Q
What Major categories of Metal Lathe?
A

There are many variants of lathes within the metalworking field. Some variations are not all that obvious, and others are more a niche area. For example, a centering lathe is a dual head machine where the work remains fixed and the heads move towards the workpiece and machine a center drill hole into each end. The resulting workpiece may then be used "between centers" in another operation. The usage of the term metal lathe may also be considered somewhat outdated these days, plastics and other composite materials are in wide use and with appropriate modifications, the same principles and techniques may be applied to their machining as that used for metal.

Center lathe / engine lathe / bench lathe

The terms center latheengine lathe, and bench lathe all refer to a basic type of lathe that may be considered the archetypical class of metalworking lathe most often used by the general machinist or machining hobbyist. The name bench lathe implies a version of this class small enough to be mounted on a workbench (but still full-featured, and larger than mini-lathes or micro-lathes). The construction of a center lathe is detailed above, but depending on the year of manufacture, size, price range or desired features, even these lathes can vary widely between models.

Engine lathe is the name applied to a traditional late-19th-century or 20th-century lathe with automatic feed to the cutting tool, as opposed to early lathes which were used with hand-held tools, or lathes with manual feed only. The usage of "engine" here is in the mechanical-device sense, not the prime-mover sense, as in the steam engines which were the standard industrial power source for many years. The works would have one large steam engine which would provide power to all the machines via a line shaft system of belts. Therefore, early engine lathes were generally 'cone heads', in that the spindle usually had attached to it a multi-step pulley called a cone pulley designed to accept a flat belt. Different spindle speeds could be obtained by moving the flat belt to different steps on the cone pulley. Cone-head lathes usually had a countershaft (layshaft) on the back side of the cone which could be engaged to provide a lower set of speeds than was obtainable by direct belt drive. These gears were called back gears. Larger lathes sometimes had two-speed back gears which could be shifted to provide a still lower set of speeds.

When electric motors started to become common in the early 20th century, many cone-head lathes were converted to electric power. At the same time the state of the art in gear and bearing practice was advancing to the point that manufacturers began to make fully geared headstocks, using gearboxes analogous to automobile transmissions to obtain various spindle speeds and feed rates while transmitting the higher amounts of power needed to take full advantage of high speed steel tools. Cutting tools evolved once again, with the introduction of man made carbides,and became widely introduced to general industry in the 1970s. Early carbides were attached toolholders by brazing them into a machined 'nest' in the tool holders, later designs allowed tips to be replaceable, and multi faceted, allowing them to be reused. Carbides tolerate much higher machining speeds without wearing. This has led to machining times shortening, and therefore production growing. The demand for faster and more powerful lathes controlled the direction of lathe development.

The availability of inexpensive electronics has again changed the way speed control may be applied by allowing continuously variable motor speed from the maximum down to almost zero RPM. This had been tried in the late 19th century but was not found satisfactory at the time. Subsequent improvements in electric circuitry have made it viable again.

 

Toolroom lathe

A toolroom lathe is a lathe optimized for toolroom work. It is essentially just a top-of-the-line center lathe, with all of the best optional features that may be omitted from less expensive models, such as a collet closer, taper attachment, and others. The bed of a toolroom lathe is generally wider than that of a standard centre lathe. There has also been an implication over the years of selective assembly and extra fitting, with every care taken in the building of a toolroom model to make it the smoothest-running, most-accurate version of the machine that can be built. However, within one brand, the quality difference between a regular model and its corresponding toolroom model depends on the builder and in some cases has been partly marketing psychology. For name-brand machine tool builders who made only high-quality tools, there wasn't necessarily any lack of quality in the base-model product for the "luxury model" to improve upon. In other cases, especially when comparing different brands, the quality differential between (1) an entry-level center lathe built to compete on price, and (2) a toolroom lathe meant to compete only on quality and not on price, can be objectively demonstrated by measuring TIR, vibration, etc. In any case, because of their fully ticked-off option list and (real or implied) higher quality, toolroom lathes are more expensive than entry-level center lathes.

 

Turret lathe and capstan lathe

Main article: Turret lathe

Turret lathes and capstan lathes are members of a class of lathes that are used for repetitive production of duplicate parts (which by the nature of their cutting process are usually interchangeable). It evolved from earlier lathes with the addition of the turret, which is an indexable toolholder that allows multiple cutting operations to be performed, each with a different cutting tool, in easy, rapid succession, with no need for the operator to perform setup tasks in between (such as installing or uninstalling tools) nor to control the toolpath. (The latter is due to the toolpath's being controlled by the machine, either in jig-like fashion [via the mechanical limits placed on it by the turret's slide and stops] or via IT-directed servomechanisms [on computer numerical controlled (CNC) lathes].)[4]

There is a tremendous variety of turret lathe and capstan lathe designs, reflecting the variety of work that they do.

 

Gang-tool lathe 

A gang-tool lathe is one that has a row of tools set up on its cross-slide, which is long and flat and is similar to a milling machine table. The idea is essentially the same as with turret lathes: to set up multiple tools and then easily index between them for each part-cutting cycle. Instead of being rotary like a turret, the indexable tool group is linear.

 

Multispindle lathe

See also: Screw machine (automatic lathe)

Multispindle lathes have more than one spindle and automated control (whether via cams or CNC). They are production machines specializing in high-volume production. The smaller types are usually called screw machines, while the larger variants are usually called automatic chucking machinesautomatic chuckers, or simply chuckers. Screw machines usually work from bar stock, while chuckers automatically chuck up individual blanks from a magazine. Typical minimum profitable production lot size on a screw machine is in the thousands of parts due to the large setup time. Once set up, a screw machine can rapidly and efficiently produce thousands of parts on a continuous basis with high accuracy, low cycle time, and very little human intervention. (The latter two points drive down the unit cost per interchangeable part much lower than could be achieved without these machines.)

 

CNC lathe / CNC turning center

Computer numerical controlled (CNC) lathes are rapidly replacing the older production lathes (multispindle, etc.) due to their ease of setting, operation, repeatability and accuracy. They are designed to use modern carbide tooling and fully use modern processes. The part may be designed and the tool paths programmed by the CAD/CAM process or manually by the programmer, and the resulting file uploaded to the machine, and once set and trialled the machine will continue to turn out parts under the occasional supervision of an operator.

The machine is controlled electronically via a computer menu style interface, the program may be modified and displayed at the machine, along with a simulated view of the process. The setter/operator needs a high level of skill to perform the process, however the knowledge base is broader compared to the older production machines where intimate knowledge of each machine was considered essential. These machines are often set and operated by the same person, where the operator will supervise a small number of machines (cell).

The design of a CNC lathe varies with different manufacturers, but they all have some common elements. The turret holds the tool holders and indexes them as needed, the spindle holds the workpiece and there are slides that let the turret move in multiple axis simultaneously. The machines are often totally enclosed, due in large part to occupational health and safety(OH&S) issues.

With rapid growth in this industry, different CNC lathe manufacturers use different user interfaces which sometimes makes it difficult for operators as they have to be acquainted with them. With the advent of cheap computers, free operating systems such as Linux, and open source CNC software, the entry price of CNC machines has plummeted.

 

CNC Horizontal Milling 

CNC horizontal machining is performed using horizontally-configured lathes, machining centers, boring machines, or boring mills. The equipment used typically consists of rotating cylindrical cutters moving up and down along five axes. These machines are capable of producing a variety of shapes, slots, holes, and details on a three-dimensional part.

CNC Vertical Milling

Vertically-oriented CNC machines utilize cylindrical cutters on a vertical spindle axis to create plunge cuts and drilled holes, as well as custom shapes, slots, and details on three-dimensional parts. Equipment used in this type of milling includes vertical lathes, vertical machining centers, and 5-axis machines.

Swiss-style lathe / Swiss turning center 

Swiss-style lathe is a specific design of lathe providing extreme accuracy (sometimes holding tolerances as small as a few tenths of a thousandth of an inch—a few micrometers). A Swiss-style lathe holds the workpiece with both a collet and a guide bushing. The collet sits behind the guide bushing, and the tools sit in front of the guide bushing, holding stationary on the Z axis. To cut lengthwise along the part, the tools will move in and the material itself will move back and forth along the Z axis. This allows all the work to be done on the material near the guide bushing where it is more rigid, making them ideal for working on slender workpieces as the part is held firmly with little chance of deflection or vibration occurring. This style of lathe is commonly used under CNC control.

Most CNC Swiss-style lathes today use one or two main spindles plus one or two back spindles (secondary spindles). The main spindle is used with the guide bushing for the main machining operations. The secondary spindle is located behind the part, aligned on the Z axis. In simple operation it picks up the part as it is cut off, and accepts it for second operations, then ejects it into a bin, eliminating the need to have an operator manually change each part, as is often the case with standard CNC turning centers. This makes them very efficient, as these machines are capable of fast cycle times, producing simple parts in one cycle (i.e., no need for a second machine to finish the part with second operations), in as little as 10–15 seconds. This makes them ideal for large production runs of small-diameter parts.

Additionally, as many Swiss lathes incorporate a secondary spindle, or 'sub-spindle', they also incorporate 'live tooling'. Live tools are rotary cutting tools that are powered by a small motor independently of the spindle motor(s). Live tools increase the intricacy of components that can be manufactured by the Swiss lathe. For instance, automatically producing a part with a hole drilled perpendicular to the main axis (the axis of rotation of the spindles) is very economical with live tooling, and similarly uneconomical if done as a secondary operation after machining by the Swiss lathe is complete. A 'secondary operation' is a machining operation requiring a partially completed part to be secured in a second machine to complete the manufacturing process. Generally, advanced CAD/CAM software uses live tools in addition to the main spindles so that most parts that can be drawn by a CAD system can actually be manufactured by the machines that the CAD/CAM software support.

 

Combination lathe / 3-in-1 machine

combination lathe, often known as a 3-in-1 machine, introduces drilling or milling operations into the design of the lathe. These machines have a milling column rising up above the lathe bed, and they utilize the carriage and topslide as the X and Y axes for the milling column. The 3-in-1 name comes from the idea of having a lathe, milling machine, and drill press all in one affordable machine tool. These are exclusive to the hobbyist and MRO markets, as they inevitably involve compromises in size, features, rigidity, and precision in order to remain affordable. Nevertheless, they meet the demand of their niche quite well, and are capable of high accuracy given enough time and skill. They may be found in smaller, non-machine-oriented businesses where the occasional small part must be machined, especially where the exacting tolerances of expensive toolroom machines, besides being unaffordable, would be overkill for the application from an engineering perspective.

 

Mini-lathe and micro-lathe

Mini-lathes and micro-lathes are miniature versions of a general-purpose center lathe (engine lathe). They typically have swings in the range of 3 to 7 in (76 to 178 mm) diameter (in other words, 1.5 to 3.5 in (38 to 89 mm) radius). They are small and affordable lathes for the home workshop or MRO shop. The same advantages and disadvantages apply to these machines as explained earlier regarding 3-in-1 machines.

As found elsewhere in English-language orthography, there is variation in the styling of the prefixes in these machines' names. They are alternately styled as mini lathe, minilathe, and mini-lathe and as micro lathe, microlathe, and micro-lathe.

 

Wheel lathe

Wheel lathes are machines used to manufacture and resurface the wheels of railway cars. When wheels become worn or compromised from excessive use, this tool can be used to re-cut and recondition the wheel of the train car. There are a number of different wheel lathes available including underfloor variations for resurfacing wheels that are still attached to the rail car, portable types that are easily transported for emergency wheel repairs, and CNC versions which utilize computer-based operating systems to complete the wheel repair.

 

Pit Lathe

A lathe for large diameter, though short work, built over a recess in the floor to admit the lower part of the workpiece thus allowing the toolrest to stand at the turner's waist height. An example is on display at the London Science Museum, Kensington.

 

Brake lathe

A lathe specialized for the task of resurfacing brake drums and discs in automotive or truck garages.

 

Oil country lathe

Specialised lathes for machining long workpieces such as segments of drill strings. Oil country lathes are equipped with large-bore hollow spindles, a second chuck on the opposite side of the headstock, and frequently outboard steadies for supporting long workpieces.

Various feed mechanisms exist to feed material into a lathe at a defined rate.

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