Sometime it is usefull to lock the spindle of the lathe (e.g. when tapping) To do this I made a simple spindle lock. It is L shaped piece of the 12x12 mm iron rod. It is fastened using holes for the gear cover. There is M6 thread in the weldment with brass screw which locks the spindle.
When the spindle is locked, the central switch should be switched off. Also FORWARD/REVERSE switch should be in the central position to prevent demage resulting from turning on the lathe.
To divide spindle turn I used gear wheels for the slide movement. Aproprite wheel is placed to the A position in the gearbox. Available are with 60, 50, 45, 40, 35 or 30 teeth.
Rest of the transmission is removed. Holder of the gears is used for locking, using a "spike" which fit between the teeth.
Carriage bolt M8 fits in to the mortice in the holder. Washers are used to set the position of the spike (5mm steel sheet) against the wheel.
By turning the holder the spike moves to engagement with the gear where is fixed by the nut.
This device is not very precise. In combination with the spindle lock it is much better. This solution is very simple with no invasion to the construction of mini-lathe.