CNC Routing Wood vs Plastics vs Metal Material guide & design considerations for custom parts
Material selection for CNC routing isn’t just about mechanical properties — it’s about how a material behaves under a spinning cutter at speed. Wood tears. Acrylic melts. Aluminium work-hardens. Get it wrong and you end up with burn marks, blown-out edges, or a ruined piece you can’t use.
Wood is the most common CNC routing material for good reason — it’s 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.
Grain direction dramatically affects cut quality. Routing against the grain causes tear-out; with it produces a clean result. Matters most for visible decorative edges.
Homogeneous — cuts cleanly in any direction. Ideal for jigs, prototypes, and painted panels. Edges absorb moisture; seal them if left exposed.
Alternating grain resists tear-out but can chip the top veneer. A compression spiral bit — upcut and downcut combined — shears cleanly on both faces.
Spindle speed, feed rate, and depth of cut all shift depending on species and sheet type. These are practical starting points — dial in from there.
Cutting Parameters
- Spindle: 18,000–24,000 RPM
- Feed: 2,500–4,500 mm/min
- Depth: 3–6 mm per pass (6 mm bit)
- MDF can often run slightly deeper
Tooling Selection
- Upcut spirals — pocketing, clears chips
- Downcut — clean top-face finish
- Compression spiral — both faces of plywood
- Replace bits before they go dull
Common issue — burn marks: Almost always caused by moving too slowly or dwelling in one spot. Increase feed rate first, not reduce spindle speed — counterintuitive if you’re new to this. A dull bit rubs rather than cuts, generating heat instead of chips.
Cutting plastics requires a mindset shift. The problem isn’t mechanical force — it’s heat. Plastics have low thermal conductivity, so heat builds fast at the cut zone. If chips aren’t cleared quickly, they re-weld to the edge and no amount of finishing will fix the result.
Cast Acrylic
- Chips cleanly rather than gumming up
- Edges polish to optical clarity
- Preferred for display, light guides
- Always specify cast for visible parts
Extruded Acrylic
- Cheaper and more widely available
- Melts more readily under the cutter
- Prone to stress cracking at hold-downs
- Acceptable for non-visible functional parts
Cutting Parameters
- Spindle: 12,000–18,000 RPM (lower = less heat)
- Feed: 1,500–3,500 mm/min
- Depth: 2–3 mm per pass
- HDPE / soft plastics: faster feed, sharper tooling
Tooling & Setup
- Single-flute or O-flute bits — plastics only
- More flutes = more heat = more problems
- Continuous air blast on the cut zone
- Leave masking paper on during cutting
Common issue — chipping and cracking at inside corners: Acrylic is brittle. Sharp 90° internal corners concentrate stress during cutting and in service. Add a fillet radius of at least 1–2 mm to every inside corner — it also improves structural performance in the finished part.
Aluminium is the most technically demanding common CNC routing material, 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 one of them and you’ll know quickly.
Cutting Parameters
- Spindle: 12,000–18,000 RPM
- Feed: 800–2,000 mm/min
- Depth: 0.3–1 mm per pass
- Finishing pass: 0.1–0.2 mm for surface quality
Tooling & Lubrication
- 2- or 3-flute carbide end mills (aluminium-specific)
- High helix angle improves chip evacuation
- WD-40 or cutting fluid — continuous, not occasional
- Shorten tool stick-out as much as possible
Common issue — chatter: The vibration that leaves wavy marks on pocket walls usually comes from too much tool stick-out, workpiece movement, or a cut too aggressive for the machine’s rigidity. Clamp hard, shorten the tool, and reduce depth of cut. A finishing pass at 0.1–0.2 mm with a sharp bit makes a noticeable difference and is almost always worth the extra few minutes.
A few principles that consistently improve outcomes, regardless of what you’re cutting. Address these at the design stage — not after the first cut.
Your cutter is round, so inside corners can never be perfectly square. Design with the tool radius in mind — or plan a secondary operation if square corners are critical to the function.
Tabs hold parts in place during profile cutting. Place them on non-critical edges that are easy to clean up — not on surfaces that will be visible or mated to another component.
For wood and composites, orient parts so the most critical edges run with the grain. This reduces tear-out and improves edge quality without any change to tooling or parameters.
CNC routing tolerances are typically ±0.1–0.3 mm depending on material and machine. A dimension drawn at 20.00 mm may come out at 19.85 mm. Design your fits with that in mind — especially in assemblies with multiple mating parts.
The most capable machine produces poor results if the part can move. Think about how each part will be held before you finalise the geometry — not after the first failed cut.
Most projects fall into one of six common requirement categories. Use this as a starting point — then refine based on your specific constraints.
Structural & Low Cost
- Plywood or MDF for panels and jigs
- Solid timber for decorative or natural finish
- Plywood where lightweight strength matters
Functional & Technical
- Cast acrylic for transparent or display-ready parts
- HDPE for chemical resistance or food-safe use
- 6061 aluminium for structural precision metal parts
- Polycarbonate where impact resistance is critical
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. Get the material and design right before you cut a single line.
GeoSaffer in Auckland works across wood, plastics, and aluminium day in, day out. Whether it’s a structural bracket, a display panel, or a custom timber component — bring your file and we’ll give you honest feedback before anything gets cut. Even a rough DXF or a sketch is enough to start.
Get a quote for your custom part →