Intro: Micro/Mini Lathe, Three Bearing Live Centre
My Sieg C0 lathe came with a fixed centre that had around 0.4mm play in it. I realised that when a need came up I would not feel comfortable using it. I also fly RC helicopters and the main blades in the large models have thrust bearings in the blade holders – that would be a good size for a small lathe live centre. I also saw this as a good opportunity to hone my lathe skills to get a good fit for the bearings!
The parts cost $6 and were enough for two live centres! This excludes the metal and screw that you will hopefully have in your scrap box.
- Some 14mm dia steel around 30mm long (for the body)
- Some 8-10mm dia steel around 40mm long (for the shaft).
- Thrust bearing (5.3 x 10 x 4) for 600 Heli (SKU GT600-T051004) $1.75 for 2
- Ball bearing (5 x 10 x 4) for car (SKU A2016T-30073) $1.89 for 2
- Ball bearing (4 x 8 x3) for car (SKU 9315000044) $2.30 for 4
- A 2mm screw.
Note these were the ideal parts for my application. You may wish to explore other sizes if your lathe is different.
The SKU references are for HobbyKing. However they should also be available from local hobby shops who supply RC helicopters and cars.
Step 1: Design for the C0 Lathe
The socket in my tailstock was10mm diameter - or a least supposed to be. It was however slightly tapered – the wrong way – maybe narrowed by the die making the external 14mm thread. So my first task was to get this cleaned up. I found someone with a decent lathe and collet that could hold the tailstock shaft and true it up with a boring tool. To my surprise opening it to 10.1mm was enough.
The tailstock socket is 18 mm deep, with a 14mm external thread. So the design emerged to have a smaller bearing at the inside end a larger bearing and thrust race in an external 14mm diameter section. Hence the choice of 8mm OD bearing for the inside and 10mm OD thrust race and bearing for the external section.
The shaft has to fit the three bearings. Note that the thrust bearing has a small clearance over 5mm (5.3mm ID).
So the shaft had:
- 1.2mm long 5.3mm OD for the outside ring of the thrust bearing.
- 6.8mm long 5mm OD for clearance for the rest of the thrust race and good fit the ID of the 10mm bearing.
- 15.2mm long 4.95mm OD so the 10mm bearing slides on easily.
- 3mm long 4mm OD for the 8mm bearing.
The holder was 27mm long of which 18mm was turned to fit the tailstock. The internals were:
- A 10mm pocket, 8mm deep for the thrust race was made in the outer end with the outer 2mm opened to 10.1mm to give clearance to the outer thrust ring.
- A 5.5mm hole drilled all the way though.
- A 4mm long pocket 8mm diameter for the smaller bearing. The bearing is only 3mm long but this arrangement means the retaining screw is recessed.
Step 2: Turning Sequence
I started at the small end for both parts before turning them around to do the outer end.
- Drill 5.5mm clearance hole down the centre.
- Face the end.
- Make a pocket 4mm deep 8mm diameter for a tight fit to the bearings. I used a 7.5mm drill to remove the bulk of the metal and a boring tool to finish it off.
- Turn the outer diameter for a tight fit with the tailstock and 18mm length.
- Part off with just over 9mm length for the outer bearing holder.
- Now turn the holder around and hold on the 10.1mm turned section.
- Face off to 9mm length.
- Make a pocket 8mm deep and 10mm diameter for a tight fit from the 10mm bearing.
- Open up the outer 2mm to 10.1mmto give clearance for the thrust bearing outer ring.
- Turn the head OD down to 14mm.
- Face off the bar end and take a Y reference. I just had enough bar protruding for the 15.2mm and 3mm sections as I worried I might get some flex if I also had the 1.2mm and 6.8mm lengths out as well.
- Turn down to 4mm dia for length of 3mm for a tight fit with the 8mm ball bearing.
- Drill and tap 2mm hole for retaining screw.
- Turn 15.2mm section just under 5mm OD.
- Move the bar out so the 6.8 and 1.2mm sections can be done.
- Turn down an 8mm length to 5.3mm/tight fit with the trust race.
- Turn down the inner 6.8mm of the above for a tight fit with the 10mm ball bearing.
- Now while the bar is still in the chuck trial fit the bearings and holder.
- Move the bar out again and part off with 10mm clear of the bearings. I had 8mm bar. If you have larger you will want this section a bit longer for the point.
- Turn the shaft around. My 5mm clearance section was close enough to the 5mm fit section that I could hold it well over these two sections. It may have helped having a collet chuck (that I would recommend).
- Make a 30 degree point using a compound slide/angle tool holder. Alternatively take steps of 0.05mm off the OD initially cutting to a length the OD mm from the end and then make the cuts 0.1mm shorter each time.
The tight fits are always a challenge. I managed in each case to get very close using calipers and fine cuts, and then a second pass possibly with light finger pressure against the tool holder to enable the part to fit. When I was getting close with the fine cuts I stopped and checked after cutting 0.5 to 1.0 mm length to make sure I was not taking too much off. I expect others with more experience will have other techniques. Some turn a bar first to check caliper readings or drill a hole in some sheet a tiny bit over-size to use as a gauge to show that one is getting close.