Flexible production with exchangable headed type of milling cutters

Sándor dr. Sipos ^1,a – Sándor Csuka ^1,b –István Tomoga ^2,a - László Orosz ^2,b

1,a - Associate Professor, Óbuda University, Bánki Donát Fac. of Mech. Eng.

1,b - Institute Engineer, Óbuda University, Bánki Donát Fac. of Mech. Eng.

2,a - Sales Manger , Sandvik Hungary Ltd.

2,b - Product Specialist for Coromant Products, Sandvik Hungary Ltd

Introduction

There are ever-increasing requirements against the biggest tool manufacturers, encouraging them to make tools with higher and higher performance and more versatile applicability. The last requirement can be met by developing the tool construction (and by reducing the extremely valuable non-productive time; this it the so called „time factor”). Tools with advanced constructions (use of inserts, use of tool cassettes or with monolite construction) are used to increase the performance; however, the real breakthrough can be achieved by using innovative tool materials. This breakthrough can be fulfilled not only by using advanced base materials (for example, gradient materials or „2 in 1” materials) but by applying even more advanced coating technologies (chemical vapour deposition at mid-temperature and physical layer deposition at always lower temperature) and new combinations of coating systems (nanocomposite nACRO³, oxynitride: nACoX⁴) as well. When increasing the performance, it is the clear aim to achieve the requested economic effect (this is the so called „cost factor”).

The present article introduces a tool construction, having an excellent performance due to its flexible applicability and universal character on the application field of mid-size end milling cutters.

1. Constructional and technological characteristics of exchangeable headed type of milling cutters

Modern end milling cutters – due to their flexible applicability and the complexity of the cutting parts - can be produced with three different constructions (Figure 1.).

The small-sized (∅0,05 – ∅10 mm) end milling cutters have solid (monolith) construction. They are made often from modern tool materials (micrograin carbides, cermets or Sialon); and very often ultra-modern PVD-coatings (μAlTiN, nanocomposite coating in different combinations) are used to increase the cutting performance. Their characteristics are as follows: due to the limited chip space only the versions with low number of teeth have spread; because of the increased load it is necessary to strive after the minimal overhang. In order to overcome the increased vibration tendencies the versions with uneven pitch and/or helix angle should be used. Recently high feed milling cutters are produced with solid construction as well.

The end milling cutters with a diameter bigger than ∅16 mm are made mainly with indexable inserted construction where usually carbide base materials are used with different coating layers (CVD, MTCVD, PVD). The reasonable limit of the types with indexable inserted construction is the diameter of ∅32… 40 mm, above this limit the cutting tasks are carried out with arbor mounted milling cutters. The indexable inserted construction allows us great latitude to equip the milling cutters, having diverse entering angles, with inserts of various shape and size. The load-bearing cross-section of the shaft, having advanced construction, is made from structural steel and it can be extended by tangential arrangement of the inserts; and, in order to increase the productivity, the inserts should be arranged in more rows.

As it can be seen from the Figure 1., the reasonable diameter values of milling cutters with exchangeable heads are between ∅10… 25 mm. The head, performing the cutting process, is exclusively made from carbide base material and mounted on the tool holder (shaft and shank), made usually from structural steel. The tool holders are available for the users with various (straight and/or taper) constructions and in some variations of length.

In case of end milling cutters the most crucial constructional problem is to mount the head and the tool holder stable, centered, with radial runout. Without this it is not possible to achieve great results or even to create strong connection between the tool parts. The very first solution to this problem was the taper draw-in bar and the straight connection coupling (Minimaster, Seco, 1988). Mounting with cylindrical thread and surface is applied widespread as well (Horn, Komet). The patented conical thread of CoroMill316 EH, developed in 2009, can be characterised by conical thread and mounting on the axial surface [1, 2]. The connection is secure, precise and appropriate rigid as

• EH is centered and oriented with the help of taper (threaded) surface,

• the location of cylindrical holder and EH on the axial surface determinates clearly the axial and radial co-ordinates, to be considered during CAM-programming, therefore

• the precision tasks can be carried out with appropriate accuracy,

• the coupling with torque key prevents overstressing the coupling,

• the necessary rigidity can be provided even in case of slot milling at large depth of cut (a_p, mm).

2. Execution of tests with EH milling cutters

The cutting tests with CoroMill 316 end mills have been carried out in the machine shop of Budapest Tech, Bánki Donát Faculty of Mechanical Engineering under the circumstances, given in the below Table 1. During the tests, three components of the cutting force have been measured on-line, in function of cutting data. The workpiece material is popular and used in production of forming tools and mould cavities; we have carried out dry machining without applying lubricating-cooling medium.

Table 1.

The test series have been supported with Design of Experiments (DoE) as – applying the same cutting speed value (v_c=150 m/min) – the effect of 3 technological data have been examined by us, they are as follows: radial width of cut (a_e, mm), axial depth of cut (a_p, mm) and feed per tooth (f_z, mm). In case of three technological data, varying them on 3 setting levels we should have carried out 27 tests, but applying our well-tried DoE method we have managed to reduce the number of the tests, to be carried out, to 6 planned tests + 3 checking tests. The cutting speed value, applied by us, and three technological data, affecting the chip size, have been determined by us so that they should fall in the upper zone of the limits of recommended value (Figure 2.).

The tools were available for us with two different diameters, with two versions, concerning the total number of teeth, and two different types of edge designs.

3. Examination of the behaviour of the milling cutters

3.1. Shoulder milling

The shoulder milling operations have been carried out with setting data, given in the Table 1., applying conventional (up) milling. Due to space limitations of the paper we can present neither the results of the measured axial and radial force nor the comparison of the changes of the registered force components during two rotations of the tools. Therefore we have modelled the measured values with the help of our well-tried power function [3] and the diagrams can be seen in below Figure 3. From this it can be seen well that the increase of the number of teeth reduces force relevant while increasing the productivity. The application of chip divider is more favourable not only from the point of view of power requirement, but it has a favourable effect on the accuracy of workpiece with thin walls or with low stability as well.

Our tests have been extended to EH tools with bigger (∅16 mm) diameter with continuous edges and with chip divider as well. From the measured force data, shown in Figure 4., it can be seen well that in case of increasing the diameter, the tool construction with chip divider (kordell construction) is less efficient from the point of view of feed force. From the point of view of axial force it can be noticed that the application of chip divider is clearly favourable as the force, affecting the work piece, reduces by one third part, or even to half part.

3.2. Slot milling

The slot milling tests have been performed – under constant cutting speed value – with an EH tool with ∅12, (a_e=12 mm); the axial depth of cut (a_p, mm) and the feed rate per tooth (f_z, mm) have been varied by us systematically. It was our aim to demonstrate the efficiency of the chip divider and to get clear picture about the performance of the tool as well.

From the test results, shown in Figure 5., it can be seen well that

• the feed force (and parallel with it, the power requirement) changes favourably when applying chip divider, especially when setting greater axial depths of cut. For example, if a_p=0,5 × D, then the feed force value is only the half part of the value, measured on „normal” edges.

• It can be seen well that upon application of chip divider the axial force develops even better as the force, affecting the work piece, is only half or just third part of the force values, registered on normal edges.

In case of slot milling the advantage of chip divider has been proved, especially if we compare the photos, taken about the chips, generated with increased feed milling (Figure 6.).

4. Technological advantages and recommendations

Tools with head are used mainly for milling operation of parts, produced in small or medium series as they can be exchanged easily (without effort) and quickly; so the differently shaped and designed heads for roughing, semi-finishing and finishing can be varied flexible. Not only the traditional shaped (3D contour milling, profile milling) but high feed milling versions are available as well.

As regards the price rates of the EH tools: it is only 27-43% of the price of similar tools with solid construction (with the same characteristics, concerning design, material and total number of teeth). It is only one third part of the price of the solid tools; it means it is worth purchasing this version also in case if the re-sharpening of the head is not possible. The usefulness of the EH tools can be noticed even better if the solid tool notched or broke due to any reason (programming mistake, inattention etc.). The re-sharpening and re-coating of EH tools become possible in the future and this fact makes the application of EH tools even more rentable.

5. Summary, further tasks

During the tests, carried out with CoroMill 316 tools, it has been proved that the use of tools with greater number of teeth and the use of head with kordell geometried chip breaker brings essential advantages - under testing circumstances. The economical dominance of the EH type against the solid (monolith) type of tools can be noticed, especially when establishing the conditions of the tool re-sharpening.

In the future we wish to examine the lasting behaviour of the heads under semi-industrial circumstances, looking for an answer to the question: How does the different design affect the wear behaviour in case of use of cooling/lubricating medium and in case of green machining (dry machining, applying cold air jet nozzle, minimal quantity of lubrication). Based on our positive experiences we intend to carry out further tests in order to compare the performance of high feed milling with indexable inserted tools and milling cutters with EH as well.

6. Literature

[1] CP09.1 – CoroMill 316 (ppt-presentation), Sandvik Coromant, 2009.

[2] CP09.2 Internal Information Material – CoroMill 316 (ppt-presentation), Sandvik Coromant, 2009.

[3] S. Sipos dr.: Cutting theory and modern tools (Lecture materials in pdf-format),

Budapest Tech, 2000-2009. (www.banki.hu)