My Speedway 7x12 Bench Lathe
Update: 2003.05.04
Note Well!
I am not a professional machinist, nor have I ever been one. Prior
to purchasing this lathe, I had used a metal cutting lathe precisely once
in my life, for about two hours; that was in 1980, and the lathe I used then
was a monster compared to this small, inexpensive machine tool. Don't
take anything written here as a recommendation -- in a machine shop, I'm not qualified to recommend
anything other than following the manufacturer's instructions and seeking competent advice.
Please ignore my thoughts in the previous entry on an alternate method of center drilling
work. I got a chance to try the method, and it doesn't work, at least
for centers approximately 1/4" to 3/8" surface diameter. Here's how
I know:
I wanted to get started on my first real turning project, a set of game pieces
that will be turned from aluminum and brass (eventually including a board
with faced squares, recesses for matching rims on the piece base, and magnets
in pieces and board squares to hold the pieces in place). Having seen
an illustration of C3 collets being made in a pair, from a single bar, that
looked like an efficient way to proceed with minimum waste of the stock (1"
6061-T6511 round extruded aluminum, $3/lb, and close to one pound per foot),
so I cut off a bar about eight inches long (two pieces, each 2.5" tall, some
waste in between, and an inch for the chuck). This I carefully chucked
in the 3-jaw, the (presumably square, factory cut) end hard against the chuck
face since it was too big to enter the center hole. First thing I found
was that, practically speaking, this was about as long a piece as my "twelve
inches between centers" lathe could work with the tailstock mounted. I
had to remove the threading indicator from the carriage and dismount the
tailstock to get a tool bit to the end of the bar -- but the piece was pretty
steady at low RPM, so I went ahead. I don't yet have a dial indicator,
but I checked the runout against a tool point, gave the high side a couple
bumps with the heel of my hand, and when it was as good as I could see I
tightened the living daylights out of the chuck. I could just get the
threading bit to the end (I'd previously checked it against the dead center
and shimmed it to center height -- extra critical, since I'd be doing what
amounted to a concave facing cut).
Okay, spin the piece at what looked like 300 RPM or so, and with the compound
set at 60°, I started to advance the tool into the end of the work, around
.120" out from center. At first it cut okay, but then it started to
crowd the end of the work off center and stopped cutting. Back off,
try again with a slower approach (I'd already found that helped to get a
clean end in facing a long part). Again, it cut okay at first, then
started crowding and stopped cutting. A close look (with a flashlight,
since I also don't yet have good light at my lathe bench -- it is
a basement, after all) showed the problem -- the cutter didn't have enough
relief, and the steel support under the carbide insert was rubbing on the
inside of the cut below the point.
Okay, plan B: I set up to drill the end of the piece to 5/32", thinking I
might be able to cut the countersink after drilling the hole. Mount
the drill chuck in the tailstock (yep, I still haven't gotten the MT2 arbor
shortened or drilled, so I had to extend the ram an extra 3/4" beyond the
zero mark). Take the tailstock off to mount the chuck, since there
isn't enough room with the work in place. Take it off again to get
the twist drill into the chuck. Run it up as closely as possible to
to the work (which was still hanging off the tail end of the ways by a fraction
of an inch), add cutting fluid, spin the work, and advance the drill. After
about five tries, using a very slow feed, I got the drill to cut on center
(essentially using the springiness of the drill's shaft to get it to cut
away the center bump left by the attempt to countersink with the threading
bit), and drilled in about 1/4" or so. Take the tailstock off again
to dismount the drill -- got the drill out of the chuck, but the chuck doesn't
want to come out of the taper. Tightening the setscrew in the tailstock
handwheel solved that problem. Now, try again to countersink the the
existing hole. No go; threading tool just won't penetrate far enough
in that small a cutting radius. I don't have a boring bar, so I have
to get creative.
Get the Dremel (a Model 275, which is approaching 30 years old -- eventually,
I'll make an adapter to mount it in the tool post). Take the tool holder
off the compound and hand guide the Dremel with the edge of the top slide,
using the smallest cylindrical cutter I have to cut the countersink (with
the work turning, so I get a figure of revolution). Yep, that seems
to have worked, and the angle looks right relative to the dead center. Remount
the tailstock, with the center in the taper, oil the center and snug up gently
to the work. Spin the work, and yep, it spins. I don't think
the tailstock is aligned quite right; the work seems to pull in toward the
operator side of the lathe when I tighten the center into the countersink,
but I haven't even got the paint off the setover ways yet (why do they paint
that part, anyway?) -- and I'll need a test bar to find out in any case,
which in turn requires I be able to cut a proper center in the work. Never
mind.
Got the work turning; faced the centered end, first, with the edge of the
threading tool (I wouldn't try this with a HSS threading tool, but with HSS
I'd grind a special facing tool); I had to coordinate the cross slide and
compound to get the tool into the work as closely as possible to the center
and then face outward; with the tool angle I had set, that gave a very nice
surface finish, but required far too much fussing to start the cut and wouldn't
permit me to face inward. Then I started a power feed truing cut with
a right hand tool, about .020" deep to get under some surface nicks. About
halfway down the piece, I notice the lathe motor laboring a bit -- slowing
down, then speeding up again. This continued intermittently for the
remainder of the cut; each occurrence accompanied by one or more grooves
around the bar as the tool cuts deeper at the slower speed (the work still
deflectts when supported by a center, it just deflects most in the middle
instead of at the free end). Get to the chuck end, disengage the power
feed before contacting the chuck jaws (which I don't have any trouble seeing,
but I can see where a distraction at this critical point could lead to a
crash), back the tool out of the work, and shut off the spindle. Run
a fingertip over the cut surface to check the finish -- hey, that's hot!
Now I understand what the books say about easing the center and relubricating
periodically -- I hadn't thought that would be a problem on a cut that
only took a couple minutes, in softish material at a low cutting speed. The
laboring of the motor was caused by binding as the work piece expanded (probably
by .010" or more in length over that 8" bar), which increased friction at
the center and expanded the work still more, and so forth. Heat from
the tool probably didn't help (I've got it angled so I can turn and face
with the same setting, which reduces the front relief to around 5° instead
of the 10° or so that's nominal for that tool -- improves surface finish,
but most likely increases heat loading a bit), but hasn't even been noticeable
on previous cuts, including a fairly heavy cut in suface hardened steel when
I tried to repair a drill press circle cutter, the very first thing I turned
on this new lathe.
I haven't had a chance yet to examine the dead center with a critical eye under good light, but I don't think
I scored it -- if so, I'll have to find a way to repair it before I can work
on this long piece again. While I'm working on the center (which, if
it did score, was likely never hardened at the factory -- not too surprising
given that even the ways on this lathe aren't hardened), I'll cut (well,
grind -- cutoff wheel in the Dremel) two grooves in the 60° point; one
that will orient to the top, for oiling while the work is turning (a groove
to carry the oil into the bearing surface of the countersink) and one on
the side to provide relief for facing to the center, when I want to do that.
As long as I chamfer the edges of those cuts they shouldn't have a
tendency to cut into the work in the countersink. Update
added May 8, 2003: The center wasn't scored. I could feel something
when I ran my fingernail over the area where it had contacted the work, but
that turned out to be aluminum plated or welded onto the hardened steel of
the center. I was able to scrape almost all of it off with a knife
blade, chucked the center by putting the shank end into the hole in the chuck
(the center is too big to go through) and closing the jaws gently on the
taper itself. I then took a soft carborundum slip stone, lubricated
it with the Mobil 1 I've been using on ways and dovetails, and held the stone
carefully against the rotating center, maintaining contact over the full
length of the 60° point. Once the surface was completely even,
I polished further with the fine side of the same stone; I might yet go back
with an India stone I bought years ago for honing my straight razor and smooth
the point even more, or I might try to get some very fine sandpaper and work
down to Flitz metal polish to put a proper polish on the center, which was
supplied with a turned finish.
Meantime, however, I need to get some drill rod and make a 60° counterink,
or I need to get some proper center drills. Maybe both...