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Posted by Joseph Gwinn on April 27, 2008, 7:34 pm
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Well, I measured the dovetails on the cross-slide, and they seem pretty
good, no more than a thousandth of wear on the male slide on the
carriage. The gib is visibly worn.
While I had the cross-slide off, I noticed two nubs that seem to ride on
the top of the male dovetail, leaving a mark. I was trying to install
the Cross Slide Shield (which goes between cross slide and the carriage,
slides back and forth, and keeps swarf off the cross slide screw), and
these nubs were interfering.
They were also preventing the cross-slide dovetail from fully seating,
which made the joint springy.
The nubs are the filed-off ends of the two hex socket flat head 1/4-20
screws that hold the Screw Cover to the cross-slide. They do not appear
original, were cut to length with a hacksaw, and are simply too long.
They are also in bad shape. Replaced both screws, filing the length so
that it does not protrude.
Put gib back in up side down (the gib is symmetric), so the worn surface
is no longer against the moving dovetail.
It does seem to help, though I have not tried to cut anything off.
I also had to tighten the compound gib.
The following was done before doing the above work on the lathe.
What I did try was to trepan through a 3.4" thick 6" by 6" piece of 6061
aluminum plate held in the four jaw chuck. The intended diameter of the
hole was 2.25". I was using a 3/16" HSS tool bit ground to suit the
purpose held in a BXA-6 toolholder.
There was some drama. At high speed (not back gear), it chattered.
With backgear, it didn't chatter but it did vibrate and generate flake
chips. The toolholder and toolpost were tilting visibly. What was
happening is that when the toolpost et al tilted, it dug the toolbit
deeper into the workpiece, in a vicious circle, so one could not gently
peel a nice chip off. You could see the toolbit itself bending. I
didn't know that HSS could be bent. Anyway, it eventually broke. Part
of the problem was the large overhang needed to avoid the jaws of the
4-jaw chuck; this overhand has considerable leverage, and together with
the height of the toolbit over the slide increases the depth of the
self-feed into the workpiece.
After working on the cross-slide, I tried face grooving again, and was
far more successful, although it still chattered, and I did manage to
break another 3/16" HSS tool bit.
I'm wondering if the whole carriage is tilting a bit, but the dovetails
are the first place to improve. I can test the carriage theory by
supporting the overhanging tool with a bar resting on the arms of the
carriage, or using a different holder.
Anyway, I did manage to cut the initial hole in the aluminum plate using
a hole saw, and had no problem boring the hole out to 2.670" diameter.
No chatter, no gouging. Nor did I have any problem facing the plate.
Joe Gwinn
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Posted by Jon Elson on April 28, 2008, 1:35 pm
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Joseph Gwinn wrote:
> There was some drama. At high speed (not back gear), it chattered.
> With backgear, it didn't chatter but it did vibrate and generate flake
> chips. The toolholder and toolpost were tilting visibly. What was
> happening is that when the toolpost et al tilted, it dug the toolbit
> deeper into the workpiece, in a vicious circle, so one could not gently
> peel a nice chip off. You could see the toolbit itself bending. I
> didn't know that HSS could be bent. Anyway, it eventually broke. Part
> of the problem was the large overhang needed to avoid the jaws of the
> 4-jaw chuck; this overhand has considerable leverage, and together with
> the height of the toolbit over the slide increases the depth of the
> self-feed into the workpiece.
>
Yes, you want the point of the tool as close to the center of
the compound swivel as possible, to prevent that positive
feedback of the force pushing the tool down making it dig in
harder. One trick is to move the toolpost to one end of the
T-slot so the compound is closer to the chuck. This may be a
problem with your chuck, a different way to mount the work might
help.
Jon
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Posted by Joseph Gwinn on April 29, 2008, 10:35 am
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> Joseph Gwinn wrote:
>
> > There was some drama. At high speed (not back gear), it chattered.
> > With backgear, it didn't chatter but it did vibrate and generate flake
> > chips. The toolholder and toolpost were tilting visibly. What was
> > happening is that when the toolpost et al tilted, it dug the toolbit
> > deeper into the workpiece, in a vicious circle, so one could not gently
> > peel a nice chip off. You could see the toolbit itself bending. I
> > didn't know that HSS could be bent. Anyway, it eventually broke. Part
> > of the problem was the large overhang needed to avoid the jaws of the
> > 4-jaw chuck; this overhand has considerable leverage, and together with
> > the height of the toolbit over the slide increases the depth of the
> > self-feed into the workpiece.
> >
> Yes, you want the point of the tool as close to the center of
> the compound swivel as possible, to prevent that positive
> feedback of the force pushing the tool down making it dig in
> harder. One trick is to move the toolpost to one end of the
> T-slot so the compound is closer to the chuck. This may be a
> problem with your chuck, a different way to mount the work might
> help.
I couldn't figure out how to do it with the tooling I have. This may
cause the purchase of new tooling -- this is the slippery slope of
machining.
I've been looking at a BXA-13L. I think DoN said he used a 13L for just
this kind of thing.
Another alternative would be a boring bar that accepts coaxial (versus
perpendicular or diagonal) bits. This would have the advantage of
adjustability, and I already have the boring bar holder (BXA-4D, holds
1", plus sleeve to accept 0.75" shanks). It would be easy to make the
coaxial bar on the lathe and mill - axial hole to accept the bit, two
perpendicular setscrews to clamp the bit in place.
The BXA-4D holds the bar *very* firmly, a 0.75" steel bar is far stiffer
than the 3/16 toolbit, and one can adjust for minimum lever arm
case-by-case, so this setup may be the best one can do.
Joe Gwinn
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Posted by DoN. Nichols on April 30, 2008, 1:06 am
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[ ... ]
> I couldn't figure out how to do it with the tooling I have. This may
> cause the purchase of new tooling -- this is the slippery slope of
> machining.
>
> I've been looking at a BXA-13L. I think DoN said he used a 13L for just
> this kind of thing.
I use a 13 for extending the support on threading tools so the
toolpost does not run into a fat live center in the tailstock.
And I postulated using a 13L for getting threading tools even
closer to the tailstock center -- but I don't have one, and I don't have
the three-dovetailed toolpost which would allow me to do that without
having to rotate the toolpost -- which is something that I try to avoid
as much as possible.
> Another alternative would be a boring bar that accepts coaxial (versus
> perpendicular or diagonal) bits. This would have the advantage of
> adjustability, and I already have the boring bar holder (BXA-4D, holds
> 1", plus sleeve to accept 0.75" shanks). It would be easy to make the
> coaxial bar on the lathe and mill - axial hole to accept the bit, two
> perpendicular setscrews to clamp the bit in place.
Hmm ... intersting.
> The BXA-4D holds the bar *very* firmly, a 0.75" steel bar is far stiffer
> than the 3/16 toolbit, and one can adjust for minimum lever arm
> case-by-case, so this setup may be the best one can do.
O.K. Good luck,
DoN.
--
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---
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Posted by Joseph Gwinn on April 30, 2008, 8:43 am
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>
> [ ... ]
>
> > I couldn't figure out how to do it with the tooling I have. This may
> > cause the purchase of new tooling -- this is the slippery slope of
> > machining.
> >
> > I've been looking at a BXA-13L. I think DoN said he used a 13L for just
> > this kind of thing.
>
> I use a 13 for extending the support on threading tools so the
> toolpost does not run into a fat live center in the tailstock.
Ahh. Now I remember. Given the axial boring bar approach mentioned
below, I will hold off on the 13L.
> And I postulated using a 13L for getting threading tools even
> closer to the tailstock center -- but I don't have one, and I don't have
> the three-dovetailed toolpost which would allow me to do that without
> having to rotate the toolpost -- which is something that I try to avoid
> as much as possible.
OK. We both have BXA toolposts, so I will eventually have the same
problem.
I suppose one could do the close-in threading using a boring bar to hold
the bit, but some toolpost rotation would be required.
This can also be used as an excuse to buy yet another toolholder.
> > Another alternative would be a boring bar that accepts coaxial (versus
> > perpendicular or diagonal) bits. This would have the advantage of
> > adjustability, and I already have the boring bar holder (BXA-4D, holds
> > 1", plus sleeve to accept 0.75" shanks). It would be easy to make the
> > coaxial bar on the lathe and mill - axial hole to accept the bit, two
> > perpendicular setscrews to clamp the bit in place.
>
> Hmm ... intersting.
>
> > The BXA-4D holds the bar *very* firmly, a 0.75" steel bar is far stiffer
> > than the 3/16" toolbit, and one can adjust for minimum lever arm
> > case-by-case, so this setup may be the best one can do.
>
> O.K. Good luck,
Thanks. I will try this. There will eventually be a report.
Joe
PS: Teenut wrote a posting on trepanning in a lathe:
<http://yarchive.net/metal/trepanning.html>. He had problems with
tipping and self-feeding in as well, judging from the part about
minimizing overhang and replacing the cross-slide with a solid block.
That said, as I dig deeper, I am finding things to tighten up in my
lathe, like the tale of two nubs that prevented the cross-slide from
resting firmly upon the carriage dovetail.
Trepanning has much in common with parting off, especially in how one
shapes the tool bit: <http://yarchive.net/metal/parting_off.html>.
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> With backgear, it didn't chatter but it did vibrate and generate flake
> chips. The toolholder and toolpost were tilting visibly. What was
> happening is that when the toolpost et al tilted, it dug the toolbit
> deeper into the workpiece, in a vicious circle, so one could not gently
> peel a nice chip off. You could see the toolbit itself bending. I
> didn't know that HSS could be bent. Anyway, it eventually broke. Part
> of the problem was the large overhang needed to avoid the jaws of the
> 4-jaw chuck; this overhand has considerable leverage, and together with
> the height of the toolbit over the slide increases the depth of the
> self-feed into the workpiece.
>