Wood CNC processing for custom furniture: what drives accuracy

• 1
  Why wood CNC processing for custom furniture differs from metal cutting
• 2
  Grain direction, cutter geometry and the cut that does not tear
• 3
  Feeds, speeds and chip load on hardwood and sheet goods
• 4
  What tolerance you can actually hold on a wood part
• 5
  Fixturing and workholding for furniture parts
• 6
  When routing is the wrong process for a furniture part
• 7
  Which process fits which furniture part

Solid wood is not homogeneous. Earlywood and latewood sit in alternating bands a few tenths of a millimeter thick, and their density differs by a factor of two or more inside the same board. A cutter crossing those bands meets a changing load several hundred times per second.

The second difference is direction. Wood severs cleanly when the knife edge lifts fibers against the grain boundary, and tears when it digs under them. That is why the same cutter at the same RPM can leave glass-smooth maple on one pass and fuzzy oak on the next.

So wood CNC processing for custom furniture is less about holding a single tight number and more about managing a range. Metal parts are checked against ±0.005 mm. A routed oak rail is checked against how it fits a mortise after two weeks in a dry room.

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  Density variationEarlywood to latewood ratio sets cutting load.
• 2
  Fiber directionClimb vs conventional cut changes surface quality.
• 3
  Moisture content6–8% MC is the usual shop-floor target.
• 4
  Fastener insertsMetal threads still need real machining tolerance.

A router bit cuts with a rotating edge. Where that edge enters the wood relative to the fibers decides whether the chip peels off or the surface lifts. Cutting with the grain, the fibers are supported by the material behind them. Cutting against it, the edge pushes unsupported fibers forward and they break instead of sever.

On a CNC, the fix is the direction of travel, not the spindle speed. For an outside profile, a conventional cut (cutter moving against the feed) usually gives the cleanest edge on the finished side. For a pocket, a climb cut on the final 0.3–0.5 mm pass reduces tear-out at the wall.

Cutter geometry matters just as much. Compression bits have an up-cut and a down-cut spiral on the same shank. The down-cut section presses the top veneer down, the up-cut lifts the bottom, so both faces stay clean in a single pass. On 18 mm plywood, that is often the difference between a visible chip-out and a part that needs no sanding at the edge.

Tool diameter sets the smallest inside radius you can cut. A Ø6 mm bit cannot enter a tighter corner than R3, and the actual finished radius is R3 plus the tool deflection under load. If the design calls for R2, either the corner gets a drilled relief or the tool changes to Ø4 mm with a slower feed.

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  Compression bitClean top and bottom in one pass on veneered board.
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  Down-cut spiralBest for laminates and thin top skins.
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  Up-cut spiralBest chip evacuation in deep pockets.
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  Corner radiusFinished radius = tool radius + deflection.

Chip load is the thickness of material each cutting edge removes per revolution. Too small and the edge rubs, heats and dulls. Too large and the tool deflects or the spindle stalls. For a two-flute cutter at 18,000 RPM and a feed of 6,000 mm/min, the chip load is 0.17 mm per tooth. That is a reasonable starting point for hardwood.

Softwoods and MDF tolerate higher feeds because they have less resistance to severing. Dense species like hard maple, wenge or ipe push back harder; drop the feed by 20–30% and keep the RPM, or keep the feed and reduce the depth of cut. Heat is the practical limit, not force.

Depth of cut is the other lever. A 0.5 × D axial cut (half the tool diameter) with a 0.4 × D radial stepover keeps most tools inside their deflection budget. Pushing to full depth in one pass on a 19 mm cutter in oak usually shows up as a tapered wall, not a broken tool, which makes it harder to notice until the parts do not fit.

Dust extraction is part of the cutting setup, not a housekeeping detail. Chips left in the kerf get recut, which doubles the heat and triples the edge wear. Good extraction at the collet also keeps the tool cooler and holds chip load closer to the calculated value.

• 1
  Two-flute hardwood18,000 RPM, 6,000 mm/min, 0.17 mm/tooth.
• 2
  Dense speciesReduce feed 20–30% or reduce depth.
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  Axial depthKeep near 0.5 × D for stable walls.
• 4
  ExtractionRecut chips are the main heat source.

Machined wood does not hold a metal-style tolerance in service, because the part itself changes size. A 300 mm oak panel can move 2–3 mm across the grain between 30% and 70% relative humidity. Cutting it to ±0.1 mm is possible in the shop and meaningless six weeks later in a dry apartment.

The practical approach is to machine the joint, not the whole part, to a tight number. Mortise and tenon, dowel holes, domino slots and metal insert pockets are cut to ±0.1 mm or better, because those features set the fit. Free edges, curves and profiles are cut to ±0.5 mm and then sanded.

Where a wood part must interface with a metal component, the metal part carries the tolerance. We cut aluminum and stainless inserts to ±0.005 mm on the same program that cuts the pocket, so the insert and the cavity come from one setup. That removes the stack-up error that appears when two suppliers cut the two halves.

Composites and plywood sit between the two. Their movement is restrained by the glue line, so a 20 mm birch plywood shelf can hold ±0.2 mm on a routed edge over a 1,200 mm span, provided the sheet was acclimatized in the shop before cutting.

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  Joints±0.1 mm or tighter on mortise, tenon and dowel holes.
• 2
  Free edges±0.5 mm, finished by sanding.
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  Metal inserts±0.005 mm, cut in the same setup as the pocket.
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  Plywood±0.2 mm over 1,200 mm after acclimatization.

A wood part is usually large, thin and flexible, which is the worst combination for workholding. Vacuum tables solve most of it. A 600 × 1,200 mm part held on a grid table at 0.6–0.8 bar stays flat enough for routing, and the whole sheet can be nested to reduce waste.

For curved or three-dimensional parts, a machined fixture is cut first from MDF or tooling board, then the actual workpiece is clamped into it. The fixture is a consumable and it is cut on the same machine, so its profile matches the part within the machine's own repeatability. This is how chair backs, armrests and shaped seat shells get located for a second operation.

Thin sections need support underneath, not more clamps on top. Clamping force bends the part and the cutter then follows a bent surface. A sacrificial backing board or a matching female mold lets the part sit in its natural shape while the tool removes material.

The 4,000 mm maximum processing size on our large-format machines covers full table tops and long rails in one setup. Splitting a 3,600 mm rail into two cuts and joining them adds a visible seam and a tolerance stack that no downstream sanding can hide.

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  Vacuum table0.6–0.8 bar holds flat panels for nesting.
• 2
  Machined fixtureCut from MDF for curved second operations.
• 3
  Backing supportPrevents clamp-induced bowing on thin parts.
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  Single setupUp to 4,000 mm without a joining seam.

CNC routing is not always the cheaper or better answer. A straight, square-edged panel is faster and cheaper on a panel saw. A long production run of identical simple parts is often better on a dedicated edge-bander and drilling line, where cycle time is seconds rather than minutes.

Routing wins when the geometry is complex, the quantity is low to medium, or the part must be repeatable across a revision. Curved rails, shaped chair components, sculpted handles, inlay pockets and parts that combine wood with a machined metal insert are all good candidates.

Veneered and laminated surfaces need care. Routing a veneered panel exposes the core at the edge and the veneer can chip if the tool is dull or the feed is wrong. A compression bit and a fresh edge solve most of it, but a design that puts a routed profile right on a veneer seam is asking for trouble.

For solid wood with strong figure, like burl or crotch walnut, the surface after routing is rarely the final surface. Plan for a 0.5–1.0 mm sanding allowance on show faces, and do not design a sharp routed edge that sanding will round off.

• 1
  Use a panel sawStraight square cuts on flat sheet goods.
• 2
  Use routingCurves, pockets, joinery and mixed-material parts.
• 3
  Veneer riskAvoid profiles that land on a veneer seam.
• 4
  Sanding allowanceLeave 0.5–1.0 mm on figured show faces.