A vast number of industries, including the automotive, appliance, electronics, bathroom fittings and other industries, and products make use of turned components, says machine tools manufacturer FAS Machine Tools CEO Peter Frow.
In a research paper written by Frow, he points out that some of these components are made in-house by the product manufacturer, while others are subcontracted to specialist repetition turning ‘job shops.’ The component type and size vary enormously, as do batch sizes – anything from ten to hundreds of thousands.
“Where batch sizes increase above 100 and particularly above 1 000, the components will typically be made on one or other type of production lathe,” he states.
A production lathe is any lathe where the tool movements are achieved by some automatic means. Thus, once set, they can produce components without any direct operator action, except for loading the raw material.
“In the production turning stakes, it is important to select the best machine for the job.
“The best machine is the one that yields the lowest cost for a particular component and a particular batch size,” says Frow.
Generally speaking, the production turning industry uses two distinct types of production lathes – Cam automatic lathes and computer-numerically controlled (CNC) lathes.
The paper highlights that the design of the cam automatic lathe was pioneered over a hundred years ago and it uses cams for achieving tool movements. The lathes are characterised by fast cycle times and long set-up times.
Further, the paper states that CNC lathes were developed in the seventies, initially as hard-wired numerically controlled machines. They soon incorporated microprocessors, hence the term CNC. These lathes have relatively long cycle times, compared with cam-operated lathes, especially for smaller components, but are far more user-friendly and have relatively short set-up times.
“Within these two broad categories, production lathes can be further broken down into various subtypes (see Figure 1).
“The diagram illustrates that there is usually something of a trade-off between cycle time and set-up time.
“Cycle time is a function of the metal removal rate (MRR) of the different tools, the number of tools that can be made to work simultaneously on the work piece, and the time taken for nonproductive or idle operations, such as the feeding of material and indexing of tools.
“Thus, to optimise the cycle time, one must maximise the MRR, maximise the number of tools working simultaneously and minimise the duration of idle operations,” says Frow.
He points out that the cam-operated multi-spindle lathe (Type A, Figure 1) works on six components in various stages of completion at any one time. Thus, it has as many as 12 tools cutting simultaneously.
“Amazingly, this type of machine was developed in the late nineteenth century and still produces the quickest cycle times of any production lathe.
“However, South Africa has less than 150 of this type of machine in regular operation.
“The greatest number (estimated at 2 500+) of cam automatics in the country, are of the single spindle type (Types B, C, & D),” he says.
Frow notes that these machines can typically have four tools working simultaneously. They make extensive use of the ‘plunge forming’ technique, where tools having a particular shape are advanced into the work piece along an axis perpendicular to the spindle axis.
“By contrast, CNC lathes use single point tools and generate the required shapes with a digitally programmed tool path. Form tools can generally be applied at the same time that end working is taking place. This technique is known as ‘overlapping’.
“In order to select the best machine for a particular application, one should plot the cost-effectiveness curve for each machine being considered. Figure 2 would be a typical set of curves for a component of less than 40 mm in diameter made from bar stock.
“Other things being equal, the batch size represented by X would be the point at which it would be more cost effective to make the component on a cam automatic than a CNC lathe. Batch size Y would be the point at which a multi-spindle would become the better choice,” says Frow.
He points out, however, that other things are not equal: In particular, there is a dwindling pool of tool setters with the skills necessary to set cam automatic lathes. Each job requires a set of job-specific cams. The number of persons capable of designing optimum cams has also shrunk.
“There is thus a tendency to run jobs on CNC lathes despite their longer cycle times where the cost effectiveness curves would suggest otherwise.
“The great majority of CNC lathes in the country are of types J and K, although there are a fair number of type H lathes.
“Viewed at a macro level, the net effect of this is a tendency to drive component prices upward, making South Africa less competitive in a global market,” says Frow.
He notes that imported type E CNC lathes have cycle times comparable with cam automatics, but unfortunately are priced such that one can purchase about three type J machines for the same figure.
Further, he says that, despite fierce competition from the East, South Africa has CNC lathe manufacturers. “Efamatic in Gauteng produces a range of type J machines, while FAS Machine Tools in Durban manufactures a type E multi-slide machine with cycle times that more than match those of equivalent cam automatics.”