How to Calculate Inches Per Minute

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Inches per minute are units commonly used in machining applications, such as when describing the feed rate of a milling machine. Calculating inches per minute can be done once you understand the basic principles involved.

Inches Per Minute Definition

In simple terms, the inches per minute definition is that it is a measure of speed, or how fast something is moving. Speed can be measured in many different units, but it is always a distance unit divided by a time unit. For example, when driving your car, your speedometer tells you your speed in miles per hour, while the speed of a sprinter might be measured in meters per second.

Simple unit conversion can change one set of speed units into another. For example, converting 25 miles per hour (mph) to meters per second (m/s) would be done as follows:

25 \Bigl( {\sout{\text{miles}} \above{1pt} \sout{\text{hour}}}\Bigr)\Bigl({1609\text{ m} \above{1pt} \sout{1\text{ mile}}} \Bigr) \Bigl({\sout{1\text{ hour}} \above{1pt} 3600\text{ seconds}} \Bigr)=11.2 \text{ m/s}

Inches Per Minute Definition Applications

The units of inches per minute are used frequently in machining applications such as welding, drilling and milling. These units may describe the rate at which a machine apparatus moves across a material (also known as the feed rate) and can be relevant to determining proper machine use as well as production rates for job quotes.

Feed Rate

Feed rate is the rate at which a cutting tool, such as a milling bit in a milling machine, moves along the surface it is cutting. The inches per minute (IPM) for a milling bit can be calculated as the product of the rate of rotation in revolutions per minute (RPM) and the inches per revolution (IPR) as follows:

\text{IPM}=\text{RPM} \times \text{IPR}

If needed, inches per revolution can first be calculated by multiplying inches per tooth (IPT), also called the chip rate, by the number of teeth, or cutting edges, in the drill (Z).

\text{IPR}=\text{IPT} \times \text{Z}

Surface Feet Per Minute

Another associated concept is surface feet per minute (SFM). This is the speed of the cutting edge relative to the surface of the material. This is different than feed rate because the cutting edges on a milling bit move in a circle around the bit axis. Surface feet per minute can be calculated from the rotation rate in RPM and the diameter of the bit, D, in feet as follows:

\text{SFM}=\text{RPM} \times \text{D}\times \pi
  • Note that if the diameter is measured in inches, you can easily convert it to feet by dividing by the conversion factor of 12 inches per foot.

A more intuitive example of how rotational rate relates to linear speed can be seen in how the tires on a car work. The tires rotate at a certain number of revolutions per minute. For each revolution, the car then rolls forward a distance equal to the tire’s circumference. The forward speed of the car in feet per minute would then be given by:

\text{speed}=\text{RPM} \times \text{tire circumference} =\text{RPM} \times \text{D}\times \pi

Where D is the diameter of the tire in feet.

If the car were held in place, and the tires were spinning out, the surface feet per minute of the tires (the speed of the surface of the tires relative to the pavement) would be that same speed.

Welding Inches Per Minute Formula

Another application that uses the units of inches per minute occurs in welding. There is often an ideal travel rate that will create the strongest weld for any given set of materials and conditions. The welding inches per minute formula is given by simply taking the length of the weld in inches and dividing by the time to complete the weld in minutes.

For any particular set up, however, the notion of a welding inches per minute formula is a little misleading. While you can certainly determine how fast a weld is being done empirically, the ideal travel rate is not found by a formula but is rather found in a table for different thicknesses and types of welding rods.

Metric Conversions

Some useful metric conversions associated with the above applications are the following:

  • To convert from inches per minute to millimeters per minute, multiply by 25.4 mm/inch.
  • To convert from surface feet per minute to meters per minute, divide by 3.28 ft/m.

References

About the Author

Gayle Towell is a freelance writer and editor living in Oregon. She earned masters degrees in both mathematics and physics from the University of Oregon after completing a double major at Smith College, and has spent over a decade teaching these subjects to college students. Also a prolific writer of fiction, and founder of Microfiction Monday Magazine, you can learn more about Gayle at gtowell.com.

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