Lead Extrusion Theory

[Linstrum]

Extrusion of lead and other materials is a very interesting process that is a bit more complicated than one would think. When I was making jewelry findings for a living some years ago, I looked into using extrusion and post-sintered powdered metal extrusion as possible methods for making my own tubing from precious metals. I could not afford to pay the rip-off prices being charged for commercially manufacted silver and gold tubing required for making some of my products because it totally wiped out my profit margin when I sold my stuff at competetive prices.

I found and read a book published in Great Britain in the 1910s (I don't remember the name of the book, I read it 20 years ago) that was the most detailed work available on lead extrusion (which would translate quite well to extruding copper jackets, as well as silver and gold). The author had taken different colors of damp clay and cut them into cubes or long square cross-sectional strips like lengths of fettuccine and then arranged them in his extrusion apparatus so they formed a checker-board in cross section before extrusion. The extrusion barrel and nozzle were split length-wise in half-sections so they could be taken apart to see what was going on inside them and the different colors of clays traced the flow patterns inside the barrel and nozzle. The photos in the book showed in pretty good detail how the material flowed inside the barrel and nozzle and the author pin-pointed problem areas in the shape and angle of the extrusion nozzle and he was able to refine the internal shape of extrusion nozzles to maximize performance.

The big thing he discovered was to avoid sharp internal angles that the lead or clay had to flow around and places where the lead or clay had to change direction radically to enter the extrusion nozzle, since these trouble areas caused non-linear "eddy current" type of flows. The eddy currents did not harm the final product but dramatically increased the force required to get the material to enter and go through the nozzle.

The main difference between what the book covered and what we are doing is the book was oriented toward extruding seemless lead tubing and we are making solid wire. In making hollow lead extrusions, the extrusion die has a torpedo-shaped mandrel inside it that forms the internal bore of the tubing, and the lead has to flow around the arms that hold the torpedo in place inside the nozzle. The arms in the example given in the book were shaped pretty much like knife blades so they would cause the lead to flow around them with minimal disturbance, but it created cuts down the sides of the tubing so it was not seemless. It was expected that the lead would "cold weld" itself back together as it flowed through the nozzle, but plain old-fashioned common sense would say that this would not happen, and it didn't. Back then lead tubing was still used somewhat for plumbing (inexpensive galvanized steel pipe was available in the 1890s) but by 1910 most extruded lead sheathing was used for things like toothpaste and ointment tubes as well as to cover phone and telegraph cables in wet, rainy places, or under water like for the very first Trans-Atlantic Telegraph cables layed back in the 1850s and 1860s, and there was a problem with the lead covering leaking at the seems where the cold weld did not form perfectly. The problem was solved by simply heating the nozzle hot enough so that the lead would solder itself back together as it went through the nozzle since quite a bit of heat was also generated as the lead flowed through the reduction area. Between being externally heated along with the heat from working the lead in the reduction area, the temperature was high enough so the lead would make a truly seemless extrusion.

I saw some photos of the lead extrusion machines used back 140 years ago and they were darn near as tall as a two-story house. The lead blank was as tall as a man, two feet in diameter, and weighed 6-2/3 tons (or 13,333 pounds)! The press that was used to power the extrusion apparatus used steam pressure to drive the piston since hydraulics were not very well developed at the time.

If some of you lead extruders would care to experiment with tubular extrusions, say for making deep hollow points, there are two ways that it can be done. One would be to make lead tubing by attaching the torpedo mandrel to the end of the piston so it moves along with the piston. The mandrel would have to be a bit longer than the piston stroke because part of the mandrel would need to to be inside the extrusion nozzle during the entire piston stroke. This would make retracting the piston back out of the extrusion cylinder a bit difficult, but would not be impossible. The other way, which I would do (and Buckshot is already doing for hollow-base Minie balls), is to make lead cups shaped pretty much like a pistol cartridge with real thick walls by having a die machined to the required body diameter, and then have a mating mandrel that would fit down inside the die with a slightly tapered hollow-pointing tip that would form the hollow cavity inside the lead slug. The finished lead cup would have to be pressed out the bottom of the die, but that isn't too hard if there is a removable plug in the bottom of the die that can be quickly pulled out before pressing the finished piece on through the die and out its bottom.

There are all sorts of things possible that can be done with lead extrusion. Have fun!


rl542

[JIMinPHX]

The commercially produced gold tubing for jewelry use is actually a welded tube. It starts as gold flat bar. It gets drawn into precision foil (down to 30 millionths tolerance). Then it goes through progressive forming dies. Then it gets tig welded in such a precise way that you can't even see the weld seam. Pretty much all of the equipment to do that comes out of Italy. The price of that tube might sound like a rip off, until you see the process that is used to produce it. It ain't real simple or easy to duplicate. The controls alone on that equipment cost a small fortune & are regulated by the DOD because they use weapons grade components.

[Jim_Fleming]

Here's a link that I found on YouTube about making tubing... It's not Gold, but the principles should be very similar:

http://www.youtube.com/watch?v=rOK90NY3qtQ

Enjoy,

Jim

[Linstrum]

What is interesting about all this for me is that shotgun and some rifle barrels are made from vacuum-welded tubing nowadays, and are not at all weak like the old Damascus crap barrels of our great-great grandfathers' era, being every bit a strong as some of the fully drilled and machined barrels. Even after the American Civil War, the old 1858 and 1861 Springfield muskets had their barrels relined down to .50 and .45 caliber with sheet metal tubing to make the first Trapdoor Springfields, of course bucked-up by the original drilled musket barrel to hold in the pressure.


The way I ended up making my own precious metal tubing was by step-brazing it, using a precious metal brazing alloy with a lower liquidus point than the tubing material liquidus-solidus phase. I had to use my didymium brazing glasses in order to carefully watch the melting point of the brazing rod. Once I got the temperature right with a propane-oxygen torch it was very fast to make, but again the cost of the brazing materials was prohibitive, so I was next into making my own brazing alloys. It was a never-ending process, getting me farther and farther away from making jewelry components.

For softer metals, like annealed Sterling silver and 14-23 karat copper-gold alloys, the exact same commercial process that is used for extruding copper plumbing and refrigeration tubing can be used quite well only when the amount of tubing needed justifies the cost of the production equipment, which is huge. But, there is no way that I would take 10 pounds of Sterling silver over to a copper extrusion factory and say: "Here, make me some silver tubing." I would probably not get back much of my 10 pounds of silver!

I attended a series of lectures on commercially manufacturing components called "The Santa Fe Symposium" back in the early 1990s and lectures on the various methods for precision welding were given. The cutting edge back then for jewelry was electron beam vacuum welding, which was nothing new at all, even back then. Electron beam welding is good for soft stuff like silver all the way up to Osmium-Iridium alloys that are as hard as tool steel and have extremely high melting points up around molybdenum, so they could handle anything in ANY industry. Steel water pipe and structural tubing are all made by vacuum welding nowadays and are just about as strong as the Shelby seamless drawn tubing developed during WW2. TIG and MIG have also been used going way back to before WW2 when Heliarc process was invented in the 1930s for welding aluminum. When my dad worked as a machinist in the aerospace industry in the late 1950s they were using TIG and MIG for attempting to weld together the fir-tree bases onto the blades of the hot-side rotor assemblies for jet engines, since forging and machining of the high temp high-strength alloys was holding back production. Didn't work well enough, though, the welds came apart, and so did the jet engines that cost about a quarter million bucks in today's money.

The beauty of welded tubing is that the size of tubing can be changed in minutes instead of having to take days to make all new extrusion dies for even minor changes in diameter, and welding is quite attractive from that standpoint alone when the world-wide consumption of precious metal tubing only a few hundred feet per day instead of in the millions of feet per day for copper. From that aspect the overhead investment in equipment is more cost-effective with welding than extrusion.

rl545

[JIMinPHX]

For softer metals, like annealed Sterling silver and 14-23 karat copper-gold alloys, the exact same commercial process that is used for extruding copper plumbing and refrigeration tubing can be used quite well only when the amount of tubing needed justifies the cost of the production equipment, which is huge.

The gold tubing that is used to make earrings & such for the jewelry you find at Service Merchandise, K-mart, Walmart, etc is nearly all of the welded variety. That quantity is enough to justify just about any common commercial process. The value of the material & the other manufacturing processes that follow & the efficiency of the whole manufacturing process in combination make the welding process more suited for this application.

the world-wide consumption of precious metal tubing only a few hundred feet per day

Not True. The above example is in excess of that.

I know next to nothing about jewelry in general, but I have intimate knowledge of the large scale production industry from the engineering & operation side.

[Linstrum]

Hi, Jim, you are absolutely right about the rather large amount of tubing used every day in jewelry like is sold at K-Mart, Wal-Mart, Best Buy, etc. For seamed tubing I would imagine that the daily world-wide consumption of that stuff could be measured by the dozens or even hundreds of miles. I guess we weren't on the same page about the type of tubing I was talking about, tubing used for jewelry where only several hundred feet per day is used is the extruded, brazed, or welded type, and is only used where an un-joined seam would cause a production process deficiency, such as forming the tubing into spherical beads, and then for most applications like that a different process is used very similar to how pistol and rifle cartridges are drawn up from flat planchets. I got my off-the-cuff estimated daily seamless tubing production figure from a Handy and Harmon representative at one of the Santa Fe Symposiums I attended. Handy and Harmon is (or was, until the Chinese and Asian-type Indians came along) one of the biggest suppliers in the world of fabricated items made from precious metals, and for seamless tubing the big hassle with them was that you had to special order things up to a year in advance, such as odd shapes and seamless tubing, and then pay blood money for it, far, far in excess if the stuff needed were instead fabricated at a local sheet metal shop from mild steel, copper or aluminum. They would not keep on-hand large stocks of those special items because the daily or even monthly demand was so miniscule, coupled with problems of inventory taxation. Commonly used stuff with high rates of turnover like brazing and solder alloys, wire, strips, plate, sheet, powder, etc, they did have in stock all the time.

In jewlery, the vast amount of tubing used every day for stuff like locket hinges, little short tubing sections in the form of heishi strands (called "liquid silver" and is a favorite that the Southwest Indians use by the dozens of yards in just one piece of jewelry) is not seamless or have the seam welded or brazed. The seam edges are simply pressed together during the rolling down and drawing process, and the seam is not visible unless etched or crushed with pliers, then it "comes apart at the seam". Seamed tubing is extremely easy to make and when I occasionally go back into the jewelry findings business to help pay bills around here, I make hundreds of feet of precious metal seamed tubing per day for my own in-house consumption for making aglets, crimps, locket hinges, heishi, etc. I make seamed tubing by taking large diameter silver, gold, or platinum wire and then roll it out to nominally 0.010-inch thickness and then pull it through my wire drawing mill, where the wire drawing dies curve up the edges and form it into a tube several hundred feet long. It takes about ten trips through the wire mill, each pass drawing it down a few 1/1000 of an inch until the edges meet. After that, one more pass through the next smaller wire drawing die compresses and cold-forms the edges together where they can't be seen. The big hassle with thin-walled seamed tubing like that is that it can't be wound up on a spool for storage in the safe because the seam allows a kink to form when it is bent around a tight radius, and at times I have had $5000 worth of gold or platinum tubing I just made lying on the shop floor waiting to get used up! Oh, such problems life dishes out!