What is the Dogbone gadget in VCarve Pro & Aspire and how do I use it for CNC joinery?

Overview

The Dogbone gadget solves a basic limitation of CNC routing: a round end mill cannot cut a perfectly sharp 90° internal corner. When two pieces need to slot together tightly (box joints, dado joints, finger joints, mortise-and-tenon), the rounded corner left by the bit stops a flush fit. Dogbone fillets fix this by overcutting into the corner, creating a small circular relief so the mating part can seat fully. The gadget is a free download from the gadget library and is installed with Gadgets > Install New Gadget.

Why Dogbones Are Needed

When a CNC router cuts an internal corner (pocket or profile), the end mill leaves a radius equal to half the bit diameter at every corner. For a 1/4" bit, every internal corner has a 1/8" radius.

If you are cutting a box with interlocking tabs:

• The male tab is cut with sharp corners (external corners cut perfectly sharp)
• The female slot has rounded internal corners
• The male tab will not fully seat into the female slot because the rounded corners block it

Dogbone fillets overcut the corner by the bit's radius, creating a small circular notch that the mating piece fits around. From the outside of the assembled joint, the dogbone is hidden. This makes it best for concealed joints.

Two-Stage Gadget Workflow

Stage 1: Dogbone Detect

1. Select the closed vectors with internal corners that need dogbones
2. Gadgets menu > Dogbone Detect
3. Set Tool Diameter. It must match the actual bit you will use for the profile or pocket cut, because it determines the size of the dogbone fillets
4. Set Interior Allowance (the allowance on interior contours) and Outer Allowance (the allowance on outer contours)
5. Set the Marker Layer name. This is the layer the gadget creates and then uses in stage 2
6. Click OK. The gadget places visual markers at all locations where dogbones could be added
7. Review the markers and delete any you do not want (for example, decorative corners that do not need clearance)

Stage 2: Dogbone Create

1. Gadgets menu > Dogbone Create
2. The form displays the options from Detect (tool diameter and the inner and outer allowances). Confirm them
3. Marker Layer should be picked up automatically. If it is not, enter the marker layer name
4. Set Profile Vector Layer, the layer this gadget will create
5. Click OK. The gadget creates the profile vector layer with the dogbone fillets
6. Switch to the Toolpaths tab and make a Profile toolpath from those vectors as usual

There is also a single-step variant, the Dogbone Toolpath Creator, which also asks for a Layer To Toolpath (the layer you want the toolpath added to) along with the tool diameter, allowances and profile vector layer.

Dogbone vs. Fillet Tool (Manual Approach)

The software also has a built-in Fillet tool in the Drawing tab that adds fillets interactively where any two spans on a contour meet. Choose a radius and a fillet type: Normal, Dog-Bone or T-Bone.

1. Select the Fillet tool
2. Choose Dog-Bone (not Normal) as the fillet type
3. Set the radius to half your bit diameter
4. Click each internal corner that needs a dogbone

When to use the Fillet tool vs. the Dogbone gadget:

• Fillet tool: fewer corners, when you want manual control over each corner
• Dogbone gadget: many corners (CNC joinery with dozens of corners), with automatic detection and batch processing

Sizing the Dogbone Correctly

The dogbone radius should be at least the bit radius (half the bit diameter) for a mating part to fit:

• 1/4" (6.35 mm) end mill: dogbone radius 0.125" (3.175 mm)
• 1/8" (3.175 mm) end mill: dogbone radius 0.0625" (1.5875 mm)
• 6 mm end mill: dogbone radius 3 mm

For a slightly easier fit, try adding a small clearance (a starting point of 0.005" to 0.010") and test on scrap first.

Placement Styles

The Fillet tool's Dog-Bone and T-Bone types place the overcut differently:

• Dog-Bone: a circular cut-out placed so the upper right part of the circle touches the original sharp corner. The manual says not to use this when the slot width and the tool are similar in size
• T-Bone: a circular cut-out that lets the slot grow out to the side. Use it when the slot width and the tool are similar in size. You choose which side by clicking on that side of the corner; clicking the corner itself places it on the longest side

Tips

• Add tabs on the profile cut-out pass so parts stay held during cutting
• Always cut a test piece in scrap and adjust the allowances before cutting a full sheet
• Box Creator makes a complete dovetail box with its own toolpaths. The Dogbone gadget is for your own designs where you need square internal corners

Common Use Cases

• Box joints and finger joints for drawers, boxes and decorative panels
• Dadoes and grooves where shelves or dividers must sit flush
• Cabinet joinery from CNC-cut plywood panels
• Puzzle joints and interlocking parts
• Mortise and tenon where the mortise is CNC-pocketed
• Slotted assemblies (for example, flat-pack furniture)