Machining and the Machine Shop

Questions and answers for students and new machinists

Tom Boyer at work in the Foundation shop. Tom is using the Bridgeport mill. Behind the red tool box in the foreground is the Shopfox 14 x 40 lathe. To the right is the Nikon Comparitor and Starrett surface table. Behind the lathe is a Deckel pantograph mill. (Click on photo to view a larger image.

What does a Machinist Do?

A machinist trained to use powerful yet precise metal cutting tools to produce accurate metal parts exactly to specifications provided to him in mechanical drawings. He must have knowledge of the properties of the material he will be cutting and the capabilities of the tool he will be using. This means he must know how much metal can be removed from a particular part using a particular tool in a given amount of time. He must also decide how the piece of material will be held on the machine while it is cut and in what order the cuts will be made. If special fixtures must be made to hold the part, he must make them as well. He must also be able to accurately measure the part while it is being made and when it is done to assure it is made to the specified limits of size tollerance.

What are the main tools in a machine shop?

The main power tools a machinist will use are a lathe, milling machine, drill press, grinding machine and bench grinder.  The main hand tools used by a machinist to shape metal are various files and other standard hand tools like hammers and pliers. The machinist also uses a bench vise and a perfectly flat ground steel or granite “surface plate” for measuring. Measuring tools include scales, calipers, micrometers, depth gauges, height gauges and dial test indicators.

What does a Lathe Do?

Shopfox 14 x 40" lathe

On a lathe, a part is held in a spinning chuck, collet or between centers and driven by the lathe’s motor. While the part is spinning, a sharp metal tool is accurately brought into the part to reduce its diameter. Long, curly chips usually come off the part as a strip of metal is peeled away from the spinning part. The cutting tool is mounted firmly in a tool post and the machinist rotates accurately calibrated handwheels that move the toolpost and cutting tool during the cut. He uses one handwheel at the right end of the lathe to drive the main leadscrew that moves the tool across the part from end to end. He uses another handwheel on the cross slide table that moves the tool deeper into the part. These handwheels are usually marked in increments of 0.001” (one one-thousandth of an inch) or 0.01 mm (one one-hundredth of a millimeter). A lathe is usually used to reduce the diameter of a part and to make parts that are round in cross-section.

What does a Milling Machine Do?

Tom at the Bridgeport mill. A Sherline 2000 mill sits on the desk at the right. A Deckel pantograph mill is behind it.

A milling machine differs from a lathe in that the part does not turn, rather it is mounted firmly in a vise or held to the mill table while a spinning cutting tool secured in the powered spindle is moved into the part. The most common cutting tool used is called an “end mill.” It looks like a drill bit, but it is sharp on the sides as well as the end, so it can cut sideways through metal to make a slot, a pocket or to reduce the thickness of a piece. If sharpened to do so, an end mill can also be plunged into a piece of material to make a hole or to start a slot or pocket in the middle of the part. A “fly cutter” is a tool that holds a single pointed cutter that spins in a larger circle. It is used to flatten an entire surface and provides a nice finish. Mills can also be used to hold a boring tool to open up the size of a hole to a precise diameter. The machinist has three handwheels to move on a mill. The “X” axis handwheel is usually on the right-hand end of the mill table and moves the table (and part) side to side. The “Y” axis handle is directly in front of the machinist and moves the saddle the supports the table in and out. The “Z” axis handwheel is on top of the spindle and moves the spinning cutting tool up and down. On some mills the Z axis  “knee” under the saddle moves the table up and down under the fixed spindle instead. Because of their versatility, milling machines do the majority of work in most shops.

What is a Rotary Table?

A rotary table can be used in a drill press or on a milling machine to provide a 4^th axis of movement called the “A” axis. When the handwheel on the side of the rotary table is turned, the table rotates. A part mounted to the table will move in a curved or circular path. This allows a machinist to mill a round part or hole, drill a series of accurately indexed holes in a circle or machine a portion of a circle on or in a part. It can be held horizontally or vertically to cut splines on a shaft or to cut teeth on a gear blank. Used in conjunction with a milling machine, those 4 axes of movement theoretically allow just about any part to be made. That means a mill with a rotary table is the minimum amount of tooling needed to duplicate itself; that is, to make another mill and rotary table. This is why, with the right tools, a good machinist can make almost anything.

What kind of accuracy must a machinist maintain?

Most manual machine tools are capable of allowing a machinist to make parts to tolerances 0.001” (one thousandth of an inch) or less. A good machinist will often be able to make parts to 0.0005” tolerances (one half of a thousandth of an inch). For greater accuracy, grinding machines are used. They can typically be expected to hold tolerances as close as 0.0001” (one ten-thoushandth of an inch). For tighter tolerances, processes like honing must be used.

What is CNC Machining?

Like most home shop machinists we use manually operated machines in our shop, but most parts machined in factories are now made using milling machines and lathes that are controlled by a computer. These are called Computer Numeric Controlled machines or CNC machines. They cut metal the same way a machinist would do, except instead of the machinist turning the handwheels himself, he writes a program to tell the machine how to move, and the machine does the cutting. Instead of regular threads on the screws that drive these machines, they use highly accurate drives called “ball lead screws” that move very accurately. Power to run them is provided by DC motors called "stepper" or "servo" motors that move in very accurately controlled segments of a rotation or steps. The code written by the machinist in a language called “G-code” is translated by the CNC machine’s control program into an appropriate number of steps. For example, 400 steps might represent one full rotation of the stepper motor. If the program sends 400 pulses, the motor will move 400 steps or exactly one rotation. If the screw it is driving has 20 threads per inch on it, that means it would move exactly 1/20 of an inch or 0.050”. If only 40 pulses are sent, it would move exactly 1/200" or 0.005”.

The main advantage of a CNC machine (in addition to speed and accuracy) is that while a machinist can usually turn only one handwheel at a time, the CNC machine can turn two or more at a time. That allows not just straight cuts, but curves or 3D surfaces to be cut. In fact, a CNC machine can cut a complicated curved path just as easily as a straight line. Remember, though, that a CNC machine is not very smart—it is just very good at following orders. If a machinist gives it incorrect information, it will not be able to make a good part.

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