On Jun 6, 10:02 pm, "Michael Koblic" wrote:
I have spent a very long and unproductive day trying to cut a 5/16-24 thread
on a cut-off piece of 5/16 bolt.

I hit the books to see if I omitted something glaring. Most of the texts I
have are big on starting *taps* straight but they did not stress the dies so
much....

Because external threads are so much easier to cut on a lathe.

...How does everyone manage to get a straight thread when die cutting?

It isn't easy, and so-called 'drunken' threads were common way back
when threads were cut by hand. Your drill press technique or some
variation on it helps. If you can turn or file down the end enough
that the die slides on or at least cuts easily it's more likely to
start straight, though not always concentric. I've used a vee block to
square the die and line up the shank of a cut-off headed bolt that was
too big for the chuck.

You might be able to sharpen the die by grinding the holes with a
Dremel. Most of my larger taps and dies came from second-hand stores
and I resharpened them until they cut satisfactorily. Carefully hand-
grinding the front face of a cutting edge, where the chips flow,
shouldn't affect the geometry of the edge enough to matter for home
use. Take a small equal cut off all edges and check, then repeat,
rather than trying to do each one completely, so they stay about the
same size. A sharp edge doesn't reflect light.

The dies sold in hardware stores are good for cleaning rust and dings
off old threads, not so good for cutting them from round stock. As
others have written, dies from an industrial supplier are much better.

See why we buy a lathe? I cut the thread part way on mine and finish
with a die, using the tailstock to start it square.

BTW, if using a drill press to start a tap in a hole how do you turn the
tap? Do you grind flat spots on your taps to get a better wrench purchase?

I have but if they are deep enough to not slip they weaken the tap
considerably. On a two-flute tap I grind shallow wrench flats into the
shank end of the threads.

I chuck the tap, loosen the belt and turn the pulley by hand to start
the tap straight. Just before (or when) it slips I switch to a tap
wrench, being careful to apply equal pressure to both ends of the
handle so the tap isn't pulled sideways until there are a few full
threads to hold it straight. The long straight types of tap wrench
that clamp in the center work best here because they fit in under the
chuck.

Another solution is a good tee-handled tap wrench that can be guided
at the upper end. The smaller ones fit into a loosened chuck, some
have a center hole and you can chuck a guide pin, there is a type with
a separate guide on the top. The trouble with them is they stack up so
high you may have to raise the head to use them and lower it to drill,
losing position.

After the Army I worked as an assembler building custom equipment for
the auto industry so I got plenty of practice locating, drilling and
tapping holes by hand on large machines. After a week or so I stopped
breaking taps and eventually learned to tap with a reversing power
drill. It just takes sharp tools and practice to learn the feel.

They made me go through all the assembly, machining, wiring and
drafting positions before promoting me to project engineer. It was a
very useful experience that unfortunately can't always be imposed on
fresh engineering graduates.

Jim Wilkins