← AskMachinist · Guides · Calculator · 🔎 Search

How to Cut a Hex on a Mill

Guide · Updated October 28, 2026

A hex is six flats at 60° spacing. On a manual mill you hold round stock in a hex collet block, spin indexer or dividing head, cut one flat, index, and repeat. The size that matters is across flats; the stock has to be big enough to cover the corners.

Check across flats

Set your mic against a gage block stack at the target size before the final pass. The free Gage Block Calculator builds the stack.

Step by step

Size the stock: Across corners = across flats ÷ cos 30° = 1.1547 × flats. Start from round stock at least that big.
Hold the part: Use a hex 5C collet block, spin indexer or dividing head so each index is exactly 60°.
Touch off the top: Touch the cutter on top of the round stock and note Z.
Cut the first flat: Mill one flat with an end mill side or face cut, leaving 0.005–0.010 in per side.
Index 180° and measure: Cut the opposite flat and measure across flats; adjust depth so both are equal.
Cut the rest: Index 60° at a time and cut the remaining flats at the same Z.
Finish and deburr: Take a light final pass on all six and break the corners.

Worked example

For a 3/4 in hex: across corners = 1.1547 × 0.750 = 0.866 in, so 7/8 in (0.875) round works. Each flat goes (0.875 − 0.750) ÷ 2 = 0.0625 in below the stock surface, measured from the top touch-off along the radius. Rough each flat 0.055 in deep, measure across flats (about 0.765 in), then drop 0.0075 in and finish all six to 0.750 in.

Collet block vs dividing head

A hex 5C collet block flips on its flats on the vise, so indexing depends on the block. A spin indexer or dividing head indexes by degrees or holes and also handles squares and odd counts. Either way, make sure the part is tight before each cut.

Common problems

Uneven flats usually come from changing Z between cuts or a part slipping in the collet. Small corners mean the stock was too close to across-corners size. Chatter on long parts means you need tailstock support.

Tips

Indexing: 5C collet block and spin indexer guides.
Divisions: dividing head for other counts.
Measuring: reading a micrometer.

FAQ

What is across corners on a hex?

Across flats × 1.1547.

How deep is each flat?

Half of (stock diameter − across flats), from the stock surface.

Can I cut a hex in a vise without indexing?

Only roughly; an indexer or collet block keeps the 60° accurate.

How do I cut a square instead?

Index 90°; across corners = flats × 1.4142.

Why are my flats different sizes?

Z changed between cuts or the part slipped.

Related guides and calculators

Build your stack in seconds

The free Gage Block Calculator finds the fewest-block combination for inch and metric sets, with wear blocks. No login.

Questions? Ask a machinist.

General shop information only — follow your machine and tooling manufacturer's instructions and your shop's safety rules.