Pluses and Minuses of High-Speed Machining

Dec. 15, 2005
It's essential to have the right tools, techniques

It is possible to use high-speed machining to mill out cavities in high-alloy tool steels, and when doing so 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 optimized tool paths, cutting data, and cutting strategies. That’s the message from Sandvik Coromant, a leading supplier of cutting tools.

HSM can be effective on forging dies, most of which are made of demanding tool steels, while many are small or moderately sized. Because of their greater stability, using short tools will result in higher productivity.

HSM works for injection molds and blow molds, too, because of their small size, making it economical to perform everything from roughing to finishing in one set up. Many of these molds have deep cavities, requiring thorough planning of the approach, retract, and overall tool paths. Often long and slender shanks/extensions are used in combination with light cutting tools.

An early HSM application was modeling and prototyping of dies and molds using easy-to-machine materials; cutting speeds are often as high as 1,500-5,000m/min with high feed rates.

A primary reason to use HSM is to cut production costs via higher productivity, mainly in finishing operations.

Equally important is the ability to increase overall competitiveness via shorter lead and delivery times.

Advantages of HSM —When HSM is used, cuts are shallow and the engagement time for the cutting edge is very short; The feed is said to be faster than the time for heat propagation.

Low cutting force gives a small and consistent tool deflection. In combination with a constant stock for each operation and tool, this is a prerequisite for a highly productive, safe process.

Because the cuts 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 the operator to use longer tools with less risk of vibrations.

HSM can be a productive cutting process for small components, because roughing, semi-finishing, and finishing requires relatively little material removal. HSM also achieves productivity in general finishing and it is possible to achieve very good surface finishes, often as low as Ra ~0.2 microns.

It’s possible to machine very thin walls. Downmilling tool paths should be used, and the contact time between edge and work piece must be very short to avoid vibration and wall deflection. Cutter microgeometry must be positive, and the edges very sharp.

The geometric accuracy of dies and molds makes them easier and quicker to assemble, and human skills cannot match CAM/CNC-produced surface texture and geometry. Also, increased time and focus on the machining phase can reduce manual polishing. Hardening, electrode milling, and EDM may also be minimized, lowering investment costs and simplifying logistics. HSM will achieve a dimensional tolerance of 0.02 mm, while EDM tolerances are 0.1-0.2mm.

Disadvantages of HSM —Higher acceleration and deceleration rates, and spindle start and stop result in faster wear of guideways, ball screws, and spindle bearings, leading to higher maintenance costs.

Also, HSM requires specific process knowledge, programming equipment, and interfaces for fast data transfer. As a result, finding, training, and keeping good operators may be a problem.

HSM may require lots of trial and error. Good process planning is necessary, as are appropriate precautions and safety equipment (e.g., enclosures.) Tools, adapters, and screws must be checked regularly for fatigue cracks. Only tools with posted maximum spindle speed should be used.