Boring Machining: Definition, Uses, and Types
Boring machining is a typical process used to create holes of varying sizes. It is one of many subtractive manufacturing processes that can be used to create finished parts. While other processes, such as drilling, can cut smaller holes into parts, boring machining is ideal for cutting large-diameter holes with exceptional dimensional accuracy. For example, boring machining is often used in the automotive industry to make parts such as engine cylinders and couplings, and in the heavy machinery industry to make parts such as hollow shafts and rods. There are many types of boring including horizontal, vertical, and precision boring machining. This article will review boring machining, discuss its uses, describe its advantages and disadvantages, and review the different methods for implementing this manufacturing process.
Boring machining is a subtractive manufacturing process used to make holes of all sizes with high accuracy. Boring typically follows other hole-making processes, like drilling, to enlarge holes to meet required dimensions and surface finishes. It is typically completed on a lathe using a tool called a boring bar, which is a cylindrical tool with a cutting insert. Because boring machining is often performed on a lathe, it is commonly, but incorrectly, referred to as “internal turning.” While the process is often performed on a lathe, it can also be performed on CNC milling machines.
The origins of boring machining date back to the 18th century. John Wilkinson invented a precision barrel-boring machine to make cylinder blocks for a steam engine developed by James Watt in 1774. The boring machine invented by Wilkinson has often been called the world’s first machine tool and was used for boring cylinders for steam engines and barrels for cannons. Almost a century later, in 1860, Francis A. Pratt invented a boring machine whose motion was controlled by a screw feed that was manually operated, instead of using a rack and pinion. World War I and World War II led to numerous advancements in boring machining tools, such as the invention of jig borers. As CNC machining became more commonplace in the 1970s, mechanically-operated boring machines gave way to computer-controlled models.
Boring machining is used when a smaller hole drilled as part of a previous process step must be enlarged to meet the required dimensions and surface finish specifications. Additionally, boring machining is used to straighten holes, rectify casting defects, and make countersinks irrespective of the hole’s diameter. While boring machining is similar to reaming, it differs in that boring uses a single-point cutting tool and offers better positional accuracy and throughput efficiency. Boring machining is ideal for all types of production volumes or when more stock material is present in the hole.
Boring machining is usually carried out on a lathe, using a boring bar fitted with an appropriate cutting insert. However, boring machining can also be conducted using CNC milling machines, with either a horizontal or vertical boring axis. Whether a lathe or a mill is used for boring machining, a hole must already have been made in the part — usually by 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 until the desired dimensions and surface finish are attained.
The process of boring machining starts with a hole that has been pre-drilled or cast into a part. These holes are enlarged using a boring bar fitted onto a lathe or milling machine. Depending on how much larger the hole needs to end up, it may not be possible to complete the boring process in one pass. It may be necessary to make several passes, cutting progressively larger holes until the final size is obtained. Boring is carried out at a moderate cutting speed, no more than 100 m/min, and a shallow cut depth. High cutting speeds lead to excess chatter and vibration and can adversely affect the hole quality, while low cutting speeds lead to inefficient cutting and can also affect hole quality. Additionally, high cut depths can increase stress on the tool and can result in poor quality.
There are several different methods for boring machining. The main approaches are listed and described below:
- Horizontal Boring: Horizontal boring is the most common type of boring machining. It is accomplished on either a lathe or a horizontal milling machine. In horizontal boring, the boring bar moves parallel to the axis of the workpiece. This approach is typically used for boring holes in long workpieces.
- Vertical Boring: Vertical boring is accomplished on a vertical milling machine, CNC router, or drill press. In vertical boring, the boring bar moves perpendicularly to the floor. Vertical boring is best suited for heavy workpieces and for large holes, up to 24” in diameter.
- Precision Boring: Precision boring machines are machines designed particularly for boring and are smaller than lathes or milling machines. Because of their small size, precision boring machines are best for boring small parts that require great surface finishes and extreme precision. For example, watch components are typically bored using a precision boring machine called a watchmaker’s lathe.
There are several tools needed in boring machining for the process to be successful. Those tools are listed and described below:
- Lathe or Milling Machine: The lathe or milling machine will guide the position of the boring tool.
- Boring Bar: The boring bar allows the cutting insert to cut the walls of the hole. If using a mill, the workpiece is fixed while the boring bar rotates to cut the part.
- Cutting Insert: The cutting insert is assembled onto the boring bar and cuts into the part to enlarge the hole.
The advantages of boring machining are listed and described in Table 1 below:
Identifying a boring machine as "the best" depends on the buyer's budget and the degree of accuracy and precision required for the job. The best boring machines are listed below:
- DBC Boring Mill: The DBC boring mill is a horizontal boring machine. It is one of the best boring machines, overall, due to its relatively low price, small footprint, and ability to precisely cut small and large holes in parts. Its large build size larger workpieces to be cut, while its 35-horsepower motor allows spindle speeds up to 3000 RPM.
- CLIMAX BB5000 Line Boring Machine: The CLIMAX BB5000 is another excellent horizontal boring machine that is both affordable and compact. Its compact design allows it to be easily broken down and set up again at a different location — making it an excellent option for part geometries that may be too restrictive for other boring machines.
- Summit® HB Series: The Summit® HB series is a horizontal boring machine that is both affordable and efficient while also highly precise. The HB Series is capable of cutting both large and small workpieces and its heavy-duty build ensures long machine life.
- Summit® Vertical Boring Machines: The vertical boring machines offered by Summit® come in various sizes to accommodate both large and small parts. Its servo motors and highly precise ball screws lead to exceptional boring accuracy.
This article presented boring machining, explained it, and discussed its types and uses. To learn more about boring machining, contact a Xometry representative.
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- Summit® HB series is a trademark of Summit Machine Tools.
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