The number of flutes on a bit is essential to calculating proper feed and speed

The number of flutes on a bit is essential to

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The number of flutes on a bit is essential to calculating proper feed and speed rates. For most applications a bit with 1, 2, or 3 flutes can be used, but the feed rate and RPM must be adjusted accordingly to maintain proper chip load. End shape: Straight and up-spiral bits come in a variety of end shapes. Square ends are most com- mon, and are a good choice for creating pockets and grooves, profile cutting, simple lettering, and drilling operations. Ball (or rounded) ends are best for 3D carving. V-carve bits are often used to create complex letters for sign making. They can also be used to chamfer edges and create countersinks for screw holes. What’s the difference between a square-end bit and an end mill? An end mill has cutting flutes that extend across the bottom (end) of the bit. It is designed for plunge- cutting as well as lateral cutting. “Square-end” is simply a description of end shape. Square-end bits are not always end mills, and end mills do not always have square ends. The first image at right shows a true end mill, and the second shows a straight-fluted, square-end bit that is not an end mill. A ramp-in must be applied to a toolpath when using this type of bit. Calculating feeds/speed with Chip Load: Chip load refers to the actual thickness of the chip cut by each revolution of the cutter. It is the mea- surement that all feed/speed calculations are based on. A spinning bit generates friction and heat as it moves through the material, and part of this heat is pulled away by the flying chips. A larger chip load pulls away more heat, but also puts more stress on the cutter. Each material has its own ideal chip load range that balances heat dissipation with cutter stress. A basic chart for common materials is available in the SB3 software. Click on Tools > Chip Load Cal- culator, then click on Chip Load Help. You can use this chart along with the Chip Load Calculator to determine a good starting speed for each toolpath. More detailed chip load charts are available online at Onsrud’s website (). When calculating feeds/speeds for a toolpath, do not rely on the defaults in your tool database. Those values are only placeholders and are not intended for any particular material. Feeds and Speeds A challenge of getting a good CNC cut is in selecting the best cutting speed (feed rate) and router/ spindle RPM (speed of rotation). Feeds and speeds are a critical part of machining and should be fully understood before deviating from recommended settings. A primary concern of machining is chip load, which is a representation of the size of the chips produced during cutting. The goal is to get the maximum chip load possible to increase productivity, reduce heat, and prevent premature dulling. When chip load is too small, bits will get too hot and dull quicker. When chip load is too high, the tool will deflect creating a bad surface finish and, in extreme cases, chip or break the bit.
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  • Fall '19
  • Tools
  • Milling cutter, High speed steel, Surface feet per minute, End mill, Carbide Tipped

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