Cutting Speeds & RPM Calculations

Milling machines are used to perform a wide variety of machining operations. There are those operations that are strictly milling operations, but we also use milling machines to perform other operations such as drilling, reaming, tapping, and boring. The rules and principles of cutting speeds and R.P.M. calculations that apply to these "other" operations being performed on milling machines are still used in the same manner. An example of this would be reaming. Reaming is done at half the speed and twice the feed as drilling. This rule still applies on the milling machine as it does on the drill press or the lathe.

Cutting Speed for Milling- Cutting speed is the speed at the outside edge of the milling cutter as it is rotating. This is also known as surface speed. Surface speed, surface footage, and surface area are all directly related. Two wheels can illustrate a nice example of this. Take two wheels, one wheel which is three feet in diameter and the other wheel which is one foot in diameter, roll each wheel one complete turn (Figure 1).

Figure 1

Which wheel traveled farther? The larger wheel traveled farther because it has a larger circumference and has more surface area. Cutting speeds work on the same principle. If two cutters of different sizes are turning at the same revolutions per minute (RPM), the larger cutter has a greater surface speed. Surface speed is measured in surface feet per minute (SFPM). All cutting tools work on the surface footage principal. Cutting speeds depend primarily on the kind of material you are cutting and the kind of cutting tool you are using. The hardness of the work material has a great deal to do with the recommended cutting speed. The harder the work material, the slower the cutting speed. The softer the work material the faster the recommended cutting speed (Figure 2).

Steel Iron Aluminum Lead

Increasing Cutting Speed Figure 2

The hardness of the cutting tool material has a great deal to with the recommended cutting speed. The harder the cutting tool material, the faster the cutting speed (figure 3). The softer the cutting tool material the slower the recommended cutting speed.

Carbon Steel High Speed Steel Carbide

Increasing Cutting Speed

Figure 3

The depth of cut and the feed rate will also affect the cutting speed, but not to as great as an extent as the work hardness. These three factors; cutting speed, feedrate and depth of cut are known as cutting conditions. Cutting conditions are determined by the machinability rating. Machinability is the comparing of materials on their ability to be machined. From machinability ratings we can derive recommended cutting speeds. Recommended cutting speeds are given in charts. These charts can be found in your Machinery’s Handbook, textbook or a chart given to you by your tool salesperson. In Table 4 you will find a typical recommended cutting speed chart.

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Milling-Machine Operations

Table 4. Recommended Cutting Speed for Milling in Feet per Minute (fpm)

Work Material	Hardness, Bhn	Cutting Speed, fpm
		High-Speed Steel	Carbide
Plain Carbon Steel, AISI 1010 to AISI 1030	to 150 150 to 200	110 100 to 140 100 80 to 120	600 400 to 900 450 300 to 700

The milling machine speed must be set so that the milling cutter will be operating at the correct cutting speed. To set the proper speed we need to calculate the proper revolution per minute or RPM setting. We stated earlier that cutting speed or surface speed would change with the size of the cutter. So to keep the surface speed the same for each size cutter we must use a formula that includes the size of the cutter to calculate the proper RPM to maintain the proper surface footage.

Calculating RPM- The RPM setting depends on the cutting speed and the size of the cutter. The RPM setting will change with the size of the cutter. As the milling cutter gets smaller the RPM must increase to maintain the recommended surface footage. Again, take the case of the wheel. Think of the cutter as a wheel and the cutting speed as a distance. A larger wheel (cutter) will need to turn fewer revolutions per minute to cover the same distance in the same amount of time than a smaller wheel (cutter). Therefore, to maintain the recommended cutting speed, larger cutters must be run at slower speeds than smaller cutters.

The milling machine must be set so that the milling cutter will be operating at the proper surface speed. Spindle speed settings on the milling machine are done in RPMs. To calculate the proper RPM for the tool we must use the following formula:

Cutting speed (CS) X 4
Diameter of cutter (D)

This simplified version of the RPM formula is the most common formula used in machine shops. This RPM formula can be used for other machining operation as well.

Lets put this formula to work in calculating the RPM for the machining example below. Use the recommended cutting speed charts in Table 5.

Milling-Machine Operations

Table 5. Recommended Cutting Speed for Milling in Feet per Minute (fpm)

Work Material	Hardness, Bhn	Cutting Speed, fpm
		High-Speed Steel	Carbide
Plain Carbon Steel, AISI 1010 to AISI 1030	to 150 150 to 200	110 100 to 140 100 80 to 120	600 400 to 900 450 300 to 700
AISI B1111, AISI B1112, AISI B1113, Steel	140 to 180	140 110 to 200	650 400 to 1200
Plain Carbon Steel, AISI 1040 to 1095	120 to 180 180 to 220 220 to 300	195 80 to 120 85 70 to 110 60 30 to 80	600 400 to 800 350 300 to 500 200 100 to 300
All Alloy Steels Having .3% or Less Carbon Content: AISI 1320, AISI 3120, AISI 4130, AISI 4020, AISI 5020, AISI 4118, AISI 9310, etc.	180 to 220 220 to 300 300 to 400	80 65 to 100 60 30 to 80 40 30 to 50	350 300 to 600 300 200 to 350 125 100 to 150
All Alloy Steels Having More Than .3% Carbon Content: AISI 1340, AISI 2340, AISI 4140, AISI 4150, AISI 4340, AISI 5140, AISI 5150, AISI 52100, AISI 8660, AISI 9260, etc.	180 to 220 220 to 300 300 to 400	80 60 to 100 55 30 to 80 30 20 to 50	325 275 to 450 250 180 to 300 100 80 to 130

A milling cut is to be taken with a 0.500 inch high speed steel (HSS) endmill on a piece of 1018 steel with a brinnel hardness of 200. Calculate the RPM setting to perform this cut.

Cutting Speed = 90 (fpm)

Diameter of Cutter = 0.500

Since the available spindle speed settings are generally not infinitely variable, the machine cannot be set precisely to the calculated RPM setting. Some judgment must be made in selecting the speed to use. Try to get to the speed which is nearest to the calculated RPM, but if you can’t consider these conditions. Are you roughing or finishing? If you are roughing, go slower. If you are finishing go faster. What is your depth of cut? If it is a deep cut, go to the slower RPM setting. Is the setup very rigid? Go slower for setups that lack a great deal of rigidity. Are you using coolant? You may be able to go to the faster of the two settings if you are using coolant. The greatest indicator of cutting speed is the color of the chip. When using a high-speed steel cutter the chips should never be turning brown or blue. Straw colored chips indicate that you are on the maximum edge of the cutting speed for your cutting conditions. When using Carbide, chip colors can range from amber to blue, but never black. A dark purple color will indicate that you are on the maximum edge of your cutting conditions. Carbide cutting tools are covered in much greater detail in other section of your learning materials.

Let’s try some more examples.

A milling cut is to be taken with a 6.00 inch (HSS) side milling cutter on a piece of 1045 steel with a brinnel hardness of 300. Calculate the RPM setting to perform this cut.

Cutting Speed = 55 (fpm)

Diameter of Cutter = 6.00

A 1-inch (HSS) drill is used on a piece of 1010 steel with a brinnel hardness of 100. Calculate the RPM setting to perform this drilling operation.

Cutting Speed = 140 (fpm)

Diameter of Cutter = 1.00

A milling cut is to be taken with a 3.00 inch carbide face milling cutter using coated inserts on a piece of 4140 alloy steel with a brinnel hardness of 200. Calculate the RPM setting to perform this cut.

Cutting Speed = 400 (avg. fpm)
Diameter of Cutter = 3.00