Dry spell

The use of fluids during machining serves several functions, such as effective lubrication and cooling at the tool/workpiece interface, as well as effective flushing of the chips. However dry or near dry machining in certain applications is proving to be a viable alternative as Solutions reveals.

One of the drawbacks in using lubricating fluids is their susceptibility to bacterial propagation and odour generation, which could cause serious health hazards. Numerous other drawbacks associated with these fluids include the cost of procurement, disposal, maintenance and labour. Stains on the part or contamination can also be problematic.

For flushing purposes, hole making operations require the use of lubrication at the contact point of the drill with the workpiece material, aiming to eject the removed material from the hole. Lack of fluids will cause the chips to adhere to the hole walls and average roughness of the machined surface might be twice as high when compared to wet operation.

In milling and turning, transfer of the heat from the cutting zone towards the removed chips is an indicator of positive machining characteristics. Good chip design enables a deflection of 85% of the generated heat away from the cutting zone, while the remaining heat flows into the workpiece or dissipates into the tool.

This heat generation phenomenon has a detrimental effect on the cutting tool in terms of tool life. During a milling operation, for example, the cutting edges tend to heat and cool as they enter and exit the piece being machined. These temperature fluctuations create a sequence of expansions and contractions that lead to fatigue stress and thermal cracks. The introduction of lubricating fluid often makes the situation even worse. 

Under debate

A long lasting debate amongst research engineers revolves around the question whether cutting fluid actually reaches the zone interfacing between the bottom side of the chip and the cutting tool. If they do their effect is limited, assuming that they cool only the shearing vicinity. This hot/cold interaction only intensifies the temperature gradients and increases the thermal stress.

With regard to economical considerations, twenty years ago the purchase, management and disposal of lubricating fluids accounted for less than 3% of the production costs. Today, in comparison, the same operations constitute 16% of the cost of the average job.

Consequently, a growing number of manufacturers are turning to dry or nearly dry machining solutions, with the intention to benefit from coolant cost savings or improved tool life. Dry machining refers to machining without any fluids, while near dry machining or minimum quantity lubrication (MQL) is the use of a minute amount of fluid that is applied directly to the cutting edge (either internally or externally). The near dry machining features fluid vaporisation during the process, leaving dry chips. The practice of dry or near dry machining has already proved in numerous machining case studies that much faster cutting conditions can be successfully applied.

High pressure coolant

High pressure coolant (HPC) is the latest technology, used particularly for exotic and stainless steel materials. High pressure creates a localised stream of coolant at pressure that eliminates the formation of vapour and assists chip breaking into small segments.

The combination of coolant at high pressure (70-140 bar) precisely targeting the cutting zone provides advantages that are not available with conventional flood coolant. The coolant is delivered with enough force to reach the cutting zone as a liquid, not a vapour. In the liquid phase, it actually lubricates the cutting process as it quenches the molten chip, shattering it into smaller, more manageable pieces. Also, because of its internal flow path, HPC keeps the entire spindle, tool, insert and workpiece cooler, leading to much longer tool and equipment life. In that sense, it works like the cooling system in an automobile engine.

Another benefit is that delivering coolant close to the secondary shear zone improves machinability of the material under high speed conditions. Higher pressure coolant, directed right at the cutting zone, creates smaller, more manageable chips, whether the material is titanium, Inconel or steel. Directing the coolant right to the cutting zone is the key. This is the ‘laser' approach to coolant deployment, not the ‘floodlight' approach.

Iscar's JetHP line of special HPC tools includes Cut-Grip grooving tools, IsoTurn and HeliTurn turning and facing tools, Tang-Grip parting and grooving tools and PentaCut grooving, recessing and parting tools.
At the extreme cutting rates involved with today's HPC turning, high pressure coolant flow is an integral part of the cutting process. Besides prevention of overheating and thermal shock in the cutting zone, the coolant contributes a ‘waterjet cutting' effect, which actually enhances cutting action that increases productivity, enhances quality and improves tool life.

Substrate and coatings

The method of new dry/near dry machining has been favourably recognised by Iscar in the past few years. The company strongly supports the adoption of green machining techniques, with the added value benefits of cost reduction. Thus, the latest technology of cutting tool materials, such as advanced coated carbide, ceramics, cermets, cubic boron nitride (CBN), and polycrystalline diamond (PCD) has been developed.

The submicron grain structure of the solid carbide substrate provides the retention of cutting edge integrity, even at high cutting temperatures, while accommodating a sufficient softening effect to combat deformation and resist cratering. The high cutting temperatures which occur during dry machining tend to slightly soften the carbide structure, which in turn increase its toughness, prevent any potential chipping and prolong tool life.

Using polycrystalline boron nitride cutting tools also provides a successful alternative to dry machining. Abrasive technology features PCBN tools for turning, boring and milling applications in a form of braze tipped or solid PCBN inserts.

The PCBN features a close resemblance to diamond hardness with a considerable amount of abrasion resistance, combining both a high degree of toughness and hot hardness. All of these properties make PCBN the most suitable cutting tool materials for machining hardened steels, cast irons and super alloys.

Iscar recently launched the new HeliTurn IS8 ceramic inserts with a moulded chipformer. These new inserts combine the advantage of the HeliTurn tangentially clamped insert system with the ceramic grade and a sophisticated moulded chipformer.

The tangential clamping is most important when using ceramic inserts, due to the ability to eliminate any bending forces exerted on the insert. The new insert can be used for high speed machining of cast iron, ranging from 500 to 1,000m/minute. The LNMX IS8 insert is an ideal solution for fast metal removal when machining disk brakes, flanges, valves and other mass production parts made from cast iron.

The bottom line

In conclusion, machining some materials, such as aluminium and high nickel based alloys essentially requires the use of cutting fluids, but dry machining and MQL methods are gaining in popularity.
Supporting the implementation of dry machining, Iscar has recently introduced the 3P SumoTec, which features new, premium tungsten carbide grades for drilling, milling, turning and grooving. The new grades extend tool and insert life as a result of innovations in the substrate and in the CVD and PVD coatings.
The PVD process is composed of Iscar's Al-Tec coating technology, while the CVD coating is based on its Alpha-Tec technology; both of which have been shown to provide good wear resistance. In an additional process, another layer is deposited on top of the PVD and CVD coatings to provide a very even, low stress layer that improves tool life further.

Iscar
www.iscar.com