LatheCalculator

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Boring head dial calculator

Measure the bore, enter the size you want, and it tells you how far to move the boring head dial.

Use mm or inches throughout. Check your head’s maker’s marking: many metric heads read 0.01 mm on radius, others are marked on diameter.

Safety: stop the spindle before adjusting the head and tighten the slide lock afterwards. An offset head is out of balance, so keep the speed down on large offsets.
How it works, guides and notes

How it works

The boring bar moves off-centre by the slide movement, so the hole grows by twice the slide movement. Halve the size change to get the radial move, then divide by the graduation. If your dial is marked on diameter, the maker has already allowed for this, so divide the full size change instead.

Worked example: 24.62 mm to 25.00 mm is 0.38 mm on diameter, 0.19 mm on radius. With 0.01 mm divisions on radius, move out 19 divisions.

For the last cut, take a light spring pass and re-measure: tool deflection and bar spring often leave the bore a few hundredths under.

Spotted a mistake? Report an error.