No help with your question, but what brand of Lathe is this? I have
been thinking about a 14 X 40 geared head lathe for some time. But I
have heard that, even though many of them come from the same place, the
quality "control", finish and details of some brands is a lot better.
Any comments?
Pete Stanaitis,
western Wisconsin, USA
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rlincolnh wrote:
I hope some old hands can offer me some advice he
I have a new Chinese 14x40 gearhead lathe, and it seems to be a
reasonably solid unit although I'm still just finding out what I can do
with it.
Question 1.
I have chucked a 500mm length of 25mm 4140 with about 80mm extended from
the jaws. I can turn 50mm of that down to 17mm in a single pass at 2000
rpm and get a good finish (see second question) with a carbide insert.
It's like a knife through butter, the machine is totally smooth and
feels and sounds like it could do it all day. After letting it cool down
a little I use my Mitutoyo 0.001mm vernier micrometer to measure the
work - ok, the absolute value is a little out, no surprise, but the
diameter is constant to within a small fraction of 1 micron i.e. approx
0.0002/0.0003mm along the length of the 50mm cut. It's a bit hard to
visually interpolate between the vernier divisions on the micrometer,
but it's *way* less than 0.001mm variation. Then, just for the hell of
it, I mounted a DTI (0.01mm per division) on the carriage and rotated
the chuck by hand. Shock and horror - I see just under 1 division (~
0.008/0.009mm) fluctuation per rev. It's totally consistent, and if I
move the carriage along the length of the 50mm the DTI behaviour remains
identical. I ran the lathe at the lowest speed (70rpm), and the DTI
response is exactly the same, right along the 50mm length. I removed the
DTI and set about measuring diameter again, rotating the chuck 30
degrees, measure, rotate another 30 degrees, measure, etc. Dead
circular. Did that at several places along the 50mm length and every
measurement is within that fraction of a micron.
So here's the question; How is it that I can produce a perfectly
circular result, yet which appears to be off-centre from the spindle
axis? For the life of me I can't visualize the conditions that would
lead to this.
For the record, I took another pass again at 2000rpm but with a 0.5mm
DOC. The result is exactly the same.
Question 2.
As I said above, the cut was taken at 2000rpm. I couldn't get an
acceptable finish at any speed below that, and it didn't matter whether
the DOC was 0.5mm or 3 - 4 mm. Even more strange (to me, at least) is
that the finish that I did get with a nominally 'finishing' insert
(larger radius tip) was worse than with a smaller radius tip that is
listed for 'medium' turning.
The inserts are TaeguTec CCMT 09T304, MT chipbreaker, TT5100 carbide,
and CCMT 09T308, FG chipbreaker, TT5100 carbide, both in an SCLCR type
holder.
The chip breakers don't work, possibly because of the speed(?). I get a
single continuous streamer of swarf flowing up and back towards the
tailstock before gravity takes over after a couple of inches and it
curls down onto the cross-slide. Inevitably it falls forward towards the
chuck and if I don't stop the carriage feed (thereby breaking the chip)
the tangle of swarf gets caught up around the workpiece and mars the
finish.
I almost don't even know what question to ask here. I would have
expected that I could get a good finish at a lower speed, and I would
have expected the chipbreaker to work. I would have expected the larger
radius insert to give a better finish, not a worse one.
I will be getting some HSS to gain experience with grinding my own
tools, but that won't happen for a month or more because my 'real' work
is consuming me night and day at the moment.
All observations/suggestions will be gratefully received.
Roger
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