Parting with the rear tool post

I vaguely recall arguments that parting off from the rear tool post with the tool upside down is beneficial, but for the life of me I don't remember why.
Naturally the swarf can escape the cut easier but I am sure there was another reason. Can anybody enlighten me please?
Cliff Coggin.
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On Wed, 24 Dec 2008 10:36:48 -0000, "Cliff Coggin"

One reason that is often trotted out is that the natural flex of the tool/toolpost under cutting loads will tend to cause the tool to move away from the work in this configuration, rather than to dig in.
Regards, Tony
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On or around Wed, 24 Dec 2008 12:26:59 +0000, Tony Jeffree

you can, presumably, get the same effect by putting the machine in reverse, for ones that have reverse, and inverting the tool in the normal toolpost.
Only thing that strikes me, the (normal) toolpost is set up in general so that the top face of whatever size tool it's designed for is somewhere near the centreline of the lathe. The parting tool I use has about a 3/4" blade, so inverting it would require the holder to approx 3/4" higher than "normal".
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Cliff Coggin wrote:

The reason is because the tip of the parting tool is on the end of a cantilever that is supported by the tool-post. When the tool is held in the conventional way, the cutting force acts downwards and tries to rotate the complete tool, tool-holder and tool-post assembly forwards about the bottom of the tool-post. This results in the cutting tip moving slightly into the workpiece. If there is any play or flexing in the assembly the tip can dig-in with the usual result of a broken parting tool. If the tool is mounted upside-down on a rear tool-post, the same forces apply but the result is to move the tip slightly away from the workpiece and minimizes dig-ins.
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Pushes the spindle down onto the bearings instead of lifting.
John S.
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I don't follow this John. Are you suggesting the spindle can move upwards more than downwards?
Cliff.
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Hmm, I had to ponder that a while. It makes sense if you assume the tool and assembly are a rigid lump that pivots only at the bottom of the tool-post, but is that a reasonable assumption? Doesn't it make more sense for the tool to flex downwards and thus away from the centre line of the work?
I ask because I want to decide if it is worth the effort and expense of fitting a rear tool-post.
Cliff.
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Oh it is Cliff , well worth the trouble. It has the added advantage it can usually be left in situ ready to use.
AWEM
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I would have agreed until I got a Q-cut. This is so good that I don't have a problem any more, and with a Dickson toolpost it only take 3 seconds to fit anyway, so IMO the risk to skin of a permanent rear tool is no longer worth it.
David
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On Wed, 24 Dec 2008 18:15:44 +0000, David Littlewood

I'll second that opinion on a Q-cut. Fit one of those and you can part entirely without trepidation. Only downside, and not really that big a deal, is that the parting insert tip is about 2.5-3mm wide.
Peter
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3 mm? That's bigger than some the whole parts I turn.
Cliff.
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Cliff Coggin wrote:

Then you also don't need a rear tool post. :-)
Nick
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Nick Mueller wrote:

I use a parting tool ground from a 6x6 mm toolbit in the rear toolpost on my Cowells 90ME lathe. It works like a charm.
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Peter Neill wrote:

I considered buying a Q-cut - but as a beginner my experiences with carbide tooling have been expensive - indeed I recall threads here on how HSS was a benefit.
I had no desire to spend money on an expensive tool if it was likely to go the way of my other parting tools.
So I recently made a rear toolpost and at the first test parted off half a dozen discs from a 1" bar which i hadn't managed to do from the front at all.
Perhaps now I'll think about a Qcut again and put it at the back.
Russell
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Russell wrote:

Now you know why they are called *parting* tools. :-))
Nick
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Cliff Coggin wrote:

I think its easier to buy a good parting knife.
Nick
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John S is the only one that got it right. A word of warning if using the rear tool post, PLEASE use a safety screen, if the tool snaps it will be coming towards you. Have seen a few one eyed turners in my working life

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I'm still not sure I understand why pushing down on the spindle is better than pushing up? Sort of implies that if the lathe was mounted upsde down then the front toolpost would be best.
What does seem certain is that one reason the rear tool post works better than the normal tool post is its rigidity. If I look at the parting tool in the normal tool post, it's quite a way to one side of the toolpost bolt (one lever) and then the tip is quite a way forward from the toolpost bolt (second lever), which seems exactly not what to do with a parting tool. The rear toolpost reduces both of these significantly.
Merry Christmas
Steve
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Steve wrote:

Makes a difference when you do have play in the spindle bearing. But who has that, or even wants it?
When the parting tool is on the rear, the spindle is pressed down by gravity and cutting force. Normal position, gravity forces spindle down, cutting force up.
Nick
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On Wed, 24 Dec 2008 18:50:34 +0100, Nick Mueller

Not true for Myfords. Vee belt tension holds the spindle UP.
Jim
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