Parting with the rear tool post

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

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.

Pushes the spindle down onto the bearings instead of lifting.

John S.

On or around Wed, 24 Dec 2008 12:26:59 +0000, Tony Jeffree enlightened us thusly:

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".

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

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.

I don't follow this John. Are you suggesting the spindle can move upwards more than downwards?

Cliff.

don't

tool-post,

Oh it is Cliff , well worth the trouble. It has the added advantage it can usually be left in situ ready to use.

AWEM

I think its easier to buy a good parting knife.

Nick

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

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

In article , Andrew Mawson writes

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

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

Jim

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

Cliff.

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

Nick

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.

There's always a little play in the spindle bearings as you still wants the spindle to be able to turn. Even the tiniest amount of play matters.

I'm certain, that my spindle bearings have 0.000 mm play. They are preloaded.

Nick