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7x12 mini lathe in fetching blue!

Turning a shaft and getting a clearance fit in an Oilite bush

When a project demands a rotating shaft in a bearing, there are usually two choices, either a ball bearing or a bronze bush. Oilite bushes are impregnated with oil, and have a large advantage over a ball bearing – they do not require a large housing. A bush with an inner diameter of 10mm has an outer diameter of only 15mm, compared with 25mm or more for a ball bearing.

The diameter of the shaft is critical to an accurate finished article. The shaft must turn freely in the bush, but not be so undersized that it wobbles. All the bearing surfaces of the shaft must be cut in one operation on the lathe to retain concentricity, so it is important to know exactly what size to make the diameter of the bearing surfaces.

So it is wise to practice!

We start with a scrap bit of silver steel and turn it down to about 0.1mm oversize and measure carefully. Use of a micrometer is vital here – calipers are too inaccurate. Make sure the micrometer is well calibrated.

Then sneak up very slowly, advancing the cross slide by fractions of a division until the micrometer reads dead on 10mm – my nominal bushing size. At this point the bearing should NOT fit on and rotate. If it does something is wrong, probably the adjustment of your micrometer.

According the Oilite technical information, for a 10mm Oilite bronze bush, we need a shaft diameter of between 0.013mm and 0.028mm under the 10mm target. So clean off the bear surface with some 1200 grit paper and measure again. The difference between success and failure is about 1 division on the micrometer (0.01mm) so you need to be careful.

Take off the smallest possible cuts until you are just over 0.01mm under, i.e. between 9.98mm and 9.99mm. Use the fastest speed you can without vibrating the lathe and advance the saddle very slowly and smoothly. To get very small advances of the tool, I ignore the cross slide and rotate the top slide to about 10 degrees. Very small advances to the top slide now translate to tiny in movement of the tool. About 0.004mm advance for each division on the dial.

A good finish is achieved by a fast speed, good centre height of the tool and a tight saddle. It is wise to tighten the nuts at the rear of the saddle so that it is fairly stiff (but still smooth) on the ways - if the saddle rocks by the slightest amount you will make an error.

At this point the shaft should slip nicely into the bearing. Prepare the surface with some 2400 and 3000 grit sandpaper to get a really smooth finish. Complete with a bit of metal polish. These final grinding and polishing actions will not reduce the diameter by much!

It is said that you should take off quite a bit on the final cut to get a decent finish, however, this is impossible to do accurately on a mini lathe and the method of creeping up to the final diameter works much better on a mini-lathe. Make sure you clean off all the polish and grit with white spirit before fitting the Oilite bearing. It is also wise to re-oil the bearings - leave them for an hour in hot (100C) oil to impregnate. A tiny touch of grease on the shaft and you've got a nice bearing.

After a few tries, you’ll have a test piece which fits the bearing really well. Now, when you make the real shaft, compare the measurements on the final job with the measurements on this test shaft – this will null out any tiny miss-calibration of your micrometer.

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