|Nigel Graham 2||16/05/2019 23:41:01|
|359 forum posts|
I wonder if anyone can help with this:
E.W. Stringer 2.5" BGSC lathe.
The cast-iron headstock is in two parts dovetail-clamped to the plain bed.
The spindle appears to be mild-steel, or at least unhardened, a plain 3/4" diameter for most of its length apart from the thrust-flange and nose. It has a fine thread also 3/4" dia. on the outside end for the bearing collars, just inboard of a reduced section that holds the first change-wheel.
The bearing surfaces are simply directly spindle to headstock bores. No separate bearings of any kind. The castings are split one side only, for adjusting against their own elasticity by a cheese-headed screw.
The poor old thing is probably of mid-50s vintage and the spindle and bores have worn enough to make the chuck jump visibly. I would like to refurbish it, but there is precious little metal spare metal I can remove safely from the headstock, and undercutting the spindle for non-split bearings will truncate the back end thread and reduce the pulley section.
Bore the headstock and turn or grind the bearing areas of the spindle just enough to allow using bushes split to correspond with the headstock...
... Bushes of...?
- Oilites (commercial items in metric and inch sizes - but still need their walls thinning drastically, from the outside of course)
- Cast iron (is a c.i. bush of perhaps only 0.50" wall thickness feasible?
- Aluminium alloy (is that feasible even - it can be machined very thin but I don't know its bearing compatibility with mild-steel)
- Bronze (I understand it is not suitable for non-hardened mild-steel shafts, concentrating the wear on them.)
- Reinforced Nylon (bit easier than PTFE but still not the easiest material to machine, and likely to be more brake than bearing if the shaft warms up.
- SRBF ('Tufnol' (Is used for bearings but I'm not sure of suitability here.)
- PTFE-lined steel (commercially available, metric-only, suitable size might not exist)
Bore the headstock to minimum for rounding; make new spindle to suit. This involves the 1MT nose end. Spindle of...?
- EN8 (probably tougher than needed but not necessarily less wear-resistant, though I expect it'll see me out!)
- Stainless-steel. (probably well over-spec for the demand).
I envisage using the Harrison L5 with boring-table mounted, to line-bore the EW headstock (and come to that, tailstock) between centres, either still on their own bed or on a dovetailed adaptor.
Might be permissible to line-bore slightly below-centre, taking most of the iron from the stronger side; using as limit the faceplate clearance (I think about 1/16" above the bed).
The tailstock is thicker-walled, so a PTFE-lined or modified Oilite bushes could be feasible, with a small reduction in barrel diameter.
The bed might like some attention too: machining-marks at the ends show it was simply fly-cut or face-milled. Whether that was original or by an overhaul I have no idea, but it is probably just short enough for the Myford VMC's bed and travel.
|Michael Gilligan||17/05/2019 00:03:30|
13823 forum posts
May I suggest an alternative approach, Nigel ?
Insert bushes, but also make your new spindle with a slightly reduced diameter.
This could be made to accept collets, which should improve the capability of the lathe.
3657 forum posts
I'd bore it and fit leaded bronze bushes. Leaded bronze, not phosphor bronze. Leaded bronze such as SAE 660 or equivalent works just fine on non-hardened spindles, eg my Drummond M-type lathe and many millions of other pieces of machinery. Bronze will work well at thin wall thickness and will not deform like plastics under load. Ditto your tailstock. Don't use phosphor bronze on non-hardened shafting though, it will wear it out.
|Nigel Graham 2||17/05/2019 14:06:43|
|359 forum posts|
Thank you very much:
Michael - yes, I think there is enough steel in the spindle for that.
I would have either to use split bushes around sections reduced in diameter only in the journals, or reduce the full length and bush the pulley and back-gears, and modify the tail thread & collars. Err, I think split bushes!
Hopper - thank you for explaining the materials difference. While at it I would make a new thrust bearing pad for behind the nose, where the spindle flange has worn a recess some .03" deep.
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