WCMX 030208EUD

Workpiece hardness correction

1,25 1,0 0,2 18,5 18 15,6

WCMT 040208E46

1,35 1,0 0,2 23,7 23 20,6

WCMT 06T308

3,0 2,5 0,25 37 36 30,5

WCMT 080412

1,4 1,1 0,25 42,8 42,2 39,5

DEFINITION OF BASIC CONCEPTSCUTTING GRADES PRAMETCHOICE OF TURNING TOOLCHOICE OF MILLING TOOLCHOICE OF DRILLINGWEAR OF CUTTING INSERTSGRADE GROUPS EQUIVALENT TABLES

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6 Drilling

6.4 Practical recommendations 6.3.2 Rotating drill

For misalignment of rotating drills it is necessary to use special eccentric chucks, by which it is possible to adjust intermediate diameters of holes up to the nearest higher standard diameter. Chucks of various products enable a misalignment in the range of around –0,2 ÷ +1,4 mm. The misalignment enables to compensate production tolerances of the drill body and cutting insert; by the diameter pre-adjustment on the machine, it is possible to improve the hole tolerance up to ± 0,1mm. By controlling the misalignment during drilling at stationary drills it is possible to carry out e.g. a hole pre-drilling (hole recessing) for threads including chamfering. The hole accuracy is dependent on the drill length; at drills with 2D ÷ 2,5D it is usually in the range of +0,2 ÷ -0,1mm. The roughness of machined hole surface usually achieves the values of Ra = 3,2 ÷ 6,4 µm.

For achievement of better roughness values of machined surface it is recommended to retain the speed at the level of double up to three-fold working feed during withdrawal the drill from the hole.

At drills misalignment it comes to a speciﬁ c balance disruption of radial components of the cutting force; therefore it is necessary to reduce feed values to the level of 0,05 - 0,1 mm.rev^-1.

Drills intended for the depth of hole up to 3D can be also used for drilling in inclined concave, convex and generally uneven surfaces. They can be also used for re-drilling of pre-drilled (coaxial) holes or also in cases of drilling other holes which are perpendicular or inclined to the axis of drilled hole. But in this case it is necessary to respect the recommendations mentioned for the two following Figures.

Drills intended for the depth of hole > 3D and drills working with a large overhang require a planar entrance surface and a homogeneous workpiece.

Provided that a drill drills in an inclined concave, convex and generally uneven surface, it is necessary to reduce the feed by 50 % up to the full drilling completion. The same is accepted for drill exit after drilling a hole.

At the re-drilling of a pre-drilled hole the diameter of pre-drilled hole must not be larger than ¼ of drill diameter. Otherwise there is a risk of drill deﬂ ection (pressing off).

At drilling of a hole with perpendicular or inclined axis towards the axis of another hole, the diameter of drilled hole must not be larger than ¼ of drilled hole diameter. In the course of drilling it is necessary to reduce the feed by 50%.

DEFINITION OF BASIC CONCCUTTING GRADES PRAMETCHOICE OF TURNING TOOLCHOICE OF MILLING TOOLCHOICE OF DRILLINGWEAR OF CUTTING INSERTSGRADE GROUPS EQUIVALENT TABLES

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6.5 Use of cutting ﬂ uids at drilling with drills with cuting inserts

At drilling of a through hole in a rotating workpiece by the stationary drill, a small disc is created after drilling completion;

it springs out by a high speed. For the safety reason it is necessary to cover up the working spot.

Important notice!

An extreme power and thermal edges load of indexable cutting inserts and a large quantity of chips which are generated in a closed compartment set above all high requirements for the quantity and pressure of a supplied cutting ﬂ uid.

The cutting ﬂ uid supply in a sufﬁ cient quantity is a necessary condition for the reliable function of drills with cutting inserts.

The most important function of the cutting ﬂ uid during drilling is the removal of generated chips from the cut place and furthermore lubricative and cooling functions.

Water emulsions of emulsifying oils upon the petroleum basis are recommended for using as cutting ﬂ uids; furthermore, half-synthetic or synthetic emulsifying oils with usual concentration of 3 ÷ 5%.

The cutting fl uid supply in a suffi cient quantity and the pressure of cutting fl uid are necessary conditions for the reliable function of drills with cutting inserts; the cutting fl uid is usually supplied directly into the cut place.

The quantity of cutting ﬂ uid and its pressure depend above all on the drill diameter, thus of the diameter of a hole to be drilled, and consequently on the material volume which is removed within unit of time; furthermore on the depth of drilled hole, on the drill position (drill in horizontal or vertical position) and on the function of chip former on the cutting insert. All these technological factors have above all the inﬂ uence on the chip disposal from the cut place.

Naturally, another no less important factor is the inﬂ uence of properties of machined material. The recommended guide values for quantity of supplied cutting ﬂ uid Q l/min and pressure P in MPa are given in the following Table.

Drill diameter Dc [mm]

Cutting ﬂ uid quantity Q [l.min^-1] Cutting ﬂ uid pressure P [MPa]

Drill length Drill length

2,5 D 3,5 D 2,5 D 3,5 D

16 ÷ 20 20 28 0,25 0,36

21 ÷ 25 21 30 0,24 0,35

26 ÷ 30 22 31 0,23 0,34

31 ÷ 35 25 34 0,23 0,34

36 ÷ 40 28 36 0,23 0,34

41 ÷ 45 30 38 0,22 0,33

46 ÷ 50 32 40 0,22 0,32

51 ÷ 55 35 42 0,22 0,32

56 ÷ 58 37 45 0,22 0,31

That is impossible to drill a sheet-metal pack by this type of tool. !!!

DEFINITION OF BASIC CONCEPTSCUTTING GRADES PRAMETCHOICE OF TURNING TOOLCHOICE OF MILLING TOOLCHOICE OF DRILLINGWEAR OF CUTTING INSERTSGRADE GROUPS EQUIVALENT TABLES

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6 Drilling

6.6 Drills with cutting inserts - troubleshooting

These values are valid for drills in the horizontal position. For drills in the vertical position it is necessary to increase the quantity and pressure of supplied cutting ﬂ uid by 40%.

At a good function of chip former, the cutting liquid quantity and pressure can be reduced by 20-30%.

On the contrary, when the chip generation is bad and there is a risk of crowding the grooves for chip disposal, the cutting ﬂ uid quantity and pressure must be increased by 40 ÷ 50%.

At assessment of the right chosen cutting liquid quantity and pressure, its cooling impact cannot be forgotten. A large heat quantity generating by mechanical energy expended for drilling should be reliably taken away by cutting ﬂ uid.

The ﬂ owing chip should not be coloured as a result of heat. Provided that the ﬂ owing chip has a blue shade or is straw--coloured, it is necessary to increase the cutting liquid quantity and the pressure. Otherwise there is a risk of reduction of edge longevity and drill body life.

It means in general that with increasing drill diameter the recommended quantity Q increases and recommended cutting liquid pressure slightly falls.

Problem Problem remedy

1. Low performance of machine driving motor (low twisting moment at spindle)

a) Reduce the cutting speed – reduce the spindle revolutions b) Reduce the feed

2. Excessive wear of edge of peripheral cutting insert

a) Reduce the cutting speed

b) Choose more wear-resistant insert grade c) Increase the cutting liquid volume and pressure

3. Crumbling – fragile failure of peripheral insert edge

a) Reduce feed during drilling (especially at an uneven entrance workpiece surface)

b) Choose a tougher insert grade c) Reduce the cutting speed

d) Choose another geometry of chip former

4. Crumbling – fragile failure of internal insert edge

a) Choose a tougher insert grade b) Reduce the feed during drilling c) Check the drill and workpiece clamping d) Choose another geometry of chip former

5. Continuous, badly formed chip

a) Increase the feed

b) Enhance the cutting speed and reduce the feed c) Choose another geometry of chip former

6. Crowding of short chips in disposal grooves

a) Increase the cutting ﬂ uid quantity and pressure b) Reduce the cutting speed

c) Choose another geometry of chip former

In case that a Table with recommended values Q and P is not available, a very approximate rule is valid that the cutting liquid quantity Q in l.min^-1. should numerically correspond to the drill diameter Dc in mm.

DEFINITION OF BASIC CONCECUTTING GRADES PRAMETCHOICE OF TURNING TOOLCHOICE OF MILLING TOOLCHOICE OF DRILLINGWEAR OF CUTTING INSERTSGRADE GROUPS EQUIVALENT TABLES The following Figure speciﬁ es the wear types of the edge according to the standard ISO 3685 together with the identiﬁ cation of their characteristic dimensions.

Time relationship between the ﬂ ank wear and face wear is displayed in the following Figure.

Kf = distance of crater wear margin KB = width of crater wear KM = distance of crater wear centre KT = depth of crater wear Sectional view A-A

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7.1 Types (sorts) of wear

DEFINITION OF BASIC CONCEPTSCUTTING GRADES PRAMETCHOICE OF TURNING TOOLCHOICE OF MILLING TOOLCHOICE OF DRILLINGWEAR OF CUTTING INSERTSGRADE GROUPS EQUIVALENT TABLES

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7 Wear of cutting inserts

In a concrete case of machining there usually occur several wear types in parallel; but their growth with the machining time is not in progress with the same intensity. According to the machining conditions, one of the present wear types usually reaches a higher intensity in comparison with the others and it becomes decisive for the tool blunting and consequently it limits the tool life.

For a certain tool couple tool material-workpiece material, the prevailing edge wear type is above all dependent on the applied cutting conditions, especially on the cutting speed and the feed.

The dependence of the prevailing wear type on the feed f and cutting speed v is illustrated in the following Figure.

The wear type caused by abrasion of ﬂ owing hard components of built-ups, prevails at the lowest values of cutting speed and feeds when there is a built-up edge. With increasing the cutting speed and feed, the cutting temperature is increasing;

fi rst the fl ank wear becomes the prevailing wear type, furthermore the cratering, then the oxidation of incidental fl ank close to the tip and fi nally at the highest cutting speeds and feeds it comes to the cutting edge plastic deformation which practically indicates the exceeding of limit cutting values.

At choosing feeds it is necessary to maintain the limit values depending on the angle insert’s nose r and radius of the nose curvature rε.

In addition to the mentioned wear types which to a large extent occur and proceed regularly, it comes at carbide tools very often to a mechanical edge failure either in the form of edge crumbling or a fracture of a part of edge or of the whole cutting insert.

These types of tools blunting arise especially doe to a strong mechanical stress of the edge (i.e. the impacts during inter-rupted cut or as a consequence of preceding edge disruption due to thermal impacts). A fragile edge failure also occurs very often when the machined material contains hard inclusions (sand etc.).

Mechanical edge damage is a type of blunting which occurs accidentally. It can occur at a sharp tool in the course of cut beginning as well as at a tool with a certain wear grade. Substrates of cemented carbide with high amount of cobalt, which increases their toughness, are more resistant to mechanical damage.

7.2 Mechanisms of wear formation

From the point of physical nature, the wear of tool edge due to abrasion is a result of the whole complex of effects including chemical and mechanical processes that proceed in contact surfaces with machined material and they fade into one anther and overlap.

Two types of phenomena, namely mechanical and chemical ones, characterize the mechanism of a tool wear.

At the mechanical wear type, it comes to the failure of surface and face by the impact of chip ﬂ owing off and workpiece material in the cut area without any change of chemical composition of these surface layers of the cemented carbide.

On the other hand at chemical wear type, to a large or small extent it ﬁ rst comes to the change of the chemical composition in the surface layer of tool material in the contact place chip-face and ﬂ ank-cut area. By this change, the mechanical properties of surface layers of tool material are usually worsened and consequently also their resistance to wear due to abrasion. In other cases it comes to a direct diffusional dissolution of structural components of the cemented carbide.

DEFINITION OF BASIC CONCCUTTING GRADES PRAMETCHOICE OF TURNING TOOLCHOICE OF MILLING TOOLCHOICE OF DRILLINGWEAR OF CUTTING INSERTSGRADE GROUPS EQUIVALENT TABLES

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abrasion

mechanical wear types adhesion

diffusion

chemical wear types oxidation

Abrasive wear is a mechanical wear type. Microscopic, very hard parts cut the tool material similarly like abrasive grains at grinding. This wear type depends on the total tool path with respect to the workpiece, on the shape, amount and occurrence frequency of abrasive particles and their hardness.

Adhesive wear is an abrasion caused by the adhesion effect (formation of micro-welds) between pure metal surfaces of cemented carbide and machined material which come each other in contact on the ﬂ ank and on the face.

Oxidation wear at higher cutting speeds, some components of cemented carbide react at higher cutting speeds either with the air from ambient atmosphere or with the cutting liquid which substitutes the air environment, or eventually with the machined material.

Diffusion wear the atoms of tool or workpiece material diffuse one another and create on the one hand solid solutions and on the other hand chemical compositions, whose properties differ from the properties of the initial tool material.

the hardness of tool and machined material (HSK/Hobr) under conditions which exist in the contact, the physical-chemical wear is above all dependent on the temperature of contact place and on the mutual chemical activity of both materials irrespective of the hardness ratio.

Processes, which directly lead to the edge wear, can be divided by the following manner:

Under certain machining conditions, all processes do not participate in the total wear alike. For a certain couple machined material–cemented carbide, the one or the other process can prevail (according to the machining conditions).

The decisive factor, which determines the prevalence the type of wear process, is the contact temperature of the toll with the workpiece.

DEFINITION OF BASIC CONCEPTSCUTTING GRADES PRAMETCHOICE OF TURNING TOOLCHOICE OF MILLING TOOLCHOICE OF DRILLINGWEAR OF CUTTING INSERTSGRADE GROUPS EQUIVALENT TABLES

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7 Wear of cutting inserts

7.3 Some undesirable edge wear types and recommended measures for their removal

Provided that some undesirable problems with application of tools with cutting inserts occur, for instance undesirable or excessive edge wear, worsened surface roughness, bad chip forming or vibrations, it is necessary to respect the following speciﬁ ed recommendations.

It is one of the main criteria for characterization of the indexable insert operating life.

It originates as a result of wear mechanisms on the tool. Its impact (intensity) can be only reduced.

Recommendation:

- Apply a tougher cemented carbide grade.

- Use a coolant or increase the cooling intensity.

- Reduce the cutting speed.

- Increase the feed if it is smaller than 0,1 mm.rev^-1 (at MTCVD coated grades).

In document Prirucnik (Page 82-89)