C3 Mini Lathe Bearing Change

A step by step guide to replacing the single row ball bearings fitted to the headstock on the SIEG C3 and similar Mini Lathes with 30206 taper roller bearings. Proper replacement will provide a higher precision finish on your work and reduce or eliminate tool chatter.

For the C3 Mini-Lathe, this project requires the use of a second lathe to make and adjust two plastic spacers in the gearbox so don't start work if you don't have the facilities! The SC2 and SC3 Mini-Lathes are direct belt drive machines so there are no plastic spacers fitted.

Chuck guard, chuck, gear cover and gear train removed

1. Start by disconnecting the machine from the mains supply. Remove chuck guard, chuck, gear cover and gear train.
(Note: It is not necessary to remove the leadscrew as shown in the picture - these photos were taken during a complete strip-down of the machine during it's initial preparation for a customer)

Belt and layshaft gear removed

2. Remove belt and layshaft gear.

C-nuts, gear, key, plastic spacer and plastic bearing cover removed from spindle

3. Remove the two C-nuts, gear, key, plastic spacer and plastic bearing cover from spindle.

Gear cover support bracket removed

4. Remove the gear cover support bracket and undo the control box but don't disconnect it.

Undo the two screws holding the spindle speed sensor
Remove the spindle speed sensor

5. Undo the two screws holding the spindle speed sensor and note how the timing wheel fits between the forks of the sensor (some C3 variants are not fitted with a spindle speed display).

Remove the 3 screws from the front bearing cover

6. Working through the holes in the spindle flange, remove the 3 screws from the front bearing cover.

Remove spindle
Spindle removed

7. Remove the spindle from the headstock using a puller. The diagram below shows a simple to construct puller made with bits from the scrap box.

Diagram of bearing puller
Remove the plastic spacer and key
Extract the front bearing using a press or mallet

8. Remove the plastic spacer and key then extract the front bearing from the spindle using a press/mallet.

Replacement taper roller bearing
Press on new bearing inner

9. Clean everything thoroughly, replace the bearing cover on the spindle and press on the new bearing inner being careful to press on the bearing race and not the cage! An 8" length of steel tube is handy here but ensure the bore is greater than 30mm to clear the spindle and the O.D. no more than 40mm to clear the bearing cage.

Refit spacer and key to the spindle

10. Refit the spacer and key to the spindle.

Extract rear bearing
Rear bearing extracted

11. Extract the rear bearing from the case.

Diagram of scrap box bearing puller
Press new front bearing outer race into place
Press new rear bearing outer race into place

12. Press or tap into place the new front and rear bearing outer races.

New plastic spacer with keyway

13. To compensate for the wider taper roller bearings it is necessary to make a new plastic spacer (approx. 0.105" wide) with keyway to fit the spindle between the timing wheel and gears.

Fit the new spacer between the timing wheel and gears
Trial fit and adjust the new spacer
New spacer fitted correctly
Check the timing disk is correctly positioned

Trial fit the new spacer and adjust as required - the spindle speed timing disk must not foul the pickup and the gears must mesh correctly.

Grease front bearing and insert spindle

14. Grease front bearing and insert spindle into headstock locating keyed disk, spacers and gears on the key.

Fit the reat plastic spacer, after modifying it to have a small step on one end

15. Fit the rear plastic spacer having first modified it by turning a smaller diameter step at one end so it clears the bearing cage.

New rear bearing greased and fitted

16. Grease the new rear bearing and pull it into place using the C-nut, gear and plastic spacer.

17. Temporarily fit the belt cover and check tumbler reverse gear alignment - they probably won't align because the black plastic spacer is now too long.

18. Shorten the black plastic spacer as required.

19. Final adjust the C-nuts using two C-spanners to remove free play and pre-load the bearings.

20. Re-fit the drive pulley, belt and belt cover complete with tumbler reverse.

Gear train, gear cover and chuck re-fitted

21. Re-fit gear train, gear cover and chuck.

Re-fit spindle speed sensor

22. Re-fit the spindle speed sensor (checking again that it clears the timing disk!) and re-fit the control box.

Providing you have no bits left over (except the old ball bearings!) and everything is tightened and adjusted correctly, your lathe is now ready to use.