The ability to machine a component-material is determined by several factors, which decide the requirements and outcomes of a metal cutting operation. On most scales, exotic materials have poor machinability. They are seen as demanding to cut — but not difficult if approached in a suitable way.

The more exotic common component-materials are classified under the ISO group of S: Heat Resistant Super Alloys (HRSA) and titanium alloys. For machining, these can all be split into several sub-groups, depending upon composition, condition and properties. The chemical nature and metallurgical composition of an S-classified alloy will determine its physical properties and machinability.

Chip control is generally demanding due to the segmentation of chips. It is not unusual for the specific cutting force (that is the direct measure of how hard it is to cut a material, which determines the cutting force and power needed) to be twice that of steel (ISO P).

The main reason that HRSA materials are demanding to cut is because they retain high strength at high temperatures. They do not soften and flow in the way most other materials do and they also work harden readily. As a result, high mechanical load and considerable heat are concentrated on the cutting edge. 

Nickel-, iron- or cobalt-based alloys are sub-groups of HRSA, having capabilities for component use in the aerospace, energy and medical industries as their advantageous properties do not change much until they are close to their melting point.

From a machinability point of view however, they need more from the machine (extra stable set-up conditions), dedicated insert grades and geometries, optimised coolant application (preferably through qualified high-pressure tooling) and the right machining method and tool approach.

Titanium

Titanium as a component material is also divided into sub-groups with varying machinability demands. Generally, machinability is rated as poor and, again, places special demands on tools and methods. 

With poor thermal conductivity and its high strength at high temperatures, highly-sheared thin chips are generated, with a tendency for galling, creating a narrow contact area on the insert rake face, with high cutting forces concentrated close to the cutting edge.

Chips can have cyclic formation, leading to variable cutting forces, and some alloys have a relatively high level of carbides that make the material very abrasive. Excessive cutting speeds can give rise to a chemical reaction between chip and tool material, resulting in sudden chipping of the cutting edge, smearing and even fracture. With these challenges, the window for successful machining of many HRSAs and titanium alloys is relatively small.

Dealing With HRSA

Turning operations dominate HRSA machining, where success has a lot to do with balancing the combined effect of the material and application factors. There are a few basic recommendations which contribute hugely to its success:

• have a sound machining strategy, preplanned in detail,
• establish the best tool approach,
• decide the best tool paths and use very stable tool holding,
• usage of dedicated indexable insert technology,
• apply the most suitable cutting data from a qualified source of recommendations,
• use spiral cutting length calculation for predicting cuts,  
• apply coolant correctly — high pressure is often advantageous.

The HRSA machining process should always be carefully planned because of the decisive factors involved, such as consideration to the state of the work piece material. Cast, forged, bar-stock, type of heat treatment, solution treatment and ageing considerably affect the component in ways that influence the selection of tools and methods.

Surface condition values vary and affect machining, as does the material’s hardness. The strategy for HRSA turning should also include the demands made from the design features on the components to be machined, as well as the various stages of machining.

 When turning HRSA-material into corners, the entering angle should be minimised for best effects.
This can be done through the use of an insert having chamfered corners as part of the geometry.

Making The Right Choices

The cutting action in HRSAs is affected by the approach of the cutting edge to the work piece. The entering angle of the cutting edge, in combination with the insert geometry, dominates performance, tool life, security and results. 

Insert shapes often have to be chosen in relation to the cut that needs to be taken. Using a small entering angle however, contributes to performance and tool life and should always be a key parameter of the application.

The choice of insert grade needs to be made partly in relation to the entering angle. Amongst other things, the entering angle influences the type of wear that forms a notch on the cutting edge, the size of which affects results and causes premature tool failure. Getting the approach part of the application right also means that an insert grade capable of higher productivity can be chosen.   

Insert grade should be influenced by the type of turning operation involved — roughing, semi-finishing or finishing — as well as the workpiece condition and the type of cut. 

Due to the hardness of HRSAs, plastic deformation of the cutting edge should always be considered as the primary risk when selecting the insert grade (while notch wear is mainly affected by the entering angle and depth of cut).

Insert grade selection should also be considered in relation to the insert shape as this is a strength issue that affects the approach, chip load and whether there is a continuous or interrupted cut. Dedicated grades are therefore vital for HRSA-machining.

Adopting New Ways To Handle Titanium

Milling operations dominate titanium machining. A correct and consistent tool path is as important as the flight path that aircraft operate on. 

Machining structures in this material, especially for aerospace applications, is demanding. However, with the right tools and tool path strategies consistently used, there are opportunities in becoming a very competitive supplier of machined Ti-parts. Focusing on the crucial points has been found to be decisive to success.

For various reasons, titanium machining has up to now been widely viewed from a traditional point of view by many machine shops. As such, it was not forced to follow the progressive development found in the machining of many other materials. 

Being a challenging material to machine, coupling that with the fact that many of the components are complex, meant that a safe approach was the order of the day. However, with the large number of titanium components needed and supplier opportunities created, a new approach to efficiency, coupled with reliability, is inevitable.

When machining titanium, detailed planning of operations is needed. Factors such as the capability values of available machine tools, holding tools, component size and features, fixturing facilities, programming, cutter paths, tool load, choice of tool and method as well as the coolant capacity are decisive to the outcome of machining.

 A high-pressure, precision jet coolant supply through nozzles at each cutting edge has elevated performance considerably.

Putting Titanium Through The Works

Sound titanium milling can be made by following a few basic recommendations:

• limit the machining temperature through the cutting speed value
• lower machining temperatures further (and raise the potential for speeds) through the correct use of coolant. High-pressure coolant with modern nozzle technology in the tool should be deployed
• use comparatively sharp cutting edges to reduce the effects of the high friction coefficient.  
• use the feed rate to optimise metal removal rate and cutting times and avoid idling during tool material engagement.
• replace cutting edges at the early stages of tool wear.
• program machining to minimise impact and stress on tool; maintain tool in cut, while also optimising process efficiency.
• ensure the application of recommended values for maximum chip thickness and feed-per-tooth.

Today, there is a wide selection of milling methods that optimise the available programming facilities. In addition to face and square-shoulder milling, there are various ways to mill profiles, cavities and grooves. 

Methods like radial milling, linear and circular ramping, plunge milling, peck milling, high-feed milling, and slicing of cavities and corners have been developed extensively along with new tool technology.  

Applying Good Practices

An example of ensuring best practice can be seen in the following example. Machining in aerospace is competitive and this is particularly the case when machining demanding materials like exotics. 

Methods, in combination with the most applicable, dedicated tools are decisive to the outcome. Such practices can be seen in the aerospace supplier industry.

Aircraft frames are largely made out of titanium alloys and entail a lot of pocketing. Roughing operations are especially focused  on as metal removal rate and tool life are decisive in achieving secure processes and optimised cycle times. 

There are many pitfalls and in the past, these operations were often labourious, placing demands on machines and causing tool life to be unpredictably short. Even today, in a modern machine shop, performance can often fall short of the available potential.

For instance, high-feed milling is often overlooked. It can be an advantageous route to go, when performed with tool paths that are effective and can spare the tool from shocks and unnecessary stresses. 

A lot can be done by establishing a good milling strategy. Smooth cutter entries and exits, along with corner machining paths and tool paths can keep the tool constantly engaged along new z-axis levels. 

Helical interpolation, for example, is ideal for a high-feed indexable milling cutter with a small entering angle (10 degrees). Very rapid machining is achieved using a modern dedicated insert-geometry and grade.

In a case of a spigot in Ta11, a review of milling strategy paid off resulting in the tool life being doubled (tool edges lasting four components as opposed to two). 

This is due to the elimination of insert chipping and cracking caused by the tool impacting the part and staying in cut. The cycle time per spigot was reduced from 8.4 to 7.2 minutes. The solution was achieved through specialist programming and a modern high-feed milling cutter.