Precision coolant advantages for machining aerospace materials

The role of coolants in machining aerospace parts has undergone somewhat of an evolution. Machine shops have for many years used coolants by directing tubes which flood the machining zone, particularly on the materials that need coolant to be machined. But now, by applying coolant with high precision accurately into the machining zone, new advantages are available. Broad access to this technology has been made possible by higher coolant-supply capacity of many CNC machines as well as by new tooling concepts.

Making a difference

If coolant is to be applied effectively and make a difference, it needs to be applied as jets at high precision, in sufficient volume and directed correctly. Just having a stream of coolant or even flooding the machining zone with coolant is not enough. Qualified application of high precision coolant can, on the other hand, make a distinct difference as regards:

• chip formation,
• distribution of heat
• smearing of workpiece material on the cutting edge,
• surface integrity,
• tool wear.

These basic machining factors in turn affect manufacturing through the objects of improvement to achieve higher competitiveness:

• productivity,
• tool life,
• chip control and chip evacuation,
• component quality.

When applied correctly, precision coolant maximizes output, increases process security and improves tool performance and component quality. The positive effects start at low coolant pressure, but the higher the pressure is, the more demanding material can successfully be machined.

The application of precision coolant can make a difference to machining in general, especially for stainless steel and low-carbon steel. But it is when machining more demanding materials, such as heat resistant super alloys and titanium alloys, that the practice makes a dramatic difference. It is of consequence then that a number of recent developments have made high precision coolant even more interesting and more readily available.

Jetbreak pioneered the concept

During the 1980s and 90s, Sandvik Coromant developed the first version of Jetbreak, a high pressure coolant system. Based on research, precisely directed jets of coolant up to ultra-high pressure (100–1000 bar) became part of cutting tools used to machine materials that were demanding as regards machinability and chip control. The coolant jets forced their way in, forming a fluid wedge between the chip and cutting edge. The contact length was shortened with a lowering of the temperature at the machining zone. Another interesting result was how the curl of the chip could be influenced, improving the control of the chip and in some cases even leading to chip breaking.

For a number of special tool applications, Jetbreak became the solution for machining materials with poor machinability and/or troublesome swarf. A lot was learnt about the effect of coolant-pressure distribution and coolant-nozzle size. By varying the jet data, a variable chip-former was achieved and it became possible to guide stringy chips in a desired direction and even improve the length of chips. Installations were made on a limited scale to solve problems particularly in the oil, aerospace and ball bearing industries. Jetbreak, however, needed dedicated installation with special tool holders and have been mainly used in vertical turning machines.

High precision coolant as standard

Many modern CNC machines have coolant supplies at pressures of 70 to 100 bar as standard or option with capable tanks and pumps. This is sufficient to incorporate high precision coolant, which makes a noticeable difference to performance and results on more commonly used machining centres, turning centres, vertical lathes and multi-task machines. Standard equipment is sufficient with easy channeling of coolant to where the jet is applied.

An essential basis for high precision coolant machining are modular tools, partly to ensure quick tool changes for minimizing machine stoppages, but also to efficiently secure coolant connections and channels from the machine to cutting edge. The modular quick-change tooling system Coromant Capto® was the basis for the Jetbreak development and is today the basis for new standard high-precision coolant tooling. This system is ideal as a modular platform, designed with internal coolant supply and also suited as the means with which to supply coolant at high precision. It is an established ISO standard and option on many CNC machines with stationary and rotating tools.

Precision over coolant precisely directed towards the cutting zone improves chip control and process security. Under coolant increases tool life and productivity, especially in applications generating a lot of heat in the insert.

A milling cutter with high precision coolant has through-coolant and is equipped with nozzles positioned and directed depending upon the tool type and the application it is intended for. Channels are connected to the machine tool or extra pump equipment for supplying coolant at pressures of around 70 bar. This supply is widely available today and although the pressure involved is not ultra high, the scope is certainly sufficient for a marked improvement on performance and results. The concept makes good use of a resource many machine shops already have or can invest in for the future.

Precision coolant for turning

Turning tools with precision coolant are equipped to give accurate coolant jets with laminar parallel flow. The jets give rise to a hydraulic wedge between insert and chip, affecting the chip form and flow and reducing temperature in the machining zone. Employing nozzles, mounted close to the cutting edge, accurately projecting the high-velocity jets, forces the chip off the insert face and cools and breaks the chips into smaller lengths, helping to evacuate them.

Benefits in finishing operations have been established even at lower pressures, down to 10 bars in material including steel, stainless steel, aluminum, as well as titanium and heat resistant super alloys. Apart from higher security brought about by better chip control, precision coolant can bring about a considerable improvement of tool life and a potential for higher cutting speed. By applying precision coolant, 50% tool life increases can often be the result.

Cutting speed affects the temperature, and thus tool wear, more than any other factor. Increasing the cutting speed in titanium outside the more limited machinability window reduces the tool life dramatically. But when the feed is increased on a similar scale, a smaller reduction in tool-life is typical. This, then, often makes the feed an attractive route to improved metal removal rate with low effect on tool life. However, high feed is not always an option in these machines because of higher cutting forces as well as the effect on chip control.

Turning of a HRSA turbine disc. Precision coolant can play a vital role in ISO S machining.

The effect of precision coolant can therefore provide a potential to raise performance by way of higher cutting speeds without the usual rise in temperature and loss in tool life. There is a clear cooling effect and not the higher cutting forces through higher feeds. For ISO S classified materials, a 20% increase in cutting speed can be achieved while maintaining the same cutting length.

Internal turning is also an area where precision coolant can provide an important role to help ensure good chip formation, as well as improving shearing properties in demanding materials such as titanium. In this way the concept adds higher security and longer tool life to boring operations. When machining relatively large, deep holes with boring bars, such as in landing gear components, modular tooling at the back as well as the front end of the tool can be advantageous. Being able to change the small cutting head on the clamped bar provides quick, easy and accurate changing, adding considerable flexibility for various cuts in one set-up. CoroTurn SL combines damped boring bars with serrated locking of heads for boring larger holes with depths of ten times the diameter and is also equipped with precision coolant facilities.

Variation in component surface integrity is affected by the temperature and forces generated during machining. Coolant certainly plays its part in controlling the temperature and consequently precision coolant has been shown to provide a more reliable surface result. Tool nozzles are aimed directly at the part of the insert in contact with the finished surface. Since the nozzles are non-adjustable a lot of the variables are eliminated, resulting in a more secure and consistent machining process.

Optimization of correctly established operations

With the ability to force a fluid wedge into the machining zone, especially in operations classified as medium to finish turning, the chip thickness is more controllable and the fluid wedge easier to apply than in roughing operations.

The application of high precision coolant machining should not be seen as a means with which to compensate shortcomings due to other application factors — such as unsuitable inserts, instability, incorrect cutting data, etc. Precision coolant is an optimizer when operations are correctly established. The concept will provide the means for shorter cycle times, improved component quality consistency and higher process security in turning and milling.

Thread turning with precision coolant.

The need to optimize various machining operations, especially when chip formation and the effects of demanding materials are prominent, make precision coolant an attractive option. The rising population of multi-task machines along with new generation vertical turning machines has highlighted the benefits of machining with precision coolant, especially from the chip control point of view. The disturbance due to swarf accumulation is critical as these machines are to a rising extent used by machine shops making aerospace components in demanding materials.