Shippo Tsunagi Inspired Cutting Board

Maple cutting board with pockets milled with a 15° v-bit and a plug of Bloodwood milled that was used to fill the pockets.

VCarve Inlay

I have made several cutting boards over the years using this technique of creating pockets in a cutting board using v-shaped router bits and filling them with a plug made of wood of another color. It had been a few years and I wanted to revisit the technique and work to solidify my understanding of the process of designing the parts using VCarve.

We typically do thje CAD/CAM work for use with our CNC router using Fusion 360, but for this inlay technique VCarve has been our go-to software tool. One challenge with our process in VCarve is that it difficult to visualize how the piece that has the pockets and the piece with the plugs will come together.

I knew I wanted to try to use this technique to design a Sashiko embroidery pattern inspired cutting board, so it was a perfect opportunity to draw some sketches and do some experimentation to reflect on the process and solidify our understanding of how the parts come together.

VCarve Strategy

We are using the V-Carve / Engraving toolpath tool for this techique. The sketch at the right was created as we worked through the concept of how to setup the toolpaths. The purple lines represent the pocket being created in the main cutting board. The red lines represent the plug (which in the top of the sketch is upside down) and is nested in the pocket as it would be in the real glue up for the inlay. Below the plug is drawn as it would sit on the CNC table as it is milled.

Note that the artwork or design that is desired for the inlay sits on different planes for each scenario. The artwork will be on the surface of the cutting board so the start depth for the V-Carve / Engraving toolpath will be 0. The artwork plane for the plug sits 0.25" below the top surface of the stock. This is because in this example we are looking for 0.25" of engagement between the plug and pocket. By designing the plug to include the artwork at the depth we want to engage the plug/pocket, we end up with the art aligned to the top surface of the cutting board when the plug is inverted and placed in the socket on the main cutting board.

There are a few things to decide on when setting up your V-carve toolpathes for the pocket piece and plug piece.

Depth of engagment (DE) between pocket/plug
Depth of glue gap (GG) at the bottom of the pocket. (The space between pocket and plug).
Depth of blade gap (BG). (The space between the top surface of the board and the surface of the plug when the two parts are mated and glued. This is the space where a bandsaw blade will slice off the excess plug stock and reveal the plugs sticking out of the pockets for final sanding of the inlay design).
Angle of the V-bit you are using. These come in 7.5°, 15°, 30°, etc.

In this design we are using a 15° v-bit. We decided that a 1/4" of engagement would give us a nice amount of inlay and still allow us to optimize how VCarve mills the pieces. We are able to use a 1/4" and 1/8" end mill bits to clear extra material so that v-bit isn't milling more than necessary (detailed areas and edges only). Limiting the depth of engagement to 1/4" allows us to clear the excess material outside the plug design in a single pass with these end mills. This simplifies the process in VCarve as it does not automatically add stepdowns to the clearing operations above the start height, which is 0.25" below the surface of the wood for the plug in this scenario.

Settings for pocket (cutting board)

V-Carve / Engraving Toolpath Settings

This is the part of the process that took the most time with which to reaquaint ourselves and understand what is really going on. It took several experiments to get to the point where we think we understand what is happening and how this process works.

V-Carve has a check box to determine whether or not a user wants a Flat Depth. If we were just engraving the wood with a design for a sign, we might not care about having a flat depth. But for V-Carve inlay we want an area below where the plug sits inverted into our main board pockets where we have room for glue to fill the gap between the two parts. For our process the Flat Depth will be turned ON for both the pocket and plug milling operations.

<- Pocket: The start depth is 0, because the art/design will be on the surface of the finished cutting board. The Flat Depth is the sum of the engagement depth (ED) + glue gap (GG). So in the example from our sketch above, 0.25" + 0.05" = 0.3"

Plug ->: The start depth is 0.25", because the art/design will be 1/4" below surface of the plug stock. This means we will end up with 1/4" of engagment of the plug into the pocket. Here the Flat Depth depth is equal to the blade gap (BG). So in the example from our sketch above, the Flat Depth is 0.1". Note, the Blade Gap must be greater than or equal to the Glue gap or there will not be enough clearance to fully seat the plug into the pocket.

Settings for plug (inlay)

Milling the parts

Milling the pockets into the main cutting board and creating the plug piece is fairly straight forward once everything is set up in VCarve. One thing we have noticed is that the V-bits we have used have a very sharp point and the bit often seems to chip ever so slightly at the tip which means the bit doesn't consistenly reach to the bottom depth as expected. We are hoping to explore using v-bits that have a small ball at the tip, which should improve the reliability of the bit's abilty to mill to the full measured depth of the bit.

Once the pieces are milled, there is some cleanup that needs to be done to remove any wood that hasn't been completely removed by the v-bit. This can be done with tweezers or dental tools. The time this process will take depends on the complexity / level of detail in the artwork / design.

Cutting board milled with Shippo Tsunagi patterned pockets.

Opposing Shippo Tsunagi patterned plugs ready to invert, glue, and clamp into the cutting board with milled pockets.

VCarve-socket2.mov

VCarve-plug_trim.mp4

Screen recording of V-carve simulations of the cutting board socket milling (left) and the plug for the inlay (right).

Glue up and assembly

After clean up, glue is liberally applied to both the pockets and to the plugs and the pieces are mated. We attempted to hold the plug into the pockets using a billet of aluminum, but we found that the plug didn't stay totally flat with this technique and a portion of the plug came out of engagement with the pocket leaving us with a not optimal fit.

Clean up and sanding

The final step is cutting off the excess stock from the plug to reveal the inlay pattern. Typically this is done either with a band saw (best case scenario) or with a CNC router using a bit to mill off the excess stock. Using the CNC router is time consuming and potentially more risky in terms of damaging the board. If the board isn't too large for your bandsaw, I definitely recommend using that tool!

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