Member postings for Ivy

I would steer clear of white metal and use bronze if I was making bearings for a lathe. They would need provision for lubrication and if the top half has a hole for oil, then an oil groove in the bore would be easy to produce. You can produce a helical groove in the bores of both bearing shells with a ball end cutter in a Dremel or similar. Not a stone for fear of leaving abrasive particals. A groove about 1/16" wide and 1/32" deep would be more than adequate starting at the oil hole and stopping just before the outside edge. Some split bearing shells have the grooves in an "X" running from the oil hole and continuing at the split line to the other shell half. My choice would be leaded gunmetal, it makes nice nuts and probably good bearings as well. Perhaps somebody could have a better bronze alloy to recommend for bearing material.

Ivy, if your spindle is getting reground, the bearings could be produced at the same time with bores between 0.0005" and 0.001" bigger and then cut in half afterwards. They would have to be made with the same outside dimensions as the old ones. The better the finish in the bores the less running in they would need. Because the S & B model A bearings are adjustable, (the third type in my earlier post) they can be run until the headstock only gets warm after 20 minutes running at top speed. With shims you could gradually tighten the caps until the bearings got above warm and then go back to the earlier shim thickness. The proceedure will take a lot of time and patience but your lathe will be as new in the spindle department. I would recommend one of the bronze types for the bearings. If there is provision for oil grooves the bearings will last longer. Also there will need to be provision for the thrust taken if using a centre in the tailstock or drilling.

bearing sizes are available to buy and fit and forget.

These are denoted as precision bearing shells, not rough cast (or turned) items which are required to be dressed to the shaft - particularly where the underlying mounting may not be perfect.

You may have not noticed that your kwaka shafts/bearings are continuosly pressure fed with oil (not a simple drip feed and total loss system like the bearings with which you are comparing. Indeed the precision plain bearings in engines generally require about 5psi (~1/3Bar) to maintain a lubrication film between shaft and bearing, to avoid rapid destruction of the very thinly constructed bearing white metal deposited onto the shell backing material.

You would not be able scrape those precision shells because the white metal is only a few thousandths of an inch thick! That construction, using far less material (particularly the expensive white metal component), superseded much thicker besrings that were adjustable (to an extent) by shim removal between the bearing halves as wear took place. Precision shell bearings are not adjustable, and when worn out often require the shaft surface to be precision ground to a different undersize to accept a new set of bearings that match the shaft size precisely.

Additionally, engine crank-case bearings (particularly those with intermediate main bearings) have to be very well aligned to avoid uneven wear (or early failure). This is a standard level of precision engineering, these days, and is far better than older designs which, while being modern in their day, was far from the precision of modern equipment.

I would fit the bearings and after making sure they were not tight, just run them for a short while with plenty of lubrication. Then dismantle and look for tight areas and scrape then lightly and repeat. The design varies a great deal, from clamped and shimmed bearing halves, tapered designs which can be adjusted by axial means and also parallel surfaces which have bushes made like er collets which can be closed down by axial adjustment. Each style needs slightly different approaches. There are certainly no traces of scraping in the spindle and bushes of the Smart & Brown model A lathe. Any scraping like bedways get would be detrimental to a rotating bearing, only the rings of tighter contact require attention.

Low speed drip fed bearings and high speed, pressure fed bearings is not am good comparison.

The IC engine bearings are fed with a copious supply of oil under pressure and easily build up a dynamic wedge of oil.

The headstock bearings of a Myford are drip fed and run at comparatively low speed, and are probably designed for much smaller clearances. (A high speed IC engine will be designed for circa 0.002 - 0.004" 0.05 - 0.1 mm clearance, to allow a good flow of oil, to provide cooling as well as lubrication).

The purpose of blueing and scraping will be to maximise the area riding on the dynamic wedge of oil; possibly boundary lubrication only a few molecules of oil thick.

A headstock with automotive clearances would be classed a badly worn machine tool..

Howard

As an example I have a Kawasaki engine that runs at 10,000 RPM all day long, the bearing sizes are available to buy and fit and forget.

But if you look at the factory rebuild manual, it will tell you how to select the correct size bearing shells from a table - look up the crank & housing dimensions (often coded on the parts with coloured dots or simliar) and it will give you the correct bearing to use to get the desired clearance. Mass produced engine components are graded during production & assembly from the graded parts to get the required running clearances is easily assured.

Not so with a one-off or low volume application, hence the need to "check and adjust" by a suitably trained fitter. A true "fitter" is not just an assembly monkey - they make parts fit together as the designer intended..

Nigel B.

Scraping is done on larger bearing halves to break the surface and create pockets for some oil retention. A smooth bearing surface and a smooth shaft which are both perfectly round will not leave room for oil between the surfaces causing excess wear and heat build up.

Ivy

Fit and alignment are critical.

Robert Butler

Isn't this more to do with poured white metal bearings from a 100 years ago?

Possibly, but is it still needed with modern machining?

I should have said my lathe is a S7B with power cross feed. As Speelwerk says the saddle and the apron are different from the earlier non power cross feed model, thus the screws will be different.

John