It SEEMS safe, sure - but if that pipe gets out of balance and jumps off the wheels at that speed, IT CAN AND WILL KILL ANYONE IN THE VICINITY. It can easily get out of balance and start jumping around, finally it starts whipping vertically like a lasso and has the power to cut a tree in half.
What you have lashed up here is a classic "Here, Hold my beer and Watch This!..." situation. Please rig up a video camera at a safe distance at least 2X the length of the pipe away, and preferably behind a Lexan barrier, so the Coroner's Inquest is a lot simpler. And after the hubbub is over, your next of kin can sell the footage to America's Funniest Home Videos.
We have seen what happens, and it is not pretty. Machine shops regularly feed long sections of bar and tube stock through a hollow lathe tailstock, turn it at machining speeds, and occasionally the free end gets away - and with a sufficient application of horsepower it'll destroy anything within range. I'll bet a dozen participants here can provide "After" pictures of bar-feeder whipping stock runaways that will curl your toes.
You just built a lathe with triple the stock length, triple the horsepower and torque available to spin the stock, and far less intermediate support. And you wonder why we are concerned...
The idea is fine, but you are turning the pipe way too fast. 10 to
30 RPM is plenty fast enough - set the PTO at the slowest speed and let the tractor run at dead-slow idle, the rotation only has to go fast enough to keep exposing all sides of the pipe as you clean up the pipe surface.And have a volunteer come out and stand by to hit the Kill Switch and call 911 just in case you get tangled up in the pipe - it happens. Bring an umbrella, buy them a pitcher of lemonade, let them read a book - as long as they are close by when the screaming starts.
To knock off the mill finish and start exposing the nice metal underneath you need bulk cutting - I'd start off with either a right-angle grinder and a metal-cutting wheel held with the face flat against the pipe, or a belt sander and a rather aggressive belt like
30 or 50 grit. Have the grinding wheel cutting face or the sanding belt face travel going /against/ the rotation of the pipe.For the final finish you'll have to experiment - You can use a running belt sander with various grit belts, a power drill with a flap-wheel, ScotchBrite abrasive nylon finishing buff wheel, or a heavy wire brush, or a right-angle grinder with a wheel or wire brush.
We can't get more specific than that without knowing what finish you want, and you probably don't know exactly what you want yet either.
Knock off the big chunks on one length of pipe, and then experiment with various sanding brushing or grinding techniques on short sections till you get a look you like.
If the power tools are electric you have to do the work dry - even with a GFCI protecting you from getting shocked, water will wreck the tool unless it's designed to be waterproof. If you use air tools applying water may help, this is another part of the "experimenting to see what works best" phase.
All the 'tractor lathe' is doing is to keep exposing the bottom side of the pipe you are buffing up, Period.
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