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How to Use a Dividing Head
Guide · Updated October 28, 2026
A dividing head turns work by exact fractions of a turn so you can cut flats, hex shapes, splines and gear teeth. Most heads use a 40:1 worm, so 40 crank turns turn the spindle once.
Check the angles you cut
Verify an angle on the finished part with a sine bar and a stack from the free Gage Block Calculator.
Step by step
- Mount and align: Bolt the head to the table and indicate the spindle parallel to X; align the tailstock to the same height.
- Do the math: Crank turns per division = 40 / N, where N is the number of divisions.
- Pick a circle: For a fraction, choose an index-plate hole circle that the denominator divides evenly.
- Set the sector arms: Set the sector arms to span the needed holes, not counting the hole the pin sits in.
- Lock and cut: Lock the spindle before each cut and unlock it to index.
- Index: Crank the full turns plus the sector holes, always in the same direction, and move the sector arms after each index.
- Check: After the last division the pin should return to the starting hole.
Worked example: 17 divisions
40 / 17 = 2 6/17 turns. Use the 17-hole circle: two full turns plus 6 holes. For 6 divisions (hex): 40/6 = 6 2/3 turns, so use an 18-hole circle with 6 2/3 = 6 12/18: six turns plus 12 holes.
Degrees instead of divisions
One crank turn is 360/40 = 9°. For 15°: 15/9 = 1 6/9 turns, so one turn plus 12 holes on an 18-hole circle.
Backlash
Always crank in one direction. If you overshoot, back off past the hole and come in again from the same side.
Tips
- Indicate the spindle like you would a vise.
- For simpler angles, a rotary table may be faster.
- Check angles with a sine bar.
- For keyways and slots, see cutting a keyway.
FAQ
What is the ratio of a dividing head?
Most dividing heads use a 40:1 worm, so 40 crank turns rotate the spindle once.
How do I calculate indexing on a dividing head?
Divide 40 by the number of divisions and use an index-plate circle that gives a whole number of holes for the fraction.
What are the sector arms for?
They mark the number of holes to move past the full turns so you do not have to count holes each time.
Dividing head vs rotary table?
A dividing head indexes divisions quickly and accurately with plates; a rotary table is better for arcs and is also used for bolt circles.
What if no plate circle works?
Use differential indexing with change gears or a CNC rotary axis.
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