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Surface Speed: This is the speed the tip of your flutes or cutting edge is moving with respect to the center of the tool. (For all you physics people, surface speed is your angular velocity while RPM is rotational speed). This is an important concept because a larger tool will cut at the same speed with a lower RPM as a small tool with a higher RPM.

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Why does this matter? When you start cutting with less than 50% of the tool’s diameter as one usually does, the actual FPT is less than the programmed FPT. In most applications this is nothing to worry about as we take a large enough radial depth of cut that the tool is not being damaged. When taking increasingly small cuts with a small FPT and increasingly large tools, issues start popping up. A half inch tool taking a .001” finishing pass at a programmed FPT of .005” results in an actual FPT of only 0.0004” – and before you ask, no, the extra zero isn’t a typo. As you can see checking the chip thinning FPT with the programmed value can be critical to tool life and preventing rubbing when taking small passes.

Often times, learning speeds and feeds is the most difficult part for a beginning machinist. With the nearly endless combination of tools and materials, getting the recipe right on the first try can seem impossible – but we’re here to tell you how it can be easy!

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Feedrate: This is how fast the machine is moving on any of its axes (essentially how fast your part is moving) with relation to the tool.

Width of Cut: Also known as radial depth of cut (WOC or RDOC), this controls the width of the chip and how much of the tool’s diameter is engaged in the cut.

Earlier we talked about Feed per Tooth, which controls the thickness of the chip and how much material each flute engages. In reality, this number is only technically correct when the tool is cutting at or above 50% of its cutting diameter – which is pretty uncommon. As you can see the image below, each chip is widest at the lagging edge of the flute (entering the cut) and gradually thins until it is cut off at the leading side (as the flute exits the cut).

If you’re a beginning machinist, you’re probably most familiar with RPM and feedrate – and that’s okay! You can easily become familiar with creating recipes based solely off of these two factors, especially when using the same sized tools in the same material. However – we strongly recommend using FPT and Surface Speed to control your cuts! Why?

Feed per Tooth: FPT controls the thickness (or approximate thickness, we’ll get into this later) of each chip whereas width of cut controls the width of the chip.

Depth of Cut: Abbreviated as DOC or ADOC, depth of cut controls the length of the flutes that are cutting the material and consequently the height of the chip.

RPM: This is  how many revolutions per minute your tool is spinning. This is how fast your tool is spinning, but not how fast the flutes are moving.