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Metalworking (rec.crafts.metalworking) Discuss various aspects of working with metal, such as machining, welding, metal joining, screwing, casting, hardening/tempering, blacksmithing/forging, spinning and hammer work, sheet metal work. |
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Concrete machine tools
"Ed Huntress" wrote in message news:... "Leon Fisk" wrote in message ... On Fri, 16 Jan 2009 16:38:02 -0500, "Ed Huntress" wrote: "Leon Fisk" wrote in message ... On Fri, 16 Jan 2009 00:25:13 -0500, "Ed Huntress" wrote: snip I would have approached this as a long-term research project years ago, except that I don't want to spend that much of my spare time, and I see no money in it. If a manufacturer could make a lot lighter frame work for say a lathe, it seems like it would be profitable. The do-it-your-selfer would buy some decent bags of redi-mix locally and pour/finish the machine in situ. First, there's too much labor for it to be commercially viable. That is, except for the sheet-metal structures with the poured-in concrete. And the polymer/granite-aggregate machines you see promoted at shows are mostly viable for special, custom machines. Second, "pouring" a machine is pretty limited, because it's not that simple to get the required tensile strength and resistance to cyclic loading. It can be done, and it doesn't require a lot of skill. But it does take some time and you have to know what you're trying to achieve. Unfortunately, concrete is not cast iron. It requires some engineering for any kind of structure that needs to handle more than compressive loads. I don't think this is what you had in mind though I wish it were that simple. Ahh... but what if you purposely built/added strategically placed threaded rods and such. I'm sure they would have to be somehow enclosed in a sleeve, a bit protected from the cement mixture. After the cement has setup/cured, slap on plates/washers over them and then add tension to provide for cyclic operations. That's the "screw-thread" approach to post-tensioning. It has been used for small projects. My first experiment with PT, over 30 years ago, was done exactly that way. Post-tensioning is usually done with plain steel rods encased in plastic tubes, and a hydraulic jack, with a strain gage to determine when the elastic limit of the steel has been reached. At that point a clamp is squeezed onto the rod, and it applies the full elastic potential of the steel to the concrete. It's tougher with threaded rod because the rod has to be aligned and kept very straight, or the threads catch and drag the plastic tube. It works OK for short spans. Even if you thread the ends of smooth rod, it's more difficult to determine the elastic limit. But it can be done. They use torque sensors on the torque wrenches used in building car engines, for just this purpose. All of this has to be done after the concrete has cured, of course, and there is some relaxation of the tension as the concrete shrinks. All of this is part of the engineering calculations. In something as small as a machine tool it's fairly trivial. The best bet is to wait a month or so before tensioning. My thinking was more towards building a light weight inner skeleton and let the assembler build the simple outside forms from whatever they like. You would simply provide suggested measurements for them to use for the forms. OK, here's a simplified description of what you're dealing with, using that approach. First, the steel has to be quite close to the surface of the concrete to do any good in terms of *ultimate failure*. However, with no pre-tension, the concrete will be vulnerable to surface cracking from cyclic loads, or just from tensile loads. The usual solution to this is to avoid putting the major loads on the concrete. Those French and Italian machines I mentioned use the concrete mostly to stabilize the steel elements, preventing them from buckling by applying mostly compressive loads to the concrete, from the sides of the steel elements. Another way to deal with it is to use fine mesh just under the concrete's surface. You get something like a ferrocement skin that way. It doesn't completely prevent cracks, but they're very fine and very shallow, and have no significant effect on the strength or stiffness of the structure. The wire mess acts as crack-stoppers, so the most you get is a little crazing of the concrete surface. Either way, unless you use multiple layers of mesh, the strength of the structure is really just the strength of the steel that's in it. That's not necessarily bad, but it takes a lot more steel in the combination to do the job that way, because you aren't taking advantage of the concrete's compressive strength, except the light loads that are employed to keep the steel elements from buckling. And the steel will not stay attached well to the concrete if the loads are high. The bond will be subject to high sheer loads. I'm guessing that most people here know how prestressed concrete works; post-tensioned works the same way, basically. Ferrocement is more like fiberglass in polyester or epoxy resin. The extremely short spans of unsupported concrete -- fractions of an inch -- do not get sufficiently loaded in tension for them to fail. The steel mesh comes into play as soon as tension is applied, because of the short spans. Unlike prestressed or post-tensioned structures, however, there is no pre-stressing on the mesh. So ferrocement has more compressive strength than tensile strength. It's strong enough, however, that thin sections of it actually can be bent and they spring back. With fuel costs/shipping most likely to start marching upwards again, seems like you could save a lot in the weight area with some careful thought/engineering. This is just off the cuff thoughts... needs a lot of revision Well, you're on the right track. Lower shipping costs is what motivated the French and Italian machining centers. Also, I think that Hardinge made some machines this way a while back. It's not a bad idea but it results in a heavy machine, once the concrete is poured, that still has a lot of steel in it. What I'm talking about is somewhat different. Stressed-steel and ferrocement produce a concrete structure, not just one that's stabilized with concrete. This book is the best on the subject. Take a look at the chapter descriptions, and you'll see what it's all about: http://www.technopress3000.com/FrameB.htm There are comparable books on prestressed- and post-tensioned structure. This one, by the same U of M prof as the one above, covers the field: http://www.technopress3000.com/FrameB.htm There is a ton of free information on the web about both, but you have to watch out for the info on ferrocement. It's become the darling of the greenie-save-the-Third-World types, who want to make it out of spit and bamboo shoots. g -- Ed Huntress Sorry, I didn't notice that those URLs were the same. Just click on the book titles and it will take you to listings of the chapter headings. Then click on a chapter and you'll see the subjects covered. It's a five-minute course in the engineering of both materials. -- Ed Huntress |
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