I'm not a pro on this and I'm sure someone can improve on it, but I'd
use a somewhat higher speed than normal and take lighter cuts with a
slower feed rate.
THEN you can always touch it up with a file (very lightly) and even
finish that off with some fine grit emory cloth.
If that doesn't do it try some rouge.
I had the same problem turning mild
steel plate into flywheels.
I received an email from a reader who suggested
grinding a tool to take a shear cut. The way
mine ended up is like a small roughing tool
with high rake angles laid on it's side.
I was able to take .005" cuts and get a beautiful
finish. I also tightend up the gibs and locked
the axis that I wasn't using. I used a slow
speed and feed, trying to make sure that I didn't
heat things up and work-harden the stock.
It takes turning it slightly oversize and then spinning it while holding
on a pneumatic belt grinder (Dynabrade type) to take it the last coupla
thou and finish it at the same time. There are special tools made for
finishing pipe but they are real spendy.
They just make pipe up from rolled flat bar, and the rolled flats are most
likely coming from remelted steel, with a somewhat loose tolerance on the
recipe, so my guess is that pipe steel will vary quite a bit. If you have a
lot of it to do, you might consider using DOM tube instead of pipe. Ex:
I wanted to make a bigger column for my cheapy drill press and found
some pipe almost the right size in the hardware store's offcut pile,
needed about .030" taken off it. Unless it's D.O.M. or seamless
stuff, pipe's made from soft strip steel rolled and welded with a
nasty seam left. Mine had 4 distinct corners on it when I started. I
ended up "machining" it on a belt grinder, round and round from end to
end, I had no pipe centers large enough to take it. Had a nice ground
finish and no lobes when I was done. Took a lot of checking and a
steady hand, most of a Sat. afternoon. So that's a possiblity for
you, roughly machine to size and grind to finish. If you use sulphur-
based cutting oil, your finish might improve as well as using sharp
HSS tools. Hone after grinding.
If it's for some load-bearing application, remember that plumbing pipe
is MADE to be bent.
It's not really a big deal to grind up a HS tool bit. The first time
takes a bit longer, but after that it's just touching up. With a bit
of radius on it I'd bet you'd get a better finish, too. I'm not saying
you have to do it for this or anything, but it's an ability well worth
picking up for home shop stuff.
Thanks for the comments everyone. The project I have in mind is a
paddlewheel type flowmeter to set on the top of an open chute. I
intend on using reed switches potted inside steel pipe triggered by
wheels with rare earth magnets that will spin on the outside of the
pipe. The wheels will have UHMW poly bushings to spin on the pipe,
There will also be an air purge comingfrom inside the pipe to form a
cushion for the wheels to float on and to keep mud out. Final version
will likely be stainless but my second and third versions will be
using plain old black 1/2" schd 40 pipe. Again, any comments will be
The black iron pipe is a magnetic shield, and will short the magnetic
field out, keeping the reed switch from working. Brass, copper,
plastic, or stainless steel would be better.
Brass and especially copper could be a problem if the paddlewheel turns
too fast, as the magnets will induce eddy currents in the metal, which
will impose a drag on the paddelwheel. (Try dropping a rare earth
magnet down a copper tube - the magnet falls very slowly.) Plastic or
stainless steel would be better.
Given that the final will be stainless, it would be best to start with
It is my intent to mill a window in the pipe directly above each reed
switch. When the internals are potted in place the window will be
filled with potting compound
Speaking of copper, a few years ago I saw something really surprising.
It was a varible speed drive using magnets and a coper disc. Magnets
were on one plate parrallel to the copper plate As one turned the
magnetic field turned the other. The speed could be changed by
increasing or decreasing the distance between the two. Looks like a
cheap and easy varible drive to me. It was capable of transmitting 100
HP to a centrifuge without any energy consumed
An eddy current drive. While at first glance it may appear to be a
lossless system, it's really just a slipping clutch. The motor always
runs at full speed producing enough torque to turn the load at a lower
RPM. The power in is proportional to torque x motor RPM; the power
out, torque x load RPM. Since the torque is the same on both sides of
the coupling, the difference between the motor RPM and load RPM
represents a loss.
???????? I'm not too sure I understand.
There is no such thing as a "loss." What we perceive as an energy loss
is actually a transformation into something else, usually heat. In other
words, a slipping clutch gets awfully hot awfully fast. So, in this system,
where does the energy go if there is a significant difference
between the drive RPM and the load RPM?
OK. What had me going was your comment that "it may appear to be a
lossless system." In the example given, 100 hp applied to a centrafuge,
I would expect the quantity of heat to be very great and so the heat loss
to be substantial and very obvious.
Well ... the stainless (if a non-magnetic alloy) will probably
work, but you really expect to get magnetic fields through the walls of
a mild steel pipe? I strongly doubt that you will get anywhere near
enough to sense with anything other than an amplified Hall-effect
sensor -- if that.
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