The Secret to No More Spring Passes

The secret to no more spring passes
Taking the stress out of Surface Integrity

Sandvik Coromant understands that the best value it can offer to help the aerospace industry to take a quantum leap in manufacturing technology is by developing total process solutions to typical aerospace component features.

They develop process knowledge in their own labs ensuring that when a new component or machine is to be implemented for the first time the optimal solution is ready to apply.

Application Center is the name given to their processing R&D locations. For aerospace they have facilities in Sweden (Sandviken) and soon USA (Fair Lawn NJ). The core development areas are: -

+ Material machinability  – grade/geometry/cutting parameter limitations
+ Machine capabilities  – high pressure coolant, 5 axis, multi-task, spindle, CAM
+ Component geometry  – typical complex feature shapes with geometry limitations
+ Surface integrity  – how the cutting parameters effect the component

Sandvik Coromant is also a partner at the AMRC (Advanced Manufacturing Research Centre) in the UK - a partnership which builds on the shared scientific excellence, expertise and technological innovation of industrial partners and the world-class research within the University of Sheffield’s Faculty of Engineering.

‘Not only is Sandvik Coromant active with industry partners with their projects at the AMRC, the AMRC runs research programs on our behalf, so it is a real win-win-win’ explains Chris Mills, Sandvik Coromant responsible for aerospace in North America ‘Because of the time consuming nature, laboratory accreditation and excellent research content for students graduating, it was a perfect solution for the AMRC to head up the surface integrity research on our behalf.’

‘We began the work in 2002’ explains Mills ‘with the goal for us to give validated cutting recommendations for critical parts and steer future tool development. We developed a test matrix to identify the effect that each cutting parameter has on the surface and to identify optimised productivity combined with attractive surface properties’

‘It is fair to say that the work carried out by the AMRC’s Dr Adrian Sharman and his group is the most conclusive published research program for turning surface integrity of Inconel 718 (aged) and titanium alloy Ti6Al4V and they won the Thatcher Brothers Prize in the UK for best paper in a manufacturing/mechanical engineering subject from the IMechE in 2004’
Surface integrity
The cutting process can affect the integrity of the final component which can ultimately lead to part distortion on thin parts or reduced fatigue life in critical rotating parts (discs and shafts).

The combination of cutting force and elevated temperatures generated during machining leads to alterations of the microstructure, which can cause changes in microhardness, plastic deformation of the grain boundaries and residual stresses in the component sub-surface. These changes in turn can cause part deflection and reduced fatigue life.

Research program
The focus of the research was aimed at finish turning of critical aerospace engine parts in materials Inconel 718 (aged) and titanium alloy Ti6Al4V.

The program was to develop optimised grade, geometry and cutting parameters to leave the component in the optimum condition. To do this a matrix of tests with various nose radii, grade, geometry, speed and feed was developed – over 50 in total. The component sub surface was measured for micro hardness, residual stress and grain boundary deformation for each parameter with both a new and worn insert edge.

Research findings
The effect on titanium and nickel alloys surface integrity is very different. These 2 alloys are often grouped in the same category, even the ISO materials groupings class both simply as ISO S. It was found that regardless of parameter change there was no real pattern of change in the sub surface structure in titanium turning although the tool life changed dramatically.

The nickel alloy Inconel 718 however was a much more interesting research material. It showed significant trend changes – the most significant being:-

1) Cutting parameters applied – cutting speed and feed. There is little effect with change of feed however an increase in cutting speed will have a detrimental effect on the sub surface characterisitic with a worn edge

2) Insert nose radius – when the radius exceeded 3mm it could be seen that the deformation depth of the component material increased due to the reduced entry angle deflecting more force into the component.

3) Type of insert wear – different grades and geometies have varying wear characteristics. The critical portion is the ‘trailing edge’ of the insert. This is the part of the edge which transmits heat into the component, and generates the finished diameter. Excessive wear at this point increases the cutting temperature and forces entering the component, resulting in tapering and component deflection, hence more spring passes

4) Grade - CVD coated grade S05F was found to give the best results due to better coating adhesion and heat barrier offered by the aluminium oxide layer minimizing the wear on the trailing edge.

The results of the research, as well as steering R&D, are now used globally by Sandvik Coromant technical support engineers to provide recommendations to give the best quality surface combined with quickest and most secure production methods.

Predictive machining programming to avoid spring pass
When finishing a critical part the cutting edge must be capable to complete one pass with acceptable wear to ensure good surface integrity throughout the pass.
To be able to ensure that the chosen insert style and grade can make one pass, Sandvik Coromant provide spiral cutting length (SCL) information. For a given diameter and length of cut the SCL can be calculated for a given feed rate – the correct speed can then be applied to guarantee making the pass with acceptable wear, resulting in good surface integrity and dimensional accuracy, preventing any need to re-cut.

Eliminating spring pass
Roughing - if using ceramic, stop within 1mm due to high material deformation
Finish with carbide – 3 passes – use optimized geometry, nose radius and grade together with  SCL to ensure the cutting length required is achievable with the selected insert and cutting data recomended
1) Semi finish  – ap 0.5mm
2a) Finish measure  – ap 0.25mm (same insert style as finish cut)
2b) Measure the component to correct tool offset for last pass
3) Finish cut to size  – ap 0.2 to 0.3mm

Optimized finishing solutions
When we classify finishing operations they can be catagorized by the material, component radius and stability. Stability is a combination of the wall thickness but also how easy it is to deflect (a flange will deform easier than a diameter).
The optimized solution for each category along with SCL capability is shown below.

HRSA – Inconel, Waspalloy:

Stable component – large radius: N123J2-0600 RO S05F
vc50m/min, fn0.35mm/rev, 20mins, SCL1000m

Stable component – small radius: CNMG 120412 MF S05F
vc50m/min, fn0.25mm/rev,
20mins, SCL1000m

Unstable component – unsupported flange: VBGT 160408 UM S05F
vc40m/min, fn0.15mm/rev
20mins, SCL1000m

Titanium Ti6Al4V:

Stable component – large radius: N123J2-0600 RO H13A
vc120m/min, fn0.3mm/rev
50mins, SCL6000m

Stable component – small radius: CNGP 120412 H13A
vc120m/min, fn0.2mm/rev
50mins, SCL6000m

Unstable component – unsupported flange: VBGT 160408 UM H13A
vc120m/min, fn0.15mm//rev
50mins, SCL6000m

These recommendations are with primary consideration of the components final condition. To combine this with the best productivity the feed rates are optimized by using the largest recommended radius. By keeping the speeds moderate good length of cut can be achieved with low insert wear ensuring minimal taper and consistant surface integrity.

Productivity with security and spring passes will be a thing of the past.