Truing a hardened half centre on a Super 7

This topic has 12 replies, 10 voices, and was last updated 6 November 2020 at 16:18 by Peter G. Shaw.

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4 November 2020 at 14:09 #505292
Mike Donnerstag
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@mikedonnerstag
Perhaps I'm nuts even attempting this. I have a half-centre that had been badly ground from a normal (full) centre by a previous owner . The centre wasn't true, nor was it at 60deg to match a centre drill.

I tried using a tool with a CCMT insert to attempt to true the centre. I quickly realised that that the centre was hardened (that's probably when I should have given up!) Anyway, I persevered… I increased the rigidity by locking the carriage and cross-slide and tightening up the gib screws on the top-slide a little. I ran the lathe at 420rpm and set a depth of cut of around 10thou, using Rocol RTD to cool and lubricate the cut.

I ran the top slide slowly from the large to the small diameter at first, though the interrupted cut soon broke the tip off the insert. I tried another insert, this time a Kennedy, as the first insert came 'free' with a cheap eBay insert holder. Again the tip soon broke with the interrupted cut.

Not wanting to be beaten I then tried a third corner (turning the Kennedy insert 180deg), this time running from the 'solid' small diameter to the larger diameter. It cut well until it got halfway up the edge, and once again the interrupted cut broke the tip. However, the centre is now usable.

I assume the ideal way would have been to use a toolpost grinder, or perhaps I could have annealed the centre and re-hardened once turned, though that would probably lead to distortion?

Looking at the cost of centres, I obviously spent more in carbide tips than the cost of a new ground centre. My question is, had I attempted the impossible, or is there anything I did wrong or could have improved? I'd really be very interested what people think.

Many thanks,
Mike
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4 November 2020 at 14:09 #20010
Mike Donnerstag
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@mikedonnerstag
4 November 2020 at 14:20 #505294
Speedy Builder5
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@speedybuilder5
I would attempt it with a tool post grinder (as you said) – even a DREMEL type tool strapped to the tool post. Also, though I am not an expert, you would need a much higher speed if you are using tool inserts especially on small diameters like your centre.
4 November 2020 at 14:27 #505296
Anonymous
No, it isn't impossible. The key is to get the shear zone red hot so that it is relatively easy to shear. So that's mistake number one; don't use coolant as it does the opposite of what you need to achieve.

One needs to run fast, at a shallow depth of cut and a decent feedrate. A while ago I posted about machining hardened silver steel. The test bar was 25mm diameter and hardened to between 60 and 65Rc. Cutting parameters were 1200rpm, DOC 0.2mm and feedrate 0.1mm/rev. The swarf came off orange and was fine, like steel wool, but crumbly. An ordinary (professional grade) carbide insert worked ok, but a CBN insert didn't even blink at the cut. Mind you the operator might blink at the cost of a CBN insert.

Andrew
4 November 2020 at 17:08 #505321
peak4
Participant
@peak4
When I did one some years ago, I used a Bosch woodworker's router in a custom holder in the toolpost
This held a conventional mounted grinding point; just make sure you purchase one what's suitably rate for high rpm, rather than a cheapo 3000rpm one intended for an electric drill.

Bill
4 November 2020 at 17:42 #505324
Dave Halford
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@davehalford22513
Some good carbide tips are NOT rated for intermittant cuts.
5 November 2020 at 00:51 #505385
Hopper
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@hopper
With the cost of centres today, you may have not quite attempted the impossible, but certainly the pointless.

Toolpost grinder would be the way to do it. Even if you struggle along and and get an insert that cuts hardened interrupted cut etc the surface finish is not going to be as smooth as grinding so friction in use will be high and probability of overheating and failure commensurate.

Give up while you are ahead.

Edited By Hopper on 05/11/2020 00:55:53
5 November 2020 at 22:30 #505528
Mike Donnerstag
Participant
@mikedonnerstag
Thanks to all who posted. Though I wouldn't do it again, I've learnt a bit more about using carbide on hardened steel, and what is possible on a Myford.

For interest, what do you chaps think about hardened vs. carbide-tipped centres?

Mike
6 November 2020 at 09:12 #505551
Clive Brown 1
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@clivebrown1
On the few occasions that I've used a hardened centre, I'm always surprised how easily they can be damaged. Although I've considered buying a carbide tipped centre, I never have treated myself to one.

In practice, a rotating centre does for me 99% of the time, i wouldn't be without one.
6 November 2020 at 12:12 #505575
Mike Poole
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@mikepoole82104
Care is needed using a dead centre, lubrication is marginal, tallow is an old solution but works as when it warms it melts to supply more lube, applying too much pressure with the tailstock will quickly ruin the centre, as the work warms up it will expand very slightly so recheck the tailstock pressure. Carbide can help but keeping it lubricated it vital. A rotating centre can be very good but cheap and good is rare. Top quality like Gepy will cost serious money but you may get lucky with a used one.

Mike
6 November 2020 at 12:29 #505577
Steviegtr
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@steviegtr
These came with the Super 7. Not wanting to seem a complete idiot, but what would you use a half centre for.

Steve.
6 November 2020 at 13:38 #505591
Anonymous
Posted by Steviegtr on 06/11/2020 12:29:01:

……..but what would you use a half centre for.

It's for when you need to get the tool close enough to the lathe axis to face the end of the work without leaving a pip. I've only ever turned between centres once (crankshafts) and even then I used a rotating centre in the tailstock. My rotating centre is low end professional (Skoda) and is fine for what I do.

I use live and dead centres a lot on the cylindrical grinder. Interestingly the dead centre isn't in the best of shape, but I can still grind parallel to better than a tenth. Another oddity is that the dead centre is plain, the live centre is carbide.

Andrew
6 November 2020 at 16:18 #505615
Peter G. Shaw
Participant
@peterg-shaw75338
Well now, much to my surprise, it seems that what I did, as an experiment a few months ago, might well be acceptable.

I have a number of centres which had become damaged. Ok, mea culpa, must remember to lubricate more! Anyway, just on spec, I mounted one of the damaged centres in a MT3-MT2 converter and plugged it into my headstock (MT3). After all, I had nothing to lose, and everything to gain. I then mounted one of Greenwood Tools NJ17 CCMT inserts, angled over appropriately, and proceeded to clean up the centre. From memory, I think I used the obtuse corner in a SCBCR type holder. And, much to my surprise, it turned beautifully, so much so that I then went on to clean up the remaining centres with complete success.

Peter G. Shaw

p.s I have no connection with Greenwood Tools other than as an occasional buyer of their inserts.
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