New methods in qualification of cutting performance during drilling operation

S. SIPOS dr. – I. SZALÓKI
Óbuda University, Donát Bánki Faculty of Mechanical Engineering

Abstract

The improvement in cutting performance of the innovative tools cannot be linked solely to one criterion. The judgement of the cutting performance, made based on some simple factors (chip formation, comparison of surface roughness, edge durability etc.), is quite subjective. During the qualification, based on instrumental measurements, effect of those factors will be considered as well which can serve as the base of the tool monitoring with on-line registration of the wear process.

In our present article we are going to introduce a qualification method, worked out for innovative drilling tools and applied successfully: it is able to characterize the effectiveness of the innovation based on some features of the cutting performance (force effect, torque demand), measured at the same time. The elaborated method makes it possible to promote data generation in order to create calculation formulas with the limiting conditions during the planning phase of the machining technology and to improve the process security.

Introduction

By the cutting performance of machining tools is usually meant only one feature, chosen arbitrarily from several others, it is usually the tool life. Indeed it is really an important factor, especially in case of technical-economical comparison. The complex expression of cutting performance is used to characterize the ability of the tools in such cases when other factors are considered as well. By speaking of the judgement of the cutting performance, the following four factors are usually mentioned: edge durability, cutting force, roughness of the machined surface and shape of the detached chip. The cutting performance of tool is considered to be favourable if the tool is able to maintain its durability for a long time, to machine with developing low force effects, the roughness of the machined surface is acceptable and the chip leaves the detachment zone as fragmented chip [1].

The qualification of the cutting performance is made with the help of instrumental measurements, carried out systematically. According to literature [2] the cutting performance can be divided into main features (wear, edge tool life) and complementary features (force effects, material removal rate, temperature, surface roughness, chip shape). Based on it, it is obviously that the qualification cannot be made based solely on one feature, at the same time it is advisable to choose a certain group of characteristics and to describe their complex behaviour.

We can establish that the judgement, taking only few simple features into consideration and not supported with measurements, is fairly subjective. The qualification, based on measurements, is able to provide much more information on the process. The qualification process can be divided into long-distance and short-distance examinations, concerning its character and time development. The firstly mentioned is the subject of the present article, while the last one is connected with the deterioration (degradation) process of the tools.

The successful tool innovation should result in the explicit improvement of the performance. In the present article we are going to introduce a qualification method, worked out for innovative drilling tools and applied successfully: it is able to characterize the effectiveness of the innovation based on some features of the cutting performance (force effect, torque demand), measured at the same time. The elaborated method makes it possible to promote data generation in order to create calculation formulas with the limiting conditions during the planning phase of the machining technology and to improve the process security.

1. New methods for the qualification of drilling tools

We recommend a new method to determine the cutting performance of drilling tools, having different geometrical and edge construction, material grade and coating system. The application of old - very often fifty-year-old - correlations (for example, Kienzle-Victor formula to calculate forces) in the present modern tooling may have anachronistic, major mistakes and by no means is able to fulfil the expectations in connection with process security of the modern production. The modified force formula, used in case of turning operation, has already been introduced by us on the previous conference [3].

The precondition of the qualification method, worked out for the hole machining, is that the feed force, developing during the machining operation, and the torque will be measured with the same instrument. This device is a rotating dynamometer, produced by KISTLER, type 9123C1111, and a measuring software (DynoWare™), developed by this company to register and to evaluate the registratums.

The qualification process has the following benefits:

  • The method is based on simple and quick determination of complementary features of the cutting performance, therefore a relatively small quantity of test material is enough to carry out the qualification process.

  • Due to the determination of the force effects, measured at the same time, the constant and exponent values of Kienzle-Victor formula become determinant. In connection with it, we can get a real picture about the values of the force components, developing during the machining operation, and it becomes possible to determine the specific cutting force values (kc1.1, qc, kf1.1, qf), being necessary to calculate cutting force and feed force values.

  • The feed force Ff is determined in some of the literatures [4, 5] in function of the cutting force Fc, in a percentage form:

Ff=cFc N (1),

where in case of general drilling the „c” value is indicated in the range of c = 0,9…1,0. With the help of the method, worked out by us, it can be demonstrated that in the wide range of materials, machined by us, the value of the „c” is much lower than the range, mentioned earlier.

  • The new method characterizes the cutting performance of innovative tools with data, gathered from measurements during the machining operation – it makes possible to compare drilling tools, originating from different stages of the development. The evaluation function of the detached chip volume (mm3), achieved with one unit of work (J) is the following in case of general drilling:

Wc=d2πnf460Pc mm3Ws (2),

where

d – diameter of the tested drilling tool, mm,

n – settled number of revolutions per minute, min-1,

f – feed value, properly chosen to the examination, mm,

Pc – the power requirement of the drilling operation, W.

5. In order to maintain the uniformity, the tests have been carried out and evaluated with fixed feed values. The exact value is the hundredth part of the tool diameter (f=0,01×d), being for sure lower than the value, causing a tool break in case of material grades, applied in a wide range.

The new examination method, recommended by us, can be used to qualify the cutting performance of drilling tool in case of short-time investigations.

2. Results of the qualification of innovative drilling tools

In the past year we have carried out several tool tests, to these we have had tools from different sources. The method, worked out by us, is able to compare, characterize according to formulas (1) and (2) and to qualify the machinability of the workpiece and the cutting performance of the tool – i.e. the data, resulting from objective measurements. Due to the space limitations we are going to demonstrate the qualification method with the help of three application examples.

The tests have been carried out on vertical machining center, produced by MAZAK, type Nexus 410A, the machining conditions have been determined in such a way that they should reflect the different degrees of machinability of the workpieces. The cutting speed value has been kept on a constant value, while the feed values have been varied on four, the hole length on three levels.

2.1 The machinability of ToolOx tool steels

The ToolOx tool steel family, as a result of a relatively new innovation, is applied in the industry as a base material of forming/molding tools in case of production of plastic components, having a great value. Based on the information leaflet, issued by the producing company, the ToolOx 33 is a tool steel in pre-tempered condition, its characteristics are: a special texture structure, a hardness of 29-30 HRC, a high degree of toughness and fracture strength (cca. 1040 MPa). The ToolOx 44 is currently the hardest (44-45 HRC) pre-hardened tool steel grade in the world. Both types can be characterized by an extremely good machinability, they are free from residual stress, therefore they have a really favourable dimension durability.

The tested drilling tool

The tested drilling tool

Figure 1. The tested drilling tool

Characteristics: Ø10 mm; z=2; point angle σ=140º

Our aim was to demonstrate the difference in the machinability of two material qualities, mentioned above, and to determine the characteristics, being necessary to calculate data in case of drilling operation [6]. The tests have been carried out with cemented carbide drills Walter Titex X.treme PLUS A3389DPL, having a single main edge (so called “three-rake-geometry” with web thinning, Figure 1.).

The change of the machinability in function of the steel grades - a.) ratio of force components, measured on drilling tool

a.)

The change of the machinability in function of the steel grades - b.) specific cutting work to characterize machinability

b.)

Figure 2. The change of the machinability in function of the steel grades

  • a.) ratio of force components, measured on drilling tool

  • b.) specific cutting work to characterize machinability

Machining conditions: vc=40 m/min; hole length: 10, 15 and 20 mm; internal cooling (lubricating emulsion of 8-10% Aquamet 260 EP, pressure: 6 bar); f = 0,06 … 0,12 mm

(varied systematically on 6 levels)

By summing up test results of the machining operation (see Figure 2.), the following conclusions can be drawn:

  • in case of pre-tempered tool steel of lower hardness, the feed force Ff is 55 percent of the cutting force Fc in average, while the hardened steel ToolOx 44 has shown a value of approx. 57 percent. It means that the values, measured by us, have shown a significant deviation from the values, mentioned in the quoted literatures,

  • the ToolOx 33 steel quality can be better machined by 24 percent than the harder version. It means that with one unit of work – almost independently from the feed – by approx. 24 percent greater chip volumen can be achieved.

  • in the technology it is always important to know what kind of effects have the machining conditions on the feed force Ff and torque demand Mc. The firstly mentioned can be calculated with the following formula in case of general drilling:

Ff=kf1.11zsinσ2qff1qfd2 N (3)

while the necessary torque can be determined with the below formula (in case of full drilling):

Mc=kc1.11zsinσ2qcf1qcd2z4000 Nm (4)

where:

kf1.1 and kc1.1 – are the main values of specific cutting force, given in Kienzle-Victor formula, under testing conditions, N/mm2 ,

qf and qc – are the exponents of the chip thickness under testing conditions,

f – applied feed, mm,

d , z and σ – the diameter (mm), the number of the teeth and point angle (º) of the drilling tool.

An other aim of the tests was to determine the constants and exponents, given in the formulas (3) and (4), being characteristical to the material pairing. The results of the tests, carried out systematically, can be seen in Table 1.

Table 1.

Characteristics / grade

ToolOx 33

ToolOx 44

kc1.1, N/mm2

1266

1555

qc

0,31

0,32

kf1.1, N/mm2

450

1023

qf

0,45

0,27

The values, indicated in the Table 1., demonstrate that

  • the main values have developed quite differently during the drilling operation, it means it is not recommended to make calculations in case of one-edged tools based on values, determined long time ago,

  • the main values of the specific cutting force are usually greater when machining harder material grade, while exponents are in the usual order of magnitude,

  • as regards the constants and exponents, the most determinant factors are the special construction of the edge geometry and the material grade of the coating of the tools.

Also this short overview draws the attention to the importance of the tests, to be carried out before the operation. With them it would be possible to avoid (or to decrease the frequency of) the unexpected break-downs, and, as a result of it, the process security of the drilling operation could improve even on the sensitive areas, like for example the case of inserts for stamping dies and plastic molds.

2.2 The cutting performance of the drilling operation, measured in case of machining tool steel ToolOx 44

In the following part of our article we are going to introduce the results of the cutting performance tests, carried out on behalf of three tool producing companies, playing a market-leading role. Considering the fact that we do not have a written approval from the companies, therefore they will be called „A”, „B” and „C” company. The cemented carbide drills with size Ø10 mm, to be compared during drilling operation of ToolOx 44 tool steel grade, are shown on Figure 3. On the photos, taken about the test tools, the characteristics of the tools can be seen well: the drill of company „A” has a convex main edge, the „B” tool has a double flank land, and the „C” tool has a “three-rake-geometry”.

The tools, to be compared - a tool

„A” tool


The tools, to be compared - b tool

„B” tool

The tools, to be compared - c tool

„C” tool

Figure 3. The tools, to be compared

The results of the tests (Figure 4.), carried out in different hole depths, have clearly demonstrated that

    • the „C” tool has usually worked with the lowest cutting force and torque. This shows a way to decrease the energy consumption as the power requirement of the drilling operation has decreased.

    • the superiority in the cutting performance of the „C” tool can be seen mostly in the necessary feed force as the decrease of the load of the main spindle may lead to the increase in the frequency of maintenance works and in the life of the device.

Results of the drilling operation, carried out with tools of three producing companies - applied torque

a.) Applied torque

Results of the drilling operation, carried out with tools of three producing companies - feed force demand

b.) Feed force demand

Figure 4. Results of the drilling operation, carried out with tools of three producing companies

Machining conditions: vc=40 m/min; Hole length: 10, 15 and 20 mm; internal cooling (lubricating emulsion of 8-10% Aquamet 260 EP, pressure: 6 bar); f = 0.06 … 0.12 mm

(varied systematically on 6 levels)

Our previous statement - whereas the tool construction can affect the force effects, developing during the operation - has been proved by this test as well, and we have not mentioned the effect of the geometry, made on the accuracy, on the concentricity or even on the surface roughness (due to the space limitations we cannot introduce them in details).

2.3 The change in the cutting performance in case of sharp and worn-out tools

The deterioration process of the drilling tools can be well characterized by the change of the cutting force and torque demand. In order to prove it we wished to know to what extent the cutting performance of the tool will change in case of machining an often applied tool steel in a pre-tempered condition (970 MPa) under the conditions of minimal quantity of lubrication (MQL, measured oil flow rate 22,5 ml/hour). The tests have been carried out with cemented carbide tool, produced by Emuge, type EF Drill Steel, with diameter of Ø7.4 mm.

Our expectations have been proved numerically with the help of results of the drilling tests. The change in the cutting performance characteristics is shown in Figure 5., in case of use of sharp and worn-out tools.

The effect of the deterioration process of the tool, practiced on the cutting performance characteristics - a.) specific main values of the cutting force

a.) specific main values of the cutting force

The effect of the deterioration process of the tool, practiced on the cutting performance characteristics - b.) characteristics of the cutting performance

b.) characteristics of the cutting performance

Figure 5. The effect of the deterioration process of the tool, practiced on the cutting performance characteristics

  • a.) the specific main values of the cutting force

  • b.) the chosen cutting performance characteristics in case of feed f=0.08 mm

Machining conditions: 40CrMnMo7 tool steel (970 MPa), minimal quantity of lubrication (material: water-soluble oil, named TrimTap NC) ; cutting speed: vc= 70 m/min

The results demonstrate that

  • during drilling operation, carried out with worn-out tool, the main value of the cutting force (kc1.1) has shown an increase of nearly 70 percent, the main value of the feed force has increased by approx. 50 percent,

  • the „c” factor of the (1) formula has changed from 0.62 to 0.73. It means that the deterioration of the tool has increased the force proportion, examined by us, by nearly 18 percent. We would like to draw the attention to the fact that the calculated values approach the values, measured in case of ToolOx 33 (but they show a significant deviation from the values, given in the quoted literatures),

  • the effect of the deterioration process of the tool, practiced on the performance of the tool, has developed according to our expectations: in case of a worn-out tool by an approx. 15 percent greater performance is necessary, and as a result of it,

  • the detached chip volumen, achieved with one unit of work, has decreased to the same extent.

The tests, carried out by us, have drawn the attention to the fact as well that the supplied power, resulting from the deterioration of the tool, should be considered and followed (with tool monitoring).

3. Conclusions, further tasks

The base of our new qualification method, used to the description of the cutting performance, is to collect data from measurements, carried out at the same time, to create with them formulas and to compare them numerically. The method is able as well to provide the well-known (force and torque calculating) formulas of the machining theory with reliable constants and exponents, being true of certain tool/workpiece pairings.

Our further tasks are to carry out extended test series, sponsored by the leading tool producing companies, with the aim to choose the most appropriate tool from the technical-technological point of view. The comparative tests, carried out by us in a hard-to-machine austenitic corrosion-resistant steel with drilling tools of different purpose, produced by two competitive tool producing companies, are drawing to a close. We will report about the results of these tests soon.

This article was presented at the International Manufacturing
Conference (Budapest, Hungary, 14-16 November 2012, organised by GTE,
Hungarian Scientific Society for Mechanical Engineering) and has been published in
electronic form, on CD as well.

References

[1] dr. Sipos, S. – Dr. Palásti-Kovács, B.: A forgácsoláskutatás néhány eredménye a szerszámminősítés területén Nemzetközi Gépész és Biztonságtechnikai Szimpózium, 2007. november 14. Budapest, BGK (ISBN 978-963-7154-68-3) CD, pp. 28.

[2] Pálmay, Z.: Fémek forgácsolhatósága Műszaki Könyvkiadó, Budapest, 1985.

[3] Sz. Biró – A. Nagy – S. dr. Sipos: Use of Cutting Inserts with Modern Geometry in Higher Education International GTE Conference Manufacturing 2010. , Budapest, 20-21 October, 2010. (ISBN 978-963-9158-30-6) CD, pp. 10.

[4] Bakondi, K. – Kardos, Á.: A gépgyártás technológiája I. Tankönyvkiadó, Budapest, 1974.

[5] Garant: Zerspannungshandbuch, Art. Nr. 110950 DE, ISBN 3-00-016882-6

[6] Szalóki I.: Drilling of the difficult to machine steels (Nehezen forgácsolható acélok fúrása), Szakdolgozat, NGC 25/2011., pp. 70. + pp. 30. (Appendix)