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Feeds and speeds are critical parameters in machining operations. Speeds concern the spindle RPM (revolutions per minute), influencing tool temperature and wear. Proper spindle speeds prevent the tool from overheating, which can soften and dull the cutting edge, resulting in poor surface quality or tool failure. Feeds refer to the rate at which the cutting tool advances through the material, typically measured in inches or mm per minute, and are essential for balancing material removal rates with effective chip clearance. Inadequate feed rates can lead to rubbing, vibration and tool breakage. Both feeds and speeds are vital for optimizing tool life, enhancing machining efficiency, and ensuring superior surface finishes.
Calculating the Spindle Speed
The spindle speed can be calculated based on the desired surface speed. Surface speed is the velocity at which the tool's cutting edge moves relative to the material's surface. It is dependent on the material and the type of cutter used. This data is also available online from the tool manufacturer or in general tables. Surface speed is a useful number because it is not dependent on the size of the endmill: all uncoated endmills in aluminium will try to have the same surface speed. To get the same surface speed smaller endmills will need higher RPM.
Spindle speed RPM is calculated with the formula: Surface Meter per Minute x 318.06 / Tool Diameter.
Surface Meter per Minute (SMM), similar to Surface Feet per Minute but using metric units,
Calculating the Feed Rate
To find the feed rate, use this formula: RPM x Feed per Tooth x Number of Flutes (or teeth) on the cutter.
Feed per tooth is the amount of material removed by each tooth of the cutter during one spindle revolution. This is usually measured in millimeters per tooth or inches per tooth. The appropriate value depends on the material being cut (such as metal, plastics, or plywood) and the type of cutter used (e.g., coated or uncoated carbide). You can typically find this information online from the tool manufacturer.
DOC stands for Depth of Cut and is the depth the tool penetrates the material along its axis, perpendicular to the surface being machined. It's usually determined by the cutting-edge length of the tool and how much of the tool is engaged with the material.
WOC stands for Width of Cut and it refers to the width of the material engaged by the cutting tool in the direction parallel to the workpiece surface. This depth is less than the tool's diameter.
The selection of the DOC and WOC must balance efficiency and the need to minimize tool wear and potential damage to the workpiece. Deep cuts in the axial and radial axis can be more efficient but might lead to higher forces on the tool and workpiece, while shallow cuts are generally less efficient but can achieve better surface finishes and extend tool life.
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This video provides a clear guide on how to calculate the feeds and speeds for a tool. The linked website offers valuable resources, including feed per tooth and surface meter per minute values, based on the tool and stock material to assist in determining the appropriate feeds and speeds.
Now that you know how to calculate feeds and speeds, you can apply this knowledge by setting up your tool in Fusion 360 to proceed with the CAM operations. The next video demonstrates how to create your own tool library in Fusion 360.
