How to Create Wood Inlays on a CNC Machine
This guide walks through the full process to create wood inlays on a cnc machine, from pattern offsets to glue-up. It is written for engineers and shop owners who need a repeatable fit, not a one-off show piece.
What matters before you cut
Designing the pocket and inlay pair
Every wood inlay on a cnc machine starts as two drawings, not one. The pocket is cut slightly larger than the inlay plug, and that difference is the glue gap. For most hardwoods a total gap of 0.05–0.20 mm works. Softwoods and open-grain species sit at the loose end of that range because they compress under clamping pressure.
Set the gap on the plug, not the pocket. In CAM, offset the pocket contour outward by half the gap and keep the plug at nominal size. This keeps the pocket toolpath predictable when you change the plug thickness later.
Corner geometry is the next decision. A 3.175 mm down-cut spiral leaves a 1.6 mm corner radius, so any sharp inside corner in the pattern will not fill. Either radius every corner in the design or plan a hand-chisel cleanup pass.
Patterns thinner than 3 mm behave like veneer. They buckle, chip and suck up glue. If the design calls for fine lines, consider a 1.5–2.0 mm depth with a shallow plug and a caul board during clamping.
- 1Offset directionGrow the pocket, keep the plug nominal.
- 2Minimum feature widthKeep lines at 2× the cutter diameter where possible.
- 3Corner radiiMatch every inside corner to the cutter radius.
Choosing cutters for wood inlays
A down-cut spiral end mill is the default for the pocket. The down-cut geometry pushes fibers down at the top surface, so the rim stays clean. The trade-off is chip evacuation: chips pack into the pocket and rub, which heats the cutter. Use a short flute length and clear chips with an air blast on every pass.
For the plug, use the same diameter cutter as the pocket. Different diameters mean different tool deflection, and a 0.02 mm difference in deflection is already 20% of a tight glue gap. If you must switch cutters, re-measure the plug and adjust the offset.
Ball-nose tools are for 3D textured inlays where the surface is meant to be carved. They leave a scalloped floor, so the glue line is thicker and less reliable. Expect to sand or scrape the top after glue-up.
Two-flute carbide is enough for hardwood. Three flutes help in dense species like maple or wenge but need a faster feed to avoid rubbing. Keep the tool stick-out short, ideally under 3× diameter.
- 1PocketDown-cut spiral, 2 flutes, short flute length.
- 2PlugSame diameter, same stick-out as the pocket tool.
- 3Textured inlayBall nose, accept a thicker glue line.
Feeds, speeds and pocket depth
Wood cuts fast but burns easily. For a 3.175 mm two-flute carbide in hardwood, a spindle speed of 16,000–18,000 rpm and a feed of 1,500–2,200 mm/min gives clean chips. In softwood, drop the speed to 12,000–14,000 rpm and push the feed to 2,500–3,000 mm/min so the tool cuts instead of polishing.
Pocket depth should be 0.5–1.0 mm deeper than the plug thickness. That extra depth is a reservoir for squeeze-out. If the pocket floor is flat and the plug bottom is flat, excess glue has nowhere to go and the plug floats up.
Cut the pocket in 2–3 mm depth passes. A single full-depth pass in a 6 mm pocket loads the cutter and lifts the rim. Leave a 0.2 mm finishing pass on the walls at full depth to clean the profile.
The plug is cut slightly proud. Leave 0.3–0.5 mm above the finished surface so you can sand or plane it flush after the glue cures. Cutting the plug flush in the machine sounds efficient, but any deflection or table error shows as a step you cannot remove.
- 1Depth passes2–3 mm per pass, finish wall at full depth.
- 2Pocket depthPlug thickness plus 0.5–1.0 mm.
- 3Plug heightLeave 0.3–0.5 mm proud for flush trimming.
Test fit, clamping and glue-up
Dry-fit the plug in scrap of the same species before you commit. A plug that drops in with light finger pressure and no rattle is correct. If it needs a mallet, the gap is too tight and the glue will not spread. If it falls out, the gap is too wide and the seam will show.
Wood moves with moisture. Cut the pocket and the plug on the same day, from stock that has been in the shop for at least 48 hours. A plug cut from a board at 8% moisture and pressed into a pocket at 6% will swell and split the surrounding grain within a week.
Use a glue with a long open time and low water content. Polyurethane and epoxy both work; yellow glue swells the joint and can push the plug proud. Apply glue to the pocket walls with a small brush, not a bead in the floor.
Clamp with a flat caul board over the whole inlay. Pressure should be even and moderate: enough to seat the plug, not enough to starve the joint. Wipe squeeze-out with a damp cloth before it cures.
- 1Dry fit firstFinger pressure, no mallet, no rattle.
- 2Same-day cuttingKeep both parts at shop moisture for 48 hours.
- 3Caul boardFlat and waxed so it does not bond to the plug.
Seven steps to cut a wood inlay
Work through these in order. Skipping the dry fit is the most common cause of a failed inlay.
- 1Prepare and flatten both boardsFace and thickness the pocket board and the plug board to the same dimension. A 0.1 mm difference in thickness changes the glue reservoir. Let both boards sit in the shop for at least 48 hours after machining.
- 2Draw the pattern and set the glue gapOffset the pocket contour outward by 0.025–0.10 mm and keep the plug nominal. Match every inside corner to the cutter radius. Check that no feature is thinner than 2× the cutter diameter.
- 3Zero the tool on the pocket boardTouch off Z on the top face at the center of the pocket. Use a probe or a feeler gauge, not a paper shim. Record the offset so the plug cut uses the same reference.
- 4Cut the pocket with a down-cut spiral16,000–18,000 rpm, 1,500–2,200 mm/min in hardwood, 2–3 mm depth per pass. Leave a 0.2 mm finishing pass on the walls. Air-blast chips on every pass to stop rubbing and burning.
- 5Cut the plug and leave it proudSame cutter, same offset. Leave 0.3–0.5 mm of material above the finished surface. Cut the plug from a board with grain running the direction you want in the final part.
- 6Dry fit in scrap and adjustTest the fit in a scrap pocket cut from the same species. Target light finger pressure with no rattle. Adjust the offset by 0.02 mm at a time until it seats correctly.
- 7Glue, clamp and flush the surfaceBrush glue on the pocket walls, seat the plug, and clamp under a flat waxed caul. After full cure, plane or sand the plug flush. Scrape with the grain to avoid pulling fibers.
Inlay method comparison
Use this to pick a method before you program the part.
| Method | Glue gap | Best for | Main risk |
|---|---|---|---|
| Flat pocket and flat plug | 0.05–0.15 mm | Straight-line and geometric patterns | Glue starves the joint |
| Tapered or dovetail inlay | 0.02–0.05 mm | Thin lines and tight curves | Plug cracks on insertion |
| Ball-nose textured inlay | 0.15–0.30 mm | 3D carved scenes | Visible glue line after sanding |
| Veneer inlay | 0.00–0.05 mm | Fine detail under 3 mm | Tear-out and buckling |
| Cross-grain decorative plug | 0.10–0.20 mm | Accent contrast on flat panels | Movement cracks the seam |
The fit is the whole job
Cut the pocket and the plug with the same tool, hold a 0.05–0.20 mm glue gap, and dry-fit in scrap before you touch the real part. Everything else is secondary.
Wood inlay questions we get from shops
How tight should the glue gap be for a wood inlay?
For flat pocket and flat plug in hardwood, aim for a total gap of 0.05–0.15 mm. Softwoods sit at the loose end, 0.15–0.20 mm, because they compress under clamping.
If the plug needs a mallet, the gap is too tight and the glue cannot spread. If it falls out on its own, the seam will show after finishing.
Why does my inlay show a line after sanding?
The most common cause is glue starvation from an over-tight fit or too much clamp pressure. The second cause is sanding across the grain, which pulls fibers from one species into the other.
Check the dry fit first. Then reduce clamp force and scrape the surface with the grain instead of sanding it.
Can I cut the pocket and the plug with different tools?
You can, but you must re-measure the plug and adjust the offset. Two cutters deflect differently, and a 0.02 mm difference in deflection is already a large share of a tight glue gap.
Using the same cutter diameter and the same stick-out for both operations removes that variable.
What spindle speed should I use for wood inlays?
A 3.175 mm two-flute carbide in hardwood runs well at 16,000–18,000 rpm and 1,500–2,200 mm/min. In softwood, use 12,000–14,000 rpm and 2,500–3,000 mm/min.
If you see burn marks or hear a high-pitched whine, the tool is rubbing. Increase the feed or reduce the spindle speed.
How deep should the pocket be relative to the plug?
Make the pocket 0.5–1.0 mm deeper than the plug thickness. That space holds squeeze-out so the plug does not float on a film of glue.
Leave the plug 0.3–0.5 mm proud of the surface so you can trim it flush after curing.
Do I need a down-cut or up-cut spiral for an inlay pocket?
Down-cut for the top edge. The geometry pushes fibers down and keeps the rim clean. The downside is chip packing, so use an air blast on every pass.
Up-cut clears chips better but lifts the top fibers and tears the pocket rim. It is a better choice for deep pockets where chip evacuation is the limiting factor.
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