Wood CNC processing service explained

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  Key takeaways
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  Which wood CNC route fits your part
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  What wood does to a CNC program
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  What tolerance a wood CNC processing service can hold
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  Tooling choices that decide your part quality
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  How to read a wood CNC processing quote
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  Where wood CNC parts go wrong on assembly
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  Step by step: qualify a wood CNC supplier

Wood is an anisotropic material. Its stiffness and cutting behavior change with grain direction, so a feed rate that works along the grain will burn or tear when you cut across it. This is the first thing a wood CNC processing service has to account for, and it is why the same drawing can produce two very different parts on two machines. A metal shop that treats wood like aluminum will overshoot on both speed and depth of cut.

Moisture content moves the part after machining. Kiln-dried hardwood typically sits between 6 and 8 percent moisture content. If the blank is not acclimated to the shop before cutting, a 300 mm wide panel can shrink or swell by 2–4 mm over the following weeks. That is far larger than any tolerance the machine can hold, so acclimation time belongs in your lead time calculation.

Grain direction also decides where you can put a screw or a joint. A mortise cut across the grain splits along the fibers. A mortise cut along the grain holds. For structural parts, tell the supplier which faces are visible and which edges carry load, and let them orient the blank before programming.

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  Acclimate firstLet blanks sit in the shop for 24–48 hours before the first cut.
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  Mark the grainAdd a grain direction arrow on the drawing so the blank is not flipped.
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  Separate tolerancesHold tight tolerance on mating features only, not on the whole profile.

On a rigid router with a vacuum table, a well-behaved hardwood part can hold about ±0.1 mm on profile cuts and ±0.05 mm on hole positions. That is the practical floor for wood. Tighter numbers exist on paper but do not survive sanding, sealing and seasonal movement. If your drawing carries ±0.005 mm across a wood part, the quote will either be wrong or the part will be rejected later.

Edge finish matters more than most drawings admit. A sharp compression bit leaves an edge that needs light sanding at 180–220 grit. A dull up-cut bit leaves fuzz that needs 120 grit and still shows tear-out under a finish. Specify the finish you expect on visible edges, and ask which grit the supplier uses by default.

Surface roughness on wood is usually quoted as a visual standard rather than an Ra value. Ra 0.8–1.6 μm is a metal figure and does not translate cleanly to a porous material. Instead, send a physical sample or a photo of an accepted edge, and let the shop match it.

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  Profile tolerance±0.1 mm is realistic on hardwood with a rigid setup.
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  Hole position±0.05 mm is achievable with a spot drill before the through hole.
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  Do not mix specsKeep metal Ra values out of wood drawings.

The bit is the cheapest part of the job and the one that changes the result most. A down-cut spiral pushes fibers down and gives a clean top edge on plywood and veneer. An up-cut spiral gives a clean bottom edge but lifts the top. A compression bit combines both and is the standard choice for double-sided panels. Ask which one the shop plans to run.

Step-down and step-over control both cycle time and finish. A 6 mm bit cutting 3 mm deep per pass at 12,000 rpm and 4,000 mm/min feed is a reasonable starting point for medium-density hardwood. Push the step-down to 8 mm and you get chatter, a tapered wall and a tool that wears out early. A quote that promises a very low price usually hides an aggressive step-down.

Fixturing is the other half of the setup. Vacuum tables hold flat parts well but lose force over small areas. For a part with a narrow footprint, expect a dedicated fixture or tabs. Tabs are cheap but leave witness marks that need sanding, so plan a sanding step in your process.

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  Compression bitBest default for panels with two visible faces.
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  Tab placementPut tabs on hidden edges or in areas that get trimmed later.
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  Tool wearCarbide bits dull after roughly 8–12 hours in hardwood.

A useful quote separates programming, material, machining, finishing and inspection. If it is one lump number, you cannot tell whether the shop planned a finishing step or skipped it. Ask for the breakdown. It also tells you whether the supplier has quoted your species before, because material pricing for walnut, maple or birch plywood varies widely.

Ask about the blank size and yield. A shop that quotes a 4,000 mm part on a machine with a 4,000 × 400 × 150 mm travel envelope may need to reposition the blank mid-cycle, which adds a joint line or a re-fixturing step. That detail should appear in the quote, not in a phone call after the first part fails.

Confidentiality is routine for production wood parts with a branded shape. Uploads are secure and confidential, and an NDA is available on request. If your part is a consumer product shell, put the NDA in place before you send the CAD file, not after the quote.

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  Line-item quoteProgramming, material, machining, finishing, inspection as separate lines.
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  Blank yieldAsk how many parts come from one sheet or one board.
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  NDA timingSign before CAD release, not after the first quote.

Most wood part failures are not machining failures. They are assembly failures caused by tolerance stack. If three panels each hold ±0.1 mm and they meet at a corner, the gap can be 0.3 mm before any seasonal movement. Designers who specify metal-grade tolerances on wood assemblies often reject good parts, because the stack was never budgeted.

Seasonal movement is the second cause. A part machined in a dry winter shop and shipped to a humid climate will swell. Plan clearance on sliding fits, and avoid press fits in wood unless you are using a stabilized or impregnated material. If you need a press fit, consider a metal insert or a composite instead.

Fastener pull-out is the third. Screws driven into end grain hold at a fraction of the strength of screws driven across the grain. If your part takes repeated load, move the fastener into a cross-grain face or add a metal insert. This is a design decision that belongs on the drawing, not in the shop.

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  Budget the stackAdd up per-part tolerance before you set the assembly gap.
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  Avoid press fitsUse clearance fits or metal inserts in wood.
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  Cross-grain fastenersEnd-grain screws hold far less load.