Grooving and Parting-off

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Every machine tool user does some grooving even if, as is often the case, the groove is normally so deep that the part separates off.

 The process is often taken for granted, regarded as either too mundane or, paradoxically, too specialised to command much time or attention.

The knowledge base needed to successfully engineer solutions for some of the more complex grooving applications is unlikely to be found within a more generalised tool development environment. Unsurprisingly there is only a small number of ‘grooving specialists’ as opposed to tooling suppliers offering a limited range of grooving tools.

The cutting edge on a grooving insert is often very narrow but subject to high concentrations of heat and load. Coatings provide a certain level of protection but ultimate durability hinges on the quality of the substrate. Tight controls on the edge grinding process and provision of consistent preparation at the extremities of the cutting edge are necessary if the effective life of the insert is to be maximised.

The physical result of this preparation is visible only through a powerful magnifier but the benefits in terms of tool life and productivity are significant.

The hollow ground format is as close to a ‘standard’ as currently exists in the grooving tool market and inserts to this pattern are now available from a number of sources. The design was perfected by Paul Horn GmbH during the 1970s, with the 312 Series triangular insert.

The cutting geometry developed for 312 has provided the basis for a range that now comprises over 40000 standard insert variations. Applications now include threading, small diameter side turning, milling and innumerable grooving ‘form’ inserts

Hollow Grinding

Basic format 312 inserts have cutting edges with clearance to the rear on either side generated by hollow grinding. This makes it ideal for grooving applications as the clearance between the sides of the groove and the flanks of the insert prevents the insert being ‘grabbed’ or worn by the material being machined.

A further advantage of the hollow ground insert in service is its equal applicability to right- or left-handed internal or external grooving operations and its suitability for use in multiple groove tool applications. Use of micrograin carbide allows generation of a very sharp cutting edge which combines successfully with positive cutting geometry for reliably high performance.

One key limitation to the development of ground inserts is that it is not possible to provide integral chip breaker forms - which can be moulded into pressed inserts - to assist with swarf control on tougher materials. Additionally, the cost of a ground insert is comparatively high due to the amount of hand finishing needed to produce it to the required standard.

Achieving this is difficult - particularly for very narrow groove widths and predicates the need for a toolholder developed to provide maximum support for the cutting edge while locking the insert firmly in position.

Combining some of the elements of self grip, such as lengthy inserts having a self-orienting cross section, with well designed screw clamping provides a system which combines the best of both worlds.

Horn double-edge inserts of Type 229 and S224 have been developed to suit this approach, and feature a lengthy hexagonal centre section capable of supporting very high clamping loads.

Rigidity assumes even greater importance when the tooling is destined for internal work where the bore diameter is small and the cutting edge is delicate. It beholds the designer to maximise bending resistance against the direction of cut - to optimise accuracy - while also maintaining enough material in the remainder of the insert structure to resist side forces without compromising what is often a tight clearance on the internal bore diameter. This is equally the case with long reach groove milling tools.

Reliable turning inserts for internal grooving work within bore diameters down to 2 mm demands an insert shank with a near-elliptical cross section to provide the necessary mechanical strength and clearance within a narrow bore.

Horn Supermini® inserts (above) are ground from a micrograin carbide blank whose basic ‘flat sided egg’ shape form furnishes the vital security of location in the holder. Though these inserts look extremely delicate they are capable of very high performance provided that the cutting data is correct and the machine tool is well maintained.

On milling work the optimum solution in long reach applications is normally provided by use of solid carbide tool shanks. Though costly they provide enhanced vibration damping and stiffness compared with steel, guaranteeing optimum performance of the insert during circular interpolation of grooves.

As well as tool rigidity it is essential to ensure that the machine tool is rigid and that an adequate supply of coolant can be supplied at the cutting edge, preferably via the tool shank. Grooving tools tend to lead an extremely claustrophobic existence which does not foster effective evacuation of heat or swarf other than by third party intervention. Especially when working at small diameters provision of thru-tool coolant is an essential facility and its availability should not be overlooked.