Friday, December 1, 2006

AV - ...but I don't have a LATHE


"...but I don't have a lathe."

Sound familiar? The truth is, you probably have any number of things around your shop that can serve as a lathe.

In a lathe, the work rotates against the cutting tool. ( In a milling machine, it's the cutting tool that rotates against the work.) The main purpose of a lathe is to produce a circular part, which is achieved by pressing the cutting tool against the work-piece as it rotates. Anything that achieves that goal with sufficient accuracy for the task at hand will allow you to do the job.

The most common thing used to rotate the work is the ubiquitous quarter-inch drill-motor. A drill press works even better. And the cutting tool may be anything from a hand-held file to an angle-head grinder fitted with a flapper wheel; even a portable belt sander can be your 'cutting tool.'

"Tried that," they usually say. "Didn't work." Odds are, they didn't try hard enough :-)

The trick to holding the work-piece in a drill-chuck is to use a coupling nut. Put the coupling nut into the chuck and tighten it evenly, taking the slack out of all three jaws. (Just put the chuck key in each hole in turn, taking out any slack. If you tighten a Jacob's chuck at only one point there will always be some slack in the other two positions, usually enough so that whatever is chucked will not run true.)

The centroid of the work-piece is then drilled to accommodate a bolt that matches the coupling nut. The standard 1/4-20 will serve for most work but coupling nuts come in all sizes. To keep the work-piece from spinning on the bolt you may use a lock washer.

When using a hand-held tool as your cutter, be it a file or even a belt-sander, the trick to producing a true edge is to use your eye to align the opposite edge of the work with a known-true reference, such as the string of a plumb-bob. Or the blade of a protractor, if you're creating an angle. Any deviation is instantly apparent and just as quickly corrected.

In most drill presses the chuck is attached to the quill with a simple taper. A taper has the advantage of providing a virtually error-free alignment but has the disadvantage of not being able to withstand much of a side-load. Even a 3M fiber-wheel (used for dressing aluminum edges) may cause the chuck to leap off those cheap Chinese drill presses. To prevent that from happening when using the drill press as a lathe, the head of the bolt you use to secure the work-piece to the coupling nut should be drilled in a cone large enough to accept a small ball bearing. Simply trapping the ball between the drill-table and the bolt will provide enough axial pressure to prevent the chuck from coming adrift.

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There are endless elaborations to this theme but the basic idea is that you probably do have a lathe, if you put a bit of thought into it. Although crude in appearance such jury-rigged tools are accurate enough to allow turning down a piece of 4130 tubing to fit a standard bearing, make circular flanging dies of any diameter and even create the plug for a 10 x 12 inch spinner for your propeller hub.


1 comment:

flybynightkarmarepair said...

Some folks are so hardware impaired they don't know what a coupling nut is.

I've also used all thread, a Tee-nut to drive the (soft) material (acting like a drive center), and a nut at the end to stick the whole thing together.

Using a ball bearing as a "Live Center" is brilliant, thanks. The quill bearing of my Delta drill press is a little sloppy so this will make my work a little more accurate.

If I ever need any more valve guides for my VW heads turned down to accept Rabbit stem seals, I'll do them myself, rather than beg for someone with a little ChinCom lathe to do it for me.