My Speedway 7x12 Bench Lathe

Update: 2003.05.03


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.


I got to spend a few more hours with the lathe last night, and learned some things (I expect to learn stuff at practically every lathe session for the foreseeable future).

First, I learned that PVC plumbing pipe cuts easily and fairly cleanly with a carbide tool bit designed for cutting steel, but that it will very easily distort if cut too thin; I also note that the finish would probably be better with much more back rake than the (I think) 5° built into the carbide bit I'm using.  Fortunately, I wasn't trying to actually make anything with the pipe, just taking some trial cuts to see how things worked and practicing at meeting a diameter from both ends after turning a piece end for end in the chuck to turn the entire length.

Second, I got some experience with work deflection, turning a shape from a 3/8" aluminum rod (sorry, I don't know for certain what alloy) extending only about 1.5" from the chuck jaws; it took four passes at the same cut settings on the cross slide to stop cutting, as I worked through the deflection of the part with successively lighter cuts (the first was only .010" in feed).  Again, this might be better with a tool specifically ground for aluminum, since that would want more back rake and the resulting sharper edge would cut with less pressure against the work surface.  The surface finish, however, was better than I'd been led to expect from brazed carbide tooling in a small lathe.  I also got to play with the power feed; at the stock setting, which amounts to 256 tpi (or a little under .004" feed per revolution, if you prefer) it wasn't hard at all to disengage the half nut at the proper location to avoid overcutting the shoulder or crashing the compound into the chuck jaws, and surface finish was beautiful.  Once I got enough shims under the tool (this nominally 1/4" tool needed .090" of shims to come to center height for a nominally 5/16" or 8 mm tool holder -- I'm guessing the tool shank is a tad under nominal, since this is a brazed carbide tool rather than a precision ground square blank), I was able to make nice facing cuts using about 5° of "drag" on the tool -- point of tool closer to chuck than shank -- and cutting from center out, as suggested in one of my 1941 machining texts.  This causes the tool to take a very thin, but wide chip and even with a very small radius corner gives an excellent finish.

Work deflection is important in producing a true cylinder (as opposed to a taper); if not accounted for, the finished part will be larger at the free end than the chuck end (which is why machining texts strongly recommend supporting the end of the work with a center for anything that isn't short and very stiff).  In addition, it's important for turning to a specific diameter, because if you measure the part at the end, where deflection has left it oversize after a relatively heavy roughing cut, you'll wind up cutting it too small, either at the chuck end where there's less distortion (and the part was smaller than at the end that was easy to measure) or after a finishing cut, which because it's lighter will deflect the work less and simply cuts more than intended.

And relative to supporting the work with a center: I don't yet have any center drills, but I thought of a way to center a piece with the tools I have.  First, I can use the 60° threading bit that came with the tool bit set I have to cut the countersink (careful attention to setting the tool angle will be needed to get proper bearing inside the countersink, but I think I can match it against the center in the tailstock -- don't have a center gage or thread gage yet, either), and then use that to keep a twist drill centered to drill the pilot at the bottom, which prevents exceesive pressure and wear on the sharp point of the center.  This can't be done accurately in the opposite order because the jobber length twist drills I have will wander and drill off center, oversize, or might even break as they're advanced in a tailstock chuck into an off center hole in rotating work.  Countersinking with a threading tool will require care to cut only on the front side of center, since the bit won't cut properly (relief is wrong, at a minimum) on the back side of the work, where rotation carries it upward instead of downward past the cutter.  I think the best way to proceed will be to set the compound to 30° and cut from the perimeter of the countersink in toward center, feeding with the angled compound.  I'll have more information here when I have a chance to try this technique.


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