Why Is Material Selection Foundational to a Machined Part?
A material that is stronger than required will be paid for in machining cost, in tool wear, in cycle time, and in delivery delay. A material that is weaker than required will be paid for in field failure, in warranty cost, and in reputational damage. The asymmetry favors conservative selection — but conservatism must be tempered with cost awareness, because over-specifying is itself a form of failure.
Material selection is also a supply-chain decision. Some materials are available from multiple mills with predictable lead times; others are constrained by alloy, by product form, or by export controls. The drawing that specifies a material the shop cannot source is a drawing that delays the project.
The processing of mechanical parts capability at Yuqing covers a wide range of standard engineering materials; the material specification should match that capability.
What Are the Main Material Families for CNC Machining?
The five families that cover the vast majority of custom CNC machined parts:
|
Family |
Strengths |
Limitations |
|
Carbon and alloy steels |
Strong, low cost, easy to heat treat |
Heavy, corrodes without coating |
|
Stainless steels |
Corrosion resistant, strong, aesthetic |
Tougher to machine, work-hardens |
|
Aluminum alloys |
Light, easy to machine, good thermal conductivity |
Lower strength than steel |
|
Titanium alloys |
Highest strength-to-weight ratio, corrosion resistant |
Very tough to machine, expensive |
|
Engineering plastics |
Light, corrosion resistant, electrical insulation |
Lower strength, thermal limits
|
Each family has multiple grades with different properties. The drawing should specify the grade (e.g., 4140, 316L, 7075-T6, Ti-6Al-4V, Delrin) rather than the family.
How Are Carbon and Alloy Steels Selected?
Carbon steels (A36, 1018, 1020, 1045) are the workhorses of mechanical structures. Alloy steels (4140, 4340, 8620) offer higher strength through heat treatment.
Selection considerations:
A36 / 1018 / 1020: low-carbon, weldable, machinable. Used for structural parts, brackets, frames. Cheap and readily available.
1045: medium-carbon, can be heat treated to higher strength. Used for shafts, gears, couplings.
4140: chromium-molybdenum alloy, deep-hardenable. Used for high-stress shafts, gears, tooling.
4340: nickel-chromium-molybdenum alloy, very tough. Used for aerospace and high-stress applications.
8620: case-hardenable alloy. Used for gears and parts requiring a hard surface with a tough core.
The cost premium for alloy steels is modest, but the heat treatment cost is significant. The drawing should specify whether the part is to be supplied as-machined, normalized, quenched and tempered, or case hardened.
How Are Stainless Steels Selected?
Stainless steels are selected for corrosion resistance, aesthetic appearance, or sanitary requirements. The 300-series (austenitic) are non-magnetic and not hardenable by heat treatment; the 400-series (martensitic) are magnetic and hardenable.
|
Grade |
Corrosion Resistance |
Machinability |
Typical Application |
|
304 |
Good |
Fair |
General purpose, food contact |
|
316 / 316L |
Better (chloride resistance) |
Fair |
Marine, chemical, pharmaceutical |
|
303 |
Good |
Excellent (sulfur added) |
Screw machine parts, high-volume |
|
410 |
Moderate |
Fair |
Cutlery, valves, pumps |
|
416 |
Moderate |
Excellent |
Screw machine parts |
|
17-4 PH |
Good |
Fair (best in H900/H1025) |
Aerospace, high-strength |
|
Duplex 2205 |
Excellent |
Difficult |
Chemical, oil & gas
|
Stainless steel work-hardens during cutting. Tools must be sharp, feeds must be positive, and heat must be managed. A shop unfamiliar with stainless machining will produce parts with poor surface finish and accelerated tool wear.
The processing of various structural parts service at Yuqing includes stainless steel; verify the shop's stainless experience before specifying 316L for a tight tolerance part.
How Are Aluminum Alloys Selected?
Aluminum alloys are the most machined metals in the world. The 6000-series (6061, 6063) are general-purpose; the 7000-series (7075) are high-strength; the 2000-series (2024) are aerospace; the 5000-series (5052, 5083) are marine.
|
Grade |
Strength |
Machinability |
Weldability |
Typical Application |
|
6061-T6 |
Medium |
Excellent |
Excellent |
General structural, fixtures, plates |
|
7075-T6 |
High |
Good |
Poor |
Aerospace, high-stress |
|
2024-T3 |
High |
Good |
Poor |
Aerospace structures |
|
5052-H32 |
Low-medium |
Fair |
Excellent |
Marine, sheet metal |
|
5083-H116 |
Medium |
Fair |
Excellent |
Marine, pressure vessels |
|
6063-T5 |
Low |
Excellent |
Excellent |
Architectural extrusions |
|
Cast 356-T6 |
Medium |
Fair |
Poor |
Cast parts, housings |
The T6 temper indicates solution heat-treated and artificially aged. The drawing should specify the temper because the same alloy in different tempers has different strength and different machinability.
How Are Titanium Alloys Selected?
Titanium alloys offer the highest strength-to-weight ratio of any common engineering metal, combined with excellent corrosion resistance. The dominant alloy is Ti-6Al-4V (Grade 5), used in aerospace, medical implants, and high-performance industrial applications.
Selection considerations:
Grade 2 (commercially pure): low strength, excellent corrosion resistance, weldable. Used in chemical and marine.
Grade 5 (Ti-6Al-4V): high strength, good corrosion resistance, machinable with care. Used in aerospace, medical, racing.
Grade 5 ELI: extra-low interstitial version, used for medical implants.
Titanium machining requires sharp tools, low cutting speeds, high-pressure coolant, and rigid setup. The chips are flammable and the springback is significant. A shop without titanium experience will produce parts with poor surface finish and rapid tool failure.
How Are Engineering Plastics Selected?
Engineering plastics replace metals in applications where weight, corrosion resistance, electrical insulation, or cost matters more than strength. Common selections:
|
Material |
Strength |
Temperature Limit |
Machinability |
Typical Application |
|
Delrin (POM-H) |
Medium |
100 °C |
Excellent |
Gears, bushings, snap fits |
|
Acetal Copolymer (POM-C) |
Medium |
100 °C |
Excellent |
Similar to Delrin, better for wet |
|
Nylon (PA 6, 6/6) |
Medium-low |
120 °C |
Good |
Bushings, wear parts |
|
UHMW-PE |
Low |
80 °C |
Good |
Wear strips, chute liners |
|
PEEK |
High |
250 °C |
Fair |
Aerospace, semiconductor, medical |
|
PTFE (Teflon) |
Low |
260 °C |
Poor (gummy) |
Seals, gaskets |
|
Polycarbonate (PC) |
Medium |
130 °C |
Good |
Guards, transparent covers |
|
ABS |
Low |
80 °C |
Excellent |
Prototypes, enclosures |
|
Acrylic (PMMA) |
Low |
80 °C |
Excellent |
Transparent covers, light pipes |
Plastic machining requires different cutting parameters than metal. Tools must be very sharp, speeds moderate, and heat must be managed. Some plastics (PTFE, UHMW) are gummy and produce long stringy chips that can wrap tools; others (acrylic) crack if tools are dull.
What Is the Cost Comparison?
Approximate relative cost per kilogram (varies with market and form):
|
Material |
Relative Cost |
|
1018 / 1020 steel |
1× |
|
4140 alloy steel |
1.5× |
|
304 stainless |
3× |
|
316 / 316L stainless |
4× |
|
6061 aluminum |
2× |
|
7075 aluminum |
3× |
|
Titanium Grade 5 |
30–50× |
|
Delrin |
4× |
|
PEEK |
30× |
The material cost is only one part of the total cost. The machinability factor (which determines cycle time) often has a larger impact:
|
Material |
Machinability Factor |
|
6061 aluminum |
3–5× faster than steel |
|
1018 steel |
1× baseline |
|
304 stainless |
0.5× (twice as long) |
|
17-4 PH stainless |
0.4× |
|
Titanium Grade 5 |
0.2× (5× as long) |
|
Delrin |
5× faster than steel |
A titanium part that takes 5× longer to machine than the equivalent steel part has a total cost dominated by machining time, not material cost.
How Are Material Certificates Used?
Material certificates document the chemistry, mechanical properties, and traceability of the supplied material. They are the legal evidence that the material meets the drawing specification.
The relevant standard is EN 10204, which defines four types of certificates:
|
Type |
Content |
Typical Use |
|
2.1 |
Statement of compliance with order |
Commercial, non-critical |
|
2.2 |
Test report from the mill |
General industrial |
|
3.1 |
Mill-issued certificate with independent verification |
Critical applications |
|
3.2 |
Mill and independent third-party verification |
Aerospace, nuclear, oil & gas |
A drawing for a structural bracket may not require a certificate. A drawing for a pressure-retaining part typically requires 3.1; a drawing for a flight-critical aerospace part typically requires 3.2.
The export machined parts service at Yuqing routinely provides material certificates with shipment; buyers should specify the certificate type at the time of order.
How Are Materials Machinability Ratings Used?
Each material has a machinability rating that compares its cutting behavior to a reference material (typically AISI 1212 steel, rated at 100%).
|
Material |
Machinability Rating |
|
AISI 1212 (reference) |
100% |
|
6061 aluminum |
200–300% |
|
Brass C360 |
100% |
|
1018 steel |
70% |
|
304 stainless |
40–45% |
|
316 stainless |
36% |
|
4140 alloy steel (annealed) |
65% |
|
Titanium Grade 5 |
20–25% |
|
Inconel 718 |
12% |
A shop uses the machinability rating to select cutting speeds and feeds. A drawing that specifies a low-machinability material in a tight tolerance must be paired with a shop experienced in that material.
What Is the Effect of Heat Treatment on Material Selection?
Heat treatment changes the material's hardness and strength, which in turn changes the machinability. The drawing should specify the supply condition (annealed, normalized, quenched and tempered) because the shop needs to know the starting condition.
A 4140 part in the annealed condition (~197 HB) is much easier to machine than a 4140 part in the quenched and tempered condition (~310 HB). The shop can rough-machine in the annealed condition, then heat-treat, then finish-machine with light cuts to reach the final tolerance.
The sequence is engineering-controlled, not shop-determined. The drawing should specify the sequence and the final condition. A drawing that simply says "4140, HRC 32" without specifying the machining sequence forces the shop to choose, often incorrectly.
How Are Material Standards Specified?
The drawing should reference a published material standard rather than describing the composition. Common standards:
|
Region |
Steel |
Stainless |
Aluminum |
Titanium |
|
USA (AISI/SAE/ASTM) |
AISI 4140, ASTM A36 |
AISI 304, ASTM A276 |
AA 6061-T6 |
ASTM B265 Gr5 |
|
Europe (EN) |
EN 10083 42CrMo4 |
EN 10088 1.4404 |
EN 573 EN AW-6061 |
EN 3468 Ti-6Al-4V |
|
China (GB) |
GB/T 3077 42CrMo |
GB/T 1220 06Cr17Ni12Mo2 |
GB/T 3190 6061 |
GB/T 3625 TC4 |
|
Japan (JIS) |
JIS SCM440 |
JIS SUS304 |
JIS A6061 |
JIS T 6401 |
Specifying "316L stainless" is ambiguous between standards. Specifying "ASTM A276 Type 316L" or "EN 10088-1.4404" is precise and allows the shop to source the correct material.
How Are Specialty Alloys Selected?
Some applications require specialty alloys that are not in the standard families. The selection should be driven by the specific requirement, not by familiarity.
Inconel / Hastelloy: high-temperature and corrosive environments (chemical processing, aerospace). Very difficult to machine.
Monel: marine and chemical applications. Difficult to machine.
Beryllium copper: high strength and electrical conductivity (molds, springs). Machinable with care (beryllium dust is toxic).
Muntz metal / naval brass: marine applications with good machinability.
Magnesium alloys: aerospace, weight-critical. Flammable chips; requires special cutting strategies.
A drawing that specifies one of these alloys should be accompanied by a note explaining the requirement, so the shop can plan its tooling, its safety protocols, and its cycle time.
What Is the Material Selection Decision Flow?
A defensible decision flow:
Define the functional requirements (strength, corrosion, temperature, weight, electrical).
Identify the candidate materials that meet the requirements.
Compare machinability, cost, lead time, and certification.
Choose the material that meets the requirements at the lowest total cost (not just material cost).
Specify the standard, the temper, the supply condition, and the certificate.
Confirm with the shop that the material is available and processable.
The decision is documented in the drawing and in the part's engineering file. A material change in production requires a drawing revision and a re-evaluation of the cost and lead time.
How Are Coatings and Platings Selected?
Some applications require a surface coating for corrosion resistance, wear resistance, or appearance. The coating is specified on the drawing as a secondary operation:
Anodizing (Type II, Type III): aluminum oxide layer for corrosion and wear resistance. Standard for aluminum parts.
Electroless nickel: uniform coating on steel, aluminum, copper. Good corrosion and wear resistance.
Zinc plating or galvanizing: corrosion protection for steel.
Powder coating: durable aesthetic finish.
Black oxide: mild corrosion protection for steel, mostly cosmetic.
PTFE or similar: low-friction coating.
Chromate conversion (Alodine): corrosion protection for aluminum, paint base.
The coating is applied after machining. The drawing should specify the coating, the thickness, the standard (e.g., MIL-A-8625 for anodizing), and any post-coating dimensional limits.
Conclusion
Material selection is the engineering decision that defines the strength, weight, corrosion resistance, machinability, cost, and lead time of a
CNC machined part. The selection should reference a published standard, specify the supply condition and the certificate, and be matched to the shop's capability. The processing of mechanical parts, processing of structural parts, and export machined parts capabilities at Yuqing cover a wide range of materials; engineers should specify the standard and the supply condition precisely so the shop can source the material correctly and machine it efficiently.
Frequently Asked Questions
What is the most common material for custom CNC parts?
6061-T6 aluminum is the most common, due to its excellent machinability, low cost, good strength, and corrosion resistance. For higher strength, 7075-T6 is used; for corrosion-critical applications, 316L stainless is used.
How is machinability rating used?
The machinability rating compares a material's cutting behavior to a reference (AISI 1212 steel at 100%). Higher ratings cut faster and wear tools less. The rating is used to set cutting speeds and feeds, which determines cycle time.
What is the difference between 304 and 316 stainless steel?
304 stainless is the general-purpose grade. 316 stainless contains molybdenum for better chloride resistance; 316L is the low-carbon version for welded structures. 316 is more expensive and slightly tougher to machine than 304.
When should titanium be specified?
Titanium is specified when strength-to-weight ratio and corrosion resistance both matter and cost is secondary. Typical applications: aerospace structural parts, medical implants, marine hardware, racing components.
What is the difference between 4140 and 4340?
4340 has higher nickel content, giving it higher toughness and impact resistance. 4140 is the standard alloy steel for shafts and gears; 4340 is the premium alloy for highly stressed parts (aerospace, racing).
Can plastics be machined to the same tolerances as metals?
Engineering plastics like Delrin and PEEK can be machined to ±0.025 mm with care. Some plastics (UHMW, PTFE) are difficult to machine to tight tolerances because they deform under tool pressure. The drawing should account for the material's behavior.
How are material certificates verified?
Material certificates include the heat number, the chemistry, the mechanical properties, and the testing standards. The shop or the buyer can verify the certificate against the mill's database or against the testing laboratory that issued it. EN 10204 defines the certificate types.
What is the lead time for special materials?
Standard materials (1018, 4140, 304, 6061) are typically available from stock with short lead times. Special alloys (titanium, Inconel, specialty stainless) may require mill order with 4–12 week lead times. The drawing should specify a standard material unless the application demands otherwise.
How is corrosion resistance specified?
Corrosion resistance is specified by the material grade (304, 316L, etc.) and by any surface coating. The drawing should also specify the environment (chloride concentration, temperature, immersion vs. atmospheric) if the application is unusual.
