New family member - drill CZ005
DRILL CZ 005
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: Just as every area of life in human society has its myths, so does the field of metalworking. One of these myths is the drilling of austenitic stainless steels. Great progress has been made in this area with the use of drills made of carbide materials recently. However, these drills cannot always be used, either due to the insufficient rigidity of the machines required by this type of drill and due to the high price of these drills, which is many times higher than the price of drills made of high-speed steels.
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.
WE DO NOT TRY TO IMPROOVE SOLUTIONS WE HAVE ALREADY KNOWN BUT WE DO IT DIFFERENTLY FROM THE VERY BEGINNING TO REACH OUR GOAL:
CUTTING SPEED v=30-35 m/min, feed f = (0,016 – 0,020) x D (mm/res.) (D = drill diameter in mm).
The geometry of the new CZ005 drill is based on the principle of an extremely thin core, which significantly reduces both axial force (up to 60%) and torque (up to 30%). This was verified both by tests and measurements of drills CZ002 and CZ004 at Brno University of Technology already in 2007, and by the deployment of more than 2,300,000 pieces of these drills in practice in companies.
The nominal diameter of the CZ005 drill was chosen within a tolerance of ⌀ Dh8 and its length dimensions (total length and slot length) were selected according to DIN 338, ie "medium length series", with regard to the largest anticipated use of CZ005 for drilling holes up to 3 x D. To ensure the rigidity of the CZ005 drill, its core is made in three mutually adjoining sections.
The first section with a core diameter (0.06 ÷ 0.08) x D (mm) in the length (1 ÷ 1.5) x D from the tip of the drill is constant and significantly reduces both axial force and torque and thus significantly reduces heat development at the place of its greatest origin. In addition, this extremely thin core, in combination with the advantageous profile and the groove angle of the drill, simultaneously extends the length of its main blade and the positive values of the main blade face angle are thus maximally shifted towards the center of the drill. This solution not only enlarges the part of the drill that cuts and reduces the part of the drill (in its center) that only forms and pushes the material towards its main blade, but also gives the possibility of regrinding the drill while maintaining its original geometry.
The second section of the core in the length (4 ÷ 5) x D made with an increase to (0.25 - 0.35) x D guarantees high rigidity of the CZ005 drill in its working part.
The third, last section of the drill core with an increase to (0.4 - 0.5) x D at the end of its groove guarantees high rigidity of the drill close to its clamping. The profile of the drill groove, the transition to the dorsal part of which is made by a chamfered edge (not a radius) together with the choice of the geometry of its tip, was chosen so that the chip is formed into the most advantageous shape allowing its fastest removal from the cut. The geometry of the drill ridge was chosen on the principle of a multi-faceted design of the so-called "roof cut", which guarantees a constant ridge angle along the entire length of the main blade. In addition, this cut is made in three stages, which not only has a significant effect on reducing heat generation due to friction of the drill bit against the bottom of the drilled hole (shape memory of stainless steels is generally known), but also allows the maximum amount of coolant to be supplied to the cut.
MAIN BLADE GEOMETRY IS REVOLUTIONARY
The original solution is the design of the first part of the cut of the CZ005 drill ridge, which creates a chamfer, the width of which in the center of the drill corresponds to the core diameter. veneers (0.03 - 0.04) x D lining the drill groove. This significantly reduces the friction between the chamfer on the back of the drill and the surface of the bottom of the drilled hole due to the increasing cutting speed along the main blade towards the nominal ⌀D of the drill. The design of the main blade, which in the first 2/3 of its length from the center of the drill passes along a convex curve to the axis of the drill so as to continue its last 1/3 in its axis, thus meets the requirement that the vector of the resulting cutting force as a tangent to the circle around the center of the drill at the appropriate point of the main blade, where the highest cutting speed is, made an ideal angle of 90º with the plane passing through the axis of the drill. In addition, the main blade in the section of the first 2/3 from the center of the drill creates an advantageous angle that the main and transverse blades form, thus creating favorable conditions for drilling the CZ005 drill and thus drilling a very precise hole. In addition, this main blade geometry solution allows the selection of an extreme drill bit angle greater than 140º, which significantly reduces both axial force and torque while maintaining high geometric accuracy of the drilled holes.
This design of the main blade in combination with the extremely thin core and the preferably selected groove profile also creates very good conditions for the cutting process, which begins right at the center of the drill, thus minimizing the drill bit face that does not cut but only extrudes into the material. This has a significant effect not only on the centering of the drill, but especially on a significant reduction in heat generation during the drilling process and thus a substantial increase in its performance and extension of its durability and life.
DRILL PROTOTYPE CZ005 WAS TESTED
The purpose of the first tests of the CZ005 drill was not only to verify the properties of its geometry to be achieved in the previous two editions, but especially to confirm the real possibility of achieving the target parameters: drilling at cutting speed v = 30-35 m / min at feed f = [(0.016-0.020) x D] mm / rev. (D = nominal drill Ø in mm).
For better forming and cutting ranges taking place during drilling in individual parts of the drill, especially in the area of its tip, the diameter D = 13.00 mm was chosen for testing as the largest diameter of the considered production (2.00 - 13.00 mm). Fig. 1 shows the tip of the first prototype of the CZ005 drill.
Main parameters of the CZ005 drill:
• nominal Ø D = 13.00 mm
• total length L = 151 mm
• helix length l = 101 mm
• helix pitch angle λ = 29.9º
• point angle Ɛ = 145º
• ridge angle α on the 1st facet along the main blade = 14º
• Core thickness:
◦ 1st section in the length of 13.00 mm - offer 0.90 mm
◦ 2nd section in the length of 58.00 mm - linear transmission to 3.8 mm
◦ 3rd section in the length of 30.00 mm - linear transmission to 4.6 mm
• helix bevel width = 0.40 mm
• high speed steel: 19852, M35
• hardness: 66HRC
The drill was made of high-speed steel M35 commonly used in the production of drills for drilling stainless steels so that it was possible to evaluate only the effect of the new geometry on the drilling process. For better readability of forming and cutting processes (reflections, bruises), the drill was equipped with a commonly used TiAlN coating.
INITIAL CONDITIONS FOR THE TEST
• drilled steel: austenitic stainless steel ČSN 417240, DIN 1.4301, X5CrNi 1810 (most used so-called food)
• plate from rolled profile 200 x 120, 20 mm thick
• through holes without drilling and pre-drilling, pitch 14.0 mm (8 rows x 14 holes = 112 holes)
• testing machine: 4-axis CNC machining center HAAS VF2
• coolant Quaker Cool 7350BFF, concentration 8%
• clamping holder CZMK 3/13 with collet
• cutting speed: v = 30, 36, 32 m / min
• gradually speed: 735, 882, 784 rpm
• feed f = 0.20 mm / rev, feed rate: 147, 176, 157 mm / min
In the first phase of testing, the cutting speed v = 30 m / min and the feed f = 0.20 mm / rev were chosen. After drilling 28 holes, the cutting speed was 36 m / min while maintaining a feed f = 0.20 mm / rev. After drilling another 28 holes, the cutting speed was reduced to 32 m / min while maintaining a feed f = 0.20 mm / rev. The drill behaved absolutely stably at all cutting speeds, which could be stated not only on the basis of its sound, but also on the basis of the % spindle load data, thus proving its ability to continue the drilling process. After drilling 126 additional holes, the drilling process was completed, especially with regard to maintaining the possibilities of diagnostics of forming - cutting processes and evaluation of its eventual wear.
AND THAT'S WHY WE CALL IT "FLASH BIT"
Fig. 2 is a front view of the tip of the CZ005 drill after drilling a total of 182 holes, drilling length = 3,640 mm. It is clear from the reflections that the drill starts to cut already in its center of the forming process, it is reduced to a small area in the second half of its transverse blade. This fact was also confirmed by measuring the chip width, which is the same as the length of the main blade. The wear along the main blade increases proportionally with increasing cutting speed without creating speeds.
The shape, integrity and quality of the chip surface (Fig. 3) also demonstrates the high rigidity, stability and cutting ability of the CZ005 drill from the very beginning of the drilling process (drilling into the material) to its end (starting from the material). The chip is formed in two main parts, which are separated after exceeding the value of the strength of its cross-section caused by the centrifugal force. The final phase of the chip in the shape of a "plug", created by extruding the bottom of the hole and then cutting it with the nominal Ø D of the drill, significantly protects its main blade from damage due to the drill coming out of the material. In addition, the edges of the holes are not pulled out - they do not have a "grot" slang.
The geometry of the holes (Fig. 4) and the quality of their surface also reach a very high quality. Inspection gauges showed that none of the 182 holes were larger than 0.025 mm.
Based on the first tests, it can already be stated that the CZ005 drill has all the realistic prerequisites to be among the most powerful drills in its category.
However, our company's development team still has a long way to go, not only completing the exact specification of geometric parameter values for each individual diameter from the entire production range of 2.00 - 13.00 mm, but also testing both the appropriate type of high-speed steel and the applied coating.
However, the first test results of the CZ005 drill are so surprising that a patent application was filed under the number PV 2021-445 to solve the geometry of its tip.