Tribological query re shafts and bushings!

The shafts and bushings on a friend's 920 lathe are showing a fair bit of wear. These are the shafts for the change wheels and the bushings are the keyed bushes which fit into the change wheels.

Chester has no stock, so I may have to make new shafts and bushings from scratch, which is not a major problem. But what to use in the way of materials? Both shafts and bushings appear to be mild steel, which could be one of the factors in the amount of wear.

I was going to make the shafts from key steel, those with 920 lathes will know why. The ends are 10 x 10 mm square which transits into 10 mm diameter round.

But what is best for the bushings? I have some 14 mm O/D by 10 mm I/D stainless tube which is a perfect fit for the bushings. At least it is a dissimilar material! or should I use brass or bronze?

Having machined most of the bronze family, I am not enthused, as I find it a difficult material to work.

I suppose the 920 lathe is definitely cheap and cheerful, so do I just use mild steel again? Suggestions would be very welcome.

Andrew

I would suggest EN16T or EN24T. They are both hardened a bit (typically 20-30 HRC). This actually makes them machine more nicely than either the annealed steel or most stainless steels. The slight additional hardness will work well to avoid damage in use and improve the bearing properties.

Most stainless steels tend not to be very good bearing materials at all.

Hopefully the mods won't object if I post a link to a supplier's site. Haven't used them, but this might be the sort of stuff that you need:-

**LINK**

I don 't know the particular lathe and its fittings, and no lathe called "920" appears on Chester's web-site now as far as I can see, so I would not be surprised at no replacement parts.

However the combination there is probably a mild-steel bush running on hardened pins, the latter possibly only case-hardened, but under controlled conditions to give sufficient depth for finish-grinding.

Normally the best candidates for bushes running on plain steel shafts are likely to be cast-iron and leaded gun-metal, but if I understand correctly, that these are the change-wheel bushes and studs, the bush is also effectively the shaft and has a key-way cut in it.

Hence two steels of different grades or different hardnesses are appropriate, and this is established practice. I have seen the bushes and keys for one lathe cut from single pieces of metal, though somewhat awkward to make.

You could make the studs from silver-steel of diameter close to the diagonal of the square (here =14.14mm)* and mill the square. That is in any case a better approach than turning from square stock as it avoids that horrible interrupted cut and you will be ensuring a full diameter. For best results, and assuming the absence of a cylindrical grinder, the finish should be lapped, using a simple split lap held in a die-holder.

You say a "fair bit of wear". How old in working time are they?

'

*As a general point, for any square of side S, the Diagonal = 1.414S.

From Pythagoras, that constant is the square-root of 2.

Given the relatively light loading and amount of lubrication the change wheel area generally doesn't get consider steel wrapped dry bearing bushes. Basically a thin steel sleeve with dry bearing material on the inside. Because the sleeve is wrapped from sheet its already split so cutting a keyway is easy. Accurate sawing with a junior saw does it for me.

Not up to date on current suppliers but I've been working through a small stash of Glacier DU ones for the last 40 odd years on moderate speed, moderate duty jobs. Haven't had to replace any yet!

Around £3 or £4 a pop in 14 mm OD. For example a quick google gives

**LINK** https://simplybearings.co.uk/shop/Bearings-Wrapped-Steel-Bushes-Wrapped-Steel-Plain-Bushes/c3_4701_4702/index.html

and

**LINK** https://budgetbearings.co.uk/shop/Bearings-Wrapped-Steel-Bushes-Plain-Wrapped-Steel-Bushes/c3_4799_4800/index.html

A bit of oil doesn't hurt.

Clive

Silver steel for the pins,mill the ends square. use leaded bronze for the bushes (do not use phosphor bronze) or try Oilite bushes if they list the size required. I would never consider steel on steel for bearings,which are only lubricated by oil can.

Chester 920 Lathe

Thanks everyone for their input,

The 920 lathe is still a current Chester offering. I have no problem in turning 10 mm key steel down to 10 mm round, leaving one end at 10 mm square This is needed for the securing U washer, to retain the gear. It is a much faster way of making the pin, much less turning is required.(You need to turn down a 15 mm round bar for the suggested method)and no second milling operation.

I understand that key steel is a relatively high carbon steel so should have some of the properties of silver steel.

The real problem is the bush. This can be bored internally to 10 mm but it cannot be turned to the 14 mm external diameter, because it has a "male" key (5 mm x 2.5 mm) on the outside, which engages the gear key way.

As to surface finish, I have seen the studs on a new lathe, to say they are rough is being kind!. They were not case hardened according to my warding file.

I can see no way of making the bushes in one piece with the kit that I have, so I will have to fabricate the bush, again no big problem. I will investigate Clive's suggestion and some of the materials that have been mentioned.

Again, thanks everyone.

Andrew.

We had fun with the apron of the Smart & Brown model A at the museum, everything was worn out. A lot of the bushings were replaced with sintered bronze ones modified or made directly from bronze bar. Three of the shafts so worn that they had to be replaced, and I used en19t or en24t, but didn't harden them. The apron has an oil pump which lubricates all the gears and bearings, plus the saddle and cross slide. With the pump restored and good old semi synthetic 5W40 there is little likelyhood of the wear repeating itself. If you cannot harden the new shafts, just make sure they are kept lubricated well in the future and you will be ok.

Could you set up the worn existing ones in the 4 jaw, bore them bigger by say 1. 5 mm diameter and then loctite a bush in. The Bush could be steel or colphos which machines very nicely

+1 on Duncan's suggestion of sleeving the original bushings. Oilite or leaded bronze/gunmetal bushings could be fitted too. Or even cast iron. All will run well on unhardened steel shafts.

Otherwise, if making whole new bushes you will have to machine a keyway into them and install a key. Sounds like wall thickness is not enough for that though if it is 10mm bore and 14mm OD? You would only have 2mm wall thickness. So you might have to go with 1mm wall thickness for any sleeve bushings you install. Machine the ID undersize and press/loctite them in then machine the final ID in situ. Bronze would work better for such thin walls than cast iron or oilite.

Edited By Hopper on 17/05/2021 05:29:08

Edited By Hopper on 17/05/2021 05:32:15

Andrew, are these what you are after, 10 mm bore, 14 mm O/D , 15.2 long with a 5 mm key? PM your address if want them.

Tony

Hello Tony,

They are indeed the bushes that I am after. A picture is much better than my poor description! I will send you a PM.

Thanks everyone.

Andrew.

How would they have been made in the factory? Diecast? Extruded? Sintered? They look an awkward and wasteful piece to machine from the solid.

Tony's kind offer solves the problem, but what's the best way of making an object like this?

My first thought was to turn a tube, then mill a slot, and braze or loctite a key into the slot:

Looks good except the joint is weak because the slot cuts into and thins the bush. The arrangement is very weak when the key is pushed longitudinally, so it's unsuitable if there's any end thrust. (I think a keyed bush made this way is fine for lathe change gears because they don't transmit much power and the thrust is all rotational.)

Could a stronger version be milled from solid by rotating the blank against a side cutter?

Although a solid bush would be stronger, I'm not sure if the finish would be good enough on the round part, and it would be harder to turn the radius accurately. Is milling from solid a good idea?

How else could keyed bushes be made? I'm wondering if the commercial items are extruded rather than machined?

Dave

Definitely not machined, on careful examination I think possibly extruded.

Tony

Or sintered & 'sized' using dies.

Tony

Extruded would be the "low cost hobby market" option most likely. Are they then aluminium or Mazak etc and not steel? Would explain why they wore out already.

SOD: I dont think there would be enough wall thickness to cut a keyway unless it was made very shallow, say 1mm. Might get away with it for change gears in low power situation. I dont think you could mill it and get a smooth diameter without making a million passes at tiny increments etc.

Edited By Hopper on 17/05/2021 11:36:21

No not ally or Mazak as it's magnetic to a degree. End structure looks porous so I'm thinking sintered with some sort of ferrous material but has a polished looking surface on all external features either from the sintering mould or a sizing die.

Tony

I think they must be sintered as they are definitely steel. Extrusion would be perhaps far too expensive for such a low cost lathe.

I was just about to start fabricating them when Tony kindly offered me the bushes he has. I have already made the shafts and with due deference to the good finish on the new bushes I will case harden the shafts. Should last a long time, hopefully.

Thanks to all, especially Tony,

Andrew.

I think they must be sintered as they are definitely steel. Extrusion would be perhaps far too expensive for such a low cost lathe.

I was just about to start fabricating them when Tony kindly offered me the bushes he has. I have already made the shafts and with due deference to the good finish on the new bushes I will case harden the shafts. Should last a long time, hopefully.

Thanks to all, especially Tony,

Andrew.