Material Selection Guide: Choosing the Right Stock

The foundation of every successful project – understanding materials transforms good machinists into great ones

Introduction: Materials Make the Difference

The material you choose for your CNC project affects everything: how it machines, what tools you need, the surface finish you can achieve, the time it takes to complete, and ultimately whether your project succeeds or fails. Yet many hobbyists treat material selection as an afterthought – picking whatever's cheap or convenient without understanding the consequences.

Here's the reality: The "wrong" material can turn a simple project into a nightmare of broken tools, poor surface finish, and dimensional problems. Conversely, the "right" material can make even complex projects machine smoothly and efficiently.

Professional machinists understand that material selection is the foundation of process planning. They consider not just the final requirements of the part, but also how the material behaves during machining, what it costs in time and tooling, and how it affects every aspect of the manufacturing process. Master material selection, and you'll transform from someone who fights with materials to someone who works with them.

Understanding Material Properties

Mechanical Properties That Matter

Hardness:
- Affects cutting forces and tool wear
- Measured in various scales (Rockwell, Brinell, etc.)
- Generally, harder materials are more difficult to machine
- Some materials work-harden during cutting

Strength:
- Tensile strength: resistance to pulling forces
- Compressive strength: resistance to crushing forces
- Affects clamping requirements and cutting forces
- Higher strength usually means more difficult machining

Ductility:
- How much a material can deform before breaking
- Ductile materials tend to form continuous chips
- Brittle materials create small, broken chips
- Affects surface finish and tool selection

Thermal Properties:
- Heat capacity: how much heat material absorbs
- Thermal conductivity: how quickly heat spreads
- Thermal expansion: dimensional changes with temperature
- Critical for controlling heat buildup during cutting

Machinability Factors

Chip Formation:
- How the material breaks away from the workpiece
- Continuous chips: good for surface finish, harder to evacuate
- Broken chips: easier to evacuate, may cause surface roughness
- Affects tool geometry selection and speeds/feeds

Work Hardening:
- Some materials become harder as they're worked
- Stainless steels are notorious for this
- Requires constant feed rates and sharp tools
- Can dramatically increase cutting forces

Chemical Reactivity:
- How materials react with cutting tools
- Built-up edge formation in aluminum
- Corrosive effects on tooling
- Affects coolant selection and tool coatings

Abrasiveness:
- How quickly materials wear cutting tools
- High-silicon aluminum is very abrasive
- Fiberglass and composites are extremely abrasive
- Affects tool material and coating selection

Wood: The Forgiving Teacher

Why Wood is Perfect for Beginners

Advantages for Learning:
- Forgiving of mistakes and poor technique
- Low cutting forces reduce machine stress
- Excellent chip evacuation with proper dust collection
- Wide variety of hardness and grain patterns to practice on
- Relatively inexpensive materials and tooling

Typical Speeds and Feeds:
- Softwoods: 18,000+ RPM, 100-300 IPM feed rates
- Hardwoods: 15,000-18,000 RPM, 80-200 IPM feed rates
- Always prioritize chip evacuation over pure speed

Wood Varieties and Characteristics

Softwoods (Pine, Fir, Cedar):
- Easy to machine with sharp tools
- Tend to "fuzz" with dull tools
- Resinous species can gum up tools
- Good for learning basic techniques

Medium Hardwoods (Maple, Cherry, Walnut):
- Excellent machining characteristics
- Good surface finish with proper tools
- Stable and predictable behavior
- Popular for furniture and decorative items

Hard Hardwoods (Oak, Ash, Hickory):
- Require more power and slower feeds
- Excellent finish when machined properly
- Can be challenging due to grain structure
- Tear-out common without proper technique

Exotic Hardwoods (Ebony, Rosewood, Teak):
- Often contain oils or toxic compounds
- May require special handling and ventilation
- Can be very abrasive to tools
- Research safety requirements before use

Engineered Wood Products

Plywood:
- Alternating grain layers reduce warping
- Edges can be challenging due to glue lines
- Use compression spiral bits to prevent delamination
- Quality varies dramatically between grades

MDF (Medium Density Fiberboard):
- Very consistent and stable
- Excellent for templates and fixtures
- Produces fine dust requiring good collection
- Contains formaldehyde – use proper ventilation

Particle Board:
- Cheap but inconsistent quality
- Very abrasive due to glue content
- Difficult to get good edge finish
- Generally not recommended for precision work

Baltic Birch Plywood:
- Premium plywood with consistent void-free layers
- Excellent for jigs, fixtures, and precision work
- More expensive but worth it for quality projects
- Machines beautifully with sharp tools

Wood Machining Challenges

Grain Direction Effects:
- End grain machines differently than face grain
- Tear-out more likely when cutting against grain
- Plan toolpaths to minimize tear-out
- Climb milling often gives better finish

Moisture Content:
- Wood movement with humidity changes
- Green (wet) wood machines poorly
- Kiln-dried lumber is more stable
- Allow material to acclimate before machining

Seasonal Movement:
- Wood expands and contracts with humidity
- Plan for movement in precision applications
- Understand grain orientation effects
- Quarter-sawn lumber is more stable

Aluminum: The Hobbyist's Metal

Why Aluminum is Ideal for Metal Beginners

Beginner-Friendly Properties:
- Relatively soft and easy to machine
- Lower cutting forces than steel
- Good thermal conductivity helps with heat dissipation
- Forgiving of technique mistakes
- Wide availability and reasonable cost

Common Aluminum Alloys:

6061-T6:
- Most popular general-purpose alloy
- Good strength-to-weight ratio
- Excellent machinability
- Available in many shapes and sizes
- Good for structural components

6063-T6:
- Slightly softer than 6061
- Primarily used for extrusions
- Good surface finish characteristics
- Common in architectural applications

2024-T3/T4:
- High strength aluminum alloy
- More difficult to machine than 6061
- Used in aerospace applications
- Not suitable for welding

7075-T6:
- Highest strength common aluminum alloy
- More challenging to machine
- Excellent for high-stress components
- Premium price for premium performance

Aluminum Machining Characteristics

Heat Generation:
- Aluminum conducts heat well
- Tools can overheat if feeds are too slow
- Flood coolant or air blast helps significantly
- Monitor for built-up edge formation

Chip Evacuation:
- Aluminum produces stringy chips
- Good chip evacuation is critical
- Use appropriate tool geometry
- Climb milling reduces built-up edge

Built-Up Edge (BUE):
- Aluminum welds to cutting edges at low speeds
- Causes poor surface finish and rapid tool wear
- Prevent with proper speeds and sharp tools
- Coated tools help reduce BUE

Workholding Considerations:
- Soft material deforms under clamping pressure
- Use soft jaws to distribute force
- Support thin sections to prevent distortion
- Consider heat effects during long operations

Aluminum Forms and Availability

Plate and Sheet:
- Wide range of thicknesses available
- Good for flat parts and brackets
- Check for internal stress in thick plates
- Marine grade (5xxx series) available for corrosion resistance

Bar Stock:
- Round, square, and rectangular sections
- Excellent for turned parts and small components
- Usually stress-relieved for stability
- Consider cold-finished vs. hot-rolled

Extrusions:
- Complex cross-sections available
- Excellent surface finish
- Limited to specific shapes
- Very cost-effective for matching applications

Cast Aluminum:
- Different machining characteristics than wrought
- Can contain porosity and inclusions
- Usually softer than wrought alloys
- Check composition for machinability

Steel: The Strong and Challenging

Steel Complexity

Why Steel is Challenging:
- Much harder than aluminum or wood
- Generates significant heat during cutting
- Requires more robust tooling and techniques
- Higher cutting forces stress machines more
- Work hardening in some grades

When Steel is Worth the Challenge:
- High strength requirements
- Wear resistance needed
- Magnetic properties required
- Cost considerations (sometimes cheaper than aluminum)
- Welding requirements

Common Steel Types for Hobbyists

1018 Cold Rolled Steel:
- Low carbon content makes it relatively easy to machine
- Good surface finish achievable
- Weldable and readily available
- Excellent for learning steel machining
- Relatively inexpensive

12L14 Free-Machining Steel:
- Lead content improves machinability dramatically
- Excellent surface finish and tool life
- Not suitable for welding
- More expensive than 1018 but worth it for machining

4140 Alloy Steel:
- Higher strength than mild steel
- Heat treatable for various hardness levels
- More challenging to machine
- Popular for high-stress components

A36 Structural Steel:
- Commonly available and inexpensive
- Variable composition affects machinability
- Good for non-critical applications
- May have scale that damages tools

Stainless Steel Considerations

304 Stainless Steel:
- Most common stainless grade
- Work hardens rapidly during cutting
- Requires sharp tools and constant feed
- Corrosion resistant
- Significantly more challenging than carbon steel

316 Stainless Steel:
- Marine grade with better corrosion resistance
- Even more challenging to machine than 304
- Gummy and tends to work harden
- Requires carbide tooling and proper technique

17-4 PH Stainless:
- Precipitation hardening grade
- Machines better in annealed condition
- Can be heat treated after machining
- High strength when heat treated

Steel Machining Strategies

Heat Management:
- Steel generates significant heat
- Flood coolant highly recommended
- Lower surface speeds than aluminum
- Take lighter cuts to manage heat

Tool Selection:
- Carbide preferred for most applications
- Coated tools provide longer life
- Sharp tools are critical
- Positive rake angles help reduce cutting forces

Cutting Parameters:
- Lower surface speeds than aluminum
- Higher feed rates help prevent work hardening
- Consistent feed prevents work hardening
- Climb milling when machine allows

Plastics: The Surprising Challenge

Why Plastics Can Be Tricky

Common Misconceptions:
- "Plastic is soft, so it's easy to machine"
- Reality: Many plastics are challenging due to heat buildup
- Different plastic families behave very differently
- Tool selection is critical for success

Heat-Related Issues:
- Many plastics have low melting points
- Heat buildup causes dimensional changes
- Melted plastic sticks to tools
- Surface finish suffers from heat

Common Plastic Types

HDPE (High-Density Polyethylene):
- Excellent chemical resistance
- Machines well with sharp tools
- Can be "gummy" and grab tools
- Good for chemical-resistant applications

Delrin (Acetal/POM):
- Excellent machining characteristics
- Good dimensional stability
- Low friction properties
- Popular for gears and bearings

Nylon (Various Types):
- Tough and wear-resistant
- Can be challenging due to heat buildup
- Absorbs moisture, affecting dimensions
- Glass-filled versions are very abrasive

PTFE (Teflon):
- Extremely challenging to machine
- Very soft and "grabby"
- Requires special techniques and sharp tools
- Excellent chemical and temperature resistance

Polycarbonate:
- High strength and impact resistance
- Can be brittle and chip during machining
- Sensitive to heat buildup
- Used for impact-resistant applications

Plastic Machining Techniques

Heat Management:
- Use sharp tools to minimize heat generation
- Take light cuts at high speeds
- Provide good chip evacuation
- Consider air blast cooling

Tool Selection:
- Single-flute tools often work best
- Very sharp edges are critical
- Large chip gullets for evacuation
- Diamond-coated tools for abrasive filled plastics

Cutting Parameters:
- High speeds, moderate feeds generally work best
- Avoid conventional milling if possible
- Maintain constant feed to prevent heat buildup
- Test parameters on scrap first

Composite Materials: The Advanced Challenge

Understanding Composites

What Makes Them Different:
- Combination of fiber reinforcement and matrix material
- Properties vary dramatically with fiber orientation
- Extremely abrasive to cutting tools
- Require specialized tooling and techniques

Health and Safety Concerns:
- Many composite dusts are hazardous
- Carbon fiber dust is conductive and dangerous
- Fiberglass irritates skin and lungs
- Requires excellent dust collection and PPE

Common Composite Types

Fiberglass (GFRP):
- Glass fibers in various matrix materials
- Very abrasive to tools
- Good electrical insulation properties
- Used in marine and electrical applications

Carbon Fiber (CFRP):
- Extremely strong and lightweight
- Very expensive material and tooling
- Conductive dust creates electrical hazards
- Aerospace and high-performance applications

G10/FR4 (Printed Circuit Board Material):
- Glass fiber with epoxy resin
- Very abrasive and hard on tools
- Used for electrical insulation
- Creates hazardous dust

Composite Machining Strategies

Tool Selection:
- Diamond-coated tools are often necessary
- Compression cutting tools reduce delamination
- Tool life is dramatically shorter than metals
- Budget for frequent tool replacement

Cutting Parameters:
- High speeds, low feeds typically work best
- Minimize heat buildup to prevent matrix damage
- Sharp tools are absolutely critical
- Test on scrap material first

Safety Requirements:
- Excellent dust collection mandatory
- Full-face respiratory protection
- Eye protection essential
- Skin protection recommended

Exotic Materials: Special Considerations

Titanium: The Aerospace Metal

Unique Properties:
- Excellent strength-to-weight ratio
- Corrosion resistant
- Low thermal conductivity (heat builds up)
- Work hardens readily
- Expensive material and tooling

Machining Challenges:
- Requires carbide tooling
- Flood coolant essential
- Very sharp tools necessary
- Consistent feed rates critical
- Fire hazard with fine chips

Inconel: The Heat-Resistant Alloy

Extreme Properties:
- Maintains strength at high temperatures
- Extremely tough and work hardening
- Very expensive
- Used in jet engines and exhaust systems

Machining Reality:
- Requires premium tooling
- Very slow cutting speeds
- Extremely high cutting forces
- Generally beyond hobbyist capabilities

Magnesium: The Lightweight Option

Advantages:
- Lightest structural metal
- Excellent machinability when handled properly
- Good heat dissipation

Safety Concerns:
- Fire hazard, especially with fine chips
- Requires special fire suppression (no water!)
- May require special handling and storage
- Research safety requirements thoroughly

Material Cost Considerations

True Cost Analysis

Material Cost Factors:
- Raw material price per pound
- Availability and minimum order quantities
- Shipping costs and lead times
- Waste and scrap factors

Tooling Cost Factors:
- Tool life with specific materials
- Tool cost for material-specific requirements
- Coolant and setup requirements
- Machine wear and maintenance

Time Factors:
- Machining time for material
- Setup complexity
- Learning curve for new materials
- Finishing requirements

Budget-Friendly Strategies

Start Simple:
- Learn on inexpensive materials first
- Build skills before tackling expensive materials
- Use practice pieces to test parameters
- Document successful approaches

Buy Smart:
- Purchase from local suppliers when possible
- Consider online metals suppliers for variety
- Buy standard sizes to reduce cost
- Consider remnants and drops for small projects

Minimize Waste:
- Plan projects to use standard stock sizes
- Design parts to nest efficiently
- Save cutoffs for future small projects
- Consider material properties in design

Material Selection Process

Define Requirements

Functional Requirements:
- Strength and stiffness needs
- Environmental conditions
- Weight constraints
- Electrical or thermal properties
- Aesthetic requirements

Manufacturing Requirements:
- Available machine capabilities
- Tooling availability and cost
- Skill level required
- Time constraints
- Budget limitations

Selection Matrix

Create a Simple Scoring System:
- Rate materials 1-5 on key criteria
- Include both functional and manufacturing factors
- Weight criteria by importance
- Consider total cost of ownership

Example Criteria:
- Ease of machining (weight: 30%)
- Material cost (weight: 20%)
- Meets functional requirements (weight: 25%)
- Tooling requirements (weight: 15%)
- Availability (weight: 10%)

Testing and Validation

Material Testing:
- Machine test pieces before committing to full project
- Test multiple cutting parameters
- Evaluate surface finish and dimensional accuracy
- Document results for future reference

Process Validation:
- Prove complete process on scrap first
- Include all operations (rough, semi-finish, finish)
- Test workholding and setup procedures
- Validate tool life expectations

Sourcing and Purchasing Materials

Types of Suppliers

Local Metal Suppliers:
- Personal service and advice
- Ability to see material before buying
- Often will cut to size
- Support local business

Online Metal Suppliers:
- Wide selection and competitive pricing
- Detailed specifications and certifications
- Convenient ordering and delivery
- Good for specialty alloys

Big Box Stores:
- Limited selection but convenient
- Good for small quantities
- Often overpriced for serious work
- Quality can be inconsistent

Surplus and Remnant Dealers:
- Great prices on odd sizes
- Good for experimental work
- Unknown composition sometimes
- Requires flexibility in design

What to Look For

Material Certifications:
- Chemistry reports for critical applications
- Mechanical property verification
- Heat treatment documentation
- Traceability for aerospace/medical work

Surface Condition:
- Cold finished vs. hot rolled
- Surface roughness specifications
- Scale and oxidation condition
- Straightness and flatness

Dimensional Tolerances:
- Standard mill tolerances
- Precision ground stock when needed
- Consider finish machining requirements
- Plan for material removal

Storage and Handling

Proper Storage

Environmental Control:
- Protect from moisture and corrosion
- Maintain consistent temperature when possible
- Good air circulation
- Protection from contamination

Organization Systems:
- Label everything clearly
- Organize by material type
- Track inventory levels
- First-in, first-out rotation

Safety Considerations:
- Proper support for long pieces
- Weight distribution for heavy materials
- Fall protection and secure storage
- Access and handling considerations

Material Preparation

Stress Relief:
- Allow material to acclimate to shop temperature
- Consider stress relief heat treatment for critical work
- Support material properly during storage
- Plan for potential movement

Surface Preparation:
- Remove scale and oxidation before machining
- Clean oil and dirt from surfaces
- Check for defects and inclusions
- Plan workholding around material condition

The Expert's Secret

Here's what will surprise even experienced CNC users: The difference between struggling with materials and working with them isn't about having the most expensive equipment – it's about understanding material behavior and matching your approach to material properties.

Professional machinists don't just select materials based on final part requirements. They consider the entire manufacturing process: how the material will machine, what tooling it requires, how it affects setup and workholding, and what the total cost of ownership will be.

The Professional Approach:
1. Always consider manufacturing requirements alongside functional requirements
2. Test materials and processes before committing to expensive projects
3. Build a database of successful material/parameter combinations
4. Understand that "cheap" materials aren't always less expensive in total cost
5. Match tooling and techniques to material properties, not the other way around

The Secret Formula:
- Start with material properties, not just part requirements
- Consider total cost of ownership, not just material price
- Test everything before committing to expensive stock
- Build knowledge systematically through documentation
- Understand that different materials require different approaches

Master these principles, and you'll choose materials that work with your capabilities and constraints, not against them.

Quick Reference: Material Selection Guide

For Beginners:

Start With: Pine, poplar, or other softwoods
Next Try: 6061-T6 aluminum
Avoid Initially: Stainless steel, titanium, composites

For Intermediate Users:

Good Choices: Hardwoods, 6061/6063 aluminum, 1018 steel
Challenging Options: 12L14 steel, Delrin plastic
Still Avoid: Work-hardening stainless, exotic alloys

For Advanced Users:

All Options Available: Match material to requirements
Consider: Total cost of ownership, not just material price
Specialty Materials: Research thoroughly before attempting

Material Properties Quick Reference

Remember: The best material is the one that meets your functional requirements while working within your manufacturing capabilities and budget constraints.

Material selection is where engineering meets reality. Choose wisely, and every other aspect of your project becomes easier.