Cutting Trapezoidal Threads
Trapezoidal threads are lesser-used threads that, while somewhat challenging to work with, could offer certain advantages to parts in the form of increased resistance to slippage and higher efficiency when engaging the thread in certain situations. When properly created and employed, trapezoidal threads can be both powerful and fast. As such, having a working knowledge of how to cut them is paramount for any milling machine operator.
The first thing to understand about cutting trapezoidal threads is the shapes and sizes of the teeth. Unlike most other threads, trapezoidal thread teeth have a wide and relatively flat profile that is similar in shape to the trapezoid geometric figure. These flats are machined so that each tooth transitions from one stepped face to the next in a saw-toothed or wavy line. The working angle and depth of the teeth dictate the amount of torque that can be applied to the thread, and the total length of the trapezoidal thread is determined by the combination of angles used to produce the thread shape.
When cutting trapezoidal threads, there are certain geometry and tooling considerations that need to be made. To start, the pitch of the trapezoidal threading will determine the diameters of the cutting tool and the thread form. While larger pitches will allow for larger diameters (since the teeth must form a complete circle in order to create the trapezoidal threads), smaller pitches will mean that smaller diameter tools must be used. The milling tool used will also be determined by the number of teeth required, which depends upon the type of thread being cut. For continuous threads, such as those found on screws, a greater number of teeth is required than for single-start threads; this will, in turn, necessitate the use of a correspondingly larger milling tool.
The approach to cutting trapezoidal threads can vary, but one common approach is to start by machining nicks into the threads in order to create the initial form. During this process, careful control of the feed rate and the tooling stage is important. Once the thread is ready, the milling machine will go through a series of passes in order to form the teeth, using the same feed rate and adding stepovers as necessary.
The milling tool used for trapezoidal threading should have a tip width that is narrower than the diameter or root of the thread. This will help ensure that the thread teeth are properly formed and that the threads are properly cut. Additionally, all cutting tools should be sharpened before each use in order to maintain the highest quality in your finished product. Depending upon the materials used, it may also be beneficial to employ coolants or lubricants to reduce the risk of overheating the tool or work surface, although this can also increase the risk of clogging the cutting surface with debris or particles.
Given the particularities of trapezoidal threading and the demands of precision machining, cutting trapezoidal threads correctly requires skill and experience. Oftentimes, a trial and error approach will be needed to ensure that the final product is the accurate and of the highest quality, with any mistakes or issues being rectified in subsequent passes on the milling machine. Following the proper procedures and using the right tooling, however, can help to ensure that the trapezoidal threading is as perfect as possible.
