Due to internal coolant ducts, innovative geometry and coatings, the machining of stainless steels, nickel-based super alloys and CrCo-alloys has become significantly simpler and more stable. This also applies to small diameter holes as well as deep bores.
The Mikron CrazyDrill range includes through coolant drills in six different working lengths: up to 6, 10,15,20, 30 and 40 x D. In addition, there is a short pilot drill for a maximum bore depth of 3 x D. With a bore depth of 20 x D these tools are available from diameters of 1mm (the longest from 2mm) up to a maximum diameter of 6mm.
Mikron has introduced these new ranges to tackle tough-elastic super alloys with poor heat conductivity. In these instances cooling through traditional round ducts, even with high pressure, is insufficient. Premature edge chipping, packing of chips and build-up of the flutes prevents a stable drilling process. This is a particular problem for industries working with valuable materials.
Markus Schnyder, head of Mikron Tool International and one of the initiators of the new tools comments: “For many years our customers reached the limit when drilling stainless steel. The process was unstable and too slow, thus machining became extremely expensive. We wanted to find a solution. The development took us several years and we depended on close cooperation with our partners, for instance, to find the right blank material or the most efficient coating.”
Whether short or long bore, the new geometry provides a chip-breaking effect at the cutting edge. The chips are short and curved as a result. Additionally, the new geometry has an open flute profile that facilitates the evacuation of microchips.
There is also a variant for long drills: in addition to the open profile, polishing flutes help the evacuation of chips from the hole and a new coating especially developed for stainless steels helps in this regard.
Performance and process stability is mainly achieved through the cooling of the tool: two spiralised, internal coolant ducts up to the drill tip guarantee a constant and high coolant flow during the entire drilling process – an important factor to prevent overheating of the tool even with high cutting speeds.
The drop shaped coolant ducts are designed with the largest possible diameter and are located in an area with minimal stress to ensure the stability of the tools is not compromised. Compared to the conventional round diameter shape, these ducts afford a four times higher coolant flow. At diameters under 3mm a power chamber in the shaft reduces the length of the ducts and provides higher coolant pressure.
The advantage of an efficient supply of coolant to the drill tip is that it increases tool life because overheating of the cutting edges is avoided right from the start. Combined with the innovative tip geometry, the cooling helps to break and flush out the chips from the flutes. All these characteristics make it possible to work with higher cutting parameters (speeds and feeds) and to drill in one single stroke.
Starting with drill for a depth of 10 x D, little by little new and longer versions have been introduced to the CrazyDrill Cool SST-Inox range including 30 x D and 40 x D.
“One single stroke is sufficient to reach the entire depth – without chip evacuation – from 6 to 40 x D,” Mr Schnyder asserts. “The same applies to the speeds and feeds which can be set equally for a depth of 20 x D. For depths starting at 30 x D these settings need to be reduced by an average of one third.”
When drilling at 15 x D it helps to have a short pilot operation to reach optimum positional accuracy. It also helps to guide a long drill to guarantee straightness. So that centring is accurate, Mikron developed a complementary pilot drill whose tolerances are specifically intended for working with long drills. If needed, the tool can also simultaneously produce a 90° chamfer. It also features internal coolant to the tip and is available in diameters from 1 to 6mm.
There are various reasons why users turn to these new drills such as time saving, cost reduction and longer tool life or simply the need for a stable drilling process in difficult materials.
According to Mikron a customer using a traditional drill with round cross section coolant channels can reach a tool life of 3,900 bores per tool. With the new drills it is possible to reach 18,000. This represents a cost saving for changing a single tool of $15,000 per year.
For the machining of a medical part (bore diameter 4mm, depth 68mm) made from chrome-cobalt one customer had difficulty finding the right drill that would guarantee a stable process.
With the new Mikron tooling he achieved drilling that was ten times faster with three times longer tool life. The hole quality was excellent in regard to cylindricity, diameter and concentricity. Surface finish quality, depending on material was in the range of Ra 0.1 to 0.3 (grinding quality).
Mikron Tools www.mikrontool.com
