CNC Machining Materials: Metals and Plastics Selection Guide

Choosing CNC machining materials is one of the earliest and most important decisions in a prototype or production project. The material affects strength, weight, dimensional stability, surface finish, corrosion resistance, machining time and total cost. A grade that performs well in service may still be a poor choice if it is difficult to source in the required stock size or if the design does not suit its machining behavior.

The best material is therefore not simply the strongest, lightest or cheapest option. It is the grade that meets the real operating requirements with the lowest practical manufacturing risk. This guide compares common metals and engineering plastics, explains the properties that matter during machining and gives engineers a structured way to select a material before requesting a quote.

How to Choose a CNC Machining Material

Start with the part’s function rather than a familiar alloy name. A useful material specification should answer six questions:

1. What loads will the part carry? Define tension, compression, bending, impact, fatigue and fastener loads.
2. What environment will it see? Include temperature, moisture, UV, chemicals, salt, vacuum and electrical conditions.
3. Which dimensions are function-critical? Tight bores, thin walls, flat sealing faces and long slender features may control the choice.
4. What weight is acceptable? Density can be decisive in aerospace, robotics, handheld products and moving assemblies.
5. What finish or appearance is required? Anodizing, plating, passivation, polishing, painting and an as-machined finish all interact with the base material.
6. How many parts are needed? A material that is reasonable for a prototype may become too expensive or slow for a larger batch.

Document the minimum performance rather than automatically specifying the highest grade. Over-specification can increase material cost, tool wear, inspection effort and lead time without improving the product.

Metal vs Plastic for CNC Machining

Choose metal when stiffness and temperature resistance dominate

Metals are usually preferred for structural brackets, shafts, housings, heat sinks, threaded components and parts exposed to elevated temperature. They generally provide greater stiffness, strength and dimensional stability than unfilled plastics. Metals also support durable threads, thin sections and a wide range of protective or cosmetic finishes.

Choose plastic when weight, insulation or chemical behavior matters

Engineering plastics are useful for electrical insulators, low-friction guides, fluid-handling components, covers and lightweight mechanisms. They can eliminate corrosion and reduce mass, but they expand more with temperature and may creep under sustained load. Heat from machining can also distort a plastic part if tools, workholding and cutting conditions are not controlled.

Do not compare only tensile strength

A material selection based on one data-sheet value can fail in the real assembly. Stiffness, notch sensitivity, fatigue, creep, moisture absorption, thermal expansion and the direction of the load may matter more than ultimate strength. Evaluate the exact material condition, thickness and operating temperature whenever the part is safety-critical.

Common Aluminum Alloys

Aluminum 6061

6061 is a versatile choice for CNC prototypes and production parts. It machines efficiently, is widely available and can be anodized. Typical applications include electronic housings, fixtures, robotics parts, brackets, covers and general mechanical components. It is often the first alloy to evaluate when a drawing needs moderate strength, low weight and good corrosion resistance.

Aluminum 7075

7075 is selected when higher strength is required without moving to steel or titanium. It is common in aerospace structures, high-load mechanisms and performance equipment. Compared with 6061, raw stock and processing are usually more expensive, corrosion behavior needs closer attention and welding is less favorable. Use it because the strength is needed, not merely because it sounds premium.

Aluminum 5052 and 5083

5052 is frequently associated with sheet metal because of its formability and corrosion resistance. 5083 is used where marine corrosion resistance and weldability are important. Both can be machined, but they do not behave exactly like free-machining aluminum. When a project combines bent sheet, welded structure and machined interfaces, maintaining one compatible alloy family can simplify procurement and finishing.

Aluminum 2024

2024 provides high strength and fatigue performance for demanding structural applications. Its corrosion resistance is lower than that of 6061, so surface protection and environmental exposure need consideration. Confirm temper and stock form because the same alloy designation does not fully define the supplied condition.

Stainless Steel Grades

Stainless steel 304

304 is a general-purpose austenitic stainless steel with good corrosion resistance and broad availability. It is used for food equipment, enclosures, fittings, instruments and industrial hardware. It work-hardens during cutting, so stable setup, sharp tooling and suitable feeds are important. It is normally slower and more expensive to machine than aluminum.

Stainless steel 316 and 316L

316 is preferred when chloride exposure or a more aggressive environment makes 304 insufficient. The low-carbon 316L version is often specified for welded assemblies and controlled applications. For medical projects, do not assume commercial 316L and implant-oriented 316LVM are equivalent; chemistry, melting route, documentation and regulatory expectations differ.

Stainless steel 17-4 PH

17-4 PH combines corrosion resistance with high strength developed through heat treatment. It is useful for shafts, valve components, aerospace hardware and demanding fixtures. The drawing should identify the required heat-treatment condition because properties and final dimensions depend on it. Decide whether critical features are machined before or after aging.

Carbon and Alloy Steels

Low-carbon steel

Low-carbon grades are economical for brackets, frames, fixtures and machine components where weight and corrosion are not the primary concerns. They weld readily and accept paint, black oxide, zinc plating and other protective finishes. The finish specification should define corrosion expectations because unfinished carbon steel can rust quickly.

4140 alloy steel

4140 offers greater strength, wear resistance and fatigue capability than basic mild steel. It is used for shafts, tooling, gears, mounts and heavily loaded components. Its machining behavior depends strongly on hardness and heat-treatment condition. Quote requests should identify whether the stock is annealed, pre-hardened or to be heat-treated after machining.

Tool steels

Tool steels are selected for dies, wear surfaces, gauges and cutting or forming tools. They can provide high hardness and abrasion resistance, but processing may require rough machining, heat treatment, finish grinding or EDM. A drawing that calls out only a tool-steel grade without the final hardness and process sequence is incomplete.

Titanium Alloys

Titanium offers an excellent strength-to-weight ratio, corrosion resistance and useful performance at temperature. Ti-6Al-4V is the most familiar grade for aerospace, medical, marine and high-performance parts. Commercially pure titanium may be chosen when corrosion behavior or formability matters more than maximum strength.

Titanium conducts heat poorly compared with aluminum, so cutting heat remains near the tool edge. Machining requires rigid workholding, controlled engagement and an appropriate tool strategy. Thin walls can deflect and spring after material removal. Use titanium when the application benefits justify the higher raw-material and machining cost.

Copper and Brass

Copper

Copper is chosen for electrical conductors, bus bars, heat-transfer components, electrodes and RF parts. Pure copper can be gummy and may create burrs, while tellurium copper is often selected when improved machinability is useful. Conductivity requirements should identify the exact grade because alloying elements that improve cutting can change electrical or thermal performance.

Brass

Brass machines efficiently and produces good detail and surface quality. It is common in fittings, valve components, electrical hardware, inserts and decorative parts. Different brass grades vary in lead content, corrosion behavior and compliance status. If the part will contact drinking water, food or consumer products, state the applicable substance restrictions in the RFQ.

Engineering Plastics for CNC Machining

ABS

ABS is economical and practical for appearance prototypes, enclosures, fixtures and functional models. It machines well when heat is controlled and can be painted or bonded. It is less suitable for high-temperature service or applications that require excellent outdoor weathering without protection.

Acetal: POM and Delrin

Acetal combines low friction, good dimensional stability and low moisture absorption. It is widely used for gears, bushings, rollers, guides and precision plastic components. Homopolymer and copolymer grades differ in properties and behavior, so use the correct commercial or generic designation rather than treating all acetal as identical.

Nylon

Nylon provides toughness, wear resistance and good fatigue behavior for gears, spacers, bearings and structural plastic parts. It absorbs moisture, which can change dimensions and mechanical properties. Drawings for precision nylon components should define conditioning and inspection conditions, especially when fit is sensitive.

Polycarbonate

Polycarbonate is tough and can be transparent, making it useful for guards, covers, light-transmitting parts and impact-resistant housings. Machining heat, residual stress and incompatible cleaners can contribute to crazing or cracking. Optical parts require a finishing plan because an ordinary milled surface will not be transparent.

Acrylic

Acrylic is selected for clear panels, light guides, display parts and cosmetic components. It can achieve excellent optical appearance after suitable polishing, but it is more brittle than polycarbonate. Internal corners, drilled holes and assembly stress should be designed to reduce cracking risk.

HDPE and UHMW-PE

Polyethylene grades offer chemical resistance, low friction and toughness. HDPE is used for fluid components, fixtures and general industrial parts; UHMW-PE is common in wear strips, guides and impact surfaces. Their flexibility, thermal expansion and tendency to move during machining make tight, metal-like tolerances impractical on many geometries.

PTFE

PTFE provides very low friction, broad chemical resistance and useful electrical insulation. It is soft and can creep under load, so designs need adequate support and realistic tolerances. Filled PTFE grades may improve wear, stiffness or dimensional stability but change other properties.

PEEK

PEEK is a high-performance thermoplastic used where temperature capability, chemical resistance, strength or sterilization performance is required. It is substantially more expensive than common engineering plastics. Medical, food-contact, bearing and filled grades are not interchangeable, and traceability requirements should be defined before material is ordered.

Key Properties That Affect Machining

Machinability

Machinability describes more than whether a material can be cut. It includes cutting force, chip formation, tool wear, heat generation, achievable finish and burr behavior. A difficult material increases cycle time and may require special cutters, coolant, toolpaths or inspection pauses.

Stiffness and residual stress

Flexible materials deflect under cutting and clamping forces. Rolled, extruded, molded and heat-treated stock can also contain residual stress that releases as material is removed. Large flat parts and thin walls may require balanced roughing, stress relief, extra stock and a separate finishing operation.

Thermal expansion

All materials change size with temperature, but plastics generally change much more than metals. Precision inspection should therefore use an agreed temperature and allow the part to stabilize. The service environment must also be considered when a plastic component mates with a metal one.

Hardness and abrasiveness

Hard materials can increase tool wear and reduce cutting speed. Filled plastics can be abrasive even when they feel softer than metal. Glass or carbon fiber reinforcement changes both material performance and the machining strategy, and exposed fibers may affect edge quality or handling.

Material Choice and Tolerances

A drawing tolerance should reflect function, feature size, material behavior and inspection method. A blanket tight tolerance applied to every dimension increases cost and can be impossible on soft or unstable materials. Use geometric tolerancing and clear datums to communicate the relationships that actually matter.

ISO 2768-1 defines general tolerances for linear and angular dimensions when individual tolerances are not indicated. The official ISO 2768-1 standard page explains its scope. Applying a general-tolerance standard does not replace specific requirements for fits, flatness, position or sealing features.

For flexible plastics, agree on whether dimensions are measured free, supported in a fixture or assembled. For heat-treated metals, plan whether final critical dimensions need grinding or finish machining after treatment.

Surface Finishing Compatibility

Anodizing aluminum

Anodizing can improve corrosion and wear resistance while adding color. The coating changes dimensions, and color varies with alloy, surface preparation, geometry and batch. Mask threaded or electrically conductive areas where needed and identify cosmetic surfaces.

Passivating stainless steel

Passivation is used to remove free iron and support the natural corrosion-resistant surface. It does not hide machining marks or create a thick decorative coating. Specify the required process and any verification instead of writing only “stainless finish.”

Plating steel, copper and brass

Nickel, zinc, tin and other coatings may provide corrosion protection, solderability, appearance or wear performance. Coating thickness affects fits and threads, while deep recesses may receive a different deposit than exposed surfaces. Include masking and post-finish dimensions on the drawing.

Polishing plastics

Mechanical polishing, vapor-based processes and flame polishing are material-specific. A method that works on acrylic may damage another plastic or create unacceptable stress. Define the viewing area, transparency target and permitted optical defects using a sample when possible.

For machining and post-processing in one supply chain, review the available surface finishing services during design rather than after parts are complete.

Compliance and Material Documentation

A grade name on a quotation does not prove compliance. Regulated programs may need mill certificates, certificates of conformity, lot traceability, independent test reports or declarations covering restricted substances. Define the required documents before purchase because traceability often cannot be reconstructed after stock has been cut.

For products sold in the European Union, the RoHS framework restricts certain hazardous substances in electrical and electronic equipment. The European Commission’s official RoHS overview provides the current scope and regulatory context. Confirm applicability with a qualified compliance professional and obtain declarations for the exact supplied material and finish.

Medical, aerospace and food-contact applications require additional care. A commercial material that shares a familiar alloy or polymer name may not provide the cleanliness, certification, biocompatibility or change control required by the final product.

How Material Selection Changes CNC Cost

• Raw stock price: titanium, PEEK and certified specialty grades cost more than common aluminum or acetal.
• Stock availability: unusual thicknesses or diameters may require a larger billet, minimum order or longer procurement time.
• Machining rate: materials that generate heat, work-harden, abrade tools or form poor chips require slower processing.
• Setup strategy: flexible or distortion-prone parts may need soft jaws, vacuum fixtures, support tooling or extra operations.
• Tool consumption: hardened steels, titanium and reinforced plastics can increase cutter cost.
• Secondary processing: heat treatment, anodizing, plating, passivation, polishing and marking add handling and inspection.
• Quality documentation: full traceability and certified testing add procurement and administrative work.
• Yield risk: expensive stock and difficult geometry make a scrapped part more costly.

A good supplier may suggest an equivalent grade that is easier to source or machine. Evaluate the suggestion against the functional specification and approve any substitution in writing.

Design Tips for Different CNC Materials

1. Use generous internal radii instead of sharp milled corners.
2. Avoid deep narrow pockets that require long, flexible tools.
3. Support thin walls and separate cosmetic requirements from structural requirements.
4. Do not apply the same tolerance table to aluminum, stainless steel, nylon and PTFE.
5. Keep threaded depth practical and use inserts where repeated assembly would damage plastic threads.
6. Allow coating thickness on fits, holes and mating surfaces.
7. Specify grain or stock direction only when it affects performance.
8. Place datums on stable surfaces that can be held and inspected without distortion.
9. Use standard stock sizes when the envelope and design allow it.
10. Ask for design-for-manufacturing feedback before freezing the material and drawing.

Complex prototypes may benefit from 3-axis, indexed or simultaneous multi-axis machining. The CNC milling service page summarizes available machining capability, while the rapid prototyping service page covers alternative processes for early design validation.

Material Selection Workflow

1. Define the operating conditions. Record loads, temperature, life, environment and regulatory constraints.
2. Separate mandatory and preferred requirements. This prevents a cosmetic preference from being treated like a safety requirement.
3. Shortlist two or three materials. Compare performance, availability, process route and finish.
4. Review the CAD model for manufacturability. Identify thin walls, deep cavities, small tools and distortion risk.
5. Request quotations using the same revision. Ask suppliers to price the preferred grade and any approved alternative separately.
6. Approve a first article. Verify fit, finish, documentation and functional behavior before the full batch.
7. Record the final material condition. Keep alloy, temper, polymer grade, color, filler, finish and certification requirements under revision control.

RFQ Checklist for CNC Machining Materials

• STEP or another clean 3D CAD file.
• Revision-controlled 2D drawing with datums and critical dimensions.
• Exact material grade, temper, hardness or polymer formulation.
• Permitted equivalent grades or a clear “no substitution” note.
• Quantity, first-article requirement and delivery schedule.
• Surface finish, color, masking and post-finish dimensions.
• Operating temperature, load and named chemical exposures.
• Required certificates, lot traceability and compliance declarations.
• Inspection method, sampling plan and report format.
• Packaging requirements for cosmetic or corrosion-sensitive parts.

Frequently Asked Questions

What is the easiest metal to CNC machine?

Free-machining aluminum and brass grades are generally efficient choices, but “easiest” depends on geometry, tolerance and finish. A material that cuts quickly may still be unsuitable for the operating load or regulatory requirement.

What is the best aluminum for a CNC prototype?

6061 is a practical default for many prototypes because it is available, machinable and compatible with common finishes. Choose 7075, 2024 or another alloy only when the application needs its specific strength, fatigue or environmental performance.

Which plastic holds tight tolerances best?

Acetal is often selected for dimensional stability and low moisture absorption. Filled PEEK and other engineered grades can also perform well in demanding conditions. The answer still depends on part geometry, temperature, conditioning and inspection setup.

Can a supplier substitute an equivalent material?

Only with engineering approval. Two standards may describe similar materials but differ in chemistry, temper, test method or documentation. List acceptable alternatives on the drawing or approve a deviation before production.

Why did my plastic CNC part change size after delivery?

Temperature, moisture absorption, internal stress and clamping during inspection can change measured dimensions. Define conditioning and measurement conditions, and allow the part to stabilize before inspection.

Should finishing be specified before or after machining?

Specify it at the quoting stage. Finish affects material choice, dimensional allowance, masking, cosmetic preparation and inspection. Adding a coating after machining is complete can make fits or threads unacceptable.

Request a CNC Machining Material Review

Effective CNC machining materials selection connects product performance with the realities of stock, cutting, inspection and finishing. When requirements are clear, engineers can avoid paying for unnecessary properties and reduce the risk of distortion, corrosion, failed fits or unsupported compliance claims.

Send your CAD file, drawing, quantity, material preference, operating conditions and documentation requirements through the Aluprototype request-a-quote page. The engineering team can review machinability, suggest practical alternatives and prepare a quotation for metal or plastic CNC parts.