I think the fundamental problem with that is that parallel threads only exert axial and not radial pressure with any great efficiency.

I don’t think you will get it to be a tight enough fit to eliminate flexing under cutting loads, without it being almost impossible to undo.

If there’s enough ‘meat’, machining off the old thread and shrinking/pressing on a sleeve to receive a new or modified nut would be a much better solution.

Edit; using the existing thread to receive a slightly tapered / ‘unfinshed’ nose extension on a ‘once-only’ basis might be possible, but will leave you more stick-out, probably not a good thing..

On 27 August 2024 at 07:17 Hopper Said:

…
Very hard to measure how much thickness you need though. Measuring the OD does not tell you that. It will be undersize for tip/root clearance, but it is the bearing surfaces on the flanks of the V groove that take the load and will need building up. …

For which reason, it might be easier to deliberately over-plate the thread thickly, and then machine it back to size with a die or lathe.  Removing metal accurately is generally more straightforward than adding it!

More fundamentally though, is this repair likely to be satisfactory?  Gut feel from the comfort of my armchair, is that a precision speeder running in the 10000 to 40000rpm range that’s sustained metal cracking damage is likely to misbehave under load.    The speeder might seem OK spun up off-load, but will it cut accurately?   Might get an idea by padding the thread more-or-less thickly with PTFE plumber tape and experimenting.

Plating will cost time and money and results in the sort of repair I would hope not to find on a second-hand tool bought from ebay!   Adam is the man on the spot though; he’s best placed to judge if plating is a brilliant fix or a horrible bodge!

Dave

Sorry to say I’m absolutely sure plating is not your answer, some sort of sleeve over the nut may work though.

Tony

On 27 August 2024 at 09:23 Tony Pratt 1 Said:

…some sort of sleeve over the nut may work though.

It is the spindle, with a male thread and tapered hole up its centre, that is cracked. Sleeving the nut, which is undamaged and has a female thread, will not achieve a great deal.

It has already been touched on twice above, but the whole idea is misguided from a mechanical point of view.

Look at the degrees of freedom of the three parts (spindle, collet and nut) and how each degree of freedom is eliminated.

Think of a lathe chuck that goes onto a threaded spindle nose. The register provides concentricity; the threads are supposed to be loose enough so that they only provide axial restraint. The collet/nut/spindle assembly works in a similar way.

If you tighten up the thread clearance so that it does attempt to provide radial as well as axial restraint, it will fight whatever mechanism is already in place to assure concentricity.

Tightening up the radial clearance of the thread will also tighten the axial clearance and could lead to skewed contact (unequal pressure) between the face of the collet nut and face of collet. You need a (relatively) rattling fit on the thread in both directions so the taper in the spindle is the only thing influencing the collet’s radial location and the face of the nut is pushing it smoothly and evenly into that taper.

Have you considered tightening a collet to open the crack, apply one of the mega-strength Loctites (or whichever brand you prefer) like 601 or 648 ( exact to be proposed by more expert than me, then swiftly remove collet to allow crack to minimise and the Loctite to (hopefully) permanently re-join the crack.

If the crack opens when the collet is pushed in this could be worth a try it’s non destructive and relatively simple non destructive and no heat

1 machine a dummy collet

2 put a flat on the taper

3 clamp in situ to open the crack

4 dribble Locktite on crack and it may wick into the crack

Edit…….opps already mentioned above. didn’t see it!

On 27 August 2024 at 13:54 Adam Harris Said:

Might be a good idea Michael – they do an SK6 collet which takes up to 6mm cutter which would be good enough for me. I could make an SK6 taper sleeve (same taper externally and internally) and glue it into the SK10 sleeve somehow. Then put some weld onto the nut nose and bore it out for the SK6 collets. What glue/anaerobic retainer would you recomend? I could hold the crack tight closed while gluing with an appropriate jubilee clip. Do you think the wall of the SK6 sleeve would be thick enough for purpose?

I would think it worth a try

… up to my ears in domestic stuff at the moment, so [sorry] can’t go searching the Loctite specs.

There is one that’s nearly as strong as 638, but’s more fluid

MichaelG.