Yesterday I was holding in my hand a grub screw. A rather ordinary
object, costing no more than a few cents, but I realized I have no
idea how they are made.
The threads are no problem, but how do the get the allen-head
indentation in the end?
I'll make this WAG based on no first hand infrmation whatsoever:
They forge the allen socket first, then place/hold that blank on a hex
spindle matching the socket while they cut or roll the threads onto it.
Then, they harden and oxide finish/plate them.
I'm reasonably sure that screws are not held on a mandrel during the thread
rolling process. The thread rolling dies are actually plates which have
linear grooves which match the profile and helix angle of the thread being
created. I believe there are cylindrical dies as well. The linear
motion/rotation of the dies against the screw blank feeds the screw through
While I've seen videos of the process, I'm not exactly sure about the
| Yesterday I was holding in my hand a grub screw. A rather ordinary
| object, costing no more than a few cents, but I realized I have no
| idea how they are made.
| The threads are no problem, but how do the get the allen-head
| indentation in the end?
I'll take another stab at it, having seen bits and pieces of the process
and similar operations over the years. Starting with a drawn roll of steel
wire, the wire end is heated (maybe, not so sure this is needed) and held in
a mandrel or clamp. At the same time a round punch forms the hole another
die shears off the other end. There is now a slug with the proper ends
(various shapes are available) which is dropped into a mill that rolls the
threads in between a couple plates. Finally a final cleanup punch with the
hex shape. You can barely see the material from the punch if you look in
When you look at the other end of non-specified grub screws (whatever
you call ones that have not been specified as having a certain point on
them) you'll see a little bit of material that has rolled over the end from
thread rolling. After the threads have been rolled then the final point or
cup gets put on it somehow or the other.
I used to be an industrial scale tech in another life and got to see how
sockets, wrenches, and other tools were forged/punched at one place (in
other words, buy the cheapest American brand you can find!) watched threads
rolled in another shop that made bolts (the machines weren't near as big as
expected,) and discovered that how the consumer is silly enough to pay more
for a different color of carpet pad that was simply called "premium" (carpet
pad is recycled foam from couches, beds, industrial processes, and so forth,
and dyed to make you want to pay more.) Very enlightening. While I'm at
it, I learned that the lowest fat hamburger is simply beef that has had less
fat added during the blending process to get to the specified amount. And
unleaded gas actually costs less to make than leaded!
email@example.com (Roy Smith) wrote in news:d3ept5$mj$ firstname.lastname@example.org:
The blanks are cold formed on a cold header. The hex is put in with a punch
in the header.
The scews are then thread rolled, heat treated, then plated or surface
treated or coated.
If you want to make your own you can make a servicable push broach out of
an old HSS drill bit. Surface grind a hex on it. Drill a hole slightly
larger than the flats on the hex in your home made screw, then use a small
arbor press or even a drill press, or the tailstock on your lathe, if you
have a robust one, to push the hex broach into the hole. On Swiss screw
machines, when we make titanium medical screws we use a wobble broach:
And pushing the wall material down into the bottom of the hole. You
can always tell a wobble broached hole, as it generally has a star of
material at the bottom. Cold headers tend to compress the material
so it doesnt show the star.
"To be civilized is to restrain the ability to commit mayhem.
To be incapable of committing mayhem is not the mark of the civilized,
merely the domesticated." - Trefor Thomas
Richard J Kinch wrote in
Actually you are broaching with a hex shaped broach in the case we are
taliking about. Almost any shape can be broached, the broach is always
the same shape as the shape you need to put into the work. So the broach
isn't tracing out the shape in the sense of profiling if that's what you
are saying. The axis that the broach spins on is kicked off at a one
degree angle. The workpiece drives the rotation of the broach. Since the
broach's axis of rotation is kicked off on an angle there is a small
point of contact at any given time. This gratly reduces the amount of
pressure it takes to push the broach into the work. The broach is still
forming the hole by being the same shape that you need to broach into the
work. The easy way to visualize what is going on is to hold a small cup
or drinking glass up against your hand so that the rim is flush against
your palm. Now tilt the glass a couple of degrees. That is the contact
that the broach has against the part. Now if you rotate the glass and
your hand together, keeping the glass tilted, you can see that in one
rotation that the entire rim of the glass will eventually make contact
with your hand, one small spot at a time. Now if you think about the
broach pusing into the hole you can see that by having less contact at
any given time, it will take less pressure to push the broach into the
hole. The main limitation is your inch per revolution feedrate. The
broach is in a sense cutting a helix the same way that a turning tool or
threading tool does. A coarse pitch thread has a grater helix angle than
a fine pitch one does. You always have to have a greater side clearance
on a threading tool than the helix angle of the thread. Otherwise the
tool will rub. The helix angle created by the broach and the rotation
can't be greater than the amount the broach is kicked off at (usually 1-
1.5 degrees) otherwise you would no longer have that single point of
contact, you would have full contact and a higer broaching pressure. I
think that Somma has a formula on their site for calculating the max feed
rate. Usually it ends up being a small value .002"-.003" per rev. If your
confused after reading all this, try the glass thing it usually turns the
The leading edge is wider than the body.
Since the tool is always at a 1 degree angle to the work, the sides of
the tool must have a 1 degree or greater draft.
Ideally the tool advances at the same rate that it cuts. So a 1/2"
diameter tool should advance at 0.009" per revolution. 1/2*sin(1). If
it advances any faster than that then the tool becomes choked, if it
advances any more slowly then you get an interrupted or zig-zag cut.
Since all work material is elastic, you would actually cut a little
less than the ideal rate just to release the load on the non-cutting
edge of the tool.
There is some spiraling of the tool as it cuts, so the bottom of the
hole may be rotated with respect to the top of the hole. Spiraling may
be undesirable because it binds the body of the tool and prevents it
from wobbling freely. One solution to this is to reverse the rotation
in mid cut causing the tool to spiral in the opposite direction.
George wrote in
Exactly right. The sin of one degree is .017452. The formula for
broaching is to take the diameter of the broach and multiply by .016 to
get the proper feed rate. For an internal hex you should be using the
distance across the corners, and not the distance across the flats.
Spiraling is nearly nil in a shallow hole or a short O.D. length. If the
brach is deep, you pretty much have to reverse the spindle. Another
trick that helps if it's allowed by the drawing, is to groove the
workpiece so the broach essentially is cutting a series of short
lengths. If you look at the valve stems for faucets in a hardware store,
you will often see the spline on the end of the valve stem where the
knob mounts, has a groove in the middle of the spline. This allows the
spline to be cut on a multi-spindle screw machine, which doesn't have
the ability to reverse the spindle. Coolant is usually not required. If
you are going to use coolant on an internal broach, then the hole in the
blank must be larger than the broach is across the flats. If that is not
possible, there are companies that make wobble broaches with a vent
hole. Coolant does not compress very well.
OK, I missed that part.
I know the groove that you are talking about. I've vaguely wondered
why it was there. But how does that help reduce spiraling?
And thanks for the excellent walk-through on this.
George wrote in
I'm not 100% sure. I've been told that it's because the groove breaks the
chip, which could be, but I can't quite figure out how that stops the
spiraling. I think it has more to do with replacing some of the length of
cut with nothing. In other words when the broack traverses the groove it
doesn't spiral because it's not cutting. The previously broached section
behind the broach keeps it lined up, then the broach starts cutting lined
up on the other side and eventually starts to spiral again. Since it's
cutting a short distance the spiral effect is small. Well that's my theory
anyway. If I remember, I'll ask someone at Somma or Slater the next time I
talk to them.
Your welcome. Every once in a while a wobble broach will show up on Ebay
cheap, you can never have enough tools.
It's truly a fascinating process to watch. Threads are cut or rolled on a
length of steel rod, just as you would expect. The threaded rod is then
cut up to the desired length. One end of the screw is then ground to the
desired shape: cup, point, flat, or whatever.
The screws are placed into racks, tightly packed, with the point end
against a flat plate. The racks are slid vertically into boxes, which
have slots in the sides that hold the racks about 1 inch apart.
Now comes the interesting part: a large number of specially bred insect
larvae are introduced to the box, and the box is sealed. Instinctively,
the larvae burrow into the exposed ends of the screws, creating perfectly
hexagonal holes, similar to the honeycombs created by honeybees.
The larva then seal themselves into the holes, and enter their pupal
stage. A few weeks later, the adult insects emerge. The adult insects
are captured for breeding, and the finished screws are cleaned up and
packaged for sale.
The insects are of the species Apis schruvus. The larval form is commonly
known as the screw grub.
Have a good weekend, all!