Why mould makers can’t treat cutting tool choice in isolation
The same radii principle is true for roughing operations, i.e., using cutter radii smaller than those of the work-piece and, while this leaves more material in corners, it helps maintain consistent and better evenly distributed cutter load for all subsequent semi-finishing and finishing operations.
Just as critical as cutter radii is cutter rigidity where cutter draft angles/tapers play a key role. Most cutters have either relief neck or tapered neck styles. Relief neck tools have a smaller diameter than the flute size above the flutes and below the cutter’s shank diameter. With tapered neck style tools, there is the cutting diameter and above that a smaller relief area and a taper up to the shank diameter. Most long-reach cutters incorporate tapered neck designs.
Less draft angle means more rigidity in the tool. But, draft is needed to avoid rubbing against mould cavity sidewalls. To select the most rigid cutter, mould cavity drafts need to also be taken into consideration.
Cutting tool drafts should never match or exceed mould drafts. For instance, if a mould cavity has a 3-degree draft on all its sidewalls, tools with drafts up to 2.5 degrees on their relief can be used.
The roles of coolants and tool holders
As far as the use of coolant is concerned in mould machining, most of today’s advanced cutter designs and geometries perform at their optimum when run dry. The exception is running with oil mist, but doing so at all times. For hard milling, the rule is absolutely no coolant use at all, unless oil mist is available.
Most cutting tool companies recommend vibration-dampening holders for mould machining and for use with today’s advanced cutters. The best tool in a below-par holder will fail to achieve optimum performance and tool life will be impaired whereas, with advanced high-performance holders, tool life can potentially be doubled.
Vibration control is critical, especially with long tool gauge lengths and in high-feed roughing operations. The key is that any vibration-dampening holder has been designed to reduce harmonics and any vibration generated by the cutting process.