Oct. 23, 2024
Boring machining is one of several subtractive manufacturing processes used to create finished parts. This method encompasses various types, including horizontal, vertical, and precision boring. This article will explore boring machining, its applications, advantages and disadvantages, and the different methods for implementing this manufacturing process.
Boring machining is a hole-making process that follows initial operations like drilling to enlarge holes to specified dimensions and achieve desired surface finishes. This method allows for the precise creation of various hole sizes with high accuracy. Boring has a long history, dating back to the 18th century. In 1774, John Wilkinson invented the first precision barrel-boring machine—often referred to as the "world's first machine tool"—to produce cylinder blocks for a steam engine developed by James Watt. In 1860, Francis A. Pratt introduced another type of boring machine that utilized a manual screw feed for motion control, as opposed to a rack and pinion system. The two World Wars spurred significant advancements in boring machining, including the development of jig borers. With the rise of CNC machining in the 1970s, mechanically operated boring machines gradually transitioned to computer-controlled models.
Typically, the boring process is performed on a lathe using a cylindrical tool with a cutting insert known as a boring bar. This is why it is sometimes mistakenly referred to as "internal turning." Boring can also be executed on CNC milling machines with either a horizontal or vertical boring axis. In all cases, a hole must already exist in the part, typically created through drilling or casting. With a lathe, boring machining works by placing the head of the boring bar into the drilled or cast hole. As the lathe rotates the workpiece, the cutting insert cuts into the edges of the hole, and the hole widens. With a mill, the workpiece is held stationary and the cutting tool rotates to enlarge the hole. In both cases, the hole is enlarged using a boring bar fitted onto a lathe or milling machine until you get the desired dimensions and surface finish.
Depending on how much larger the hole needs to be, the boring process may require multiple passes to achieve the desired size. Each pass cuts progressively larger holes until the final dimensions are met. Boring is typically conducted at moderate cutting speeds—no more than 100 m/min—with shallow cut depths. Excessively high cutting speeds can lead to chatter and vibration, negatively impacting hole quality, while low speeds can result in inefficient cutting and also affect quality. Furthermore, deep cuts can increase stress on the tool, leading to subpar results.
Boring machining is employed when a smaller hole, created during a prior process, needs to be enlarged to meet specific dimensional and surface finish requirements. It is also used to straighten holes, correct casting defects, and create countersinks, regardless of the hole's diameter. While similar to reaming, boring uses a single-point cutting tool, providing superior positional accuracy and throughput efficiency. It is suitable for various production volumes, especially when additional material is present in the hole.
Boring produces smooth surface finishes on the interior of holes and is a highly accurate process, as boring tools follow the hole's centerline rather than its overall position, as seen with drills and reamers. The technique can be performed on various machines configured for horizontal and vertical boring, and the same tool can be used to create different hole sizes. Additionally, cutting inserts can be quickly replaced, enhancing efficiency.
However, boring is not ideal for enlarging blind holes due to the geometry of the boring bar and the arrangement of cutting inserts, which can leave a taper at the bottom of the hole. Furthermore, operators require more technical expertise to perform boring compared to drilling or reaming.
The three main types of boring machining include:
1. Horizontal Boring: This is the most common type, performed on a lathe or horizontal milling machine. In this process, the boring bar moves parallel to the workpiece axis, making it ideal for boring holes in longer workpieces.
2. Vertical Boring: Conducted on a vertical milling machine, CNC router, or drill press, vertical boring involves the boring bar moving perpendicularly to the floor. This method is best suited for heavy workpieces and can accommodate large holes, up to 24 inches in diameter.
3. Precision Boring: These specialized machines are smaller than lathes or milling machines and are specifically designed for boring. Precision boring machines excel at creating small parts that require exceptional surface finishes and extreme accuracy. For instance, watch components are often bored using a precision boring machine known as a watchmaker’s lathe.
Successful boring machining requires several essential tools, as described below:
1. Lathe/Milling Machine: This machine guides the positioning of the boring tool, ensuring accurate hole enlargement.
2. Boring Bar: The boring bar holds the cutting insert and is responsible for cutting the walls of the hole. In milling applications, the workpiece remains fixed while the boring bar rotates to perform the cutting.
3. Cutting Insert: Attached to the boring bar, the cutting insert actively cuts into the part to enlarge the hole to the desired dimensions.
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