Change Gears and Pitch

When cutting threads on the lathe we need to look at the change gears and pitch. The change gears on the back of the headstock that connect the headstock rotation to the leadscrew rotation along with the leadscrew pitch determine the pitch you will cut.

hobbymat change gears

The gears shown here on a Hobbymat MD65.

The spindle is driven with a belt and pulley system with an intermediate reduction pulley that can be seen on the left hand side of the image.

A gear is located on the spindle and this has a key to locate it on the main shaft.

The set of gears here have a large reduction as the system is set up for a fine traverse.

The pillar at the bottom of the image has a lever that operates the dog-clutch to engage and disengage the leadscrew.

The number of teeth on these four gears determines the ratio between the spindle and the leadscrew.

The pitch of the leadscrew determines how far the saddle moves for each rotation of the leadscrew.

lathe change gears rotated

The calculators have been gradually moving online rather than embedded in an excel workbook. Let us know whether this works better or if you would like both options.

Workshop Calculations

The workshop calculations download now has a worksheet for change gears. This offers the online calculator as per above, but also a combinations calculator.

change gear combinations calculator

Enter the thread pitch you want to cut, leadscrew pitch and then 3 gear options for each of the positions in the change gear train. This then looks at all combinations and highlights the exact match in green. Matches with 2% are highlighted amber. This also finds options for TPI threads with a metric leadscrew and vice versa.

2 thoughts on “Change Gears and Pitch”

  1. Just testing the Thread Cutting Calculator Nigel – I would expect to be able to select either the [TPI] or [mm] radio button and have either the TPI or pitch in mm displayed but was somewhat dismayed to see 2mm shown as the pitch when 32tpi was produced (0.79375mm is correct).

    Changing to gears which would create a 48tpi thread, the pitch shows 1.33333mm ??? that should be 0.529167

    • Hi JG, the calculator on the page only works in TPI or mm for Leadscrew and Thread. If you want to cut a metric thread with an imperial leadscrew then you will need the Workshop Calculator that can be downloaded from my pages. I will eventually update the online calculator to give both options, however, the syntax for this plugin is not that friendly. Best regards, JG


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