CNC Routing Wood vs Plastics vs Metal: Material Guide & Design Considerations for Custom Parts

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CNC Routing Wood vs Plastics vs Metal: Material Guide & Design Considerations for Custom Parts

You've got a custom part to make. You've got a rough design. Now comes the question that trips up even experienced makers: what material should I use, and will the router actually cut it cleanly?

Material selection for CNC routing isn't just about mechanical properties — it's about how the material behaves under a spinning cutter at speed. Wood tears. Acrylic melts. Aluminium work-hardens. Each one demands a fundamentally different approach to feeds, speeds, tooling, and design geometry. Get it wrong and you end up with burn marks, blown-out edges, chatter marks, or a ruined piece you can't use.

This guide breaks down the key differences across the most common CNC routing materials, gives you practical parameters to work from, and helps you make smarter decisions before you cut a single line.

Wood: The Forgiving One (Until It Isn't)

Wood is the most common CNC routing material for good reason — it's relatively cheap, easy to source, and cuts quickly. But "wood" covers an enormous range of materials, and they don't all behave the same way under a cutter.

Solid Timber vs Sheet Goods

Solid timber (pine, macrocarpa, rimu, oak) has grain direction that dramatically affects cut quality. Routing against the grain causes tear-out on edges; routing with the grain produces a much cleaner result. This matters most for decorative work where edge finish is visible.

MDF is the opposite — homogeneous, cuts cleanly in any direction, ideal for CNC. The downsides are the dust (fine and genuinely nasty — wear a proper respirator) and the fact that edges absorb moisture and swell if left unsealed. Great for jigs, prototypes, and painted panels.

Plywood sits somewhere in between. The alternating grain layers resist tear-out but can cause chip-out on the top veneer if your upcut spiral bit pulls fibres upward. A compression spiral bit — upcut and downcut combined — solves this by shearing cleanly on both faces. If both sides of the panel will be visible in the final product, it's worth the extra cost of the bit.

Wood Parameters to Work From

  • Spindle speed: 18,000–24,000 RPM for most timbers
  • Feed rate: 2,500–4,500 mm/min depending on density
  • Depth of cut: 3–6mm per pass for 1/4" (6mm) bits in hardwood; MDF can sometimes be run deeper
  • Tooling: Upcut spirals for pocketing (clears chips well), downcut for surface quality, compression for plywood

Common Issue: Burn Marks

Burn marks on wood almost always mean you're moving too slowly or dwelling in one spot. The fix is usually increasing feed rate, not reducing spindle speed — counterintuitive if you're new to this. A dull bit is the other common culprit. A worn edge rubs rather than cuts, generating heat instead of chips. If the bit looks fine but burns keep appearing, increase your feed rate first.

Acrylic and Plastics: Where Heat Is the Enemy

Cutting plastics — acrylic (Perspex), HDPE, polycarbonate, nylon — requires a mindset shift. The problem isn't mechanical force, it's heat. Plastics have low thermal conductivity, so heat builds up fast at the cut zone. If chips aren't being cleared quickly, they re-weld to the edge and you end up with a fused, melted mess that no amount of finishing will fix.

Acrylic (Cast vs Extruded)

Cast acrylic cuts better. It chips cleanly rather than gumming up, and edges can be polished to optical clarity after routing. If you're making display pieces, light guides, or anything where edge finish matters, always specify cast.

Extruded acrylic is cheaper and more widely available, but it melts more readily and is prone to stress cracking around hold-down points. For functional or visible parts, the price difference is usually worth paying.

Plastic Parameters to Work From

  • Spindle speed: Lower than you'd expect — 12,000–18,000 RPM. Higher speeds generate more heat, which is exactly what you're trying to avoid
  • Feed rate: 1,500–3,500 mm/min — fast enough that chips carry heat away from the cut
  • Depth of cut: Shallow passes, 2–3mm, especially for harder plastics
  • Tooling: Single-flute or O-flute bits designed specifically for plastics. More flutes means more heat means more problems
  • Air blast: A continuous blast of compressed air on the cut zone makes a real difference. It's cheap, it works, don't skip it

Common Issue: Chipping and Cracking

Acrylic is brittle, especially at inside corners. Sharp 90° internal corners concentrate stress — during cutting and later in service. The fix is simple: add a small fillet radius to inside corners. Even 1–2mm helps. It also reduces the chance of cracking when the part is loaded in use, so it's good design practice regardless of how it's being made.

Leave the masking paper on during cutting if possible — it reduces chipping and keeps the surface scratch-free.

For HDPE and softer plastics, the challenge flips: the material deforms rather than chips. Sharper tooling and faster feed rates are your friends here.

Aluminium: Slow Down, Think Carefully

Aluminium is the most technically demanding of the common CNC routing materials, but it's very achievable on a capable machine with the right setup. The three things that matter most: rigidity, chip clearance, and lubrication. Skip any of them and you'll know about it quickly.

Alloy Matters

6061-T6 is the standard for machinable aluminium. Strong, cuts cleanly, and widely available in NZ in sheet and plate form. 5052 is softer and tougher — good for forming, but more prone to built-up edge on the cutter. Avoid 1000-series alloys for routing; they're too gummy and will drive you mad.

Aluminium Parameters to Work From

  • Spindle speed: 12,000–18,000 RPM (lower end for deeper cuts)
  • Feed rate: 800–2,000 mm/min — aluminium needs to be cut, not rubbed
  • Depth of cut: Conservative — 0.3–1mm per pass depending on machine rigidity
  • Tooling: 2- or 3-flute carbide end mills designed for aluminium. High helix angle helps chip evacuation
  • Lubrication: Non-negotiable. WD-40 or a dedicated aluminium cutting fluid applied continuously prevents built-up edge and extends tool life considerably. Don't skip this step

Common Issue: Chatter and Poor Surface Finish

Chatter — the vibration that leaves wavy marks on pocket walls — usually comes from too much tool stick-out, workpiece movement, or a cut that's too aggressive for the machine's rigidity. Shorten tool stick-out as much as possible, clamp the workpiece hard, and reduce depth of cut. A finishing pass at 0.1–0.2mm with a sharp bit makes a noticeable difference to surface quality and is almost always worth the extra few minutes.

At GeoSaffer, we run aluminium parts regularly and have dialled in our feeds and hold-down systems to handle everything from thin 1.5mm sheet to 10mm+ plate — which matters when you need reliable, repeatable results across a production run.

Design Considerations That Apply Across All Materials

A few principles that consistently improve outcomes, regardless of what you're cutting:

  • Inside corner radii: Your cutter is round, so inside corners can never be perfectly square. Design with the tool radius in mind, or plan for a secondary operation if square corners are critical
  • Tab placement: Tabs hold parts in place during profile cutting. Put them on non-critical edges that are easy to clean up afterwards
  • Grain and fibre direction (wood and composites): Orient parts so the most critical edges run with the grain
  • Tolerance stack-up: CNC routing tolerances are typically ±0.1–0.3mm depending on material and machine. Design your fits accordingly — a dimension drawn as 20mm may come out at 19.85mm. That gap matters in an assembly
  • Workholding: The fanciest machine in the world produces rubbish results if the part can move. Think about how each part will be held before you finalise the design — not after

Choosing the Right Material for Your Project

Requirement Material to Consider
Structural, lightweight, low cost Plywood or MDF
Decorative, natural finish Solid timber
Transparent / display-ready Cast acrylic
Chemical resistance, food-safe HDPE
Structural, precise, metallic 6061 Aluminium
Impact resistant, stiff Polycarbonate

Getting It Right First Time

The biggest cost in custom CNC routing isn't machine time — it's iteration. A part that goes through three rounds of revisions because of an avoidable design issue or the wrong material choice costs far more than a decent conversation upfront.

If you're planning a custom part — one-off prototype or short production run — talk to someone who cuts these materials day in, day out. The team at GeoSaffer in Auckland works across wood, plastics, and aluminium regularly and can give you honest, practical feedback on your design before anything gets cut.

Structural bracket in aluminium, display panel in acrylic, custom timber component — whatever you're after, get in touch for a quote and bring your file. Even a rough DXF or a sketch. We can work with it.