Understanding Feed and Speed Rates in CNC Milling
Every CNC milling operation requires a cutting tool. In fact, the cutting tool is what chips at the material in a controlled and methodical manner to achieve the desired final part. The rate at which the cutting tool runs affects the accuracy and precision of the project. It also impacts on the lifespan of the tool itself.
Before machining commences, it is important to understand the tool cutting speed and feed rates as these parameters are not constant for every project. Cutting speed and feed rates are dependent on the operation, the material being machined as well as the cutter diameter. Today, we take a technical approach to understanding how to determine the ideal speed and feed rates for your CNC milling operation.
What does Cutting Speed and Feed mean?
Cutting speed is simply defined as the differential speed that exists between the tool speed and workpiece speed. It may also be called the surface speed and is a measure of the distance that can be covered over time while machining. Surface speed is measured in SFM (surface feet per minute). It is dependent on the various characteristics of the material to be machined and the cutting tool’s diameter.
The cutter speed is one of the most important parameters in machining as it is used in calculating the ideal feed rate of the cutter. The cutter feed rate is measured by expressing the amount of material (inches) that the cutter can get through in a minute. It is measured in Inches per minute (IPM). The formula for calculating the Surface speed is given as:
Cutting Speed (SFM) = Cutter diameter * Speed (RPM)
The Speed defined in RPM (rotations per minute) is the amount of rotations that the tool can make in a minute. The 3.82 value is an industry standard value. In order to accurately express the cutter feed rate, other factors such as Chip load per tool and Chip load per tooth must be accurately estimated.
What is Chip load per tool and per tooth?
The Chip load per tool, measured in inches per revolution (IPR) is the right amount of material to be chipped off by all cutting edges of a tool in one revolution. The chip load per tooth on the other hand is the appropriate amount of material that would be removed by just one cutting edge of the tool in one revolution.
The cutter feed rate is therefore given as:
Feed rate (IPM) = Speed (RPM) * Chip load per tool (IPR)
These calculations help to benchmark the appropriate range in which your speed rate and cutter feed rates should fall. As materials and operations differ, the cutting speed and feed rate would have to be calculated on each occasion.
Other factors affecting Speed and Feed Rates
Because tools differ in type and size, it is not practical to expect a cutter diameter. The angle and depth of cut also affect this cutter diameter. For instance, a tool with a shank diameter of 0.500’’ and a tip diameter of 0.090’’ will deliver its cutting action somewhere in between the shank diameter and the edge of the tool. In cases such as this, calculating the average diameter along the chamfer with the consideration of both shank and tip diameters will be effective.
To calculate the average diameter, we take the sum of the major and minor diameters and divide by 2.
The equation may be given as Average diameter = (Major Diameter + Minor Diameter)/2.
For the tool used in our example, our average diameter will be 0.295’’.
Speed Rate Limits
After calculating the appropriate speed rate for your cutter using the equation above, the answers might yield unrealistic SFMs. Because the speed rate id affected by the material, the tooling and cut diameter, certain speed rates that are unattainable without the use of high-speed air spindles may result from the computation. In instances such as these, the best approach is run the tooling at the machine’s highest speed or at the highest speed that the machinist is comfortable with.
The correct chip load that matches the cutting diameter should be maintained. Doing this will bring a close-to-optimal performance based on the highest speed that can be obtained. As a rule of thumb, control always trumps high-speed rates, ensure that you operate your machine and operation at the highest speed you can control irrespective of the highest speed returned by your calculation.
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