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turning hardened steel

Drive shaft

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Kenneth Deighton18/12/2018 19:44:54
67 forum posts

I have been asked to reduce the diameter of a motor cycle drive shaft from 20mm to 18mm , as you will guess the shaft is hardened steel. Can I soften the shaft with heat treatment or how do I reduce it by normal turning in my Myford ?.

Thanks .

Andrew Johnston18/12/2018 20:10:40
5410 forum posts
627 photos

If you soften it you'll need to reharden afterwards, if possible.

I don't know if the Myford will have enough grunt, but it is possible to turn hardened steel with carbide inserts, although it's a one off use of the insert. Or you can use a CBN insert, but sit down before you look at the price. In both cases you need to run fast, with shallow depths of cut and a sensible feedrate. See this thread for real results on hardened silver steel:


The basic idea is to get the shear zone to high red heat, so the metal is locally soft, without affecting the bulk material.


Clive Foster18/12/2018 20:16:51
2154 forum posts
73 photos

Best to find out the exact process used to make it hard before starting work. Depends on the age and breed of the machine. On anything reasonably old a drive shaft is more likely to have been case hardened, nitrided or similar surface treatment rather than through hardened by heat treatment.

Through hard components tend not to deal well with millions of torque reversal and have a short critical crack length also if a crack does start rapid failure is likely. Not good for a drive shaft.

Generally surface layer hardening treatments can't be (simply) undone. I have ripped a case hardening layer off by seriously leaning on the component with a carbide tool. Tool was pretty much ruined in the process but it got under the skin allowing the core to be trimmed up and a hard tube loctite fitted to repair. All at customers own risk!

If it is truly hard I doubt that a Myford will cope. Best to find someone with grinding facilities really.

if its modern no telling what it is or how its made. Some very sophisticated metallurgy on the mass production lines these days.


Edited By Clive Foster on 18/12/2018 20:17:37

Neil Lickfold19/12/2018 07:01:32
593 forum posts
102 photos

If you have the TNMG16 tool holder, and get a tngg160402r-s pr1125 insert from Kyocera.

Another good insert is CCGT09T302MP-CK for the 09 holders or the smaller 06 insert CCGT060202MP-CK I have the tnmg16 holders and the ccgt06 holders and inserts.

These inserts will cut steel to 60 Rc at 30m/min surface speed and feed rate of 0.05mm /rev (2 thou) and a depth of cut max at 0.2mm Ø0.4 and finish passes at Ø0.05mm at 400 rpm to 480rpm . Use cutting oil or coolant.

If you have the 1/3 hp motor you may need to make the cuts at 0.1mm deep Ø0.2mm but keep at the 2 thou per rev feedrate.

These inserts can handle the interupted cutting as well.


Grizzly bear19/12/2018 18:33:50
232 forum posts
8 photos


What's the reason for reducing the drive shaft?

Are you talking about the whole shaft or the splines?

Sounds drastic.


Kenneth Deighton19/12/2018 19:29:23
67 forum posts

Hi Grizzly Bear, the job is for a friend who is building a "special" motor cycle and does not want the splines touching., I don't even know what make of bike it is from. I think that it will be cut in half and welded to another bikes drive shaft.


Vic19/12/2018 20:58:39
2494 forum posts
14 photos

I turned some HSS rod with a TNMG insert a while back, finish could have been better though! blush

It was asking a bit of my 8 x 14 lathe though.

duncan webster19/12/2018 23:43:58
2543 forum posts
49 photos

If it is hardened and then your friend is going to 'cut it in half and weld....' he is asking for trouble. Welding harden-able steel often gives rise to a brittle heat affected zone unless the welder really knows what he is doing and follows a rigorous controlled heat-up/cool-down procedure

Edited By duncan webster on 19/12/2018 23:58:10

John Reese21/12/2018 00:14:58
836 forum posts

I have cut hardened steel and even Stellite on a 9" South Bend lathe. The first time I did it was with brazed carbide tools. Later I did similar work with carbide inserts. The South Bend wasn't really rigid enough for that work, but it got me by. Most inserts for hardened steel are honed to a fairly blunt edge. That is definitely not what is needed on a hobby sized machine. A sharp cutting edge is desirable to keep the cutting forces low. Unfortunately a sharp edge in hard material results in short tool life.

Howard Lewis22/12/2018 14:39:49
3146 forum posts
2 photos

Turned down some spacers for a milling arbor. Inch and a half diameter. originally. Looked like they were case hardened.

Used a CCMT0604 insert and ran at about 500 rpm with 0.025" cuts. The swarf came off as a continuous red hot wire!.

The theory behind carbide tooling, is, I believe, to get the metal so hot that it softens to the point where it can be cut.

Hence the blue and brown swarf!


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