Learn the Basics About Metal Cutting Parameters
Conventional metal-cutting processes involve metal reduction by single point, multiple point, or abrasive tools. The word "conventional" is used to distinguish these traditional machining processes from nontraditional or unconventional machining processes which are more involved with chemical, electrical, or thermal energy. Conventional metal-cutting is the outwardly simple process of removing metal on a work piece in order to get a desired shape by relative movement of the work piece and tool, either by rotating the workpiece (as in a lathe) or by rotating the tool (as in a drilling machine). But behind this simple process lie numerous parameters that play their roles, from a small to a big way, in deciding many things in the act of metal-cutting, including the speed of doing the job, the quality and accuracy of the finish, the life of the tool, the cost of production, and so on.
Some parameters involved in the metal-cutting process are in fact closely related with some other parameters in the metal-cutting process; playing with one will have an influencing effect on another. Thus, even after several years of experience, process planning engineers may find difficulty in confidently declaring themselves as experts in metal-cutting!
In this series of articles, we shall first list the major conventional metal-cutting parameters and learn a few basic things about them. In subsequent articles, we shall delve deeper into how they contribute their roles in relation with others in the conventioanal metal removal process.
1) Material machinability:
The machinability of a material decides how easy or difficult it is to cut. The material’s hardness is one factor that has a strong influence on the machinabilty. Though a general statement like "a soft material is easier to cut than a harder material" is true to a large extent, it is not as simple as that. The ductility of a material also plays a huge role.
2) Cutting Tool Material:
In metal-cutting, High Speed steel and Carbide are two major tool materials widely used. Ceramic tools and CBN (Cubic Boron Nitride) are the other tool materials used for machining very tough and hard materials. A tool’s hardness, strength, wear resistance, and thermal stability are the characteristics that decide how fast the tool can cut efficiently on a job.
3) Cutting speed and spindle speed:
Cutting speed is the relative speed at which the tool passes through the work material and removes metal. It is normally expressed in meters per minute (or feet per inch in British units). It has to do with the speed of rotation of the workpiece or the tool, as the case may be. The higher the cutting speed, the better the productivity. For every work material and tool material combo, there is always an ideal cutting speed available, and the tool manufacturers generally give the guidelines for it.
Spindle speed: Spindle speed is expressed in RPM (revolutions per minute). It is derived based on the cutting speed and the work diameter cut (in case of turning/ boring) or tool diameter (in case of drilling/ milling etc). If V is the cutting speed and D is the diameter of cutting, then Spindle speed N = V /(Pi x D)
4) Depth of cut:
It indicates how much the tool digs into the component (in mm) to remove material in the current pass.
5) Feed rate:
The relative speed at which the tool is linearly traversed over the workpiece to remove the material. In case of rotating tools with multiple cutting teeth (like a milling cutter), the feed rate is first reckoned in terms of “feed per tooth," expressed in millimeters (mm/tooth). At the next stage, it is “feed per revolution" (mm/rev).
In case of lathe operations, it is feed per revolution that states how much a tool advances in one revolution of workpiece. In case of milling, feed per revolution is nothing but feed per tooth multiplied by the number of teeth in the cutter.
To actually calculate the time taken for cutting a job, it is “feed per minute" (in mm/min) that is useful. Feed per minute is nothing but feed per revolution multiplied by RPM of the spindle.
6) Tool geometry:
For the tool to effectively dig into the component to remove material most efficiently without rubbing, the cutting tool tip is normally ground to different angles (known as rake angle, clearance angles, relief angle, approach angle, etc). The role played by these angles in a tool geometry is a vast subject in itself.
To take away the heat produced in cutting and also to act as a lubricant in cutting to reduce tool wear, coolants are used in metal-cutting. Coolants can range from cutting oils, water-soluble oils, oil-water spray, and so on.
8) Machine/ Spindle Power:
In the metal-cutting machine, adequate power should be available to provide the drives to the spindles and also to provide feed movement to the tool to remove the material. The power required for cutting is based on the metal removal rate – the rate of metal removed in a given time, generally expressed in cubic centimeters per minute, which depends on work material, tool material, the cutting speed, depth of cut, and feed rate.
9) Rigidity of machine:
The rigidity of the machine is based on the design and construction of the machine, the age and extent of usage of the machine, the types of bearings used, the type of construction of slide ways, and the type of drive provided to the slides. All play a role in the machining of components and getting the desired accuracy, finish, and speed of production.
Thus, in getting a component finished out of a metal-cutting machine at the best possible time within the desired levels of accuracy, tolerances, and surface finish, some or all the above parameters play their roles. As already mentioned in the beginning, each of the parameters can create a positive or negative impact on other parameters, and adjustments and compromises are to be made to arrive at the best metal-cutting solution for a given job.
We shall try to learn more of these details in the subsequent articles.