On 2013-10-15, Tom Gardner Mars@Tacks wrote:
I have a W&S #2 and I need to thread the stem on an End Brush (see link)
to 1/4-20. The stem is about 1" long.
External thread, I presume.
I guess I need a Geometric Die Head. I know little about them or other
methods. What size head should I be looking for? What inserts do I
want? Is there one on someone's shelf gathering dust?
For 1/4" threads, if you aren't planning to go much larger, the
5/16" die head would be the best choice, but you could probably use up
to a 3/4" die head as long as you aren't going any smaller. A 1/2"
would be pretty reasonable if you can find them. Some sizes are hard to
find these days, and as I got all of mine from eBay auctions, I don't
know about commercial availability.
For a turret lathe (not to be confused with an automatic screw
machine, with which I have no real experience), you would likely need to
make an adaptor collar from the shank of the die head to the socket in
the turret. Mine has 1" turrets, and most 3/4" die heads come with 1"
shanks already.
The die head has "chasers" not "inserts". It looks like a 4-jaw
chuck, except that the faces of the jaws are formed to cut threads.
Depending on the material, you will want a different grind on
the chasers (rake and taper at the entry), and if you are going into
serious production, probably a second set of chasers (per thread you
want to cut), and a sharpening fixture for the chasers to be used on a
surface grinder.
The four jaws are closed to the cutting diameter by a lever,
usually hand operated on a simple turret lathe, or operaed by cams on
automatic screw machines, and fancy power-cycled turret lathes. When
the chasers contact the spinning workpiece, they start cutting the
threads, and pull the head and the ram of the turret in at the proper
speed, until you hit a stop set for that station on the turret. At this
point, the shank stops, but the chaser part of the hed is pulled a bit
forther in, releasing a lock which allows the hand lever to spring to
the "open" position, and the chasers to withdraw from contact with the
workpiece. This allows the turret to withdraw the Geometric die head
while the workpiece is still spinning. (There are also versions which
have a push to trip setup, if there is room for something for it to hit
in the lathe. Some can be converted fairly simply between the two
release techniques.
The force needed for that pull by the threads is the primary
limit on using smaller threads in larger die heads. The force required
scales with the heads.
Note that above I said that it was closed to the proper diameter
for the threads. Some have a small lever to switch between a roughing
and a finish pass, so you feed it on twice. These are useful if you
have a tougher material to machine. (Or, if you have stations to spare
on the turret, you can have one set to roughing, and another set to
finish to operate successively on the workpiece.)
I don't know what else you may be doing. I'll assume that the
workpiece will start out a bit oversized for the thread, so you will
want a roller box tool on the preceding station, to turn it down to the
proper diameter (just barely oversized) to accept the Geometric die
head. (You start off a little oversized, and you can check the settings
for pitch diameter by simply measuring over the completed thread,
because the thread form is complete.
Oh yes -- the chasers can have a shallow taper if you don't need
full threads up to a shoulder, or can have a much sharper taper if you
do need the full threads. When you buy the chasers, a good source will
ask you for the workpiece material and other things so they can supply
chasers sharpened to work optimally with that material.
The chasers are changed by pulling out a spring-loaded stop pin,
and allowing the closing lever to move past its normal stop, at which
point all four chasers can be slid out of their ways. (The ways and the
chasers are numbered, and *must* be installed in the proper places to
produce a proper thread.)
I need to do about 200/hr. The threading will be done before the cup is
filled with wire.
I think that it could handle that -- with a good fast chucking
system. An automatic screw machine could certainly do it.
I use mine (most often) on brass, involving turning down a bit,
knurling the part not turned down, threading 5/8-27 on the turned down
part, drilling the center, and tapping for a center mounting thread, and
then parting off -- and I have to stop and reverse the spindle for the
tapping at least, and I get about thirty to forty in a couple of hours,
with brass bar stock fed through a collet. You are just threading (as
far as you have said), so it mostly will be a function of how quickly
you can chuck and unchuck the workpieces.
http://www.ebay.com/itm/WEILER-10028...em33817d 2fa6
Looking at that, I'm assuming that the major turning is being
done on another machine. A proper collet to accept the cup (ideally
with a stop to keep the depth consistent) would work well -- but you
would have to stop the spindle for the change of workpiece.
Good Luck,
DoN.
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