Feeds and Speeds
The Pocket NC mill removes material using a tool with a series of small rotating cutting edges. A tool with 2 cutting edges will make two cuts for each revolution. The material that is removed is referred to as a chip. The thickness of a chip that the Pocket NC is capable of producing will vary depending on what material is being removed. The thickness of a chip(also called the chip load) is adjusted using two variables, feed and speed. Feed refers to the linear rate in inches/minute or millimeters/minute that the cutting tool moves through a material. Speed refers to the rotational speed in RPM (revolutions per minute) of the cutting tool. To increase chip load, speed can be decreased or feed can be increased.
The ideal chip load for some materials is 0.001”. This means, with a spindle speed of 10,000 RPM and a linear progression of 0.002” per revolution (2 cutting edges X 0.001”) gives a feed of 20 inches per minute. It is important to keep in mind that the calculated chip load is only a guideline or starting point, the user should adjust the speed and feed to what sounds good and feels good. Feed and speed can be adjusted on the fly using the Pocket NC user interface.
Machining also has two more variables, side load and depth of cut. These two variables will change the width and length of the chip being removed from the material. Dimension A in the image below represents the amount of material removed with the cutting tool in the X/Y direction or side load. Dimension B in the image below represents the amount of material removed with the cutting tool in the Z direction, also known as the depth of cut.
Above, the image on the left shows a tool path with a side load of 0.08” and the image on the right shows a toolpath with a side load of 0.04”. If the material being removed was soft like wax or wood, the tool path on the left would be much more appropriate as the machine will be able to handle that amount of side load. If the tool path from the image on the right was used with the same material, the user would have spent twice as much time and worked the spindle half as hard.
To adjust side load and depth of cut in Fusion 360 see the images below.
Feed and speed and be adjusted from the Tool tab for each tool path. Below, spindle speed is marked with a blue arrow and feed is marked with a red arrow.
With that said, we have posted a general guideline for the 4 variables below in a number of materials. Note that side load and depth of cut will be expressed as a percentage of tool diameter and feed will be represented as feed/tooth. For example, the end mill pictured above has 4 cutting teeth. To calculate feed, use the following equation.
Use this as a starting point for your feed and speed and make adjustments as you go.