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How to Cut a Square on a Mill

Guide · Updated October 28, 2026

A square on the end of a shaft drives a handle, a crank or a socket. On a manual mill you hold the round stock in a square collet block, a spin indexer or a dividing head, mill one flat, rotate 90° and repeat. The math is simple: each flat sits half the across-flats size from the center.

Check across-flats with a mic and blocks

Measure each pair of flats and confirm your mic on gage blocks at the target size. The free Gage Block Calculator builds the stack.

Step by step

Check the stock is big enough: The round diameter must be at least the square diagonal, which is across-flats × 1.414, or the corners will be rounded.
Hold the work: Use a square 5C collet block, a spin indexer or a dividing head. Indicate the stock so it runs true.
Find the center height: Touch off the top of the round, then note the center is one radius below.
Calculate the depth: Depth per flat = (stock diameter − across flats) ÷ 2, measured down from the top of the round.
Mill flat one: Use an end mill side cut or face cut; take roughing passes and leave about 0.005 in.
Rotate 90° and repeat: Turn the block or index 90° for each of the four flats.
Finish and measure: Measure across both pairs of flats, adjust depth, then take the finish passes on all four.

Worked example

Cut a 0.500 in square on 3/4 in (0.750 in) round. Diagonal check: 0.500 × 1.414 = 0.707 in, smaller than 0.750, so the corners will be sharp with a little round left on them. Depth per flat from the top = (0.750 − 0.500) ÷ 2 = 0.125 in. Rough to 0.120 in deep on all four flats (leaves 0.510 in across flats), measure, then finish each flat at 0.125 in. If the first pair reads 0.502 in, go 0.001 in deeper on each side of that pair.

Square vs hex

A hex uses 60° indexing and a depth of (diameter − across flats) ÷ 2 the same way; see the hex guide for the 1.155 diagonal factor.

Common problems

Unequal pairs mean the stock was not centered or the depth changed between flats. A rhomboid (not square) shape means the index was not 90°. Chatter on a long stickout means the part needs a tailstock or shorter hold.

Tips

Holding: 5C collet block makes 90° indexing easy.
Hex: cut a hex on a mill.
Check: check squareness.

FAQ

How deep do I cut each flat?

(Stock diameter − across-flats size) ÷ 2, measured from the top of the round.

What size round do I need?

At least across-flats × 1.414 for full sharp corners.

Can I use a vise?

Yes with care: mill one flat, rotate the part onto it and repeat, but a collet block is more accurate.

How do I check it is square?

Measure both pairs of flats and check angle with a square or on a surface plate.

Why are my pairs different sizes?

The part was off center or a depth setting changed between flats.

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