User:Three-quarter-ten/Draft:Automatic lathe
An automatic lathe is a lathe (usually a metalworking lathe) whose actions are controlled automatically. Because of the historical path of development of machine tool technology, the natural language terminology used to name automatic lathes, at least in English, is not hierarchical in quite the way that a manufacturing layperson might expect. Retronymy (and, in other respects, lack of retronymy) have shaped the nomenclature. However, it is easy enough to understand once a bit of history is known (as explained below).
General nomenclature
[edit]The term "automatic lathe" is generally used in manufacturing to mean non-CNC lathes. The first kinds of automatic lathes were mechanically automated ones (whose control is via cams or tracers and pantographs). Thus, before electronic automation via numerical control, programmable logic controllers, and so on, the "automatic" in the term "automatic machine tool" always referred implicitly to mechanical automation. The earliest mechanically automated lathes were geometric lathes. These included Rose engine lathes and others.
In industrial contexts during the Machine Age, the term "automatic lathe" referred to mechanical screw machines and chuckers, of which a large variety of types, brands, and models were built.
Since the maturation of CNC, the implicit dichotomy of "manual versus automatic" still exists, but because CNC is so ubiquitous, the term "automatic" has lost some of its distinguishing power. All CNC machine tools are automatic, but (perhaps therefore) the usage of the machining industries doesn't routinely call them by that term. The term "automatic", when it is used at all, still usually refers implicitly to cam-operated machines.
Small- to medium-sized cam-operated automatic lathes were (and still are) usually called screw machines or automatic screw machines. These machines work on parts that (as a rough guide only) are usually less than 50 millimetres (2.0 in) in diameter and less than 250 millimetres (9.8 in) long. Screw machines almost invariably do bar work, which means that an entire length of bar stock (anywhere from 2 to 12 feet in length) passes through the spindle and is gripped by the chuck (which is usually a collet chuck). As the part is being machined, the entire length of bar stock is turning with the spindle. When the part is done, it is "parted off" from the bar, the chuck umclamps, the bar is fed forward, and the chuck then closes again, ready for the next cycle. The bar-feeding can happen by various means, including pulling-finger tools that grab the bar and pull or roller bar feed that feeds the bar from behind.
Larger cam-operated automatic lathes were (and still are) usually called automatic chucking lathes, automatic lathes, automatic chuckers, automatics, or chuckers. The "chucker" part of the name comes from the fact that the workpieces are usually discrete blanks, held in a bin called a "magazine", and each one takes a turn at being chucked (gripped by the machine for being worked on) and cut. (This is analogous to the way that each round of ammunition in the magazine of a semi-automatic pistol gets its turn at being chambered.) The blanks are either individual forgings or castings, or they are pre-sawed pieces of billet. However, some members of this family of machine tools turn bar work or work on centers (for example, the Fay automatic lathe). Many of these machines are multispindle (more than one main spindle). Well-known brands of such machines have included National-Acme, Acme-Gridley, New Britain-Gridley, Davenport, and others. Some of these machines and brands are obscure outside of narrow industry niches, such as automotive parts suppliers. This is because the market for machine tools to crank out gargantuan production volumes with extremely high degrees of automation is fairly small, because it does not include the neighborhood job shop or tool and die shop.
CNC lathes began as conventional designs (on the form factors of engine lathes, turret lathes, and traditional screw machines) retrofitted with CNC control. The term "turning center" was sometimes applied to them, in direct parallel to the "machining center" name for milling machines equipped with CNC and automatic tool changers (ATCs). However, it was the advent of live tooling (lathe tools with rotary axes) that gave the term "turning center" a truly distinct meaning from a "mere" CNC lathe (if one chose to enforce the nomenclatural distinction).
In recent decades CNC lathes have evolved into new classes of machine tools that are often called by names other than "lathe", such as "turn-mills", "mill-turns", and "rotary transfer machines". These machine tools are neither lathe nor mill (as classically defined) but a hybrid of both. Decades of analyzing and synthesizing machining processes have made clear that no particular form factor need be taken for granted in machine design. This development, in tandem with the continual advance of CNC, has tended to add more and more spindles and axes to machine tools, to the point that CNC programming (such as with G-code) requires CAD/CAM (as opposed to "manual" programming) to be practical and competitive in the market segments served by these classes of machines. As Smid says, "Combine all these axes with some additional features, and the amount of knowledge required to succeed is quite overwhelming, to say the least."[1] At the same time, however, programmers still must thoroughly understand the principles of manual programming and must think critically and second-guess aspects of the software's decisions.
Screw machines
[edit]
Screw machines, being the class of automatic lathes for small- to medium-sized parts, are used in the high-volume manufacture of a vast variety of turned components. The nomenclature of screw machines is somewhat nonintuitive due to less-than-consistent retronymy over the years.
Choice of machines for small or medium work
[edit]Mechanical screw machines have been replaced to some extent by CNC lathes (turning centers) and CNC screw machines. However, they are still commonly in operation, and for high-volume production of turned components it is still often true that nothing is as cost-efficient as a mechanical screw machine.
In the hierarchy of manufacturing machines, the screw machine sits at the top when large product volumes are needed. An engine lathe sits at the bottom, taking the least amount of time to set up but the most amount of skilled labor and time to actually produce a part. A turret lathe has traditionally been one step above an engine lathe, needing greater set-up time but being able to produce a higher volume of product and usually requiring a lower-skilled operator once the set-up process is complete. Screw machines may require an extensive set-up, but once they are running, a single operator can monitor the operation of several machines.
The advent of the CNC lathe (or more properly, CNC turning center) has blurred these distinct levels of production to some extent. The CNC turning center most appropriately fits in the mid-range of production, replacing the turret lathe. However, it is often possible to produce a single component with a CNC turning center more quickly than can be done with an engine lathe. To some extent too, the CNC turning center has stepped into the region traditionally occupied by the (mechanical) screw machine. CNC screw machines do this to an even greater degree, but they are expensive. In some cases they are vital, yet in others a mechanical machine can match or beat overall performance and profitability. There are many variables involved in answering the question of which is best for a particular part at a particular company.
Automatic chucker
[edit]An automatic chucking machine is very similar to an automatic screw machine, except it is not bar-fed, but rather is fed by a magazine full of blanks (pieces of stock), each of which gets a turn at being chucked (gripped by the machine for being worked on). (This is analogous to the way that each round of ammunition in the magazine of a semi-automatic pistol gets its turn at being chambered.) While a screw machine is limited to around 3.5 inches in practice, automatic chuckers are available that can handle up to 12" chucks arranged in the same way that a screw machine would arrange multiple spindles. The chucks are air-operated.
Some of these machines can also be bar-fed. The Fay automatic lathe was a variant that specialized in turning work on centers.
Design
[edit]
An automatic lathe may have a single spindle or multiple spindles. Each spindle contains a bar or blank of material that is being machined simultaneously. A common configuration is six spindles. The cage that holds these six bars of material indexes after each machining operation is complete. The indexing is reminiscent of a Gatling gun.
Each station may have multiple tools that cut the material in sequence. The tools are usually arranged in several axes, such as turret (rotary indexing), horizontal slide (linear indexing), and vertical slide (linear indexing). The linear groups are called "gangs". The operation of all these tools is similar to that on a turret lathe.
By way of example: a bar of material is fed forward through the spindle. The face of the bar is machined (facing operation). The outside of the bar is machined to shape (turning operation). The bar is drilled or bored, and finally, the part is cut off (parting operation).
In a single-spindle machine, these four operations would most likely be performed sequentially, with four cross-slides each coming into position in turn to perform their operation. In a multi-spindle machine, each station corresponds to a stage in the production sequence through which each piece is then cycled, all operations occurring simultaneously, but on different pieces of work, in the manner of an assembly line.
Operations
[edit]Form tools
[edit]For the machining of complex shapes, it is common to use form tools. This contrasts with the cutting that is performed on an engine lathe where the cutting tool is usually a single-point tool. A form tool has the form or contour of the final part but in reverse, so it cuts the material leaving the desired component shape. This contrasts to a single-point tool, which cuts on one point at a time and the shape of the component is dictated by the motion of the tool rather than its shape.
Threading
[edit]Unlike on a lathe, single-point threading is rarely if ever performed; it is too time-consuming for the short cycle times that are typical of screw machines. A self-releasing die head can rapidly cut or roll-form threads on outside diameters. A non-releasing tap holder with a tap can quickly cut inside diameters but it requires single spindle machines to reverse into high speed in order for the tap to be removed from the work. Threading and tapping speed (low speed) is typically 1/5 the high speed.
Rotary broaching
[edit]Rotary broaching is another common operation. The broach holder is mounted stationary while its internal live spindle and end cutting broach tool are driven by the workpiece. As the broach is fed into or around the workpiece, the broach's contact points are constantly changing, easily creating the desired form. The most common form made this way is a hexagonal socket in the end of a cap screw.
History
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The history of geometric lathes is discussed elsewhere, and concerns such applications as engraving and printing (especially of paper currency), jewelry making, and artistic pursuits.
The history of automatic lathes in industrial contexts began with screw machines, and that history can only be truly understood within the context of screw making in general. Thus the discussion below begins with a simple overview of screw making in prior centuries, and how it evolved into 19th-, 20th-, and 21st-century practice.
Humans have been making screws since ancient times. For most of those centuries, screw making generally involved custom cutting of the threads of each screw by hand (via whittling or filing). Other ancient methods involved wrapping wire around a mandrel (such as a stick or metal rod) or carving a tree branch that had been spirally wrapped by a vine.
Various machine elements that potentially lent themselves to screw making (such as the lathe, the leadscrew, the slide rest, gears, and "change gear" gear trains) were developed over the centuries, with some of those elements being quite ancient. However, it was not until the era of 1770-1800 that these various elements were brought together successfully to create a new type of machine tool, the screw-cutting lathe, which for the first time took screws and moved them from the category of expensive, hand-made, seldom-used hardware into the category of affordable, often-interchangeable commodity. (The interchangeability developed gradually, from intra-company to inter-company to national to international).
Between 1800 and 1840, it became common practice to build all of the relevant screw-cutting machine elements into engine lathes, so the term "screw-cutting lathe" ceased to stand in contradistinction to other lathe types as a "special" kind of lathe. The 1770-1840 development arc was a tremendous technological advance, but later advancements would make screws even cheaper and more prevalent yet again. These began in the 1840s with the adaptation of the engine lathe with a turret-head toolholder to create the turret lathe. This development greatly reduced the time, effort, and skill needed from the machine operator to produce each screw. Then, in the 1870s, the turret lathe's part-cutting cycle (sequence of movements) was automated by being put under cam control, in a way very similar to how music boxes and player pianos can play a tune automatically. According to Rolt (1965),[2] the first person to develop such a machine was Christopher Miner Spencer, a New England inventor. All of the above machine tools, from screw-cutting lathes to suitably equipped engine lathes to turret lathes, were sometimes called "screw machines" during this era (logically enough, given that they were machines tailored to screw making).
Charles Vander Woerd may have contemporarily independently invented a machine similar to Spencer's. Spencer patented his idea in 1873; unfortunately, his patent attorney failed to protect the most significant part, the cam drum, which Spencer called the "brain wheel".[3] Therefore many other people quickly took up the idea. Later important developers of fully automatic lathes (large and small) included S. L. Worsley, who developed a single-spindle machine for Brown & Sharpe[2]; Edwin C. Henn, Reinhold Hakewessel, and George O. Gridley, who developed multiple-spindle variants and who were involved with a succession of corporations (Acme, National, National-Acme, Windsor Machine Company, Acme-Gridley, New Britain-Gridley)[2];[4] F.C. Fay and Otto A. Schaum, who developed the Fay automatic lathe; Ralph Flanders and his brother Ernest, who further refined the Fay lathe and who developed the automatic screw thread grinder; and many others. Meanwhile, engineers in Switzerland were also developing clever new manually and automatically controlled lathes during this same era. The technological developments in America and Switzerland flowed rapidly into other industrialized countries (via routes such as machine tool exports; trade journal articles and advertisements; trade shows, from world's fairs to regional events; and the turnover and emigration of engineers, setup hands, and operators). There, local innovators also developed further creative tooling for the machines and built clone machine models.
The development of numerical control was the next major leap in the history of automatic lathes—and it is also what changed the paradigm of what the "manual versus automatic" distinction even meant. Beginning in the 1950s, NC lathes began to take over the jobs that formerly were done by manual lathes and cam-op screw machines, although the displacement of the older technology by CNC has been a long, gradual arc that even today is not a total eclipse. By the 1980s, true CNC screw machines (as opposed to simpler CNC lathes), Swiss-style and non-Swiss, had begun to make serious inroads into the realm of cam-op screw machines. Similarly, CNC chuckers were developed, eventually evolving into rotary transfer machines. Very few people outside of automotive manufacturing are familiar with these machine tools, because they simply have no economic reason to come into contact with this segment of the machine tool industry. Today CNC lathes and their offspring (turn-mills, mill-turns, rotary transfers) are technological wonders with a blizzard of axes and accessories under CNC control. Their sophistication, accuracy and precision, metal-removal speed, tool-changing speed, degree of automation, and degree of networking with the rest of the enterprise are formidable.
References
[edit]Bibliography
[edit]- Rolt, L. T. C. (1965), A Short History of Machine Tools, Cambridge, Massachusetts, USA: MIT Press, OCLC 250074. Co-edition published as Rolt, L. T. C. (1965), Tools for the Job: a Short History of Machine Tools, London: B. T. Batsford, LCCN 65080822.
- Rose, William (1990), Cleveland: the making of a city, Kent State University Press, ISBN 9780873384285
- Smid, Peter (2008), CNC Programming Handbook (3rd ed.), New York: Industrial Press, ISBN 9780831133474, LCCN 2007045901.
Further reading
[edit]- Roe, Joseph Wickham (1916), English and American Tool Builders, New Haven, Connecticut: Yale University Press, LCCN 16011753. Reprinted by McGraw-Hill, New York and London, 1926 (LCCN 27-24075); and by Lindsay Publications, Inc., Bradley, Illinois, (ISBN 978-0-917914-73-7).