March 2003 Newsletter
	BASICS OF MACHINING Machining is the science (sometimes art) of material removal through the use of tools. We've all used hand tools such as drills, saws, files, scrapers and such. We've all bumped up against stuff we couldn't drill, couldn't cut, couldn't shape. We've all cut too short, too long, crooked and on and on. The intent of this introductory talk is to highlight some of the fundamental aspects of machining. CUTTING The basis of machining is cutting. If we think about cutting something, we can envision splitting the item with a knife, or slicing off pieces. Most machining is like slicing off pieces. Typically we call the part we are cutting the work piece and the knive the cutting tool. Cutting tools come in many many shapes, sizes and materials. TYPES OF MACHINES Conceptually there are only a few basic types of machine tools. The lathe in which the work piece turns, and mill in which the cutter rotates are the two primary machine tools. To that we add saws, grinders, drill presses, and many more variations on the above. In all cases the important thing to focus on is the interaction between the work piece and the cutter. SPEED We all know from flying that too fast can cause trouble and too slow can bring on a stall. Machining is the same old story. If we try and cut too fast we burn the cutter, if we cut too slow we stall the machine. Almost all machining involves rotation and there is a simple formula which you should keep in mind which will work in all cases. Speed is like velocity in a car. In our case we are generally interested in determining what speed to cut or the rotational speed in rpm. A simple formula to determine that is : Rpm = 3.82 * SFPM / DIA First simplify the formula and change 3.82 to 4 SFPM is "SURFACE FEET PER MINUTE" DIA is the diameter For a lathe the diameter is the diameter of the cut. For example, if the piece rotating in the lathe is 6" diameter, then 6" is the number to be used. However, it is important to know that as you whittle the diameter down say from 6" to 3" you need to increase the speed of rotation as you get closer to center. For a mill / drill and most other tools the cutter is the diameter to use. This makes calculations somewhat easier since the cutter diameter stays the same. SFPM is "SURFACE FEET PER MINUTE" Surface speed per minute pertains to how fast you can cut various materials. There is a lot of theory involved, but as a rule of thumb we want the maximum surface speed possible which gives us reasonable tool life and provides adequate safety both for ourselves and the workpiece. Like a car, we can drive around in first gear all the time and won't get far. If we get going too fast, we are likely to get into trouble with the law or worse, have an accident. The theoritical surface speed depends on material we are cutting and the type and material of the cutting tools. There are numerous charts relating surface speed to the various tool manufacturers. FEED Feed is how hard you push. A heavy feed takes a big chip and cuts at the highest rate. A light feed creates small chips and may or may not be easier on the machine. Too much feed may stall the machine. Too little feed may cause problematic chips and can cause burning in materials like stainless steel CHIP REMOVAL The fundamental calculation relating power and performance is chip removal. Simply stated the chip removal rate is the amount of chips you cut over time. There are theoretical rates for materials, but typically the chip removal rates pertain to the type of machine tool, it's horsepower and rigidity. HEAT Thermodynamics Conservation of Energy states that all energy equations balance. When we machine a part, we say we put work into the part. It takes power to do that work which is typically represented by the electrical power used to operate the machine. Since all of the power or energy must balance it's handy to know where the energy goes. Basically the energy used to machine a part goes into the part itself and more importantly into the chips. If we can cause more energy to be put or diverted into the chips then we have less problem with too much heat into the part. There are of course other ways to cool the part while machining, the most obvious being coolant sprayed of flooded on the part. MATERIALS Materials have properties such as hardness, elasticity, strength, ductility, heat conduction, and on and on. These properties are what dictate the ability to machine. Hard materials are typically harder to cut. Stainless is particularly difficult to cut do to his high strength, toughness and heat conduction. SAFETY This is probably the most important aspect to always keep in mind while machining. When things are going well, machining may seem effortless. When things go bad safety or lack of safety may come to the forefront. Machine tools are strong because they have to be to machine metals. A simple fractional horsepower drill press can wrap a human around it's spindle without slowing down. Machines can be very dangerous. The process of cutting creates chips which fly off at cutting speeds. Not only can these chips be sharp, but they may contain a lot of the process heat which means they can burn. Count the fingers of most machinests you run across and then think to yourelf how attached you've become to your own fingers, and eyes and so on. Common sense is the operative. First wear protective clothing including glasses, gloves and proper appeal (no loose sleeves, etc.). Second, expect the operation to throw off chips and manage that with baffles, or enclosures. Keep the area clear so if you need an escape route, it's not blocking your way. TOOLING Typically pertains to the cutters and attachments used in the machining process. It is common knowledge in the industry that the tooling often costs as much as the machine tool. Tooling comes in many forms, drill bits, grinders, lathe bits, milling cutters, saw blades. Most tooling is designed to wear which means it is expendable or doesn't last forever. With the advent of high speed machining a whole line of carbide and ceramic insert tooling was created. Now the expendable part of the tooling is the insert which comes in many varieties. FIXTURING This is the process of holding onto the work for machining. Essentially you can't hold onto a part and machine all sides at one time so fixtures are very handy in multiple part operations to located the part in a repeatable aspect. Fixturing may take longer to make then the initial part. THREADING Most thread cutting is done in a lathe or machine that functions like a lathe such as a screw machine.
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