Thanks Fred,
I had wondered if the length inside the spindle could be a problem,
which was why I made a point of mentioning the length. But I hadn't
heard any signs of distress as I spun it up through different speeds on
the way to 2000, so I didn't worry too much about it. I'll try all this
again with a shorter piece.
Is coolant per-se going to affect the chip/chipbreaker interaction
(maybe by cooling the chip?), or are you saying that using coolant may
help in achieving the appropriate DOC and feed rate to make the
chipbreaker work?
Roger
ff wrote:
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.
Ok, that 500mm long length of stock inside the headstock; when it's
spinning at 2000rpm, it is resting against one side of the tube due to
centrifugal force which cocks the 50mm long part you are turning
slightly the other direction. Your tool turns it straight under those
conditions. When you stop the lathe, the whole bar straightens back out
and when you turn it slowly with the dial indicator on it, you see the
runout that was there at 2000rpm before you cut it straight.
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
Steel cuts cleaner at higher rpm. At lower rpm it's tearing more than
cutting. If the chipbreaker is not working, the feedrate is not enough
or it could be too much depth of cut for the feedrate. Without flood
coolant it's going to be hard to get good cutting conditions without
cooking your part.
Fred
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