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# Dogbone vs T-Bone Fillets: Check CNC Inside Corners

> Compare diagonal dogbone and side T-bone relief, inspect the extra material each removes, and separate ideal corner clearance from a verified CNC fit.

Source: https://perfectvector.com/blog/cnc-dogbone-vs-tbone

Language: en
Author: Irene Kim
Published: 2026-10-20T08:00:00+09:00

For a square inside corner, a diagonal **dogbone relief** extends the pocket beyond both adjoining walls. A side **T-bone relief** concentrates that extension along one wall. Choose by where you can afford the extra removal, the receiving tool, and the mating geometry. Neither choice establishes physical fit by itself.

This differs from adding an ordinary rounded fillet. A normal fillet changes the intended corner to a curve; corner relief deliberately opens extra space around it. [Vectric's Create Fillets documentation](https://docs.vectric.com/docs/V12.5/Cut2DDesktop/ENU/Help/form/Create%20Fillets/index.html) distinguishes Normal, Dog-Bone, and T-Bone options.

## Why a sharp SVG corner can leave rounded material

A circular cutter moving inside a pocket cannot occupy a sharp inside corner while staying entirely within both nominal walls. **Nominal** means the intended reference geometry before relief or an allowance is added. Moving the cutter farther toward the corner removes some material beyond that reference.

[Mekanika's corner-relief guide](https://www.mekanika.io/en/blog/cnc-milling/what-is-a-dog-bone) explains the resulting interference with a square mating corner and illustrates diagonal and side relief. The relevant question is which surrounding area your design can give up.

![Conceptual illustration of sharp and rounded inside-corner references with a circular cutter footprint](https://perfectvector.com/blog/cnc-dogbone-vs-tbone/corner-clearance.webp)

*Conceptual illustration of the sharp-corner and round-cutter conflict. It is not the measured SVG study, a toolpath, or a cut result.*

For the broader source-to-CAM sequence, use the [CNC vector preparation guide](https://perfectvector.com/blog/image-to-svg-for-cnc). Here, the decision is the local relief around an inside corner.

## Choose which wall can accept the relief

| Situation                                         | Candidate to examine                                    | What to check                                                            |
| ------------------------------------------------- | ------------------------------------------------------- | ------------------------------------------------------------------------ |
| Both adjoining walls can accept a small extension | Diagonal dogbone                                        | Extra area on both sides, neighboring features, and visible recesses     |
| One wall must retain its nominal boundary         | Side T-bone directed toward the other wall              | Available material on the relieved side and the complete cutter approach |
| Slot width is close to tool diameter              | A destination-supported side relief or a revised design | Whether the selected construction is valid for that slot and cutter      |
| The mating corner can be redesigned               | A rounded mating corner or different joint              | Agreed geometry and allowance for both parts                             |

The narrow-slot row is consequential: [Vectric's V12.5 documentation](https://docs.vectric.com/docs/V12.5/Cut2DDesktop/ENU/Help/form/Create%20Fillets/index.html) warns against its Dog-Bone option when slot and tool widths are similar, and describes T-Bone as the side-growing option for that situation. Its interface also allows choosing the side. These are documented destination rules, not a test of every CAD or CAM implementation.

A recess may be hidden by the assembled part, but check the actual overlap. Its direction alone does not establish invisibility, joint strength, or adequate remaining material.

## An original single-corner geometry comparison

We authored three SVGs describing an ideal two-dimensional circular-cutter sweep. The nominal pocket was 18 × 14 coordinate units, and cutter radius `r` was 3. These are illustrative units, not recommended tool sizes or machining allowances.

The unrelieved model expands a cutter-center rectangle from `(3, 3)` to `(15, 11)` by a radius-3 disk. For the two alternatives, only the **top-left corner** changes. Coordinates increase rightward and downward from that nominal corner at `(0, 0)`:

* Diagonal dogbone: the center extends from `(3, 3)` to `(3/√2, 3/√2)`.
* Side T-bone: the center extends from `(3, 3)` to `(3, 0)`, toward the top wall.

Each extension includes the whole round-capped segment swept by the disk. We reopened the saved SVG coordinates and checked the resulting regions with numerical cross-section integration and an independent point-distance grid. Actual renders of all three saved files were also checked at 15 known pixels.

| Top-left construction          | Nominal corner area left uncovered | Extra removed area outside the nominal pocket | Maximum extension beyond left / top wall |
| ------------------------------ | ---------------------------------: | --------------------------------------------: | ---------------------------------------: |
| Unrelieved                     |                1.9314 square units |                                             0 |                              0 / 0 units |
| Diagonal dogbone               |                                  0 |                           5.1372 square units |                    0.8787 / 0.8787 units |
| Side T-bone with full approach |                                  0 |                          14.1372 square units |                              0 / 3 units |

The diagonal option removes less extra area **in this declared model**, while the T-bone keeps the left boundary intact and takes a deeper bite beyond the top wall. That makes the trade-off concrete without declaring a universal winner.

For the same construction at radius `r`, the checked formulas are `r²(π/2 − 1)` for diagonal extra area and `πr²/2` for side extra area. The diagonal extension beyond each wall is `r(1 − 1/√2)`; the side extension beyond its chosen wall is `r`. These formulas describe this ideal right-angle geometry, not arbitrary corner angles or application settings.

### A circle touching the corner is not the whole sweep

We also tested the T-bone endpoint disk by itself, combined with the unrelieved pocket. Although that disk touches `(0, 0)`, it leaves **0.3905 square units** of the top-left nominal corner uncovered. The full approach segment removes that remaining crescent in our model.

This is why drawing a circle near the corner is an incomplete check. Inspect the complete removed region and how the cutter reaches it. A tangent endpoint does not prove that every point between the original rounded pocket and that endpoint has been cleared.

### Zero missing area is still zero nominal clearance

Here, **clearance** means extra room beyond a touching fit. Both modeled reliefs merely touch the sharp reference corner. Their nominal clearance at that point is **zero**. No allowance for the mating part, actual cutter diameter, runout, material movement, or machining error was tested.

The other three pocket corners remain rounded in all three files. Consequently, this single-corner result does not show that an entire square part fits the pocket. The SVGs describe ideal swept regions using round strokes; they are not machining-boundary export files, and we did not import them into CAM or cut a part.

A destination may require a different allowance. For example, [Shaper's Dogbone Rectangle documentation](https://support.shapertools.com/dogbone-rectangle) says its dogbone diameter should be slightly larger than the intended bit diameter. That reinforces the need to use the actual destination's construction and fit requirements rather than copying this zero-clearance example.

## Keep relief, rounding, and compensation separate

A [rounded SVG path corner](https://perfectvector.com/blog/round-svg-path-corners) changes the artwork boundary. A corner relief adds space beyond a nominal pocket near a concave corner. [Kerf compensation](https://perfectvector.com/blog/svg-kerf-compensation) concerns offsets for a cutting process; it is a separate decision from choosing a local relief shape.

Preserve the nominal master so you can identify which geometry is intentional artwork, which is relief, and where the destination applies compensation. Check the actual imported boundary and generated operation together before accepting either.

## What this looks like with PerfectVector

If a suitable raster motif is all you have, [prepare an editable SVG with PerfectVector for the CNC workflow](https://perfectvector.com/svg-for-cnc). Compare its silhouette, openings, and closed boundaries against the source, preserve the accepted SVG, then verify its dimensions in the receiving CAD or CAM tool.

That step recovers inspectable artwork geometry. The destination workflow still owns relief construction, cutter selection, allowances, toolpaths, and machining verification. If you already have a dimensionally authoritative vector or CAD model, work from that master instead of tracing an image of it.

Before committing the design, check every inside corner that meets a mating part. Review the relief's complete approach, which walls it extends past, and nearby features. Confirm the intended clearance in the actual model, inspect the generated operation, and validate the joint with a suitable test cut before treating it as a production fit.

## FAQ

**Is a dogbone fillet the same as a normal fillet?**

A normal fillet rounds the intended corner. A dogbone relief deliberately extends beyond a nominal inside corner to make extra room for a circular cutter. Check the destination's actual construction rather than relying on the command name alone.

**Does a T-bone always remove less material?**

No. In our radius-3, right-angle model, the side T-bone removes 14.1372 square units outside the nominal pocket, versus 5.1372 for the diagonal dogbone. The side option preserves one wall's boundary while extending farther past the other.

**Does a relief touching the sharp corner prove the joint fits?**

No. Our tangent constructions have zero nominal clearance, and only one corner was relieved. Whole-part fit also requires checking the remaining geometry, agreed allowances, actual tool and material, and the machined result.

## Sources

1. [Vectric V12.5 — Create Fillets](https://docs.vectric.com/docs/V12.5/Cut2DDesktop/ENU/Help/form/Create%20Fillets/index.html) — Distinguishes normal, Dog-Bone and T-Bone constructions and documents its narrow-slot restriction and side choice.
2. [Mekanika — What Are Dogbones and How to Create Them?](https://www.mekanika.io/en/blog/cnc-milling/what-is-a-dog-bone) — Explains circular-cutter interference at inside corners and diagonal versus side relief.
3. [Shaper — Dogbone Rectangle](https://support.shapertools.com/dogbone-rectangle) — Describes the destination template's geometry controls and diameter guidance relative to the intended bit.

***

If raster artwork is your starting point, [create and inspect its SVG candidate](https://perfectvector.com/svg-for-cnc), verify the silhouette, openings, and imported dimensions, then build and check the required corner relief in the destination workflow.

## Explore

- [Read the HTML article](https://perfectvector.com/blog/cnc-dogbone-vs-tbone)
- [LightBurn Cuts Outside First: Check Groups and Layers](https://perfectvector.com/blog/lightburn-inner-shapes-cut-order.md)
- [FreeCAD Image to Sketch: Build Editable Geometry](https://perfectvector.com/blog/freecad-image-to-sketch.md)
- [Product facts](https://perfectvector.com/product-facts.md)
- [FAQ](https://perfectvector.com/faq.md)
- [Pricing and credits](https://perfectvector.com/pricing.md)