when planning for machining.
The Cutting Fluid or Coolant
The metal cutting process is one that creates friction between the cutting tool and the workpiece. A cutting fluid or coolant is necessary to lubricate and remove heat and chips from the tool and workpiece during cutting. Water alone is not sufficient because it only cools and does not lubricate; it will also cause rust to develop on the machine ways and table. Also, because of the heat produced, water vaporizes and thus compromises the cooling effect. A mixture of lard-based soluble oil and water creates a good coolant for most light metal-cutting operations. Harder materials, like stainless steel and high alloy composition steels require the use of a cutting-oil for the optimum results. Advancements have been made with synthetic coolants as well. Finally, the flow of coolant should be as strong as possible and be directed at the cutting edge to accomplish its purpose. Some machine tools are equipped thru-spindle/tool and high pressure coolant that really aid in cutting zone cooling and removal of chips. Programmers and machine operators should research available resources like the Machinery’s Handbook and coolant manufacturer data, for information about the proper selection and use of cutting fluids for specific types of materials. The manufacturer data will include the coolant mixing ratio requirements and PH-level checking parameters.
The Workpiece and the Work Holding Method
The material to be machined has a definite effect on decisions about what tools will be used, the type of coolant necessary, and the selection of proper speeds and feeds for the metal-cutting operation.
The shape or geometry of the workpiece affects the metal-cutting operation and determines the type of work holding method that will be used. This clamping method is important for CNC work because of the high performance expected. It must hold the workpiece securely, be rigid, and minimize the possibility of any flex or movement of the part.
The Cutting Speed
Cutting speed is the rate at which the circumference of the tool moves past the workpiece in surface feet (sf/min) or meters per minute (m/min) to obtain satisfactory metal removal.
The cutting speed factor is most closely related to the tool life. Many years of research have been dedicated to this aspect of metal-cutting operations. The workpiece and the cutting tool material determine the recommended cutting speed. The Machinery’s Handbook is an excellent source for information pertaining to determining proper cutting speed. If incorrect cutting speeds, spindle speeds, or feedrates are used, the results will be poor tool life, poor surface finishes, and even the possibility of damage to the tool and/or part.
The Spindle Speed
When referring to a milling or a turning operation, the spindle speed of the cutting tool or chuck must be accurately calculated relating to the conditions present. This speed is measured in revolutions per minute, r/min (formerly known as RPM), and is dependent upon the type and condition of material being machined. This factor, coupled with a depth of cut, gives the information necessary to find the horsepower required to perform a given operation. In order to create a highly productive machining operation, all these factors should be given careful consideration. Refer to the formulas below needed to calculate r/min.
| For Inch Units: | For Metric Units: |
 |
 |
where:
CS = Cutting Speed from the charts in Machinery’s Handbook
π = 3.1417
D = Diameter of the workpiece or the cutter
Many modern machine controllers have a feature that allows automatic calculation of feeds and speeds that is based on appropriate operator input of the cutting conditions. Often when using Computer Aided Manufacturing (CAM), feeds and speeds data can be extracted from available machining libraries.
The Feedrate
Feedrate is defined as the distance the tool travels along a given axis in a set amount of time, generally measured in inches per minute in/min (formerly known as IPM) for milling or inches per revolution in/rev (formerly known as IPR) for turning. This factor is dependent upon the selected tool type, the calculated spindle speed, and the depth of cut. Refer to the Machinery’s Handbook and cutting tool manufacturer data for the chip load recommendations and review the formula below that is necessary to calculate this aspect of the metal-cutting operation.
F = R × N × f
where:
F = Feed in in/min or mm/min
R = r/min calculated from the preceding formula
N = the number of cutting edges
f = the chip load per tooth recommended from the Machinery’s Handbook
The Depth of Cut
The depth of cut is determined by the amount of material to be removed from the workpiece, cutting tool flute length or insert size, and the power available from the machine spindle. Always use the largest depth of cut possible to ensure the least effect on the tool life.
Cutting speed, spindle speed, feedrate, and depth of cut are all important factors in the metal-cutting process. When properly calculated, the optimum metal-cutting conditions will result. Refer to the Machinery’s Handbook, tool and insert ordering catalogs, and online applications from the tool and insert manufacturers for more information on recommended depths of cut for particular tooling.
Certain steps must be followed in order to produce a machined part that meets specifications given in an engineering drawing or blueprint. These steps need to be organized in a logical sequence to produce the finished part in the most efficient manner. Before machining begins, it is essential to go through the procedure called process planning. The following are the steps in the process:
1. Study the engineering drawing or blueprint.
2. Select the proper raw material or rough stock as described in the engineering drawing or blueprint.
3. Study the engineering drawing or blueprint and determine the best sequence of individual operations needed to machine the required geometry.
4. Transfer the information onto planning charts.
5. While the part is still mounted on the machine, use in-process inspection to check dimensional values as they are completed.
6. Make necessary corrections and deburr.
7. Perform a 100% dimensional inspection when the part is finished and log the results of the first article inspection on the quality control check sheet.
8. Take corrective action if any problems are identified.
9. Begin production.
