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Feeds and Speeds
For all metal-cutting processes, "speeds and feeds" are important parameters. The colloquial term "speeds and feeds" refers to the speed, feed, and depth of cut of a metal-cutting process. To describe these parameters, we will be using the turning process. The figure below shows the important geometry. The speed (Labeled on the figure as V.) is the cutting speed, which is a measure of the part cut surface speed relative to the (here, stationary) tool.

Speed is a velocity unit, which is typically listed in terms of feet/min, inches/min, meters/second, or meters/min.

Feed (Labeled on the figure as fr), is the amount of material removed for each revolution or per pass of the tool over the workpiece. Feed is measured in units of length/revolution, length/pass, length/tooth, length/time, or other appropriate unit for the particular process.

The depth of cut, DOC (Labeled on the figure as d.), represents the third parameter for metal cutting. For turning, DOC is the depth that the tool is plunged into the surface. The DOC is half of the difference in the diameters Da and Db,

the initial and final diameters, respectively.

Below is a summary list of the terms used.

Speeds and feeds are important since they are critical input parameters which determine the output of a machine tool. Other inputs include the material of the part, the shape of the raw stock and final part, and the geometry of the machine tool used. Outputs include the surface finish, the time to machine each part, the temperature of the cutting tool and cut part, the residual stresses left in the part, and warping of the part.
Speeds and feeds typically take much experience and experimenation to determine accurately, but a good place to start is a table of recommended speeds and feeds.

The MRR, or the Metal Removal Rate, is listed at the end of the above relation list. For turning, MRR values range from 0.1 to 600 in3 per minute. Most processes have MRR's that can be expressed as the volume of metal removed divided by the time needed to remove it:
MRR = (volume of cut)/(cutting time).
MRR can be used to estimate the power required to sustain the cutting operation. With turning, the cutting time can be expressed as the following:

Where the symbols have the same meanings as defined earlier on this page and the allowance is an estimation factor which is added to the L term to allow for the tool to enter and exit the cut.

A single-point tool is a cutting tool that cuts only at a single edge or area of the cutting tool. Turning and shaping are examples of single-point cutting. Multiple-point cutting processes are milling and drilling.


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