parts.
Adding teeth: 2-3 weeks, minimal cost. Profile shift: 3-4 weeks, +25-40% cost. New pressure angle: 4-6 weeks, custom tooling required.
Check if your bearings can handle higher radial forces. Ensure all mating gears use the same pressure angle. No other major changes needed.
Yes, but put undercut gears in lower-torque stages where reduced strength won’t affect system reliability.
Test prototypes at 150% of expected torque for 100 hours. If performance stays consistent, you’re likely in the safe zone regardless of calculations.
