Machining hard materials with geometrically defined cutting edges field of applications and limitations

Machining Hard Materials with Geometrically Defined Cutting Edges - Field of Applications and Limitations W.. Hitherto, a grinding process has generally been used to finish materials w

Machining Hard Materials with Geometrically Defined Cutting Edges -

Field of Applications and Limitations

W Konig (1) M Klinger, R Link;

Lehrstuhl fur Technologie der Fertigungsverfahren, WZL, RWTH Aachen/Federal Republic of Germany

Received on January 15,1990

Summary

The paper deals with means of reducing production time and costs by machining hard materials The cutting processes described are turning, milling, drilling and broaching Tool materials for these tasks require extreme hardness and high compressive strength combined with adequate high-tempera- ture resistance and a stable chemical structure Surface quality and accuracy-to-size are extremely high, dispensing with the need for finish grinding Keywords: Hard machining, CBN , carbides, ceramics

1 Introduction

Highly-stressed steel components are frequently hardened to increase their

strength and wear resistance Impermissible distortions must be expected,

particularly in the case of geometrically complex parts Where there are high

demands on workpiece quality, i.e on surface finish and accuracy-to-shape-

and-size, the part has to be finished in a highly-tempered or hardened state

Hitherto, a grinding process has generally been used to finish materials with

hardness values in excess of 60 HRC, but improved knowledge of processes

and the consistent exploitation of modern cutting materials now enable cutting

processes with a geometrically defined edge to he employed

For the first time, for example, the broaching of surface-zone hardened

components offers a means of finishing complex internal profiles at economic

cost The turning, milling and drilling processes are particularly advantageous,

since there are no shape restrictions on the corresponding tools, rendering

them suitable for a variety of machining tasks and workpiece geometries and

in many cases permitting complete processing in a single fixture

Higher machining rates as compared to grinding often allow a reduction in

production times and costs

saving through elimination of heat treatment This assumes, however, that the

material can be hardened directly using the forming heat

3 Chip Formation and Surface Integrity

In order to specify the potential and requirements of this new production technology in greater detail, it will be necessary to discuss the technological distinctions between hard machining and the machining of soft materials

This entails an initial analysis of the chip formation mechanism Owing to the negative tool rake angle, high compressive stresses are created both on the

cutting edge and in the material As a result, the material is parted by cracking

and plastification, and chips are formed (Fig 2)

The chip root in the figure represents a chip segment which has just been produced Owing to the brittleness of the material, the high compressive stress initially leads not to a material flow but to formation of a crack This crack releases the stored energy and thus acts as a sliding surface for the material seement, allowing the segment to be forced out between the parting surface

2 Alternative Production Sequences by Means of Hard Machining

In addition investment costs can be saved and door-to-door times shortened

by redesigning production sequences, as illustrated in the case of large roller

bearing rings (Fig 1)

Fig 2: Chip Forming of Care- Hardened Steel (63 HRC)

Simultaneously, plastic deformation and heating of the material occur at the leadingedge ofthe cutting tool.Once the chipsegment hasslid away, renewed cutting pressure results in formation of a fresh crack and chip segment The temperature increase needed for plastification of a small section of the chip material issupplied by the heat created in the cuttingprocess.The temperature may be calculated from the applied power, i.e the product of cutting force and speed, and the dissipated heat The latter is determined by the thermal conductivity of the tool and workpiece materials and of the ambient medium The individual chipsegments are linked by the small proportion of the material which is plastically deformed and heated to a high temperature A continuous

fig 1: Convewiond and optimhed producjon squnCes for (he P&ction

of Roller Bearing Rings

In this instance, the redesigned finishing process requires fewer production

steps than the conventional process, with the additional advantage of energy

chip is formed In most cases, the temperature required for rehardening on the

underside of the chip is created only by friction on the face

The characteristic chip formation process influences the residual stress state

of the machined surface This factor is of particular interest, since, in conjunc-

tion with accuracy-to-shape and roughness, it decisively determines compo-

nent behaviour Formation of the chip produces high restoring forces These

induce a compressive residual stress state in the workpiece surface zone

(Fig 3) An increase in feed raises the compressive residual stress maxima and

feed:

Fig 3: Residual S t m Curves of H a r d - T m d Won4piecec

deepens the affected tone This effect is largely attributable to the high

mechanical stress on the workpiece Feed force and friction-induced tempera-

ture risewith increasingwear,resultinginaresidual tensile stressin thevicinity

of the workpiece surface and an increase in compressive stress in the deeper

surface tone

4 Process-Specifie Requirements and Results

Important appraisal criteria for the finishing performance of a cutting process

are attainable surface quality and tool wear, which directly influences the

process forces and indirectly affects tool life It is therefore necessary to select

the optimum cutting material for each specific application

4.1 Turning

CBN-basedpolynystallinecutting materialsare now available for turning (and

also for milling and drilling) in a number of variants and compositions There

are significant differences between individual products in terms of hard ma-

terial component, intermediate phase and structure These affect wear beha-

viour and attainable surface quality for the different cutting materials An

additional question is the extent to which the behaviour of these cutting

materials differs from that of mixed ceramic in the fine turning of hardened

material

@

work material: 100 Cr 6, hardness 61 - 63 HRC

cutting speed: v, = 120 m/min

depth of cut: ap = 0.4 mm

f e d : f - 0.08 mm

no coolant

inseft

@

tool cuning edge anglex, - 48O

4' 4' 32' 1 6 m

tool life criterion

120

min

100

80

Ra - 0,s I-

e m

-

- 0

' 40

20

0

-

ceramic

Ii:,"~ialll CBN I I CBN with carbides

cutting chamfered chamfered chamfered chamfered

I edge 11 0,2x20° 11 sharp ~ 0 1 ~ 2 0 ~ I 1 0 0 5 ~ 2 0 ~ ~ 0 2 ~ 2 0 ~ I

In comparison to conventional CBN, which possesses good toughness charac- teristics, a new CBN-based cutting material with a high percentage of metal

carbide exhibits a 60 Yereduced thermal conductivity, in the same range as for

the mixed ceramics It also possesses higher edge strength owing to its fine- grained structure (F$ 4)

in fine turning tests on hardened 1OOCr6 roller bearing steel, this high metal-

carbide cutting material (indexable tip with soldered CBN comer) proved significantly superior to conventional CBN (solid indexable tip) and mixed

ceramic In a dry cut at existing machining parameters, the width of wear land

is approximately 50 9below that of the cutting ceramic used in the tests, with

a value of 0.15 mm after roughly two hours' cutting time At the same time, the required surface quality for the large roller bearing rings was not exceeded

In addition, a comparison between a chamfered and a sharp-edged cutting edge showed the chamfer to be unfavourable in terms of attainable surface quality For mixed ceramic, which is generally chamfered in order to stabilize the relatively brittle cutting edge, a 0.05 x 20" chamfer represents the closest approximation to a sharpedge

a

P

3 pm

' 0 3

z

r l

z 6

F 4

g 2

.i - 0

0 40 80 120 min 200

g 0.4

f mm { 0.1

g 0.04

g 0.01

- L

B

1 4 20 min 200

g 0.4

f mrr { 0.1

g 0.01

g 0.01 I I ' 1 '

- L

I I r C B N w i t h ~ ! b ~ ~ s

1 4 20 min 200

B mixedmamk I

cutting time tc

47.6 Hard Turning with High Sped Steel

achieved after tool lives of 120 minutes With mixed ceramics, cutting edge breaksuts lead to extremely rapid wear

A distinct influence on the wear state of the tool can be demonstrated metal- lographically in the case of surface zone influences in hard turning A change

of 2-3 p m in the surface zone, not observable when a cooling lubricant is employed, is detectable with a width of wear land of YBmu = 0.3 mm (Fk 7)

Fig 4: Hard Turning with CBN and Ceramic

Fig 7: Structural Change in the swfoce Zone of a Had-Turned Workpke

Fig 8 Milling Cmehardened Slots with CBN

4 2 Milling

Casehardened slots can also be milled with CBN In this case, however, use of

a cooling lubricant very rapidly causes cutting edge break-outs (Fig 8) Even

in a dry cut, end of tool life is determined by a break-out near the soldered

edge; wear is still extremely low at this stage A stable cutting edge, especially

in the comer zone, is, however a prerequisite for the successful use of CBN

In addition, high cutting speeds are necessary, entailing considerable machine

effort whensmall-diameter tools areused, owing to the high speeds of rotation

For the application d m i d above, carbides, which are cheaper than CBN,

can be used cost-effectively A lower cutting speed of vc = 100 d m i n can be

selected The best results are achieved with micrograin carbides without

cooling lubricant (Fig 9) No surface zone changes occurred with this material

0.2

mm

0.1

E

B I

0.05

0.02

om-

Fig 9: Increming Tool Life for the Milling of Cawhardened Steel Using

Mictvgrain Carbides

cutting length if

4 3 Broaching

Broaching is the most recently-introduced hard machining process using a

geometrically defined cutting edge Cutting edge stresses are similar to those

encountered in milling Owing to its great toughness, micrograin carbide has

proved to be especially suitable for broaching martensitically-hardened ma-

terials (Fig 10) 'Ihe high impact loading at cutting edge contact must be

OEIDU broaching leng(h

Fig 10: Broaching Cmehardened S h - Influence of Coating

allowed for by limiting the undeformed chip thickness to 1Opm A calibration piece without a lead is needed to improve accuracy-to-size A considerable reduction in wear is achieved by coating the cutting edges Cutting speeds of

vC = 5 -20 d m i n are generally used for broaching non-hardened materials

In order to attain the results outlined above, however, a cutting speed of at least 40 Wmin is essential This cannot be realized on the majority of existing broaching machines Under these optimized conditions, components with surfaces finished to grinding quality can be produced

4.4 Drilling The use of indexable tip technology in conjunction with CBN and adapted drilling tools hasenabled holes to be drilled in hardened steelswith workpiece hardnesses of approximately 62 HRC A modified indexable tip drill with negative wedge geometry has proven its value in this application It possesses

two rhomboid tips secured to the tool by means of countersunk screws The internally-cooled tool achieves drilling depths of up to 1.5 times its diameter

of D = 34 nun

Tool life behaviour when drilling differing steel materials was investigated Identical cutting parameters were selected to enable the influence of various hardness mechanisms and material structures on the machinability of the materials to be appraised (Fig 11)

c a s e - h a r d e n i n g s t e e l surface hardened mmensite maerial hardness : ca 62 HRC

depth 01 drilling without coolant

: I - 2 rnm

r e o l d w o r k s t e e m

1 I I i~ii-irdened martensite i

n i t r i d i n g s t e e l carbonltrided ca 0 5 mm sudace contents carLndes

n ! ! ! s 5 m -

-m -m

Fig I I : Influence of Maerial Structure on Drilling with CBN

Maximum tool life was achieved with a 16MnCrSE casehardening steel The depth of the purely martensitic casehardened zone was roughly 2 nun, and hardness measurements of the zone yielded values of up to 62 HRC at the workpiece surface Taking casehardening depth into account, a drilling depth

of 1 = 2 mm or 1 = 1.5 nun (31CrMo12) was selected for the surface-zone hardened materials This ensured that exclusively hard structures were ma- chined A drilling depth of 2 mm was necessarily also selected for the fully- hardened X 100CrMoV5 1 and X210CrW12 cold work steels At a cutting speed

of vc = 200 d m i n and a feed off = 0.02 mm, a tool life of roughly 100 mm was achieved with X1oocrMoV51 As carbide content and the proportion of coarse carbides increase, maximum tool life is reduced The wear criterion of

V B m u = 0.5 mm or cutting edge fracture is reached after only about 20 mm

in S6-5-2 high speed steel The machining of thin, ultrahard layers of 31CrMo12 nitride steel is problematical In this case, the hardness increase is produced by the incorporation of carbon atoms (casehardening or carburiza- tion) and nitrogen atoms (nitriding) in the metal lattice of the steel structure Owing to the specific parting mechanism involved in the machining of such structures, the negative cutting edge geometry may lead to spalling and break-

outs on the centre edge of the drill

cutting speed lo vC = Oat the centre are especially evident when drilling hard

materials The centre tip can withstand the high compressive stress only if

cutting speed IS sufficient For each material there will therefore be ;in opti-

mum cutting speed: the softening of the material caured hy the cutting rpeed

or cutting temperature will he adequate for the stahilityof the centre tip, while

the abrasive wear L'fl occurring on the peripheral tip remains minimal

Ilnder the conditions described above, high surface qualities and good con-

centricity values can he achieved for the drilling of fully hardened materials

(Fig 12) The required minimum cutting speed is dependent on the type and

Y

work material X 210 CrW 12 (ca 60 HRC)

cuning material CBN

tool inserted drill D = 34 mm

drilling depth I = 1 x D

leed rale I = 0 02 m m

cunlng edge geometry

{r %per k e n

E

m

z

300

200

100

0

275 285 300 350 cunlng speed (m.mini

Fig 12: Drilling f h ~ w r d SIC(,/ with CRN

formation of the material structure It is approximately vc = 2x5 m/min for

X210CrWI2 cold work steel At lower speeds the centre edge fails due to

hreak-out, while at higher values the abrasive wear on the peripheral tip rises

sharply Cooling lubricant is essential to cool the tool shaft and transport the

chips The lubricant affects wear, however, as demonstrated by the drilling of

Ih,blnCrS (Fig 1.3) No differences are detectable in the wear diagram with

wear values ranging from 0.54 to 0.56 m m after identical drilling lengths At

lower drilling depths, i.e for the removal of hard surface zones, a dry cut is

therefore feasible

cutting material CBN cutting s p e e d vc = 200 rnirnin

material 16 MnCr 5 E feed f = 0,02 rnrn

hardness c a 62 HRC width of wear land

drilling d e p t h I = 2 rnrn without coolant V h a x = 0 56 rnrn

V h a x = 0 54 mm

with coolant

mm 1989

Fig 13: Influence of Cooling Fluid on Cuffing Edge U k i r

In general the machining of thin, hard layers of the kind encountered with

carbonitriding is problematical The depth of hardnesr in 31C:r41(112 steel is

less than 0.5 mm Whisker reinforced ceramic fails after a few steps due t o

fracture This may be attributed to the stress relationship on penetration of the

hardened zone (drilling depth I = 1.S mm, Fig 14) The use of tough CBN

withaut carhides ir similarly characterized by extremely short tool lives

Carbide tools (Fig 1.5) are hetter suited to this application The figure presents

tool lives for the drilling of various curface-zone hardened materials Drilling

depth was 30 mm i.e machining continucd in the transitional structure and

the soft substrate structure after the hardened zone had been penetrated At

a cutting speed of 50 m/rnin and a feed off = 0.08 (douhlc cutting drill:f, =

0.04 mm), break-outs occurred at the corners after a drilling length of approxi-

2

m

>

material 31 CrMo 12 ( c a 60 HRC )

diameter D= 34 mm

drilling depth I = 2 rnm feed I = 1.5 rnm

cutting speed vc

'Cczat989

Fig 14: Cornpurism of Cutting Marerialr for llrilling 31CrMo12

mately 1.1 m Flank wear VAwas not measurable No damage was detected on

!he chisel edge Material-specific optimi7ation of the cutting parameters en- abled tool live\ for the drilling of this type of material structure with carbide- tipped drills of this kind to he prolonged

Fig I S : Drilling Sutfuce-Zone Ilardened Mnterinlr with Curhidc

5 Literature

;I: Ackerschott, G.: Grundlagen der Zerspnnung einsatzgehiirteter Stiihle mit geometrisch bestirnmter Schneide doctoral thesis, RITII Aachen

1989

i2: Nakayama K Arai, M., Kanda, T.: Machining Characteristics of Hard

Materials, Annals of the CIRP Vol 37/1/1988 pp 89-92

/3/ Momper, F.: Mischkeramiken und Bornitrici-Schneidstoffe I d - A n z

14/IYXX, s 26-29

/4/ Konig, W., Komanduri, R., Tiinshoff, H.K., Ackerschott G.: Machining

of flard Materials Annals of the CIRP, Val 33/2/1984 pp 417-427

?5! K(inig W Iding, M Link K.: Feindrchcn u n d Rohren gehirteter Stahlwerkstoffe IDR 1/89, S 22-33

:6: Abel R.: Harthearbeitung mit Schneidkeramik und Homitrid dima YIXX,

S SO ~ 60

17: Ohtani, T Yokogawa, 11.: The Effects of Workpiecc Hardness on Tool Wear Ch:ir:icteristics, Dull Japan Soc of Prcc Eng., Vol 22 S o 3 (Scpt

IYXX), pp 229-231,