Feed Rate

Feed Rate

Feed rate is the velocity at which the cutter is fed, that is, advanced against the work piece. It is expressed in units of distance per revolution for turning and boring (typically inches per revolution [ipr] or millimeters per revolution). It can be expressed thus for milling also, but it is often expressed in units of distance per time for milling (typically inches per minute [ipm] or millimeters per minute), with considerations of how many teeth (or flutes) the cutter has then determining what that means for each tooth.

Feed rate is dependent on the:

§  Type of tool (a small drill or a large drill, high speed or carbide, a box tool or recess, a thin form tool or wide form tool, a slide knurl or a turret straddle knurl).

§  Surface finish desired.

§  Power available at the spindle (to prevent stalling of the cutter or work piece).

§  Rigidity of the machine and tooling setup (ability to withstand vibration or chatter).

§  Strength of the work piece (high feed rates will collapse thin wall tubing)

§  Characteristics of the material being cut, chip flow depends on material type and feed rate. The ideal chip shape is small and breaks free early, carrying heat away from the tool and work.

§  Threads per inch (TPI) for taps die heads and threading tools.

When deciding what feed rate to use for a certain cutting operation, the calculation is fairly straightforward for single-point cutting tools, because all of the cutting work is done at one point (done by "one tooth", as it were). With a milling machine or jointer, where multi-tipped/multi-fluted cutting tools are involved, then the desirable feed rate becomes dependent on the number of teeth on the cutter, as well as the desired amount of material per tooth to cut (expressed as chip load). The greater the number of cutting edges, the higher the feed rate permissible: for a cutting edge to work efficiently it must remove sufficient material to cut rather than rub; it also must do its fair share of work.

The ratio of the spindle speed and the feed rate controls how aggressive the cut is, and the nature of the formed.

Formula to determine feed rate:-

This formula can be used to figure out the feed rate that the cutter travels into or around the work. This would apply to cutters on a milling machine, drill press and a number of other machine tools. This is not to be used on the lathe for turning operations, as the feed rate on a lathe is given as inches per revolution.

FR = RPM X T X CL

Where:

§  FR = the calculated feed rate in inches per minute or mm per minute.

§  RPM = is the calculated speed for the cutter.

§  T = Number of teeth on the cutter.

§  CL = the chip load or feed per tooth. This is the size of chip that each tooth of the cutter takes.

Depth of cut:-

Cutting speed and feed rate come together with depth of cut to determine the material removal rate, which is the volume of work piece material (metal, wood, plastic, etc.) that can be removed per time unit.