This month I will discuss parameters that determine the effectiveness of a cutting operation. Whether making one part or thousands, an effective operation produces the desired output at the lowest cost. The sharpness of a cutting tool is a key piece of the machinist’s puzzle. The parameters discussed in this column include the friction between a cutting tool and a workpiece, as well as the hardness, geometry and strength of a cutting tool.
Friction between the tool and workpiece will dull the cutting edge over time. A dull tool will produce parts that are out of spec because of poor surface finish or geometry, or both.
Figure 1 B.Taylor
Heat in a cutting edge allows friction to dull the edge. The useful life of an edge is a function of how hard it is at its operating temperature. As temperature goes up, hardness goes down.
Various techniques are used to improve the life of tools.
Materials that operate at higher temperatures allow faster metal removal, and that is why there are carbide and ceramic inserts.
The rake angle of a tool influences how much power is consumed. More positive rake, less power consumed, which makes for less heat generated. But there is a tradeoff. More rake makes a weaker edge that may fracture. Carbide manufacturers have improved the fracture strength of their materials, so positive rake cutting tools have become the norm.
Coatings like titanium nitride have lower coefficients of friction than the cutting tool material, and less friction means less heat generated. That’s a win win. I am familiar with titanium nitride, but there are a variety of other coatings that may perform better for your application. Reach out to insert makers for advice. They are the experts and are always glad to help. There are many insert manufacturers and I would advise you to contact a few. Research and development are always going on in that world and it is good to have a sure solution from a reputable company before you start making parts.
Coolant is an effective solution to longer tool life. I used flood coolant in my shop, but now high-pressure coolant with toolholders that direct the coolant at the cutting edge are available. That is a sure way to productivity. Your return on investment will depend on your volume. Trade-offs, trade-offs, trade-offs — that’s where we live.
What is used for a coolant is important. I like water-based solutions for fast metal removal because water has higher heat capacity and is not toxic. But for cutting threads I use Tap Magic products. The health of the cutting edge is important, but so is the health of the human. I’m 76 years old now and still walking a couple of miles a day, so I know a few things about that part too.
Figure 1 shows a titanium nitride-coated threading insert made by Valenite.
The hardness, geometry, and strength of a cutting edge along with the friction between the cutting edge and the workpiece help determine the potential effectiveness of a cutting operation — and how much money you make.
A whole year of Machinist's Corner
Brandt Taylor, the author of Cutting Tool Engineering magazine’s regular column Machinist’s Corner, will begin writing the column on a monthly basis beginning January 2025. If you have a subject that you would like Brandt to write about, feel free to let him know. “I don't know everything,” says the 76-year-old Brandt, “but I have been a machinist for 48 years now and there is a lot of data in my memory bank.” Brandt can be reached via email at gbrandttaylor@gmail.com.
Related Glossary Terms
- coolant
coolant
Fluid that reduces temperature buildup at the tool/workpiece interface during machining. Normally takes the form of a liquid such as soluble or chemical mixtures (semisynthetic, synthetic) but can be pressurized air or other gas. Because of water’s ability to absorb great quantities of heat, it is widely used as a coolant and vehicle for various cutting compounds, with the water-to-compound ratio varying with the machining task. See cutting fluid; semisynthetic cutting fluid; soluble-oil cutting fluid; synthetic cutting fluid.
- hardness
hardness
Hardness is a measure of the resistance of a material to surface indentation or abrasion. There is no absolute scale for hardness. In order to express hardness quantitatively, each type of test has its own scale, which defines hardness. Indentation hardness obtained through static methods is measured by Brinell, Rockwell, Vickers and Knoop tests. Hardness without indentation is measured by a dynamic method, known as the Scleroscope test.
- rake
rake
Angle of inclination between the face of the cutting tool and the workpiece. If the face of the tool lies in a plane through the axis of the workpiece, the tool is said to have a neutral, or zero, rake. If the inclination of the tool face makes the cutting edge more acute than when the rake angle is zero, the rake is positive. If the inclination of the tool face makes the cutting edge less acute or more blunt than when the rake angle is zero, the rake is negative.
- tap
tap
Cylindrical tool that cuts internal threads and has flutes to remove chips and carry tapping fluid to the point of cut. Normally used on a drill press or tapping machine but also may be operated manually. See tapping.
- threading
threading
Process of both external (e.g., thread milling) and internal (e.g., tapping, thread milling) cutting, turning and rolling of threads into particular material. Standardized specifications are available to determine the desired results of the threading process. Numerous thread-series designations are written for specific applications. Threading often is performed on a lathe. Specifications such as thread height are critical in determining the strength of the threads. The material used is taken into consideration in determining the expected results of any particular application for that threaded piece. In external threading, a calculated depth is required as well as a particular angle to the cut. To perform internal threading, the exact diameter to bore the hole is critical before threading. The threads are distinguished from one another by the amount of tolerance and/or allowance that is specified. See turning.
- titanium nitride ( TiN)
titanium nitride ( TiN)
Added to titanium-carbide tooling to permit machining of hard metals at high speeds. Also used as a tool coating. See coated tools.
Author
Brandt Taylor is owner of Berlin, Massachusetts-based Taylor Engineering, a machine shop and manufacturer of lathe chuck jaws.
