Ep 14, Dremel on a lathe: replacing swingarm bearings

The winter months are a great time to catch up on bike maintenance, so I have finally removed the rear suspension from my Cagiva Elefant 750. I bought the bike secondhand a few years ago and was prepared for the worst when I finally got round to this task.

And the worst I did find. Bone-dry bearings in the linkage had deeply scored the hardened steel bush/spacer/shaft that runs through them. And being from Cagiva, this part is nowadays ‘unobtainium’ – you can’t buy it anywhere. Spin one up on a lathe, then? Well, once the part is made the steel needs to be hardened – not something that’s really feasible in a home workshop, at least not if you want the part to last.

Thankfully I found out that some Ducatis have a similar shaft, albeit longer. Trent, the Garage Night elder, confirmed this by checking on his Monster.

So, how to cut it down to fit my bike? You can’t turn hardened steel on a lathe, you can’t cut it with a hacksaw. You need to grind down hardened steel using an abrasive disc, and hitting it with an angle grinder would not allow the required precision.

After much searching I found this diagram of a simple bracket to attach a Dremel grinder to the toolpost of a lathe. Trent agreed to set this up on his own lathe – he really is a great friend to have.

This video shows the successful results. My swingarm’s just been powdercoated, the bearings have been ordered and we’re planning our next video of the process.

– Waz

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16 thoughts on “Ep 14, Dremel on a lathe: replacing swingarm bearings

  1. Ouch! If you’re toolpost grinding on the lathe, *protect the ways* – quickest way to wreck the poor thing, even if the bed’s hardened, is to bombard it with grinding dust (a mixture of abrasives from the wheel and steel, nice) so next time you run the carriage up it starts wearing away all those nice precise bearing surfaces…

    Just lay a piece of cardboard or something where the sparks land next time!

    Another thing, with that setup you should really be running the lathe spindle in reverse (if you have it!) so the work and the grinding wheel or disc are moving in opposite directions where they meet, saves breaking (quite so many) wheels :o)

    Dave H.
    (The engineer formerly known as Homeless)

  2. Dave and JW2, thanks for the reminder about lathe cleanliness. Yes, I did think about counter-rotating the lathe chuck with the grinding disc – but on the day i forgot to ask Trent about it.

  3. Spin one up out of 4140, which is NOT that hard to harden (pun NOT intended) or use what we call (in the USA) O-1, W-1 or A1 drill rod – Think you guys call it Silver Steel (as opposed to BMS). Anyway, and of those is fairly easily heat treatable in a well equipped basement shop (as you guys seem to be)

  4. Well, at least around here, Ducati parts are hard to get, and it was more meant as a "heat treating isn't as bad as you might think"

    Here in the US, we have a steel called 1144 Stressproof – I'm NOT sure what the equivilent is over there, but it machines VERY well, does NOT warp when milled (hence the 'stressproof' name), and hardens like a dream – heat it up red hot (to the point a magnet doesn't stick – there is a trick for you – when carbon steels are heated to the point a magnet doesn't stick, they are above the critical temp), and plunge into warm oil, then temper as needed

    The "drill rod" (O-1, W-1 over here) steels also harden very easily, but are a bunch harder to machine

    Just trying to bring out other options for a small shop when heat treating MIGHT be necessary (BTW, O-1 etc will get hard enough to make cutters out of, if you need to)

  5. @KG2V, Fair enough, and it’s good stuff to know for an old Cagiva owner because for us some parts are unobtainium no matter where you live!

    The main problem I see, though, is how to dress back the steel once you’ve quenched it so that you get the right bearing clearance and a good enough surface for a bearing. As I understand it, when you harden steel in this way you are forcing carbon into the surface molecules and slightly enlarging the piece, so you need some way to grind it back to the size you require, and to polish it up to a good enough surface for bearings to run on. This seems to me the most technical part of the operation.

    Interested to hear more from you.

  6. There will be SOME change in diameter, and it depends on how tight a tolerance you really need to hold, the change isn't that much. Carbide tools can cut harded steel, believe it or not, OR you can use a spindle grinder – which, belive it or not can be jury rigged out of… A Dremel in the lathe (well, not a very good one, BUT

    For outside work, you might just chuck it up, and polish with a bit of sandpaper

    If you want to learn ALL sorts tricks, go to our news stand and start reading Model Engineer. GB is probably the "Mother Country" to the hobby of Home Shop work – Geo Thomas wrote some great books on how to do all sorts of operations on the lathe/mill, often making the tools yourself. Unfortunately for those of us 'over here', the books are quite expensive by the time you ship them

    All I can say is "try it", particularly when you have some spare time, and on some non critical pieces first

  7. id say you started losing the wheel because of the inside (you have a direct right angle)of the edge of the bush was eating the cutoff wheel….nothing to do with the hardness/surface just the angle of the egde

  8. Thank you very much Andrew, made my day. I have heard about this bike before and seen a few pics around the place. I think it may even have made its way into an Aussie bike mag.

    I have two Elefants now: my 750 and a 900IE. I don’t think I’ll ever get either of them looking that good, but I’ll keep working on it!

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