Calculate Speeds and Feeds for 1/2" (0.5 in) 2 flute end mill in Mild Steel at cutting speed = 100(ft/min), Chip Load=0.001(inch per tooth)

When you have manufacturers' data simply find your tool in the catalog and cross-reference the cutting speed and chip load against the tool diameter:

Machining operation in which material is removed from the workpiece by a powered abrasive wheel, stone, belt, paste, sheet, compound, slurry, etc. Takes various forms: surface grinding (creates flat and/or squared surfaces); cylindrical grinding (for external cylindrical and tapered shapes, fillets, undercuts, etc.); centerless grinding; chamfering; thread and form grinding; tool and cutter grinding; offhand grinding; lapping and polishing (grinding with extremely fine grits to create ultrasmooth surfaces); honing; and disc grinding.

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BIG KAISER’s MEGA Synchro drives taps securely and compensates for the small synchronization errors common with many CNC machine tools.  Image courtesy of BIG KAISER Precision Tooling.

Process of both external (e.g., thread milling) and internal (e.g., tapping, thread milling) cutting, turning and rolling of threads into particular material. Standardized specifications are available to determine the desired results of the threading process. Numerous thread-series designations are written for specific applications. Threading often is performed on a lathe. Specifications such as thread height are critical in determining the strength of the threads. The material used is taken into consideration in determining the expected results of any particular application for that threaded piece. In external threading, a calculated depth is required as well as a particular angle to the cut. To perform internal threading, the exact diameter to bore the hole is critical before threading. The threads are distinguished from one another by the amount of tolerance and/or allowance that is specified. See turning.

Runs endmills and arbor-mounted milling cutters. Features include a head with a spindle that drives the cutters; a column, knee and table that provide motion in the three Cartesian axes; and a base that supports the components and houses the cutting-fluid pump and reservoir. The work is mounted on the table and fed into the rotating cutter or endmill to accomplish the milling steps; vertical milling machines also feed endmills into the work by means of a spindle-mounted quill. Models range from small manual machines to big bed-type and duplex mills. All take one of three basic forms: vertical, horizontal or convertible horizontal/vertical. Vertical machines may be knee-type (the table is mounted on a knee that can be elevated) or bed-type (the table is securely supported and only moves horizontally). In general, horizontal machines are bigger and more powerful, while vertical machines are lighter but more versatile and easier to set up and operate.

Yet others contend that thread milling is more accurate and almost as fast. Plus, there’s no risk of the tap breaking inside the workpiece, which often results in scrapping the part.

What are Machining Speeds and Feeds One of the primary tasks machinists must learn to perform is a calculation of speeds and feeds required for milling, drilling, and turning. It starts with knowing what workpiece material you have and what tooling and how you will be using to machine it.The combination of these two factors determines your initial Cutting Speed and Chip Load that you can put into the speed and feed formulas to calculate the cutting tool RPM and feed rate. Cutting Speed is the speed at which the tip of the tool travels through the material. It is commonly expressed in Surface Feet per Minute (SFM) or Surface Meters per Minute (SMM).  Chip Load is the advancement of each tooth per revolution of the tool. In other words, Chip Load is the thickness of the material that each tooth removes per each revolution. So how do you find the Cutting Speed and Chip Load for your tool? Tool manufacturers often post Cutting Speeds and Feeds for their tools for various materials and cutting conditions. Most experienced machinists simply remember cutting speeds and chip loads for materials they machine most often. Here are commonly recommended cutting speeds and chip loads for carbide tools for a couple of materials: Aluminum: 300SFM, 0.7% of the diameter (for example fz = 0.5"dia x 0.007 = 0.0035in/tooth) Annealed Tool steel: 150SFM, 0.4% of the diameter (for example fz = 0.5" x 0.004 = 0.002in/tooth) When you have manufacturers' data simply find your tool in the catalog and cross-reference the cutting speed and chip load against the tool diameter: Since cutting speeds can be in either Imperial (SFM) or Metric (SMM or m/min) units, you have to use two formulas to calculate the RPM. Imperial Speed and Feed Calculation Code RPM= 12 x SFM = Please enter Speed and Diameter 3.14 x in Feed Rate = RPM x x in = Please enter RPM, number of teeth, and chip load (in/min) Metric Speed and Feed Calculation Code RPM= 1000 x m/min = Please enter Speed and Diameter 3.14 x mm Feed Rate = RPM x x mm = Please enter RPM, number of teeth and chip load (mm/min)

Regardless of the workpiece material and preferred threading method, what’s most important is that the job is tooled and programmed properly. Here’s a list of pointers to help you do that:

Machining operation in which a tap, with teeth on its periphery, cuts internal threads in a predrilled hole having a smaller diameter than the tap diameter. Threads are formed by a combined rotary and axial-relative motion between tap and workpiece. See tap.

Machine designed to rotate end-cutting tools. Can also be used for reaming, tapping, countersinking, counterboring, spotfacing and boring.

The formula is used for milling and drilling applications. Please note that some tool manufacturers provide their recommended feed rate as feed per revolution. In such cases do not multiply by the number of teeth.

One of the primary tasks machinists must learn to perform is a calculation of speeds and feeds required for milling, drilling, and turning.

Cylindrical tool that cuts internal threads and has flutes to remove chips and carry tapping fluid to the point of cut. Normally used on a drill press or tapping machine but also may be operated manually. See tapping.

When tapping, there’s no way to adjust the thread’s pitch diameter except by changing to a different “H” size tap. This requires machine downtime and a large tool inventory. When thread milling, size adjustment is a simple offset. What’s more, the thread mill you applied on the 1⁄4"-20 job last week can be used on the 9⁄16"-20 job today, or any other 20-pitch thread that comes along.

Machining operation in which metal or other material is removed by applying power to a rotating cutter. In vertical milling, the cutting tool is mounted vertically on the spindle. In horizontal milling, the cutting tool is mounted horizontally, either directly on the spindle or on an arbor. Horizontal milling is further broken down into conventional milling, where the cutter rotates opposite the direction of feed, or “up” into the workpiece; and climb milling, where the cutter rotates in the direction of feed, or “down” into the workpiece. Milling operations include plane or surface milling, endmilling, facemilling, angle milling, form milling and profiling.

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Cutting Speed is the speed at which the tip of the tool travels through the material. It is commonly expressed in Surface Feet per Minute (SFM) or Surface Meters per Minute (SMM).

Flexible-sided device that secures a tool or workpiece. Similar in function to a chuck, but can accommodate only a narrow size range. Typically provides greater gripping force and precision than a chuck. See chuck.

Cone-shaped pins that support a workpiece by one or two ends during machining. The centers fit into holes drilled in the workpiece ends. Centers that turn with the workpiece are called “live” centers; those that do not are called “dead” centers.

As a rule, thread mills are limited to threads no smaller than 1⁄8"(3.175mm), and because of the high cutting forces and resulting tool deflection, are generally limited to threading holes about 3 diameters deep. On the other hand, taps for #000-120 watch threads are readily available, and an extension tap can reach roughly 20 diameters deep.

Secures a cutting tool during a machining operation. Basic types include block, cartridge, chuck, collet, fixed, modular, quick-change and rotating.

Since cutting speeds can be in either Imperial (SFM) or Metric (SMM or m/min) units, you have to use two formulas to calculate the RPM.

Workholding device that affixes to a mill, lathe or drill-press spindle. It holds a tool or workpiece by one end, allowing it to be rotated. May also be fitted to the machine table to hold a workpiece. Two or more adjustable jaws actually hold the tool or part. May be actuated manually, pneumatically, hydraulically or electrically. See collet.

It starts with knowing what workpiece material you have and what tooling and how you will be using to machine it.The combination of these two factors determines your initial Cutting Speed and Chip Load that you can put into the speed and feed formulas to calculate the cutting tool RPM and feed rate.

CNC machine tool capable of drilling, reaming, tapping, milling and boring. Normally comes with an automatic toolchanger. See automatic toolchanger.

Kip Hanson is a contributing editor for Cutting Tool Engineering magazine. Contact him by phone at (520) 548-7328 or via e-mail at kip@kahmco.net.

For turning applications, we do not need this formula since Spindle Speed is usually given in Constant Surface Speed (CSS), which uses SFM value directly. But if you still want to use the RPM formula, then the diameter value is the actual diameter of the workpiece.

Microprocessor-based controller dedicated to a machine tool that permits the creation or modification of parts. Programmed numerical control activates the machine’s servos and spindle drives and controls the various machining operations. See DNC, direct numerical control; NC, numerical control.

What about hard materials such as heat-treated 17-4 PH or D2 tool steel? There’s little chance of successfully thread milling metals much above 45 HRC. And if you’re going to try to tap them, be prepared to duck the flying shrapnel when the tap explodes! Internal thread grinding and, on rare occasion, orbiting sinker EDMing are about the only ways to produce good threads in materials harder than 45 HRC.

For many, the former is the preferred method of threadmaking on a CNC machining center. It’s fast, easy to program and can even be done offline—on a drill press or with a tapping arm—if the CNC machine is better utilized another way.

Taps suitable for threading a fire hydrant hose are also available, but good luck driving a tool that big. Most CNC machine spindles struggle with tap diameters 1⁄2" (12.7mm) and larger.