CNC Feeds and Speeds Calculator

Calculating the correct feeds and speeds for your CNC machine involve a balance of the physical forces of the cut itself. Calculating the correct feeds and speeds involve considering each of the following variable: the diameter of the cutting tool, the type of material that is being cut, the type of coating on the cutting tool, and the depth of engagement between the tool and the workpiece. Each of these variables can impact the others, so a calculator is often used to test the impact that small changes to any of the variables will have on the others.

A calculator for calculating feed and speeds for CNC machines are useful in that it can turn each of these many rules into a single location where the operator can test the parameters for the cut before beginning the machining operation. Each of the variables that go into the feed and speed calculator are based upon the physical limits of the cut. For instance, the number that is entered for the surface feet per minute (SFM) rate is a means of indicating the speed at which the material can be cut before the cutting tool begin to overheat.

How to Calculate Feeds and Speeds for CNC Machines

The chip load per tooth (CLT) rate is a measure of the amount of material that each flute of the cutting tool can remove; if the CLT rate is too low, the cutting edge will rub against the workpiece instead of shearing the workpiece. Radial and axial depth are variables that determine the amount of the cutting tool that is engaged in the material at one time; these two variables impact the material removal rate of the cutting tool. Finally, the factors related to the cutting tools rigidity and its coating can be used to adjust the recommended speeds up or down from the rates suggested by the tool diameter, the workpiece material, and the depth of engagement; these factors relate to the ability of the tool to handle the cutting loads.

These factors are automatically accounted for in the calculator’s RPM and feed rate suggestion for the cutting tool. It is possible that a person can make mistake when using the calculator. For example, the surface feet per minute and the rpm variables are often treated as the final answer to the feed and speed calculation; in reality, each cutting spindle have a maximum rpm limit for that spindle, and if the rpm calculated by the tooling calculator exceeds that limit for the spindle, it is necessary to recalculate the feed rate for that spindle.

Using the theoretical rpm calculated by the tooling calculator instead of the actual rpm limit of the spindle may lead to chatter mark on the workpiece or a poor finish of that workpiece. Another potential error may be to ignore the radial chip thinning that may occur within the workpiece; radial chip thinning occurs when the radial stepover distance for the tool is small in relation to the diameter of the cutting tool; radial chip thinning results in the amount of material that can be cut in a single pass decreasing for the cutting tool. As a result, the cutting tool may rub against the workpiece.

Such automatic adjustment for radial thinning prevent the programmed parameters for the cutting tool from differing from the actual physical results of cutting the workpiece. The type of material that is being cut can impact the variables related to cutting speed. For example, materials like aluminum can be cut at high rate of surface feet per minute because aluminum naturaly clears chips out of the workpiece if the flute geometry of the cutting tool is set up correctly.

Stainless steel, however, produces more heat than aluminum when being cut; thus, it is harder for stainless steel to clear chips out of the workpiece, and the workpiece can work harder if the cutting tool is in contact with the workpiece for long periods of time. Finally, materials like titanium tend to retain heat at the cutting edge of the tool, so it may be necessary to reduce the engagement of the cutting tool when machining materials like this. Although the tooling calculator will provide suggested value for each parameter, the decision to use those values is up to the operator of the CNC machine.

Furthermore, while the calculator may provide a program that can be used for the CNC machine, it is still necessary for the operator to review that program to ensure that each parameter fall within the actual limits of that machine. For example, the material removal rate (MRR) and horsepower estimates can help the operator to ensure that the machines spindle can handle the chosen rpm. The estimated cut time can help the operator to plan the machining process; however, that estimate is based off the assumption that the cutting tool is always engaged in the workpiece, and ignores air cuts.

Finally, the calculator provides the verdict text as another suggestion to the operator regarding the adjustment of parameters like axial depth, radial stepover, or tool rigidity. In addition to the recommendations of the calculator, an operator can achieve even better results by first establishing a conservative setup of cutting parameters with the cutting tool; after setting up the cut in this conservative manner, the operator can make a short test cut with the CNC machine. If the results of the test cut are acceptable, the operator can use the set of parameters for the workpiece; if not, the operator can adjust the setup according to those test results.

Thus, each of the parameters that are entered into the cutting tool feed and speed calculator establish a narrow window for the parameters to be set. However, the calculator cannot account for the observations of the actual cut; thus, the operator can use the results of making a test cut to adjust the parameters entered into the calculator for the cutting tool. You should of checked the results more carefully to avoid errors.

Its important to recieve accurate data for teh calcualtion.