CNC Project From Woodworking to Metal

• 1
  Key takeaways
• 2
  What actually changes when the material becomes metal
• 3
  Judging whether the machine can make the part
• 4
  Workholding is the step woodworkers underestimate
• 5
  Tolerance and surface finish: where cost jumps
• 6
  DFM mistakes that come from wood habits
• 7
  Wood job vs metal job: what to change

A wood CNC project and a metal CNC project share one thing: a tool path driven by coordinates. Everything else shifts. Wood cuts at 15,000 to 24,000 rpm with a single-flute cutter and a 6 mm depth of cut, and the material springs back or tears out without much consequence. Aluminum 6061 cuts at 3,000 to 8,000 rpm, feeds at 0.05 to 0.15 mm per tooth, and turns a dull tool into a scrapped surface finish within minutes.

The physics behind that gap is cutting force. Wood needs roughly 5 to 15 N of force per millimeter of cut width. Aluminum 6061 needs 300 to 700 N/mm, and 304 stainless climbs past 1,500 N/mm at the same chip load. That force has to go somewhere: into the frame, the spindle bearings, the workholding and the part itself. When it does not, the tool deflects, the wall goes thin on one side, and the size drifts.

Heat is the second difference. Wood carries heat away in the chip and never gets hot enough to matter. Aluminum conducts heat into the part and the fixture, so a long roughing pass can grow the workpiece 0.02 to 0.05 mm before finishing even starts. Steel concentrates heat at the edge and burns tools. Coolant or high-pressure air is not optional on metal.

So the honest answer to "can I cut this on my wood router?" is a question back: what tolerance and what material? A decorative aluminum sign at ±0.5 mm, yes. A bearing housing at ±0.02 mm, no.

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  Cutting force scaleWood 5–15 N/mm, aluminum 300–700 N/mm, stainless over 1,500 N/mm.
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  Speed scaleWood 15,000–24,000 rpm, aluminum 3,000–8,000 rpm, steel 400–1,500 rpm.
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  Thermal driftA warm aluminum part can measure 0.02–0.05 mm larger than it will when cool.

Spindle power is the first filter. A 1.5 kW wood router can take a 6 mm cutter 2 mm deep in aluminum if you slow down, and it will finish a small bracket. Push it to a 12 mm cutter at 6 mm depth and the spindle stalls or the tool snaps. As a rule, keep aluminum cuts under 0.5 kW per 1 mm of radial engagement on a light machine.

Frame stiffness matters more than spindle power once you leave aluminum. A bolted extrusion gantry deflects under load. The deflection shows up as chatter marks, a tapered wall, or adimension that measures correct at the top and 0.1 mm small at the bottom. Cast iron or welded steel frames with linear guides hold the tool where it was commanded.

Tooling is where most wood-to-metal conversions fail quietly. A two-flute upcut router bit for plywood has a 10° helix and no corner radius. Aluminum wants a three-flute cutter with a 35° to 45° helix and polished flutes for chip evacuation. Steel wants four or five flutes with a coating such as AlTiN and a corner radius of 0.4 to 1.0 mm to stop edge chipping.

If the part needs five-sided access, undercuts, or a single setup for concentric features, a 3-axis router cannot get there no matter how it is tuned. That is the point where the job moves to a simultaneous 5-axis machining center.

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  Light router limitKeep aluminum radial engagement under 25% of cutter diameter.
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  Aluminum cutter3 flutes, 35–45° helix, polished flutes, 8–12 mm diameter for roughing.
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  Steel cutter4–5 flutes, AlTiN coating, 0.4–1.0 mm corner radius.

Screwing a blank into a spoilboard works because wood is light and forgiving. Metal is neither. A 200 × 200 × 20 mm aluminum plate weighs about 2.2 kg, and a roughing pass at 300 N of cutting force will slide it, lift it, or launch it if the clamp is only hand tight. The first change on any conversion is a real vise, toe clamps, or a fixture plate.

Thin parts need support under the cut, not just around the edges. A 1.5 mm aluminum panel, which is a normal thickness in wood, will drum and vibrate. The usual fix is to leave a 0.5 to 1.0 mm sacrificial web, cut the profile, then face off the web in a second operation. Vacuum fixtures work here, but only with a proper gasket and a part that is stiff enough to seal.

For parts with two or more sides to machine, soft jaws machined to the actual part profile remove the repositioning error. On a 5-axis center, a dovetail or a zero-point system lets the same part come back to the same position within 0.01 mm. That repeatability is what makes a second operation predictable instead of a re-measurement exercise.

Long parts are a separate problem. GreatLight machines can travel up to 4,000 × 400 × 150 mm, but a part that long in aluminum bends under its own weight and under cutting load. Support it in the middle, or split the design.

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  Clamp forceHand-tight is not enough. Plan for cutting forces in the hundreds of newtons.
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  Thin wallsLeave a 0.5–1.0 mm web and remove it in a second setup.
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  Repeat setupSoft jaws or zero-point systems hold position within 0.01 mm.

Woodworking drawings usually carry a general note like ±0.5 mm. Metal drawings often carry ±0.05 mm on all dimensions, which is written without thinking about cost. Not every dimension needs it. A mounting hole pattern needs tight position; a clearance slot does not. Marking only the functional dimensions keeps the price down and the lead time short.

GreatLight holds ±0.005 mm on critical features and ±0.0002 in for customers working in inches. That number is achievable on a rigid machine with temperature control and a probe, but it is not achievable on every feature of every part. Deep bores, thin floors and long unsupported walls move regardless of machine class.

Surface finish follows the same logic. As-machined aluminum lands at Ra 1.6–3.2 μm. Fine finishing reaches Ra 0.8–1.6 μm, and mirror-level work on aluminum reaches Ra 0.2–0.8 μm. Each step adds a finishing pass and sometimes a second tool. If the drawing says Ra 0.4 μm on a non-sealing face, that note is paying for nothing.

A practical rule from the shop: specify the finish on sealing faces, bearing bores, sliding surfaces and visible cosmetic faces. Leave the rest as machined.

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  Functional onlyTight tolerance on bores, holes and mating faces. Loose on clearance.
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  Finish tiersAs-machined Ra 1.6–3.2 μm, fine Ra 0.8–1.6 μm, mirror Ra 0.2–0.8 μm.

Wood designs use sharp internal corners because a router bit leaves them. A rotating end mill always leaves a radius equal to its own. If the drawing calls for a sharp internal corner, either the corner becomes a 2 to 3 mm radius or the part needs EDM, which changes the price and the schedule. Adding the radius in CAD costs nothing.

Deep pockets are the second common issue. A pocket 40 mm deep with a 6 mm cutter needs a tool that is over six times its diameter in length, and that tool will deflect. Wood does not care. Aluminum does. Either widen the pocket or accept a stepped floor and a rougher finish. A depth-to-diameter ratio above 4:1 is where trouble starts.

Engraved text is a small detail that catches people. Wood takes 0.5 mm deep lettering without complaint. On metal, laser marking needs a minimum character height of 1.5 mm to stay legible after anodizing. Anything smaller becomes a gray smudge.

Finally, check the material choice against the function. A part that was plywood because it was stiff and cheap may not need aluminum at all. Sometimes the right answer is steel for stiffness, or a plastic such as POM or PEEK where the load is light and weight matters.

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  Internal cornersAlways a tool radius. 2–3 mm is a normal minimum.
• 2
  Pocket depthKeep depth-to-diameter under 4:1 where possible.
• 3
  MarkingLaser marking needs 1.5 mm minimum character height.