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There are three factors that make up the cutting conditions: cutting speed, depth of cut and feed rate. In this information sheet we will concentrate on the depth of cut and the factors which influence it. Therefore, to determine the depth of cut we must first select the proper cutting tool, the proper machine, and a suitable setup.
Roughing tools should be used whenever large amounts of materials need to be removed. The serrated flute design allows the roughing endmill (Figure 1) to remove three times as much material as a plain, helical fluted, endmill.
Figure 1: Tool types play an important role in determining cutting conditions.
A stagger tooth milling cutter, for horizontal milling applications, is a much better choice for removing large amounts of material than a straight tooth plain milling cutter (Figure 2). Know the limitations of the cutting tools and make your choice as to the type of cutting tool based on the amount of material to be removed.
Figure 2: Stagger Tooth Milling Cutter
Use the largest tool possible that will perform the job. Larger tools are capable of larger metal removal rates. In some cases more than one tool will need to be used. It may mean changing tools, one tool for roughing and one tool for finishing. The profile of the part maybe such that you need to rough with the larger cutting tool and finish with the smaller tool. In some cases this may make the best economical sense.
What is the surface finish requirement? You may need to rough with one style of cutter and finish with another. The amount of material to be removed and the surface finish requirement may determine that you need to change tools.
|Selecting the Proper Machine|
The machine type and size will influence your depth of cut. How rigid is this machine? The easiest way to determine the rigidity of the machine is to look at the size of the spindle taper. The larger the spindle taper the greater the rigidity of the machine. Another determining factor is the frame style. How many component parts does it take to get from the column of the machine to the tool? An example of this would be the C-Frame and Ram style vertical milling machines (Figure 3). The Ram type vertical milling machine is a much less rigid style of machine than the C-frame style of milling machine. Make sure your metal removal rate decisions account for the style and size of the machine.
Figure 3: Different Styles of Vertical Milling Machines
Does the machine have the available horsepower? The horsepower that is available at the spindle is not the same as the horsepower of the main motor. The available horsepower at the spindle can be as low as 50 to 80 percent of the main motor horsepower. To calculate
For maximum metal removal rates you will need to calculate horsepower requirements. Horsepower calculation can be found in your text and also in the carbide cutting tool section.
|Rigidity of the Setup|
How the part is clamped to the table and how far the tool is extended from the spindle make up the majority of the setup. If the tool is extended and the workpiece can only be clamped in a limited number of places, the depth of cut and the feedrate will be directly influenced. The greater the rigidity of the setup, the higher the metal removal rate. The closer the tool is to the support, the higher the metal removal rate. The rigidity of the setup should always be maximized for productivity sake, but more importantly for safety sake.
|Depth of Cut|
Now that we have looked at the factors influencing the depth of cut and metal removal rates, lets look at actual depth of cut scenarios.
|High Speed Steel Endmills|
When using a plain high-speed steel endmill (Figure 1) the rule of thumb is the depth of cut should not exceed 1/2 the diameter of the cutter. Maximize the feedrate and the depth of cut while leaving the R.P.M in the calculated range. When maximizing the depth of cut, use a slower feedrate with an acceptable chip load factor. If the feedrate needs to drop below an acceptable chip load, decrease the depth of cut instead. Too low of a feedrate will prematurely dull the endmill. Milling cutters with fewer teeth will allow a greater depth of cut. Fewer teeth will allow for larger teeth on the tool and therefore more chip clearance. Use cutting fluids whenever possible. Cutting fluid serve to dissipate heat. When a smooth accurate finish is needed, always take a roughing and a finishing cut.