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CHAPTER NUMBER: Section 1, Chapter 3 |
Chapter 3-Materials for metalworking
Some good materials to work with
As with many of the other sections of this book, I am purposely staying away from engineering data and only giving the practical information you need to get started. The book I would recommend for technical information would be Machinery's Handbook. This book is the best book ever written on metals and the machining of metal (See chapter 1 of this section.) As a beginner, I would also avoid complex parts that require heat treating. If you try to accomplish this with a torch, you will find it an excellent way to destroy perfectly good parts. This can be heartbreaking to novices because they haven't "started over" enough in their life. I recommend using "leadloy" 12L14 for soft steel parts. "Stress proof" steel is the next real improvement in steels. It is a better material that is a little harder to machine. Use 4130 or 4140 steel for extra strong or tough parts, 6061-T6 for aluminum parts, and 303 for round stainless steel parts. These materials are readily available and work quite well. Ordering stock sizes that aren't readily available can be a waste of time. If you only have one part to make, it may take just a few minutes to cut a piece of stock to its proper starting diameter, but it could take an hour on the phone to find the perfect size.
(Photo caption) A good cutoff saw is an important time-saver in any shop. Miniature machine tools weren't meant to remove a lot of metal in a hurry, so getting a rough part as close to size as possible with a cutoff saw saves a lot of time and wear and tear on your machines.
(Photo caption) A treasure trove of raw materials can be found at most scrap yards. Knowing the properties of the material you are looking for is helpful, because often it is not marketed.
Eliminate machining time by cutting material close to size first
A good investment for your miniature machine shop is a cutoff saw. There is a saw available at many discount tool stores that is sold for less than $200 (1998) that is just right for home shop use. It uses a band saw design and can cut off diameters to four inches (100mm). It can also be used in a vertical position making a standard band saw out of it. You need to have a way to cut metal to length in a pleasant fashion, and, believe me, trying to saw off a large piece of stock with a hacksaw isn't pleasant. It will drive you out of the hobby faster than anything I know. When you are working with small machines it is very important to eliminate as much machining as possible by cutting stock to its proper length before starting. A little cutting oil now and then will help the process and keep blades sharper longer. If you have a friend with a good saw it is almost as good, but eventually you'll want your own.
Cut off tool cautions
A cutoff tool or "parting off" tool shouldn't be used in place of a cutoff saw. Cut your material to size with the cutoff saw, and only use the cutoff tool on the lathe for separating the finished piece from the blank. By the way, don't attempt to use a cutoff tool holder and tool on a lathe at any place other than close to the three-jaw chuck. It will bind in the material and get ripped out of its holding device and may end up damaging your machine.
(Photo caption) Brass is easy to work with and polishes up nicely so it is a popular material for model work. The chips are like little splinters, so work with it carefully.
Dealing with minimum orders and small quantities
The choices for metal seem endless, but there is a problem. It's called the "minimum order". Let's say you want to start on a project that contains several types of steel, brass, aluminum, and cast iron. Chances are each of these products will be sold by a different supplier; each with their own rules when it comes to extra charges for small orders. You should also understand that most bar material used in machine shops comes in twelve-foot lengths and material used in construction and fabrication shops comes in twenty-foot lengths. It may cost as much as $25 to have a single bar cut to length. If you call up one of these suppliers and try to order a four-inch piece (100mm) of half inch (12mm) aluminum they will probably ask if you are joking. You will then find they will not deliver orders this small and that it would be cheaper to buy the whole bar rather than paying the cutting charge. These companies exist to service manufacturing companies that buy in very large quantities, not for the home hobbyist.
Sources other than the big industrial suppliers
You will have to order from a supplier that caters to the hobby market. We have several listed on our web site and we wish them well because they provide a service that is well worth the extra cost. You can order all your materials from one source at one time. This allows you to spend your time building things, not talking on the phone to somebody who considers your order not worth the effort. If you are a novice, aways buy enough material to make three parts in case you have to start over.
(Photo caption) Wood can also be machined on miniature machine tools with the use of a tool post to support the hand held cutting tools. Small items like pens made from exotic woods are popular items as gifts. Kits are available for the working parts of the pen. Because these projects require such small pieces of material you can afford to use beautiful and exotic woods. Low material cost is one of the big advantages of working on small projects.
If you are lucky, there could be a salvage yard in your area that sells bar stock. Bring your own hacksaw because they will not cut it for you without a high cutting charge. If you find a good surplus yard that has useable materials, it might be wise to anticipate your needs and buy for future use if the price is right. Surplus yards don't have a standard inventory, and just because they may have a good assortment of material today, it doesn't mean it will be there tomorrow. What is available quite often are bar ends. These are pieces left over after a bar of stock has been cut up which are too short to use for their particular part. The problem is knowing what the material is.
Identifying various aluminum materials in a scrap yard
Material is usually color coded with paint on the end of the bar. I wish I could put a chart of colors vs. material in this book, but for some idiotic reason, each producer has its own colors. Aluminum will have the grades printed on the entire length of the bar. The grade I recommend is 6061- T6. The "T" indicates the hardness. Another grade you may find in a salvage yard is 2011-T3. This has a texture similar to cast iron and it was developed for making round parts on automatic machines. The chips are splinters and will not tangle a machine up with long, stringy chips. Softer grades of aluminum don't machine very well and lack the stiffness required in mechanical parts. The extruded aluminum shapes sold in hardware stores are usually a soft, gummy aluminum such as 6063-T3. Another type of aluminum available is the 2000 series that is usually found in extruded shapes such as rectangles and squares. The last one worth mentioning is the 7000 series. This grade is commonly used for aircraft parts and is available in a hardness to T8. This grade can have a surface hardness equal to some steels so it can be very useful for tooling.
(Photo caption) Aluminum grades are normally printed on the bar stock. Color coded ends also identify each type of aluminum, but color coding varies depending on who produced it.
Buying scrap steel is a little tougher
A whole new set of problems arises when you try to buy steel at surplus yards for it isn't labeled. If it is rusty, it probably is "hot rolled", which is a terrible material to work with for small parts. These are fabrication materials that, as an example, are good for making a wrought iron gate. The best steel material to machine is called "leadloy" or 12L14. It is available in round and square cross sections and can be case hardened. Using 12L14 will ease much of the pain of machining steel as it is a pleasure to cut. Many of the parts of Sherline tools are made of this material. Excellent finishes are easy to attain. The chips break as they are machined off, eliminating the danger of long, sharp chips.
A material called "stress proof" would be the next real improvement over leadloy. It isn't too expensive and machines better than cold rolled. Cold rolled steel is miserable stuff to machine by comparison. It is tough and gummy and has a low cutting speed, but it is slightly better for case hardening than leadloy. Normally you would have to grind it to get a good finish. For some reason that I don't understand, you can cut cold rolled and similar hard-to-machine steels at cutting speeds way above the recommended speeds (as much as four times) and get a mirror finish. The catch is you have to use carbide insert tools. These tools can cut machine times in half and are a basic cutting tool in a modern machine shop. (See Chapter 6 on Cutting Tools in this section.) The chips are very hot and care should be taken to protect yourself.
Tools steels can be hardened completely, not just the outside as in case hardening, and can be very expensive and hard to machine. They are used when high strength or holding a sharp edge is important. A stamping die would be a good example. I would recommend that you purchase these materials only when the material is clearly identified. It takes too long to make parts out of this material to have them ruined in heat treating. Because of the many uses of steels and the many kinds available, I'm limiting the information I provide on these materials. I would end up with a book filled with charts and tables that would be just like the books that line the shelves of your library's engineering section. Before starting on a project that requires heat treating, get the advice of a local heat treat shop.
The positives and negatives of brass, copper and bronze
Brass is usually sold in a half-hard condition and is very easy to machine. I don't like to machine it because the chips are like small splinters that stick in your hands and break off. However, the parts always look nice and can be easily polished and plated. Copper can be machined but the surface has a tendency to "tear" as it is being cut making for poor finishes. Diamond tools are used to cut copper in a production environment if good surface finishes are a must. Some grades of bronze can also be difficult to machine because they will wear tools, even carbide, at an alarming rate.
(Photo caption) With the addition of the wood tool rest, small wooden parts are easy to turn on a Sherline lathe. Here, exotic woods are used to make a handsome key fob and a small flute. Model ships and dollhouse miniatures also require small turned wooden parts.
Cast iron can be purchased as bar stock or cut out of an old piece of junk that contains cast iron. It is easy to machine, but also dirty. The chips look like powdered coal, and you should clean up your equipment after cutting it. The good part about cast iron is it is very stable and will not warp as it is machined. It is surprising how much some materials can warp when cut. Manufactures of complex, close tolerance parts will have materials "normalized" several times between machining operations to counteract these forces. The bigger the part the bigger the problem if you are not using cast iron.
Woods and plastics
Wood and plastics can be easily machined with tools designed to cut metals. For best results machining wood, use a very hard, fine grained wood such as maple. Soft woods will crush rather than cut. This causes poor finishes and splintering. Use two-fluted end mills when machining these materials. The problem with plastic is "melting". You can't allow the chip to clog up the cutting action when machining any type of plastic. For example, if a drill feed is low and the RPM is high, the chips will be numerous and thin. They pile up in the drill's flutes and not only create heat from friction but also work as an insulator. The plastic melts and you have ruined your part. Plastic has a very high temperature expansion rate when compared with other materials and it might be wise to take the close tolerance cuts when the part has cooled down to room temperature. Use high feed rates and sharp tools to try and eliminate these problems.
(Photo caption) A plastic mold used to make a housing for the new digital readout for a Sherline mill. It can take a very expensive mold to produce what looks like a simple part. The longer the machine runs, however, the cheaper the parts become.
(Photo caption) A selection of plastic raw materials. The dak brown block in the upper left is phenolic. Next to it is black Delrin. The red block is fiberglass. At the far left is a piece of white Delrin and next to it is a block of white Nylon. The two piles are new clear beads and recycled black chips used for injection molding.
Don't knock plastic...sometimes it's the best material for the job
I was at a trade show once when a youngster in his teens stopped by our booth. He brought with him a complex part used in the front end of a radio controlled model car. It was made out of plastic and the part was constantly failing in crashes. He wanted to buy a machine to build this particular part out of metal and save money. He thought he was getting ripped off because the plastic parts cost $3.00 each. I told him I could not build a part to replace the one he showed me in less than six hours and a metal part probably would not last any longer, because metal will not spring back to its original shape like the plastic part. I don't think he ever understood why it would take me six hours to make a three-dollar part.
Plastics have changed the way we think about value. To injection mold a complex part, it takes a complex mold, yet the machine cycle time and the skill to run this plastic injection machine remain constant. The machine time is based on the thickest cross-section, not the complexity of the part. In order for the mold to work it must be made with very tight tolerances, even if the part itself has liberal tolerances. With a good mold, plastic parts can be manufactured so inexpensively we consider them disposable, but don't think for a moment that all plastic parts are junk.
Take for example your new cellular phone. The molded forms of the body would be very difficult to produce in any material other than plastic. They are sturdy, light in weight and have the desired color molded in so they don't have to be painted. If they were made out of metal they would be heavier, more expensive and no more attractive. Plastics can be formulated to achieve just about any desired characteristic from flexibility to heat resistance to color or clarity. The development of plastics in our lifetime is probably the most important advance in materials since the discovery of how to work with metals many centuries ago.
Buying plastic for molding and machining
Plastic can be purchased in granular form for injection molding or in bar stock for machining. For machining I prefer Nylon where strength is important and Delrin for general work. In bar form, plastic can be very expensive. A two-inch diameter Delrin rod may cost over one dollar per inch compared with two dollars a pound in granules for injection molding (1998). Injection molded products are the preferred choice for use in manufacturing. As always, the tooling costs are high in order to produce a low cost part. If you are thinking about a new product to be produced in high volume you have to consider using plastic.
Machining plastic can be fun. You don't need coolant and the chips are easy to control and clean up. Use an RPM that allows heavy feed rates and use sharp tools. This will keep the plastic part from melting as it is being machined. When drilling holes use a very fast feed rate to keep the drill flutes from clogging. We ran a very profitable screw machine part by drilling a hole in Delrin at 10 times the suggested rate. The plastic would extrude out the flutes without generating any heat yet the drilled hole had a better finish and tolerance than our previous slower method. Remember that the surface of plastic is slippery and softer than metal. You have to consider this when work is held in a vise or chuck. Thin sections can be easily deformed when clamped. Temperature is also a factor. Plastic has a high coefficient of expansion, and a two-inch part can vary several thousandths of an inch from a hot day to a cool day.
Galling of materials in close fits
In summary, I find aluminum the nicest material to work with. It is clean, strong and rust free. You should also be aware that aluminum can "gall", which is the surface of one part sticking to another of the same material. This usually happens when you check a fit by putting a shaft in a hole made of the same material. Stainless steel can be just as bad as aluminum when it comes to galling. If you have to do a lot of work in these two materials plan to use an anti-galling agent on close fits. It is available in automotive shops.
(Photo caption) This 7" high display model of a 1911 "Baby" model airplane engine by Edwin Teachworth was built entirely of styrene and painted to look like metal. I is on display in the San Diego Aerospace Museum.
Novices believe that some catalog somewhere will always have what they need, and all they have to do is order it. This is not always the case. Many catalogs are filled with sizes that won't actually be produced unless they get a substantial order. When you try to use "off-the-shelf" items, you often must compromise your design. Of course, I'm not suggesting that you make things like nuts or bolts, but you might have to make that special washer. There are plans around for interesting projects that were drawn with the materials that were available then. (Try and buy a BSA screw at your local hardware store and you will understand the problem.) Unless you are building "super scale", you can save yourself a lot of grief and use the materials and fasteners available today. Fortunately, you have chosen a hobby that gives you an alternative if the exact part you are looking for can't be found. You can always make it yourself.