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Posted by Steve R. on June 6, 2008, 11:50 pm
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>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.
>
<Big Snip>
The best method that I have used is to hold the die in a lathe. Make or buy
a die holder for the cross slide. Set up the lathe for screw cutting, at the
same pitch as the die. The die can be set square by running it up to a face
plate, and of course it must be at centre height. Mount the workpiece in the
chuck. Engage the leadscrew, and crank the lathe by hand. Very often, you
can just grasp the chuck to do this. with a good hefty chuck, the key can be
used as a handle. Use plenty of cutting oil, and don't forget to back off
every quarter turn to clear the swarf. The steel in most bolts is intended
for rolled threads, and may not be good for threading with a die or
screwcutting. Better to find some better stuff to practice on.
Steve R.
--
Reply address munged to bugger up spammers
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Posted by RoyJ on June 7, 2008, 12:17 am
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Are you sure you don't have a hardened bolt? If the die won't get a
bite on the rod/bolt with a good chamfer, either the bolt is hard or the
die is soft.
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 knew things were not going to go well when I could not get the
> *never-previously-used* die to even bite. I chamfered, then chamfered again,
> nothing. eventually I made the tip almost conical. I got some purchase but
> a) the effort to cut was disproportionate and b) the end result was a thread
> which was skewed - and not subtly!
> I tried again with another piece of 5/16 rod - same result.
>
> 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.
>
> In the end I borrowed the tap technique: I gripped the rod in the chuck of
> my drill press. I clamped the die in my drill press vise (there is a little
> shoulder that allows it to rest flat). I raised the table and centered and
> clamped the vise with the rod touching. I turned the rod with a pair of
> vise-grips while simultaneously feeding the spindle (it would be really nice
> to have three hands!). After the first turn the feed took care of itself.
> This way I cut a reasonably straight thread but the effort was much harder
> than I expected. Also, the vise-grips make a mess of the rod.
>
> I tried it again with the cut-off bolt. This time I turned the spindle of
> the drill press directly by a makeshift lever improvised from the same stock
> and inserted into the chuck key holes. It was even harder to cut this time
> (I checked the diameter etc. - all identical).
>
> I should mention that the whole workshop was swimming in Rapid Tap at the
> end - no dry cutting here.
>
> I suspect that the die is crap and I will go and get another one but the
> whole process brought up some interesting questions. The most important of
> them is:
> How does everyone manage to get a straight thread when die cutting?
>
> I think the method I use is OK provided the workpiece *can* be held in a
> chuck but if not, I am stumped.
>
> 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?
>
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Posted by Richard J Kinch on June 7, 2008, 2:06 am
Please log in for more thread options Michael Koblic writes:
> How does everyone manage to get a straight thread when die cutting?
Chuck work in lathe. Position die in holder by hand at the end of the
work. Push up against opposite side of die with tip of closed drill chuck
in the tailstock, so it keeps the die "normal" to the work. Turn die
handles back and forth by hand while feeding tailstock.
This holds the die in two angular degrees of freedom, "pitch" and "yaw", if
you will, and in the Z axis. Does not constrain the "roll" (which you want
to spin by hand anyway), nor the X and Y, which are roughly constrained by
the self-centering tendency of the die around the work.
Reality is always 6 degrees of freedom that must submit to your will via
tools.
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Posted by Jim Wilkins on June 7, 2008, 9:37 am
Please log in for more thread options > 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
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Posted by Michael Koblic on June 7, 2008, 7:34 pm
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> See why we buy a lathe?
The HoT 7x12 lathe just gone on sale from $999.0 to $750.0. I suspect it is
too small, though. Furthermore, it would be a really big step in that I
would not really know what I am buying.
I am looking at local colleges for a course on lathe/mill machine use. I
have a long and relatively successful history of teaching things myself but
I have feeling this would be one of those activities where time and money
might be saved by learning on the actual instrument rather than from books
(come think of it, there might be DVDs, too).
Of course, if my Better Half returned from Toronto and found a lathe in the
garage I just might get my gonads removed.
Now here is a very basic question: I need a new drill press. Rather than
buying a bigger and more expensive one, would it make sense to add to the
pot and buy a mill? I.e. will mill do what drill press does as well as the
milling?
--
Michael Koblic,
Campbell River, BC
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>thread on a cut-off piece of 5/16 bolt.
>