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How to Use a 5C Collet Block

Guide · Updated October 28, 2026

A 5C collet block is a square or hex body that holds a standard 5C collet. Clamp it in a vise or on a plate, and you can rotate round work in exact 90° or 60° steps to mill flats, cross-drill or grind without a dividing head.

Check across flats

Measure the finished flats with a mic checked against gage blocks. The free Gage Block Calculator builds the stack.

Step by step

Pick the block: Square blocks give 4 positions (90°) and hex blocks give 6 (60°); many sets include both.
Clean everything: Wipe the collet, the bore and the block faces; chips here throw off concentricity and squareness.
Load the part: Use the collet sized to the stock and tighten the drawnut firmly with the block on the bench.
Set it in the vise: Seat a block face on parallels or the vise floor, with the jaws on two opposite faces.
Indicate the part: Check the part top for level along its length; collet blocks can be off a few tenths.
Cut the first feature: Mill the flat, drill the hole or grind the face.
Index: Loosen, rotate the block to the next face, re-seat it firmly and repeat.

Worked example

Milling a 0.500 in square on the end of a 0.750 in round shaft: each flat goes (0.750 − 0.500) ÷ 2 = 0.125 in deep. Cut the first flat 0.120 in deep, rotate the square block 180° and cut the opposite flat, then mic across: say 0.508 in. Lower the cutter 0.004 in and finish both flats to 0.500. Turn 90° and repeat for the second pair, checking with the mic each time.

Square vs hex blocks

Square blocks also give you 180° for opposite flats and keyways. Hex blocks are for six-sided features like hex heads or six evenly spaced holes. For other divisions, use a spin indexer or dividing head.

Accuracy checks

Indicate across the top of a ground pin in the collet, then index 90° and indicate again; any change is block error. Mark the best face of each block as the reference.

Tips

Other indexing: using a dividing head and spin indexers.
Vise: setting up a milling vise.
Collets: using a collet chuck.

FAQ

What is a 5C collet block used for?

Holding round work and indexing it 90° or 60° to mill flats or drill holes.

How accurate are collet blocks?

Typically within a few tenths per face; indicate to check yours.

Can I use a collet block on a surface grinder?

Yes; it is a common way to grind square or hex features.

Square or hex block?

Square for 4 or 2 positions, hex for 6 or 3.

How tight should the collet be?

Firm; tighten on the bench so the block does not shift in the vise.

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