High speed machining (HSM) offers benefits to a range of applications. It is possible to apply HSM-technology for milling of cavities in qualified, high-alloy tool steels up to 60-63 HRc.
When milling cavities in such hard materials, it is crucial to select adequate cutting and holding tools for each specific operation: roughing, semi-finishing and finishing. It is also important to use optimised tool paths, cutting data and cutting strategies.
HSM can be utilised in a productive way for die casting dies, as most are made of demanding tool steels and have a moderate or small size. Most forging dies are also suitable for HSM due to shallow geometry. Short tools always results in higher productivity due to less bending (better stability). Maintenance of forging dies (sinking of the geometry) is very demanding, as the surface is very hard and often has cracks.
HSM can also be applied to injection moulds and blow moulds because of their small size, making it economical to perform all operations (from roughing to finishing) in one set up. Many of these moulds have deep cavities, requiring thorough planning of approach, retract and overall tool paths. Often long and slender shanks/extensions are used in combination with light cutting tools.
Milling of electrodes in graphite and copper are also an excellent area for HSM. Graphite can be machined in a productive way with TiCN- or diamond coated solid carbide endmills. Manufacturing of electrodes and employment of EDM is steadily decreasing while material removal with HSM is increasing.
One of the earliest areas for HSM has been modelling and prototyping of dies and moulds using easy to machine materials such as non-ferrous, aluminium, and kirkzite. The cutting speeds are often as high as 1500-5000m/minute, with feeds also high.
HSM is also often used in direct production of small batch components for prototypes and pre-series in Al, Ti, Cu for the aerospace, electronics, medical and defence industries.
One of the main targets with HSM is to cut production costs via higher productivity, mainly in finishing operations and often in hardened tool steel.
Equally important is the ability to increase overall competitiveness through shorter lead and delivery times. The main factors enabling this are the production of dies or moulds in a single set-up, improvement of the geometrical accuracy of the die or mould via machining, and increase of machine tool and workshop utilisation via process planning with CAM systems.
Advantages of HSM
In HSM applications, cuts are shallow and the engagement time for the cutting edge is extremely short. The feed is said to be faster than the time for heat propagation.
Low cutting force gives a small and consistent tool deflection. This, in combination with a constant stock for each operation and tool, is one of the prerequisites for a highly productive and safe process.
As the depths of cut are typically shallow in HSM, the radial forces on the tool and spindle are low. This saves spindle bearings, guide-ways and ball screws. HSM and axial milling is also a good combination, as the impact on the spindle bearings is small and it allows longer tools with less risk for vibrations.
HSM allows for a productive cutting process in small sized components. Roughing, semi-finishing and finishing is economical to perform when the total material removal is relatively low.
HSM also achieves productivity in general finishing and it is possible to achieve extremely good surface finishes, often as low as Ra ~ 0.2 microns.
The machining of very thin walls is also possible. Downmilling tool paths should be used, and the contact time between edge and work piece must be extremely short to avoid vibrations and deflection of the wall. The microgeometry of the cutter must be very positive and the edges very sharp.
Geometrical accuracy of dies and moulds provides easier and quicker assembly. Human skill levels cannot compete with a CAM/CNC-produced surface texture and geometry. Additionally, increased time and focus on the machining phase can reduce time-consuming manual polishing work as much as 60-100%.
Production processes as hardening, electrode milling and EDM can also be minimised, lowering investment costs and simplifying the logistics. Fewer EDM equipment also means the requirement of less floor space. HSM also gives dimensional tolerance of 0.02mm, while the tolerance with EDM is 0.1-0.2mm.
The durability, and tool life of the hardened die or mould can sometimes be increased when EDM is replaced with machining.
EDM, if incorrectly performed, can generate a thin, re-hardened layer directly under the melted top layer. The re-hardened layer can be up to ~20 microns thick and have a hardness of up to 1000Hv. As this layer is considerably harder than the matrix it must be removed, requiring a time-consuming and difficult polishing process.
EDM can also induce vertical fatigue cracks in the melted and resolidified top layer. These cracks can, during unfavourable conditions, lead to a total breakage of a tool section.
With HSM, design changes can be made quickly using CAD/CAM, particularly in cases where there is no need for new electrodes.
Disadvantages of HSM
Higher acceleration and deceleration rates, and spindle start and stop result in faster wear of guide ways, ball screws and spindle bearings, leading to higher maintenance costs.
HSM also requires specific process knowledge, programming equipment and interfaces for fast data transfer needed. Consequently, find suitably trained staff can be difficult.
HSM can involve a considerable "trial and error" period. Good work and process planning is necessary, along with significant safety precautions and safety enclosing (bullet proof covers).
Tools, adapters and screws need to be checked regularly for fatigue cracks. Only tools with posted maximum spindle speed can be used.
Cutting fluid in milling
Modern cemented carbides, especially coated carbides, do not normally require cutting fluid during machining. GC grades perform better with regards to tool life and reliability when used in a dry milling environment. This is even more valid for cermets, ceramics, cubic boron nitride and diamond.
High cutting speeds results in a very hot cutting zone. The cutting action takes place with the formation of a flow zone, between the tool and the work piece, with temperatures of around 1000 degrees Celsius or more. Any cutting fluid that in the vicinity of the engaged cutting edges is instantaneously converted to steam and has virtually no cooling effect at all.
The effect of cutting fluid in milling is only to emphasise the temperature variations that take place with the inserts going in and out of cut.
In dry machining, variations do take place but within the scope of what the grade has been developed for (maximum utilisation).
Adding cutting fluid will increase variations by cooling the cutting edge while being out of cut. These variations or thermal shocks lead to cyclic stresses and thermal cracking. This results in premature ending of the tool life.
Article courtesy of Sandvik Coromant .