Product developers often over-specify material hardness, driving up CNC costs without functional benefit. With experience machining aerospace and medical parts, we know exactly where performance meets manufacturing reality.
HRC 40 is the practical limit for standard CNC tooling, while HRC 50+ requires specialized setups that double machining time and cost.
Learn the real hardness thresholds and design strategies that get you the strength you need without manufacturing penalties.
Table of Contents
What hardness level makes CNC machining difficult?
HRC 40 for steels, HRC 35 for stainless steel, and HRC 45 for tool steels mark the thresholds where CNC machining becomes challenging. Above these levels, expect specialized tooling, slower speeds, and significantly higher costs.
Here are the practical limits we work within daily:
Carbon/Alloy Steels (4140, 1045): Standard machining up to HRC 35, challenging above HRC 40 Stainless Steels (304, 17-4 PH): Efficient up to HRC 30, difficult beyond HRC 35
Tool Steels (A2, D2): Machinable through HRC 45, specialized setups above HRC 50 Aluminum Alloys (6061, 7075): No hardness limitations for CNC machining
Most structural applications perform well within these limits – HRC 35 steel provides adequate strength for brackets, housings, and machine components. Reserve higher hardness only for wear surfaces, cutting edges, or high-cycle fatigue applications where material performance is truly critical to function.
From our machining experience, exceeding these thresholds leads to accelerated tool wear and extended cycle times. Parts that machine efficiently at lower hardness levels require significantly more time and frequent tool changes when specified above these ranges. This translates to longer lead times and higher production costs across both prototype and production quantities.
Design Takeaway: Use these hardness limits as design constraints. If your application requires higher hardness, consider surface treatments on softer base materials or alternative alloys within the machinable range.
How do I choose the right material hardness for my part?
Start with your actual performance requirements, not safety margins. Most parts need far less hardness than designers initially specify – focus on the minimum hardness that meets wear resistance, fatigue life, or strength requirements, then add only a small buffer for manufacturing variation.
Use this decision framework to avoid over-specification:
For structural applications: Focus on yield strength rather than surface hardness. Static components like brackets, housings, and frames rarely need high hardness levels – adequate strength often comes from proper geometry and moderate hardness ranges.
For wear applications: Consider the type of wear – sliding surfaces need different hardness than impact-resistant parts. Abrasive environments may justify higher hardness, while toughness often matters more than maximum hardness for impact resistance.
For operating environment: Static components, protective housings, and one-time-use fixtures rarely justify high hardness specifications. Reserve maximum hardness for parts with specific performance requirements like repeated loading, abrasive wear, or critical fatigue life.
We regularly see designers specify excessive hardness as a safety margin, adding manufacturing cost without functional benefit. The key is separating actual engineering requirements from conservative “just to be safe” thinking.
Design Takeaway: Define your minimum acceptable performance first, document why specific hardness is required, then specify the lowest level that meets those requirements. Reserve high hardness only where material performance directly impacts product function.
What materials provide high strength with good machinability?
4140 steel at HRC 28-35 and 17-4 PH stainless steel at HRC 32-38 offer excellent strength-to-machinability ratios for most applications. For aluminum applications, 7075-T6 provides the highest strength while remaining highly machinable across all common CNC operations.
Choose 4140 steel for: Structural frames, brackets, general machinery, automotive components, and applications where corrosion isn’t a primary concern.
Choose 17-4 PH stainless for: Medical devices, marine environments, food processing equipment, and any application requiring both strength and corrosion resistance.
Choose 7075 aluminum for: Aerospace components, weight-critical applications, electronic enclosures, and parts requiring high strength-to-weight ratios.
These materials represent proven performers in CNC machining. 4140 steel machines cleanly with standard carbide tooling across a wide range of geometries. 17-4 PH stainless steel maintains good machinability in the precipitation-hardened condition while offering superior corrosion resistance. 7075-T6 aluminum machines exceptionally well with high-speed cutting, excellent surface finishes, and minimal tool wear.
We machine these materials daily without specialized setups, extended lead times, or unusual tooling requirements, making them reliable choices for both prototyping and production runs.
Design Takeaway: These three materials cover most high-strength applications while remaining CNC-friendly. Start your material selection here before considering more exotic or harder-to-machine alternatives.
What materials offer good wear resistance but are easy to machine?
Precipitation-hardened stainless steels, case-hardened carbon steels, and bronze alloys provide excellent wear resistance while remaining CNC-friendly. These materials offer superior performance compared to standard grades while maintaining good machinability.
Choose precipitation-hardened stainless for: Sliding components, pump parts, valve stems, and applications requiring both wear resistance and corrosion protection.
Choose case-hardened carbon steel for: Gears, pins, bushings, and mechanical components where surface wear is critical but you need a tough core.
Choose bronze alloys for: Bushings, thrust washers, wear plates, and applications requiring self-lubricating properties with excellent machinability.
These materials are readily available from standard suppliers, making them practical choices for both prototyping and production. Bronze alloys are particularly forgiving for complex geometries since they don’t work-harden during cutting and provide natural lubricity that reduces friction. Precipitation-hardened stainless steels offer corrosion resistance that carbon steels cannot match, while case-hardened steels provide the toughness needed for high-load applications.
From our machining experience, these materials consistently deliver reliable surface finishes and dimensional accuracy without the tool wear and cycle time penalties associated with through-hardened alternatives.
Design Takeaway: These materials offer practical compromises between performance and manufacturability. Start here when wear resistance is important before considering harder-to-machine alternatives.
What surface treatments add hardness to soft materials?
Nitriding, case hardening, and hard anodizing can significantly increase surface hardness while preserving the machinable soft core. These treatments let you optimize manufacturing by machining parts soft, then adding hardness only where functionally required.
Choose nitriding for: Steel components requiring deep surface hardness with excellent fatigue resistance and minimal part distortion.
Choose case hardening for: Carbon steel parts where you need very hard surfaces for gear teeth, pins, or heavily worn areas.
Choose hard anodizing for: Aluminum parts requiring increased surface hardness and wear resistance while maintaining corrosion protection.
Plan treatment areas during initial design – adding them later requires design changes and additional costs. Case hardening typically adds about a week to project timelines, while nitriding and anodizing services are available at most heat treat shops. Avoid sharp corners in areas that will be case hardened, as they can create stress concentrations during the treatment process.
Consider the sequence carefully: machine all features first, then send for treatment. Some treatments may require post-treatment machining for critical dimensions, so factor this into your design tolerances and project timeline.
Design Takeaway: Specify surface treatments early in your design process and design geometry with treatment requirements in mind. This approach lets you achieve hard wearing surfaces without the manufacturing penalties of pre-hardened materials.
What's the cost difference between HRC 35 vs HRC 50 for my application?
Expect 150-200% higher machining costs when specifying HRC 50 versus HRC 35 materials. The increase comes from slower cutting speeds, frequent tool changes, specialized tooling requirements, and extended cycle times that significantly impact both prototype and production pricing.
The cost escalation begins around HRC 40 and accelerates rapidly above HRC 45. Materials at HRC 50+ require carbide tooling, flood coolant systems, and significantly reduced feed rates to prevent premature tool failure. What machines in one setup at HRC 35 may require multiple operations at HRC 50 due to tool access limitations and thermal management needs.
Lead times also extend substantially – parts that complete in days at moderate hardness levels can require significantly longer at higher hardness due to slower machining parameters and potential tool replacement delays. The cost difference remains consistent across volume ranges, making it important for both prototype and production budgeting.
Consider surface treatments as alternatives – nitriding or case hardening soft materials often costs less than machining pre-hardened stock while achieving similar performance in many applications.
Design Takeaway: Evaluate whether your application truly requires maximum hardness. The cost premium for HRC 50+ materials should be justified by clear performance requirements that cannot be met through alternative approaches.
What design features should I avoid with high-strength materials?
Avoid deep narrow slots, sharp internal corners, thin walls under 0.060″, and complex undercuts when specifying hard materials above HRC 40. These features create tool access problems, increase chatter risk, and present manufacturing challenges that may require alternative approaches.
Deep holes and narrow slots: Standard tooling deflects more in hard materials. Consider larger diameters or specify reaming operations for critical hole dimensions.
Sharp internal corners: Specify generous radii (minimum 0.015″) to improve tool life. If sharp corners are required, consider EDM operations or press-fit corner inserts.
Thin walls: Maintain adequate thickness to prevent deflection and chatter. Consider ribbing or local thickness increases in critical areas.
Complex undercuts and deep pockets: May require specialized tooling or alternative manufacturing methods. Redesign as separate components or specify alternative joining methods when possible.
Parts with length-to-thickness ratios over 8:1 or large flat sections are particularly challenging in hard materials due to deflection and thermal management issues.
Design Takeaway: Design for standard tooling and simple setups whenever possible. When complex features are required, consider alternative manufacturing methods or design approaches that achieve the same function with simpler geometry.
Does heat treatment cause part distortion after machining?
Yes, heat treatment typically causes some distortion, especially in thin sections, long parts, or components with varying wall thickness. Plan for potential movement on critical dimensions and consider post-treatment machining for tight tolerance features.
Distortion occurs because heat treatment relieves internal stresses created during machining while introducing new thermal stresses. Parts with non-uniform geometry experience uneven heating and cooling, causing predictable dimensional changes. Thin walls, long shafts, and asymmetrical designs are most susceptible to movement.
Plan for this during initial design by leaving stock material on critical surfaces for post-treatment machining when tight tolerances are required. This approach ensures dimensional accuracy while gaining the benefits of improved material properties.
Consider your part geometry risk factors: length-to-thickness ratios over 10:1, large flat sections, or significant wall thickness variations indicate higher distortion potential. For complex geometries, consider designing in relief cuts or balanced features to minimize stress concentrations.
Evaluate whether your tolerances truly require post-treatment machining, as many applications can work with standard heat treatment dimensional variation.
Design Takeaway: Plan for heat treatment distortion during initial design. Focus post-treatment machining only on features where tight tolerances are functionally critical to minimize complexity while maintaining performance.
Conclusion
Tolerances tighter than HRC 40 significantly increase machining costs and lead times for most applications. Focus on minimum hardness requirements and consider surface treatments on softer materials for optimal performance-to-cost balance. Contact us to explore manufacturing solutions tailored to your hardness and strength requirements.
Frequently Asked Questions
Around HRC 45-50 is where machining costs typically increase 150-200% compared to moderate hardness levels. This threshold requires specialized carbide tooling, slower cutting speeds, and extended cycle times. Most shops can handle HRC 40 with standard equipment, but above HRC 50 requires specialized setups.
Focus on your actual performance requirements – wear resistance, fatigue life, or strength needs. HRC 35 provides adequate strength for most structural applications. Reserve HRC 45+ only for wear surfaces, cutting edges, or high-cycle fatigue applications where material performance is truly critical to function.
Heat treatment typically causes some distortion, especially in thin sections or complex geometries. Plan for potential movement and leave stock material on critical surfaces for post-treatment machining when tight tolerances are required. Many applications can work with standard heat treatment variation.
Only for applications requiring titanium’s unique properties – high strength-to-weight ratio, biocompatibility, or extreme corrosion resistance. For most high-strength applications, hardened steel or precipitation-hardened stainless steel provides adequate performance at significantly lower machining costs.
Yes, through surface treatments like nitriding or case hardening on softer base materials. You can machine parts at HRC 25-35, then achieve surface hardness of HRC 58-62 through heat treatment. This approach often costs less than machining pre-hardened stock while delivering similar wear resistance.
HRC 40 for steels and HRC 35 for stainless steel represent practical limits for standard CNC shops. Above these levels, you’ll need specialized machining capabilities, which limits your supplier options and increases lead times significantly.
