Depth Of Cut – Its effects and optimum value.
Depth of cut is the thickness of metal that is removed during one cut of the tool. It is the perpendicular distance between the machined surface and the uncut surface of the workpiece.
It is the same distance that tool bit moves into the work. It is usually measured in thousandth of an inch or in milimeters.
Depth of cut is usually given in the third perpendicular direction (velocity, feed and depth of cut usually act in mutually perpendicular directions).
Mathematically, it is half of difference of diameters.
Formula of depth of cut is given below:
d1 = diameter of workpiece before machining.
d2 = diameter of workpiece after machining
Depth of cut is inversely proportional to the cutting speed.
While considering depth of cut, it is important to remember that for each thousandth depth of cut, the work diameter is reduced by two thousandths.
General machine practice is to use a depth of cut five times the feed rate.
For Example – a rough cutting stainless steel using a feed of 0.020 inch per revolution and a depth of cut of 0.100 inch which would reduce the diameter by 0.200 inch.
Depth of cut should be reduced when the chatter marks or the machine noise develops.
Effects of depth of cut:
The value of depth of cut affects overall machining performance and machine economy. The most common effects of depth of cut are listed below:
1) Increase in depth of cut increases material removal rate (MRR). MRR is proprtional to speed, feed and depth of cut.So productivity of machining can be enhanced by employing larger depth of cut and hence machining cost can be reduced.
2) Cutting force is dependent on chip which is itself proportional to depth of cut. Hence larger depth of cut can increase cutting dorce which can hamper machining performance by inducing vibration.
3) Increasing the depth of cut too much can also break the cutting tool catastrophically.
4) Depth of cut also impacts chip thickness, type of chip production and shear deformation which are indication of machinability.
Selecting optimum value for depth of cut:
As depth of cut is an important parameter that influences overall machining performance as well as machine economy, so an optimum value of depth of cut must be choosen wisely after considering relevant factors .Usually depth of cut varies from 0.1 to 1.0 mm.
Selection of optimum value of depth of cut is dependent on the folloeing categories:-
1) Workpiece Material Strength:
A lower value of depth of cut is recommended for workpiece made of hard and brittle materials. If higher value of depth of cut will be used, the cutting tool may break.
2) Quality of cut required:
A lower value of depth of cut should be provided for finish cut and higher value of depth of cut should be provided for higher value can be used to shorten machine time.
3) Machine Tool Type:
Since depth of cut increases the cutting force and vibration, so higher depth of cut can be used for machine tool made of hard material and lower depth of cut should be used for machine tool made with materials which are not that hard.
4) Machining Operation:
Value of depth of cut should also be varied based on the machine operation to be performed. A large depth of cut can be implied on tilling operation with side and face milling cutter while large depth of cut should not be used in knurling operation.
5) Production required:
As material removal rate is (MRR) is expressed by the multiplication of cutting velocity, feed rate and depth of cut. So increase in depth of cut can increase material removal rate (MRR).
When material removal rate increases the machining time decreases and hence the productivity increases.
So large depth of cut should be used when higher productivity is required and lower depth of cut can be used when lower productivity is required.
6) Hardness of top layer of workpiece:
When cutting uncut or cast iron surface, the depth of cut should be increased to avoid cutting impure hard layer with the tip of cutting edge which prevents chipping and abnormal wear.
For milling operation,there are two types of depth of cut:
1) Radial Depth of Cut (RDOC):
The distance a tool is stepping over into a workpiece. It is also referred as Stepove, Cut Width, or XY.
2) Axial Depth of Cut (ADOC):
The distance a tool engages a workpirce along its centerline. It is also referred as Stepdown, or Cut Depth.