Effective and efficient machining is always an interplay of countless factors, which in turn entail complex interrelationships. The path to the optimum chip therefore requires both sound and holistic knowledge of the properties of the machine tools used and the possibilities and limits of the relevant processes. In this context, our blog series on manufacturing technology in no way claims to replace the relevant technical literature. However, with our insights into manufacturing technology, we would like to arouse interest and curiosity in the fascinating world of metal cutting. This time we are talking about turning.
Turning for high accuracy and best surfaces
Turning is one of the metal-cutting processes which, according to DIN 8580, are assigned to the main group “cutting”. Accordingly, the change in shape takes place with local elimination of the material cohesion, more concretely through the separation of material particles in the form of chips. The high flexibility of the cutting processes with regard to geometry generation and the possible manufacturing accuracies (dimensional, form and positional precision) as well as the achievable surface qualities result in a wide range of possible applications, especially in the field of finish machining.
IT ALL DEPENDS ON THE CUTTING EDGE
Basically, machining processes are differentiated into those with geometrically defined cutting edges and those with geometrically undefined cutting edges. For cutting with geometrically defined cutting edges, a tool is used whose number of cutting edges, geometry of the cutting parts and position of the cutting edges relative to the workpiece are defined, as is the case with turning or milling. Cutting with geometrically undefined cutting edges, on the other hand, is a cutting process in which a tool is used whose number of cutting edges, geometry of the cutting parts and position of the cutting edges relative to the workpiece are undefined, as is the case with grinding). In terms of industrial use, the group “cutting with geometrically determined cutting edges” is the most significant.
TURNING ACCORDING TO DIN
DIN (Deutsche Industrie Norm) defines turning as “machining with a closed, usually circular cutting motion and any feed motion transverse to the cutting direction”. The axis of rotation of the cutting motion is workpiece-bound, i.e. it retains its position relative to the workpiece regardless of the feed motion. The objective in turning is to produce the workpiece with a precise geometric shape and dimensions in accordance with the technical drawings. The shape of the workpiece is determined by the geometry of the tool and the relative movements between the workpiece and the tool, which is called the working couple.
DIFFERENTIATION OF TURNING IN DETAIL
The relative movements during machining generally consist of a cutting movement and one or more feed movements. In turning, the workpiece usually performs the rotary cutting motion and the tool performs the required feed and infeed motions. The resulting movement from the cutting and feed movements is called the effective movement. In turning, the main cutting motion is thus performed by the workpiece to be machined. The chip removal itself takes place by means of a rotating workpiece against which a tool is guided that executes the feed and infeed movements. Consequently, the cutting speed, the feed rate and the depth of cut at which the tool engages the material are the cutting data that require particular attention.
NOT ALL TURNING IS THE SAME
But not all turning is the same. Technically, the turning process is also differentiated according to the characteristics and shape of the surface into facing, round turning, screw turning (including thread turning), hobbing, profile turning and form turning. A further differentiation is made according to the position of the surface into external and internal turning and subdivided according to the surface quality into rough turning, finishing and fine turning.
FORM TURNING FROM MANUAL TO CNC
The bridge between tradition and the future of machining is built in particular by form turning – even if the term hardly plays a role in the general linguistic usage of the practice. The meaning becomes obvious with the reference that the shape of the workpiece is generated during form turning by the control of the feed or cutting movement. The cited bridge function is documented in the type of control of the movements according to which form turning is classified. In so-called “free-form turning”, for example, the feed movement is controlled by hand, which requires specific knowledge and, from today’s point of view, almost extraordinary skills in order to be able to produce the shape of the workpiece in accordance with the drawing. By contrast, in postform turning or copy turning, the feed motion is controlled by a two-dimensional reference die. In so-called “kinematic form turning”, the feed motion is then controlled (kinematically) with the aid of a mechanical gear.
Finally, “NC form turning” represents the current state of modern turning technology in the broadest sense and also offers the greatest potential from DMG MORI’s perspective. Here, the workpiece shape is generated by controlling the feed motion by means of entered data and using numerical control.
CNC turning between tradition and modernity
The immense progress in turning processes, precision tools and control technology has always interacted with the innovations in machine tool construction. Above all, the advancing possibilities of NC controls and programming technology have opened the window to complete machining beyond turning. As a result, even many standard lathes now have at least the option of being able to use driven tools. Thus, it is hardly possible to distinguish them from turning machining centers.
CNC TURNING CENTERS FOR COMPLETE MACHINING
In turning centers, turrets with additional Y-axis and driven tools as well as spindles with C-axis for angular positioning for face and circumferential milling and drilling operations enable complete machining of even complex and no longer rotationally symmetrical components in one setup. Additional systems for tool monitoring, workpiece measurement, and tool and workpiece changing devices are usually integrated for qualitative process design. This paves the way for the hybrid top class of turn & mill centers.
WITH INTEGRATED TECHNOLOGISTS IN THE TOP CLASS OF MACHINING
A characteristic feature of the turn-mill machining centers is the independent milling spindle, which can be swiveled around a B-axis and with which even free-form surfaces can be produced with 5-axis simultaneous axis movements. Combined manufacturing processes such as turn-mill enable a defined surface structuring of the workpieces in addition to a significant increase in the metal removal rate. Furthermore, grinding tools (with geometrically undefined cutting edge) can also be used in the milling spindle for even more precise finish machining in the same setup, while simultaneously reducing the throughput time.
CNC lathes for every application
With over 150 years of experience in the field of turning, DMG MORI has a holistic technological know-how that is reflected in around 20 series of different CNC lathes: From the versatile universal lathe to innovative turn & mill centers to highly economical production turning, a wide range of applications is covered.
CNC UNIVERSAL TURNING WITH HIGH-TECH FROM DMG MORI
The range of CNC-controlled universal lathes from DMG MORI extends from the low-cost turret lathe to flexible universal lathes with direct-drive tools, counter spindle or tailstock. In combination with forward-looking control systems, these machines form the universal backbone of countless machining operations.
MAXIMUM PRODUCTIVITY THROUGH TURN & MILL CENTERS
Where universal lathes reach their limits, the integrated turn & mill centers from DMG MORI are just getting started. As multifunctional machines, this top class of machining enables turning and 5-axis milling as well as simultaneous milling with five axes in one workspace. It is used wherever demanding workpieces need to be completely finished effectively and efficiently. Meanwhile, the necessary flexibility in the area of tool availability is provided by extensive chain magazines whose capacity can be expanded as required within the limits of economic efficiency.
So where different machine tools used to carry out several process steps in metalworking, today only one turn & mill center is required. This eliminates setup times, while reducing throughput times and – since manual reclamping operations are no longer required – increasing production quality. Implants in medical technology and turbine blades in the aerospace industry are among the potential areas of application for turn & mill complete machining, as are chassis parts in automotive engineering or large crankshafts in shipbuilding.
HORIZONTAL PRODUCTION TURNING IN ITS MOST PRODUCTIVE FORM
DMG MORI’s production lathes feature up to four turrets with four Y-axes and up to four integrated B-axes. Thanks to these up to four tool carriers, the horizontal lathes are ideal for series production of both bar stock and chuck components. Furthermore, versions with three turrets without B-axis as well as with two turrets are available.
In the version with two turrets, the TWIN concept is to be highlighted within the production lathe series. This unique workspace concept with two turrets and the transverse stroke of the counter-spindle/tailstock combination enables parallel machining in two independent workspaces. On the one hand, long workpieces such as shafts can be machined in 4 axes with the support of the tailstock. In addition, this enables collision-free face machining of long parts on the main and counter spindles.
AUTOMATION IN ALL POSITIONS
DMG MORI adds additional productivity to its lathes, turning/milling machining centers and production lathes with holistic automation solutions. The offering here ranges from bar loaders and integrated robots for automatic workpiece removal to gantry loaders and a series of flexible robotic applications for autonomous loading and unloading. The Robo2Go models, the MATRIS Light and the IMTR are worth mentioning here. Automation is a profitable building block for remaining competitive in the global market in the long term.
Turning process
Not all turning is the same. Technically, this manufacturing process is again differentiated according to the characteristics and shape of the surface:
- Facing (creating a flat surface perpendicular to the axis of rotation of the workpiece),
- Circular turning (creation of a circular-cylindrical surface coaxial with the axis of rotation of the workpiece),
- Screw turning (creation of screw surfaces with a profile tool),
- Turning (generation of rotationally symmetrical rolling surfaces, with the profile tool performing a rolling movement simultaneous with the feed movement),
- Profile turning (the profile tool is mapped in the workpiece),
- Form turning (workpiece form is created by controlling the feed and cutting motion).
This content was originally published on the DMG Mori website.
