How do they make grub screws?

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?
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Roy Smith wrote:

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.
Jeff
--
Jeffry Wisnia

(W1BSV + Brass Rat '57 EE)
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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 the dies.
While I've seen videos of the process, I'm not exactly sure about the details....
Regards,
Robin
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| 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 the end. 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!
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snipped-for-privacy@panix.com (Roy Smith) wrote in

The blanks are cold formed on a cold header. The hex is put in with a punch in the header. http://www.mastertask.com/Nat2D3B_lo.htm
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: http://www.sommatool.com/manuscripts/broaching.asp and broach while the spindle is turning.
--

Dan


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D Murphy writes:

Do I follow that action correctly: the cutting tool is tracing out an internal hex path by spinning axially and wobbling at the same time?
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On Tue, 12 Apr 2005 01:13:06 -0500, Richard J Kinch

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.
Gunner
"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
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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 light on.
--

Dan


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D Murphy writes:

OK, then, does this 1 to 1.5 degrees of tilt and wobble result in a slightly conical hole being broached, or is perhaps the tool tapered so that the leading edge is wider than the body, or what?
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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.
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wrote:

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.
--

Dan


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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.

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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.
--

Dan


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George wrote:

I'll second that. An excellent explanation of the process.
Bob
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On Mon, 11 Apr 2005 17:23:49 -0400, Roy Smith wrote:

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!
-Ron
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