V-carve inlay: start depth, flat depth and the glue gap
Cut the design as a V-carve pocket in the background board, then cut the same design, mirrored, as a V-carve in the inlay board with a start depth. The start depth makes the plug shorter than the pocket is deep, so it wedges on its sloped walls and never bottoms out. Glue, cure, then plane the whole plug board away until the inlay is flush.
Why a V-bit inlay fits so well
Both halves are cut with the same V-bit, so the sloping walls match exactly. As the plug descends into the pocket, the two slopes meet and wedge. A small error in size just means the plug stops slightly higher or goes slightly deeper, and the joint still closes with no visible line. That self-correcting quality is what makes V-carve inlay so much more forgiving than a flat-bottomed inlay, where the fit has to be exact.
The three depths
Everything about the fit comes down to three numbers. Get the relationship between them right and the rest is craft.
| Setting | Female (pocket, background board) | Male (plug, inlay board) |
|---|---|---|
| Start depth | Zero | The glue gap. The V starts this far below the surface, so the plug's walls sit lower than the pocket's and it never reaches the pocket floor. |
| Flat depth | How deep the pocket floor is | How much the plug stands proud once seated. This becomes the sacrificial material you plane off. |
| Vectors | As drawn | Mirrored, because the plug goes in face down. Invert the region too: the male is the background carved away around the design, not the design itself. |
| Bit | The same V-bit for both, with the same angle entered in CAM | |
The one relationship that must hold: the male start depth must be less than the female flat depth. The difference between them is the void under the plug where glue and any pocket-floor debris can go. If the start depth is equal to or greater than the flat depth, the plug bottoms out on the pocket floor before its slopes touch the pocket walls, and you get a visible line all the way round.
The process
- Cut the female pocket in the background board with a V-carve toolpath at start depth zero and a flat depth. Use a flat clearance tool on the wide areas if the design has them.
- Mirror the vectors and invert the selection so the male toolpath carves the background away around the design.
- Cut the male plug in the inlay board with the same V-bit, start depth set to your glue gap, flat depth set to the proud height.
- Leave the plug board thick. Do not profile the inlay out. The whole board is the plug, and it gets planed away after glue-up.
- Dry fit. It should wedge and sit proud by about the male flat depth. If it rocks on the pocket floor, the start depth is too large for the flat depth you cut.
- Glue and clamp hard. Even pressure across the whole inlay. Squeeze-out is expected and is a sign the joint closed.
- Wait for full cure, not just set. Flattening a joint that is still green pulls it apart.
- Plane, surface or band-saw the plug board away until the inlay is flush with the background. A surfacing pass on the CNC does it in a few minutes and leaves it dead flat.
Where it goes wrong
| Symptom | Cause | Fix |
|---|---|---|
| Visible line around the inlay | Plug bottomed on the pocket floor before its slopes met the pocket walls: male start depth too large, or female flat depth too shallow | Reduce the male start depth, or deepen the female flat depth |
| Plug seats but sits far too proud, and the joint opens when planed | Slopes met higher than intended: bit angle in CAM does not match the real bit, or the two halves were cut with different bits | Measure the bit angle, use one bit for both halves |
| Design is backwards after glue-up | Male vectors were not mirrored | Mirror before toolpathing the male |
| Fine details missing from the inlay | V-bit angle too wide for the detail | Use a narrower angle for both halves |
| Inlay planed through in places | Male flat depth too small, so there was nothing proud to remove; or flattening went too far | Increase the male flat depth, flatten in light passes |
| Joint opened after a few weeks | Wood movement across the grain | Match species and grain direction where you can, and let stock acclimate |
| Colour bleed into the background | Dark glue or dark dust in light wood pores | Use a matching glue, and blow out the pocket before glue-up |