A 1/4 inch cutter is 6.35 mm across, so R is 3.175 mm — exactly twice the radius of a 1/8″ bit. Every linear number on this page is therefore double the 1/8″ equivalent.
This is the most common bit on hobby routers, and it is also the one that most often fails the edge-length check, because 6.35 mm of cutter eats a surprising amount of a small pocket wall.
The numbers for R = 3.175 mm
Minimal dogbone bulge is 3.175 × (1 − 1/√2) ≈ 0.930 mm. Edge consumption per corner is √2 × 3.175 ≈ 4.490 mm, so two minimal dogbones plus the 0.05 mm tolerance need 9.03 mm of wall.
Full corner overcut is 3.175 mm and consumption is 3.175 mm per corner, so two full corners need about 6.40 mm of wall. A T-bone consumes 6.35 mm on the wall it faces and nothing on the other.
The tool keeps millimetres internally and the unit toggle only changes the labels, so typing 6.35 while the display says inches gives you a 161 mm cutter. Watch the output size line under the buttons.
Which slots pass and which fail
A 16 × 50 mm slot with minimal dogbones on all four corners passes: 16 mm clears the 9.03 mm requirement on the short walls.
An 8 × 50 mm slot with both ends enabled does not. The tool names the short edge and tells you how many millimetres you are short rather than exporting a self-intersecting contour.
That same 8 mm wall does fit two full corners, which need only 6.40 mm. Alternatively, run T-bones along the 50 mm axis and the 8 mm walls stay untouched entirely. Disabling one corner per short wall is the third option, and often the right one for a dado that only needs relief at one end.
Choosing a style for plywood boxes
Plywood carcasses cut with a 1/4″ upcut on a Shapeoko, X-Carve, or similar machine almost always want minimal dogbones. A 0.93 mm bulge vanishes inside a 12 mm dado and nobody will ever see it.
Shop jigs and fixtures are the natural home of the full corner: nothing is on show, and the extra clearance tolerates dust and finish build-up.
Reach for the T-bone when one wall is a thin tongue you cannot afford to nibble. Auto axis picks the wall with leftover slack after the opposite corner's relief is reserved, which is not always the longer wall — it depends on what the other corner already claimed.
Importing at the right scale
Take the SVG if your CAM reads millimetre width and height, or the DXF if you would rather set units in the import dialog. Both files describe the same geometry after one shared normalisation.
Then pocket with a 6.35 mm tool. Do not draw a 6.35 mm offset "for the bit" on top of this contour — the contour is already the finished pocket, and CAM will add cutter compensation on its own.
For a 1/4″ bit, type 0.25 in or 6.35 mm consistently in both places. Mixing 6.35 in the generator with 0.25 mm in CAM produces a hairline groove instead of a pocket.
When a 1/4″ bit is simply too big
If the part is small and a 1/4″ cutter is all you own, the honest answer is a smaller bit, not a smaller dogbone. Geometry cannot invent clearance that the cutter diameter forbids: the relief radius is the cutter radius, and shrinking it below that leaves material the bit cannot reach.
A 1/8″ bit halves every consumption figure on this page and turns most failing pockets into passing ones. It is slower and more fragile, but it is the only real fix when the walls are short.