Exotic and Light Alloy Machining

 

General



Usefulness is matched by the difficulties of processing these alloys. Nickel alloys and titanium are both tough and ‘sticky’, combining high tooth loadings with a propensity to build up residual deposits on the cutting edge.

 

Aerospace grade aluminium alloys tend to be abrasive, causing excessive flank wear along with build up on the edge.


Horn’s experience is that the remedies for both types of material are similar, though for different reasons.

 

A combination of a micrograin carbide substrate with true positive edge geometry provides part of the solution.

 

Micrograin substrates can tolerate grinding of a very sharp cutting edge which is then relieved very slightly to allow the application of coatings.

 

Relevant Applications

 

Nimonic - Side Turning

 

Super Alloy - Milling and Turning

 

Super Alloy - Part-off

 

Aluminium - Internal Profiling

 

 

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Super Alloys

 

Key Considerations:

Positive Geometry/Edge Sharpness; Coatings/Coolant/Light Feedrates/Controlled cutting speed

 

 

Horn six-tooth cutter insert

 

For super alloys the Horn TF grade, Futura coating applied using the PVD process works well due to its capability to be applied over an edge without compromising sharpness.

Application to a ground insert is the key factor as a moulded/sintered edge simply cannot provide the same level of sharpness.
As well as benefits for chip formation the sharp edge minimises stress introduced to the material when machining. Work hardening due to this phenomenon is at the root of many of the problems experienced when machining exotics.

 

To that end cutting data, the rigidity of the tooling system and the ability to deliver coolant right at the cutting edge are equally important.

 

On exotics lighter feed rates combined with tight control of cutting speeds work well, whilst on groove milling or threading applications a six tooth cutter is sometimes preferable as it supports higher stock removal rates without increased tooth loadings and material stress.

 

High pressure coolant is desirable, principally for its enhanced ability to penetrate to the cutting edge and evacuate swarf which itself assists in keeping cutting zone temperatures under control.
 

 

 

 

 

Aluminium alloy

 

Key Considerations

Aggressive positive geometry/Edge sharpness/Abrasion resistance/High speeds and feeds/Chip evacuation


 

With aluminium a more aggressive positive rake geometry and higher surface speeds are recommended. The positive geometry has proved instrumental in providing a burr-free finish in many applications and assists chip forming. Again the desirable very sharp edge can only be achieved with a ground insert.

 
For many aluminium machining applications an uncoated insert can be specified but for higher volume machining Horn offers the CD.05 diamond coating which significantly enhances tool life and machining speed at a fraction of the cost of PCD. Where the material is highly abrasive - a property shared by some alloys used for automotive piston manufacture and aerospace applications - then PCD is usually the best solution.


This also holds true when maximising productivity is important. Whereas uncoated carbide can operate at 180/200 m/min and diamond coated carbide can sustain 300/500 m/min, a PCD tipped tool will function at 600/800 m/min.


At high machining speeds and feedrates light alloy chip evacuation becomes a major issue, so coolant again becomes important. Tool form factor is also an important consideration as avoidance of clogging is vital, particularly on internal work.

 

Horn would recommend a 116 Series single edge or three tooth (300 Series) cutter for roughing during interpolative groove milling of aluminium, though six-tooth (600 Series) cutters are known to perform well in finishing applications.

 

Where entry bore diameters permit or external access is less restricted then 380 Series cutter bodies used with 312 Series triangular inserts offer a convenient method of achieving high surface speeds with moderate spindle rpm.


Delivery of coolant at maximum available pressure is desirable.