New family member

Is there anything else to change on high-speed steel spiral drills? I strongly believe following post will answer this question.

Company NÁSTROJE CZ, s.r.o. has set itself a goal of developing spiral drill for machine drilling of stainless steel, especially the austenitic one. There would be nothing strange about it unless the requirement is not to drill in cutting speed = 30-35 m/min and feed s = (0,16 – 0,20) x D/rev. (D= drill diameter in mm) while being made of high-speed steel, thus the speed unthinkable for the use of those drills so far.

Dispelling myths

Let‘s go back to the beggining:

So what else could be changed in the geometry of a twist drill to create even better conditions for the drilling process? To answer this question we must go back to the very beginning and describe the most important properties of austenitic steels.

These steels are characterized by several features that have a significant negative effect on the drilling process. One of the most important is the low thermal conductivity of these steels, which is up to 4 times smaller than ordinary carbon steels. This in turn means that the heat generated during the drilling process and which must be removed from the cutting point tends to pass into the tool and not mainly into the chip. At first glance, this feature resulted in a simple solution, namely to design the geometry of the drill so that heat generation is minimized during the drilling process itself, because its subsequent removal from the cutting point is already very complicated with respect to the previously mentioned.

Another equally important feature of austenitic stainless steels is their high adhesion. This causes the formation of growths, the so-called "blue effect". This phenomenon can be successfully minimized by choosing a high cutting speed, for which heat generation is again limiting, and we are again in the design of drill geometry.

Another no less interesting feature of austenitic stainless steels is their plasticity. It is up to 60% higher than conventional carbon steels. This feature implies the requirement to select a large cutting edge angle so that the resulting geometry is sharp and easy to cut.

Austenitic stainless steels have yet another important property, and that is the great tendency to strain hardening. This causes a hard layer is formed at the bottom of the hole after the drill bit has been initially drilled. This feature implies the requirement to select large feeds so that the cutting edges of the drill move below this layer. This implies a requirement for a geometry requirement to ensure the rigidity of the drill.

A very important requirement for the geometry of the drill is also that it creates an optimal chip shape and then quickly discharges it from the cutting point and at the same time allows the supply of as much coolant as possible to the cutting point.

Geometry design is a priority

You may have noticed that I have not yet mentioned at all the choice of the type of high-speed steel from which the drill should be made, as well as the possibility of using the coating of the drill. This is because I consider it a priority to design the drill geometry to meet the above requirements. If this geometry is applied with the choice of high-quality high-speed carbon steel, the influence of the drill geometry will be further enhanced.

The requirements have been defined and now remain to be met.

The new type of drill is another member of the "CZ" series of drills and was given serial number 005.

The geometry and parameters of the new CZ005 drill can be found in the continuation of this article in the next issue.

For the company NÁSTROJE CZ, s.r.o. Ing. Emil Nečesánek

NÁSTROJE CZ, s.r.o., Riegrova 399/2, 697 01 Kyjov
Obchodní kancelář Dobruška: Křovická 1047, 518 01 Dobruška; Tel.: +420 491 615 785; GSM: +420 777 581 299; E-mail: objednavky@nastrojecz.cz; URL: obchod.nastrojecz.cz