Milling Materials & Cutting Speeds

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Milling machines are used for a large amount of differing tasks. Some of these tasks are what would come to mind when you think “milling” but the machines are also capable of drilling, boring, tapping, reaming and other jobs. There are rules you can following to determine what cutting speed and RPM should be used depending on what you are doing and what kind of material you are working with. For example, reaming is generally done at half the speed and double the feed of drilling.

Milling machine

Milling machine

First consider your cutting speed. On a milling machine “cutting speed” refers to the speed of the outside edge of the machine’s cutter while the cutter is active. Cutting speed is sometimes referred to as surface speed. When determining surface speed keep in mind that it is directly related to surface footage and surface area. Say you have a one inch cutter and a three inch cutter and you roll each of them by one full turn. The three inch cutter will have traveled further because it has a larger surface area. This principle is used when determining cutting speeds. If you have two cutters that are different sizes and move at them both at the same RPM the larger one will have greater surface speed. Surface speed is measured as surface feet per minute (SFPM). Cutting tools all work using this principle. Your cutting speed will mostly depend on the type of cutting tool you are using and the kind of material you are cutting. How hard the material is determines what cutting speed you should use. Harder materials require slower cutting speeds and vice versa. If you were cutting steel and switched to iron you would want to speed your machine up. If you were cutting aluminum and switched to iron you would want to slow your machine down. The cutting speed you should use will also be greatly influenced by the material your cutting tool is made out of. Cutting tools made of harder materials allow for faster cutting speeds. Feed rate and the depths of the cuts you are making will affect speed as well but not as much as the hardness of the cutting tool and the material. Cutting speed, feed rate and the depth of the cut are called cutting conditions when you group all three together. Your cutting conditions are determined by machinability rating (comparing materials on how well you can machine them).
A milling machine must be set up so that its cutter will operate at the right cutting speed. To find out what the right speed is you need to calculate the RPM (revolutions per minute). The equation for this is (cutting speed * 4) / (diameter of cutter). There are many charts available on the Internet and in machinability books that you can use to find the hardness of materials. These charts make good cheat sheets for machinists who are still learning or for people who do not have the time to look up the information on material hardness frequently. Say that you are using a quarter inch cutting tool on a material with a SFPM of 110. That would be 70 * 4 (which is 280) divided by 0.25. This equals out to 1120 RPM. There will be many times where you cannot set your speed settings to the exact number you can but try to get as close as you can. When you have to decide whether to go a bit faster or a bit slower than the number you calculated think about things like how deep the cut will be, if you are roughing or finishing your project and whether or not you are using coolant. What you want the end result of the project to be should influence your decision. There are also software programs available online that you can use to make the calculations you need. Just be sure to have access to all the numbers you need for your calculations, regardless of whether you are making the calculations yourself or are using software.

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Milling Materials & Cutting Speeds
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Milling machines are used for a large amount of differing tasks. Some of these tasks are what would come to mind when you think “milling” but the machines are also capable of drilling, boring, tapping, reaming and other jobs.
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