Lathe (metal)
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- See also: Lathe (tool)
A metal lathe is generic description for a rigid machine tool designed to remove material from a workpiece, through the action of a cutting tool. They were originally designed to machine metals; however, with the advent of plastics and other materials, and with their inherent versatility, they are used in a wide range of applications, and a broad range of materials.
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[edit] Construction
The machine has been greatly modified for various applications however a familiarity with the basics shows the similarities between types. These machines consist of, at the least, a headstock, bed, carriage and tailstock. The better machines are solidly constructed with broad bearing surfaces (slides or ways) for stability and manufactured with great precision. This helps ensure the components manufactured on the machines can meet the required tolerances and repeatability required.
[edit] Headstock
The headstock (H1) houses the main spindle (H4), speed change mechanism (H2,H3), and change gears (H10). The headstock is required to be made as robust as possible due to the cutting forces involved, which can distort a lightly built housing, and induce harmonic vibrations that will transfer through to the workpiece, reducing the quality of the finished workpiece.
The main spindle is generally hollow to allow long bars to extend through to the work area, this reduces preparation and waste of material. The spindle then runs in precision bearings and is fitted with some means of attaching work holding devices such as chucks or faceplates. This end of the spindle will also have an included taper, usually morse, to allow the insertion of tapers and centers. On older machines the spindle was directly driven by a flat belt pulley with the lower speeds available by manipulating the bull gear, later machines use a gear box driven by a dedicated electric motor. The fully geared head allows the speed selection to be done entirely through the gearbox and be a dogo
[edit] Bed
The bed is a robust base that connects to the headstock and permits the carriage and tailstock to be aligned parallel with the axis of the spindle. This is facilitated by hardened and ground ways which restrain the carriage and tailstock in a set track. The carriage travels by means of a rack and pinion system, leadscrew of accurate pitch, or feedscrew.
[edit] Feed and lead screws
The feedscrew (H8) is a long driveshaft that allows a series of gears to drive the carriage mechanisms. These gears are located in the apron of the carriage. Both the feedscrew and leadscrew (H9) are driven by either the change gears (on the quadrant) and or an intermediate gearbox known as a quick change gearbox (H6) or norton gearbox. These intermediate gears allow the correct ratio and direction to be set for cutting threads or worm gears. Tumbler gears (operated by H5) are provided between the spindle and gear train along with a quadrant plate that enables a gear train of the correct ratio and direction to be introduced. This provides a constant relationship between the number of turns the spindle makes, to the number of turns the leadscrew makes. This ratio allows screwthreads to be cut on the workpiece without the aid of a die.
The leadscrew will be manufactured to either imperial or metric standards and will require a conversion ratio to be introduced to create thread forms from a different family. To accurately convert from one thread form to the other requires a 127-tooth gear, or on lathes not large enough to mount one, an approximation may be used. Multiples of 3 and 7 giving a ratio of 63:1 can be used to cut fairly loose threads. This conversion ratio is often built into the quick change gearboxes.
[edit] Carriage
In its simplest form the carriage holds the tool bit and moves it longitudinally (turning) or perpendicularly (facing) under the control of the operator. The operator moves the carriage manually via the handwheel (5a) or automatically by engaging the feedscrew with the carriage feed mechanism (5c), this provides some relief for the operator as the movement of the carriage becomes power assisted. The handwheels (2a, 3b,5a) on the carriage and its related slides are usually calibrated, both for ease of use and to assist in making reproducible cuts.
[edit] Cross-slide
(3) The cross-slide sits atop the carriage and has a leadscrew that travels perpendicular to the main spindle axis, this permits facing operations to be performed. This leadscrew can be engaged with the feedscrew (mentioned previously) to provide automated movement to the cross-slide, only one direction can be engaged at a time as an interlock mechanism will shut out the second gear train.
[edit] Compound Rest
(2) The Compound Rest (or top slide) is the part of the machine where the tool post is mounted. It provides a smaller amount of movement along its axis via another leadscrew. The compound rest axis can be adjusted independently of the carriage or cross-slide. It is utilized when turning tapers, when screwcutting or to obtain finer feeds than the leadscrew normally permits.
[edit] Toolpost
(1) The tool bit is mounted in the toolpost which may be of the American lantern style, traditional 4 sided square style, or in a quick change style such as the multifix arrangement pictured. The advantage of a quick change set-up is to allow an unlimited number of tools to be used (up to the number of holders available) rather than being limited to 1 tool with the lantern style, or 3 to 4 tools with the 4 sided type. Interchangeable tool holders allow the all the tools to be preset to a center height that will not change, even if the holder is removed from the machine.
[edit] Tailstock
The tailstock is a toolholder directly mounted on the spindle axis, opposite the headstock. The spindle (T5) does not rotate but does travel longitudinally under the action of a leadscrew and handwheel (T1). The spindle includes a taper to hold drill bits, centers and other tooling. The tailstock can be positioned along the bed and clamped (T6) in position as required. There is also provision to offset the tailstock (T4) from the spindles axis, this is useful for turning small tapers.
The image shows a reduction gear box (T2) between the handwheel and spindle, this is a feature found only in the larger center lathes, where large drills may necessitate the extra leverage.
[edit] Types of metal lathes
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.
[edit] Center lathe
A center lathe or engine lathe may be considered the basis for the metal lathe and is the type most often used by the general machinist or hobbyist. The construction of a center lathe is detailed above, but depending on the year of manufacture, price range, or desired features, even these lathes can vary widely between models. For convenience, the center lathe may be considered a useful starting point.
[edit] Capstan (Ram-type) lathe
The modern name for a capstan lathe used in industry is a ram-style turret lathe. A capstan lathe is a production machine that combines the features of the basic lathe along with a capstan style tailstock. This tailstock has a short slide upon which sits the hexagonal capstan head and a set of depth stops, one for each turret face. The main body is fixed to the bed in the required position and all longitudinal movement is via the short slide. The stroke of the capstan is short but the sequence can be rapid as the tooling indexes automatically at the end of the stroke. This indexing and movement is performed by turning a large levered handwheel as required. This type of Lathe is ideal to perform multiple matching operations with high accuracy and superior surface finish. Modern capstan lathes are referred to as ram-style turret lathes. They have a short spindle-to-turret distance, usually in the range of twenty to thirty inches. It is rare to find a capstan lathe with a lead screw or a taper attachment, as lead screw threading is usually far too slow for production. A die head is used instead. 1
[edit] Turret (Saddle-type) lathe
A turret lathe is a production machine that to all appearances is the same as the capstan, however the turret slides directly on the bed rather than being fixed. Some turret Movement of the turret can therefore be anywhere along the bed.1 Because of this feature, saddle-type turret lathes are far longer than the ram-type turret lathe. It is common to find saddle lathes longer than 200" for sale. Oil country and hollow spindle lathes are often of the saddle turret type.The main advantage associated with this type of lathe is that different machining operations such as drilling, reaming, boring etc., can be performed in a single tool setting by using different tools in a hexagonal turret. This considerably reduces tool setting time.
[edit] Multispindle lathe
A multispindle lathe , commonly called a screw machine, is another production machine that is of high capacity. These machines may have up to 12 spindles that all operate simultaneously, each spindle will have its own tooling and the completion of one full cycle usually produces one part, although this depends on the number of operations required to produce the part. The sequence of events is programmed into the machine by adjusting stops, cams and levers and requires a highly skilled machine setter to perform the process efficiently. The setting and operation of the machine was quite often performed by different people. Two other machines fall into the screw machine category, chucking machines with a higher diameter capacity, and rotary transfer machines, capable of end-to-end parts production of both milled and turned parts.
[edit] Combination lathe
A combination lathe may introduce drilling or milling operations into the design of the lathe. These machines utilize the carriage and topslide as the x and y axis of the machine. These are exclusive to the home market, as the machines are not rigid, accurate, or large enough to be useful in a machine shop. They may be found in smaller, non-machine oriented businesses where the occasional part must be turned.
[edit] CNC lathe / CNC Turning Center
CNC lathes are rapidly replacing the older production lathes (multispindle, etc) due to their ease of setting and operation. They are designed to use modern carbide tooling and fully utilize modern processes. The part may be designed by the Computer-aided manufacturing (CAM) process, 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 has evolved yet again however the basic principles and parts are still recognisable, the turret holds the tools and indexes them as needed. The machines are totally enclosed, due in large part to Occupational health and safety (OH&S) issues.
[edit] Swiss Style Lathe / Swiss Turning Center
For work requiring extreme accuracy (sometimes holding tolerances as small as a few tenths of a thousandth of an inch), a Swiss style lathe is often used. 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's 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 also available with CNC controllers to further increase its versatility.
Most CNC Swiss style lathes today utilize two 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 (or parted off) and 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 operation (I. E. no need for a second machine to finish the part) in as little as 10-15 seconds. This makes them ideal for large production runs of small diameter parts.
[edit] References
- Capstan and Turret differences by (pictures) also with reference to "Operators' Hand-book, fourth edition - H.W.Ward & Co. Ltd, Machine tool makers, Dale road, Selly Oak, Birmingham, published 1938
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