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.
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
Austin Shackles. www.ddol-las.net my opinions are just that
Travel The Galaxy! Meet Fascinating Life Forms...
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.
Regards, Gary Wooding
(To reply by email, change feet to foot in my address)
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.
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.
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.
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
Perhaps now I'll think about a Qcut again and put it at the back.
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
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.
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.
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