Inlay techniques

An inlay sets one material into another and flattens the two to one surface. In wood, the board that shows is the base (or female) board, and the piece set into it is the plug (or male). Two contrasting species, such as purpleheart into maple, are the usual pairing.

The four approaches, easiest first

	Resin inlay	Straight-flute (flat) inlay	V-carve inlay	Tapered ball-nose inlay
How it works	Engraved pocket filled with tinted epoxy, then flattened	Flat bottomed pocket and a plug cut to match, both with a straight bit	Matching V-shaped pocket and plug that wedge together	Like V-carve, but cut with a tapered ball-nose bit with a tiny tip
Fit	None needed at all	Set by the clearance you program: snug, not tight, not loose	Forgiving. The slopes self correct.	Self correcting like V-carve, but small machine or bit errors show
Detail possible	Whatever you can engrave	Limited by the bit diameter, usually 1/4 or 1/8 inch	Very fine, including small lettering and dates	Finer still, and a deeper inlay
Corners	As sharp as the engraving	Rounded to the bit radius	Genuinely sharp	Very nearly sharp
Difficulty	Easy to machine, takes care to finish	Easy. The usual first wood inlay.	Moderate: more settings, firm clamping, an accurate machine	Hard: tricky programming and a precise machine
Best for	Color, stars and logos, voids, cracks, river effects	Big letters, simple shapes, long grain boards	Lettering, logos, fine artwork	High-end, highly detailed pieces

Resin inlay

Engrave or pocket the design with a V-bit or a straight bit, fill it with epoxy, let it cure, and flatten it back. No fit problem at all, because the filler takes the pocket's shape exactly, and mica powders give you almost any color. It works well on long grain boards.

• Seal the pocket first in porous wood, or the resin wicks into the grain and leaves a halo.
• Overfill. Epoxy shrinks slightly and it settles into the grain, so fill proud and flatten after.
• Mind the bubbles. Warm the resin slightly, pour slowly, and pass a heat source over the surface briefly.
• Full cure before flattening, not just set. Green epoxy gums the cutter.
• Flattening is a mixed material cut. Wood and resin have very different hardness, so light passes and a sharp surfacing bit. See epoxy and composites.

Straight-flute (flat) inlay

A flat bottomed pocket in the base board and a plug cut from the other board, both with an ordinary straight or spiral bit. It is the easiest wood inlay to understand and hard to get badly wrong, which makes it the natural first one. The catch is that a round tool cannot cut a sharp inside corner:

• Detail is limited by the bit. With a 1/4 or 1/8 inch bit, small features and the points of a star come out rounded. Choose designs that look right that way: a single large letter, a simple shape, then a name once you have the fit dialed in.
• The pocket has rounded corners. The plug must have matching rounded corners, or it will not seat.
• Cut the pocket inside the line and the plug outside the line, with a small allowance to tune the fit. Snug is the target: tight enough to hold, loose enough to go in.
• Test on scrap. A few thousandths decides between a joint that will not close and one with a visible gap.
• Depth matters. The plug should sit slightly proud so you can flatten it flush after glue-up.

V-carve inlay

The technique most worth learning, because the geometry does the hard work for you. Both halves are cut with the same V-bit (with a straight bit clearing the wide areas), so their walls slope at the same angle and the plug wedges into the pocket. A small size error just means the plug seats slightly higher or lower, and the visible joint still closes tight. You are no longer limited by the bit diameter, so tight corners and small dates are possible.

It asks more of you than a flat inlay. The toolpath settings take more understanding, the plug has to be pressed in with firm, even pressure (an inlay press makes that repeatable), and a machine that is slightly off leaves gaps. The full method, including the glue gap and what each setting does, is in V-carve inlay.

Tapered ball-nose inlay

The same angled male and female approach as a V-carve inlay, cut with a tapered ball-nose bit instead of a V-bit: the kind of bit used for 3D carving, here with a very small tip, as small as about 1/64 inch. Its taper is much narrower than a V-bit's angle, so the walls are steeper, the inlay goes deeper for the same width, and points come out very nearly sharp.

• Programming is harder. You need the bit's real tip size and taper, and the same nominal bit from two brands can cut differently.
• The machine has to be precise. The male and female are two separate programs that must match at a very small scale, over and over.
• It is optional. It gives slightly finer and deeper inlays than V-carve, which matters on expensive pieces. You can make beautiful inlays for years without it.

Five things that ruin inlays

1. Cheap practice material. Pine, MDF and plywood chip and break at fine points, so a failed practice piece teaches you nothing. Dense hardwoods such as walnut, maple and cherry hold an edge, move less and give you a real chance of a clean result.
2. Grain orientation. Long grain tears and chips at small details. End grain cuts much more cleanly, which is why detailed V-carve and tapered ball-nose inlays are usually done in end grain boards. Long grain is fine for straight-flute and resin inlays. See grain direction.
3. A board that warped. Store blanks so air reaches both faces evenly (on edge, or stickered in a stack) or neither face (wrapped). A board that dries from one face cups, carves to uneven depths and leaves gaps in parts of the joint.
4. Toolpath settings. Most failed inlays trace back to the CAM side: one wrong value or one box not ticked. Check every setting before cutting, against the three depths for a V-carve inlay.
5. The machine and the bit. A machine that cannot repeat precisely cannot make two matching halves, however good the settings. A dull bit is effectively a slightly different bit: a little smaller in diameter or a little off its angle. Parts that fit early in a batch stop fitting later, so cut inlays with a sharp bit.

Which one to start with

• First inlay: resin, or a straight-flute inlay of one big letter or shape.
• Comfortable with your CAM and your machine is accurate: V-carve inlay, on a small end grain piece first, so a mistake is cheap.
• V-carve inlays are routine: try tapered ball-nose, if you want the extra detail.

Material pairing

• Contrast is the point. Walnut into maple, maple into walnut, or a light species into a dark one.
• Match density where you can. Very different hardnesses flatten unevenly.
• Mind wood movement. Two species moving at different rates across the grain can open a joint months later. See wood movement.
• Watch for bleed. Dark dust in light pores is a permanent grey halo. Blow the pocket out before glue-up and use a glue that matches rather than contrasts.