Parting off is one of the most common operations in turning applications, but at the same time, it can also be one of the most difficult. Christer Richt* reports.
IN the past, efficient machining and successful parting of components, were not synonymous in metalworking applications, but with advances in tooling technologies and the development of modern indexable-inserts, many challenges have been overcome to make the process of parting off reliable and trouble free.
Successful parting operations rely on solutions that are dedicated to the cut in question – more so than other turning operations.
Careful selection of the best suited type of tool, type of insert, cutting data and method is crucial to overcome the challenges of and to optimise particular parting operations. A completely all-round tool for broadly different parting applications is unpractical as well as very uncompetitive.
In response to this, today’s leading parting and grooving system is in the form of alternative one-edge or two-edge insert versions with a long rail-form seat focused on general parting but with a lot of room for dedication.
There is also a three-edge shallow-depth parting version, suitable for large-volume cuts, as well as a front-mounted boring bar version for internal precision work.
The defining part
Parting has a lot to do with minimising tool deflection – downwards and sidewards – as well as vibration tendencies during an operation. The operation is a primarily defined by: tool overhang (ar); width of tool (la) and; feed rate (fn).
Additional application factors include: the type of toolholder (straight adjustable blade or reinforced blade); insert (geometry especially) and; quality of the set-up (tool positioning and coolant supply).
For example, in parting, a maximum overhang of eight times the width of the tool can be accepted but it is vital that the overhang is minimised according to the particular component or bar diameter.
This maximum, long tool overhang is more than what is accepted in other machining operations, unless vibration dampening measures are in place (tuned tools).
In the majority of parting operations an overhang of this length is never needed. To generally use a tool with a constantly set long overhang to accommodate component variations will considerably limit the potentials of operations.
Getting these factors right will make for a secure operation with scope for optimisation through capable insert grades and higher cutting data.
A clear overview of capable tool programmes and application know-how will also determine how well the challenges of parting operations can be tackled.
Tool selection approaches should also differ depending upon whether machining is part of mixed, small batches or mass-production. But whatever the type of manufacturing, to achieve competitive results, there has to be some tool dedication in parting.
Firstly, a suitable insert width (la) should be established to suit the cutting depth (ar) as determined by the component or bar diameter. Recommendations are at hand for different widths which also indicate the most suitable insert geometry for each width.
Secondly, the type of operation and machining conditions (good, average or difficult) should affect the selection of feed rate (low, medium or high) leading up to the choice of insert geometry.
Further considerations for parting is the selection of whether the front angle of the insert should be straight or angled and also the size of the insert corner radius.
The zero-degree edge angle gives a straighter cut, best chip formation but leaves a pip on the bar from which the part has been cut off.
The 5-15° angled-edge on the other hand, faces off the pip and minimises burs as well as generating lower cutting forces.
However, it does generate a poorer surface finish and has a tendency for the tool to change direction.
Regarding insert-corner radius, the consequences of selecting a corner radius on the larger side are added edge-strength and security – but with somewhat higher cutting forces being generated.
Selecting the insert grade is usually the least dedicated part of the parting tool. The grade choice affects the cutting speed, tool-life and to some extent the material capability.
However, modern, first-choice grades provide broad capability for different operations, varying cutting data and materials.
The all round grade provides a solution for starting many applications thanks to a combination of high wear resistance for the medium to low cutting speeds of parting and also the safety provided by a high level of toughness level.
In parting operations, it is usually beneficial to start off with an insert grade on the tougher side and then optimise with a more wear resistant grade.
The modern grade range includes a very hard, wear resistant CVD-coated grade that can even machine hard materials, to the toughest possible alternative in the form of a PVD-grade, which can cope with most severe, interrupted cuts
Parting off is one of the most common operations in turning and its efficiency influences the total cost-efficiency of the component manufacturing. As such, it is worth ensuring that it is being performed to the best levels possible with the latest means.
*Christer Richt is the technical editorial manager at Sandvik Coromant, Sweden. For more information about the company in Australia, phone 1300 360 938 or visit the website at www.coromant.sandvik.com/au.