Screw machine

A screw machine is a metalworking machine tool used in the high-volume manufacture of turned components. Screw machines are fundamentally a type of lathe that is specialized for the automated production of small parts.

Speaking with reference to the normal definition of the term screw machine, all screw machines are fully automated, whether mechanically (via cams) or by CNC (computerized control), which means that once they are set up and started running, they continue running and producing parts with very little human intervention. This has been true since the 1870s. Mechanical automation came first, beginning in the 1870s; computerized control (via first NC and then CNC) came later, beginning in the 1950s.

The name screw machine is somewhat of a misnomer, because screw machines spend much of their time making things that are not screws and that in many cases are not even threaded. However, the archetypal use for which screw machines were named was screw-making.

The definition of the term screw machine has changed with changing technology. Any use of the term prior to the 1840s, if it occurred, would have referred ad hoc to any machine tool used to produce screws, that is, there would have been no established differentiation from the term screw-cutting lathe. When turret lathes were developed in the 1840s, the term screw machine was applied to them in overlapping usage with the term turret lathe. In 1860, when some of the movements, such as turret indexing, were mechanically automated, the term automatic screw machine was applied, and the term hand screw machine or manual screw machine was retronymously applied to the earlier machines. Within 15 years, the entire part-cutting cycle had been mechanically automated, and machines of the 1860 type were retronymously called semi-automatic. From that time on, machines with fully automated cycles were usually called automatic screw machines, and eventually, in the usage of most people in the machining industries, the term screw machine no longer was used to refer to manual or semi-automatic turret lathes, having become reserved for one class of machine, the fully mechanically automated type. This narrow meaning of screw machine remained stable from about the 1890s until the 1950s. (Brown & Sharpe continued to call some of their hand-operated turret lathe models "screw machines", but most machinists reserved the term for automatics.) Within this class called screw machines there were variations, such as single-spindle versus multispindle, horizontal-turret versus vertical-turret, etc.

With the advent of NC, screw machines diverged into two classes, mechanical and NC. This distinction continues today with mechanical screw machines and CNC screw machines. However, in shop-floor jargon, the term screw machine by itself is still often understood in context to imply a mechanical screw machine, so the retronym mechanical screw machine is not consistently used.

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.

A screw machine may have a single spindle or multiple spindles. Each spindle contains a bar 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 operation of 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.

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.

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 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.

The history of screw machines 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)^[1], 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".^[2] 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^[1]; 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)^[1][3]; 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.

Small- to medium-sized cam-operated automatic lathes were (and still are) usually called "screw machines" or "automatic screw machines", while larger ones are usually called "automatic chucking lathes", "automatic lathes", "automatic chuckers", "automatics", or "chuckers".

The development of numerical control was the next major leap in screw machine history. 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. Today CNC screw machines 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.

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.

Rotary transfers can be relatively small to very large, CNC multi-station milling and turning centers. It utilizes between 6 and 24 and more turret stations, with each turret face holding a chuck or collet. The parts are loaded by a parts bin into the holding fixture, and the part is then taken through whatever number of stations for machining. Each station can be fitted with various CNC turning, milling,slotting and grinding fixtures, and the holding stations are fully indexable. Rotary transfer machines can forgo turning operations entirely, and perform pure milling operations on bar stock. These are the singlemost expensive machine tools available today, and the highest throughput capability. One manufacturer has reported that a 12-station cnc rotary transfer machine has taken the production load off six CNC screw machines, and several CNC lathes and mills used in secondary operations. There is a Swiss manufacturer of Transfer Machines who claims production output of up to 450 parts per minute performing up to 8 machining operations on each part. These machines are often multiple tooled i.e. several parts are machined at the same time. Rotary Transfer Machines, also called, Dial Index Machines are used wherever metal components require multiple machining operations and large volume production output. Modern Transfer Machines can hold very close tolerances and achieve astounding surface machining finishes.

• 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

• 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).

• YouTube video showing a 1965 cam-op screw machine in action.
• YouTube video showing another cam-op screw machine.
• Video showing a CNC screw machine cycle.