A lathe center, often shortened to center, is a tool that has been ground to a point to accurately position a workpiece on an axis. They usually have an included angle of 60°, but in heavy machining situations an angle of 75° is used.[1]
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The primary use of a center is to ensure concentric work is produced; this allows the workpiece to be transferred between machining (or inspection) operations without any loss of accuracy. A part may be turned in a lathe, sent off for hardening and tempering and then ground between centers in a cylindrical grinder. The preservation of concentricity between the turning and grinding operations is crucial for quality work.
When turning between centers, a steady rest can be used to support longer workpieces where the cutting forces would deflect the work excessively, reducing the finish and accuracy of the workpiece, or creating a hazardous situation.
A lathe center has applications anywhere that a centered workpiece may be used; this is not limited to lathe usage but may include setups in dividing heads, cylindrical grinders, tool and cutter grinders or other related equipment. The term between centers refers to any machining operation where the job needs to be performed using centers.
A center is inserted into a matching hole drilled by a center drill. The hole is conical near the surface and cylindrical as it gets deeper.
Dead center (and live center)
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Revolving (live) center (top)A dead center (one that does not turn freely, i.e., dead) may be used to support the workpiece at either the fixed or rotating end of the machine. When used in the fixed position, a dead center produces friction between the workpiece and center, due to the rotation of the workpiece. Lubrication is therefore required between the center and workpiece to prevent friction welding from occurring. Additionally the tip of the center may have an insert of cemented carbide which will reduce the friction slightly and allow for faster speeds. Dead centers are typically fully hardened to prevent damage to the important mating surfaces of the taper and to preserve the 60° angle of the nose. As tungsten carbide is much harder than steel, a carbide-tipped center has greater wear resistance than a steel center.
Turning between centresWhen turning between centres, a 'dead centre' is used in the headstock as well as the tailstock. As the one in the headstock revolves with the work, this centre is known as a live centre.
Soft center
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Soft centers are a special version of the dead center in which the nose is deliberately left soft (unhardened) so that it may be readily machined to the correct angle prior to usage. This operation is performed on the headstock center to ensure that the center's axis is aligned with the spindle's axis.
Running or revolving center
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A revolving center, also known as a rotating center or running center in some countries, is constructed so that the 60° center runs in its own bearings and is used at the non-driven or tailstock end of a machine.[2] It allows higher turning speeds without the need for separate lubrication, and also greater clamping pressures. CNC lathes use this type of center almost exclusively and they may be used for general machining operations as well. Spring-loaded centers are designed to compensate for center variations, without damage to the work piece or center tip. This assures the operator of uniform constant tension while machining. Some live centers also have interchangeable shafts. This is valuable when situations require a design other than a 60° male tip. A live center, which may be hard or soft, is a plain center placed in the revolving mandrel; it moves and is therefore live.
Pipe center
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A pipe center, also known as a bull nose center is a type of live center which has a large diameter conical nose rather than a sharp point. This allows the center to be used in the bore of a pipe or other workpiece with a large interior diameter. While a pipe center ensures the workpiece remains concentric, its main advantage is that it supports the workpiece securely, and can be used for parts whose larger inner diameter prevents the use of a normal pointed center. Thin-walled material such as pipes easily collapses if excessive force is used at the chuck end.
Cup center
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There are two types of cup centers. The woodworking variety is a variation of the traditional live center. This type of cup center has a central point like a normal live center and also has a ring surrounding it. The ring supports the softer material around the center point and prevents the wood from splitting under pressure from the central point. A different variety of cup center is used for metalworking. The metalworking variety of cup center has a tapered hole rather than a conical point. It supports the part by making contact with the outside diameter of the end of the part, rather than using a center hole.
Drive center
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A drive center, also known as a grip center, is used in the driving end of a machine (headstock). It is often used in woodworking or where softer materials are machined.
It consists of a dead center surrounded by hardened teeth, which bite into a softer workpiece allowing the workpiece to be driven directly by the center. This allows the full diameter of the workpiece to be machined in a single operation, in contrast with the usual requirement where a carrier is attached to the workpiece at the driven end. The use of modified shell end mills in a drive center, instead of hardened pins, enables better gripping and prevents breakdown time due to pin stop.[citation needed]
Spring center
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A spring center is a metalworking lathe center for maintaining a cutting tool like a reamer or a tap, in axial alignment with a hole being worked on. It consists of a point backed by a spring to push the cutting tool into the workpiece.
References
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*Updated February 3, 2021
CNC turning centers and lathes may seem identical, but they generally refer to slightly different machines. Lathes generally only turn while turning centers can be more advanced. These machines can be capable of facing, threading, knurling, drilling, boring, reaming, and taper turning. Horizontal CNC turning machines are more common, but vertical machines utilize gravity. Between CNC turning, CNC turning centers, the different types of CNC machines, and more, it can be quite confusing. Within this article, we will focus on the basics of CNC turning to give you a strong foundation and answer the most commonly asked questions.
Feel free to skip around this guide if there is a particular question you have:
CNC turning is a subtractive manufacturing process in which pieces of material are rotated as a machine tool is fed to the piece, removing material to create a finished piece. It is primarily used to manufacture parts that are cylindrical in nature.
Close up photo of CNC Turning in action. [Source]
There are many, many materials that can be used with CNC turning, as shown in this guide. However the most common are metal alloys like aluminum.
CNC lathe machines, or CNC turning machines are machine tools that rotate a bar of material, allowing the cutting tool to remove material from the bar until the desired product is remaining.
The material itself is secured to, and rotated by, the main spindle, while the cutting tool can be moved along multiple axis. The types of parts created by a CNC lathe are often cylindrical, or symmetrical around an axis.
You can break CNC machines down into the following parts, and if you would like to explore them further, this article can help:
CNC machining is far better than manual machining for a few specific reasons:
Accuracy
: Programming a CNC to complete a job creates a consistent and accurate cut to ensure product quality. It can be repeated over and over in the exact same manner.Cost Effectiveness
: The cost of hiring a person to manually machine parts at scale is far more expensive than having a machine do it, as a machine will not tire, does not need lunch breaks, and can, theoretically run 24/7.Speed
: CNC turning can churn out hundreds and hundreds of parts while retaining a consistent end product.While CNC turning rotates the part for cutting, CNC milling rotates the tool to cut the part.
Going further, CNC turning will typically be used for cylindrical parts, as you rely on the part itself to rotate and create the friction to remove material when the cutting tool is placed against it.
On the other hand, CNC milling can create more complex parts as it is the cutting tool (rather than the material) that is spinning.
The difference between turning and milling in CNC machining. [Source]
NC’s are numerically controlled machines, whereas CNC’s are computer numerically controlled machines. Although they both are automatic machines that, in most cases, work on metal parts and are programmed to enable a desired operation, they differ in operation.
Numerical Control (NC) Machines are controlled with punch cards that are fed into the machine, making the programs difficult to modify. Even to make a small change to the program, the tape would have to be removed, modified, and fed back into the machines.
Computer Numerical Control (CNC) Machines have a built in computer to store the programs and modify the program via the control on the machine.
The CNC turning process involves material being secured to a spindle within a CNC machine, and a cutting tool moving in linear fashion to act upon the material and remove undesired material until he finished component is all that is left.
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