The machining process : cutting

Transcription

1 The machining process : cutting Citation for published version (APA): Hutchins, P. (1988). The machining process : cutting. (TH Eindhoven. Afd. Werktuigbouwkunde, Vakgroep Produktietechnologie : WPB; Vol. WPA0638). Eindhoven: Technische Universiteit Eindhoven. Document status and date: Published: 01/01/1988 Document Version: Publisher s PDF, also known as Version of Record (includes final page, issue and volume numbers) Please check the document version of this publication: A submitted manuscript is the version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. People interested in the research are advised to contact the author for the final version of the publication, or visit the DOI to the publisher's website. The final author version and the galley proof are versions of the publication after peer review. The final published version features the final layout of the paper including the volume, issue and page numbers. Link to publication General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. Users may download and print one copy of any publication from the public portal for the purpose of private study or research. You may not further distribute the material or use it for any profit-making activity or commercial gain You may freely distribute the URL identifying the publication in the public portal. If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the Taverne license above, please follow below link for the End User Agreement: Take down policy If you believe that this document breaches copyright please contact us at: openaccess@tue.nl providing details and we will investigate your claim. Download date: 11. Mar. 2019

2 THE MACHINING PROCESS CUTTING wi- BY PATRICIA HUTCHINS WPA 0638 OCTOBER, 1988 Eindhoven University of Technology Eindhoven, The Netherlands 28, October, 1988

3 THE MACHINING PROCESS : CUTTING According to J. Kaczmarek, the process of cutting used in the machining of materials is done with cutting tools. These tools have surfaces called the rake face (or cutting face) and the flank (Fig 1.). In the cutting process, wear takes place on the cutting tool. Kaczmarek describes this result as the wearing action of a tool, the ability to lose volume and mass gradually during the cutting process(5:76). Thomas J. Drozda and Charles Wick discuss that the wear of the tool may be caused by abrasion, adhesion, diffusion, chemical processes, or oxidation(2: ); Kaczmarek adds to this list mechanical forces(5:75). According to Drozda and Wick, abrasion occurs when hard particles from the chip slide on the tool face and remove tool material. Adhesion occurs when two pieces of material are brought together by high temperature and high pressure. Diffusion occurs when atoms move from an area of "high atomic concentration" to an area of low atomic concentration. The chemical processes that can cause wear begin when the tool and workpiece are used "in an environment of suitably active chemicals." Oxidation results at very high temperatures which cause the structure of the tool to weaken{2: ). The mechanical forces that lead to wear mentioned by Kaczmarek, may be mechanical abrasion, ultimate strength wear, or fatigue strength wear{5:75). As said earlier, in diffusion, atoms move from one lattice point of high atomic concentration to one of low atomic concentration, and Paul H. Black points out that solid-state diffusion is a cause of flank wear and crater wear in cutting tools(i:127). Drozda and Wick define flank wear as a form of abrasion that occurs between the workpiece and the flank of the tool (Fig 2.). The rate of flank wear increases as the cutting speed increases.

4 2, L ~_~_~,,::::_,..-..,, fig. 't. c.~oss S"Er..""(loN of \AlOU,", t"~f!>\oe j1iol S Kco"",AlC> J:LANt. w'''a.. A14D c.r"~ll. IIVttAt\. O"S~,p L.4HE " ORIG-INA" ou"t"l'ne. Also shown in Fig 2., is crater wear, defined by Drozda and wick as a combination of adhesion, abrasion, and chemical diffusion that occurs on the rake face of the tool(2:3.1-2). According to Black, as the chip moves over the rake face, it rubs on the face of the tool, thus creating a crater; Black believes the following method is how a crater is formed: (l) "As the chip glides over the cutting face of the tool, it is at high temperature as a result of heat generated at the shear plane and frictional heat from its passage over the tool face. Local adhesion of a pair of contacting asperites will take place in the absence of an absorbed film, and the pair of asperites will be temperature-welded together to form a junction. Continued movement of the chip will cause failure of the junction at its weakest point and a particle of the chip is transferred to the tool. (2)"Diffusion will now occur between the transferred particle of the chip and the adjacent particle of the tool. Conditions of high temperature and large diffusion gradient at the junction are favorable to diffusion and allow migration to the chip at the expense of the lo~alized region of the tocl, so that the tool material adjacent to the transferred chip particle becomes weakened. (3}"The particle, which may have accumulated additional chip material, will now be 'wiped off' the tool face by the oncoming chip. Since the weakest section of the particle, on account of diffusion, now lies within the surfaces of the tool, when the particle leaves the tool face it will carry with it a microparticle of the tool."(1: ) Drozda and Wick ref ere to the resislance of any of these wear mechanisms, so that the cutting tool will have a long life and it will keep its shape and cutting effeci~ncy, as a requirement of the tool(2:3.1). Kaczmarek defines the tool life as the "working time of the tool point, or cutting time under constant cutting conditions until conventional dulling of the tool point [the loss of the tool's capacity to machine workpieces under given conditions] occurs." He also explains that as the cutting speed is varied within a wide range, the tool life may be considerably increased or decreased(5:82).

5 3 John L. Feirer and Earl E. Tatro describe the three kinds of chip that may be formed during the cutting process. These kinds of chip are the discontinuous chip, the continuous chip, and the continuous chip with a built-up edge. The discontinuous chip, or broken chip (Fig 3.), is a small chip produced when the workpiece is compressed in front of the cutting tool until a piece of the workpiece is broken away. The discontinuous chips are each individually separate. a. b c d e f \ ~ ~. Howa discontinuous chip is formed. The edge of the cutting tool in a and b is compressing the metal. The chip in c and d is breaking away from the metal, and the chip in e is now completely separated. The continuous chip (Fig 4.) is produced the same way as the discontinuous chip, except the piece of workpiece that is broken off is continuous, ie. not separate pieces(3:8-10). f\~ 4. A continuous chip is H ideal way of machining metal.

6 4 According to Black, because of little time for the chip to aquire a film of oxide from the atmosphere, the newly formed chip will scrub the surface of the cutting tool so that "asperites of the tool and chip will be in contact at high temperature and pressure", and as a result, particles from the chip may adhere to the surface of the tool. Other particles roay then combine with those from the chip that have adhered to the tool, creating a built-up edge (Fig 5.). 'F \ G. ~. A continuous chip with built-up edge results in a roughe surface. Because the particles of the chip have more time to adhere to the tool at low cutting speeds, the built-up edge forms more quickly. In the machining at high cutting speeds, friction at the tool-chip interface is lower, resulting in slower formation of the built-up edge. High cutting speeds are therefore favored because of low wear and because of a good finish of the surface. The built-up edge damages the workpiece in two ways: (1) "By the change in shape of the active cutting edge due to the pressence of the built-up edge (2)"By the presence of fragments dislodged from the built-up edge as they pass into the tool-workpiece interface."(1:ll7) To show the relationship between Wear of a cutting tool and time, five tool ~aterials witt their corresponding diffusion coefficients and predicted wear rates, given by P.O. Hart~ng and B.M. Kramer, were used(4:77). The equations for the time of diffusion and wear length are the following: Time of Diffusion t: 1- t = x 2D

7 5 where x is the given distance, coefficient. and D is the diff~si~n Wear Length L: L = 1 R ( 2 ) where R is the predicted wear rate. A graph of length vs. time was plotted to compare the results (Fig 6.). A second graph was also plotted using LOG values of the first graph as another form of co~parison (Fig 7.). As can be seen in Graph 1., a linear relation can be obtained from the five materials, excluding WC. Likewise in Graph 2., a linear r2lation between ZrC, TiC, NbC, and VC can be obtained with WC resul~ing in an unlike manner. Values of in Table 1. the results of Eq~ations (1) and (2) can be seen "\ r '-' J( we. t-r- C 1 \ i.'-\ ~ i, i) ~ ') - (..; d ~ 8-:. v J /0 - - q-; - ~ \; I 11 I( j :J " ~ 10 '-\ ; I~ ~ Y :;) '" I 'J \.it.10\ q Co a t;; "! I, Y " x l":; 1'-'! Q. "\, ') i.) J ~ ~ele- \. Q.ESVLTS Df! "Tl'E E ~~I"NS OF "'~f,'m, or \\t=fvslojol "... 0,.".. Wf "12. It "'6rT~. '" 0,; 1&

8 :i;::::l:~:j::~:~~':: ::T~:"C!:=:l::::e~:::~:J.,.::::':'.....':!-,.e: 1. ~... ~= ;::t=::: T::::: :':T:C: '=S::'::: :/" :>~: ::=:L-=:.:... 1: ' :=:~:::... '.':.. (:...=-."1

9 [- - - n L i r- f-----'---~---+_----l--_t_--+_--+, t----l-----c-+_- t _-!----c-_+_~, i - ---r--- --, I : ; 't : ~ l l----j:-...,lb~_i_ ---.:_-+_..:.:.'-..:C+-,-~---.:-+---.:_= ' t-l---':_-'-l-' ~---_-_'-_:_-:-+t-r_-~;~~-l--:l~. "'::_:1-1:-_ _.-...:-_...:-~-l_r_--_ ~ _-...:._:+_;~---...:-_ _-~-'-i--'---f :--.!-'~ ~--- i 1----" ----'1-- _'._.-~!_' il~,'. '+'---+-'--::":"'-l--'---'---l--'--'--4--'---'--f---+--'. '-_:~~-_-_-...:!_-_-_-...:-_C-_'.l-j-_'-_-_ -_-_-_l:..-'...:1)...; _' ----:--... ~ _~ ' l --f ' i _. -'- ---: ~~ ~.._~--...:..j-i~... ~----+-'---=--'--~...:----.: :-'-l--~.:-l: i i.1;., --! ' --jl_ -_-_-._:-_-_'-1+1 '-w. -~+~-:-_l-~-i-::i '-... L-.~. I 'j ~ ~--~~~---~----~----'--- 1 I ~--+---'-'----'-+-~-+-tli~-...:.:.:...:...:+--'-:...:...:~~-+-..:..:..:...-'--t-..:..:..:...~~~...:---.:+--'-:..:.:...:-=+-'--'--'-+--'-'--". --CLi- - ~- -t--...::~:...:-l-~~l --r ~ T"j t-: -..,--t----:-----+~.~+_-'-'-_j...:...:_'_'+_~'--+~~~:...:...:+ '_'-----_j...:...:.:.:r_:-'-'--'--'--=.~f '----'--- +i_, :----1 ~ -~! I ~ : j h-- L--t "t-... -~r ~_+_.J -..:~~-.--.l-.-.--l. ---<., I. r. I ;,._.~:1 ~---:--i r-~r :+~~~~~~~+ :~~~~--+--_rr f r ~-+_~~~f!~u~~.. ~.. ~-~----+-~ ~ r -.~ + I ~j~_ u.1 I r 1 : i ' '-----'"-- --i'! I -I i I -' : 1 ~-...:.. ", I n --.-:_

10 LIST OF REFERENCES 1. Black, Paul H. Theory of Metal Cutting, McGraw-Hill, IJ.Y Drozda, Thomas J. and Charles Wick. Tool and Manufacturing Engineers Handbook. McGraw-Hill, Michigan, Feirer, John L. and Earl E. Tatro, Machine Tool Metal Working, McGraw-Hill, N.Y Hartung, P. H. and B. M. Kramer. Tool Wear in Titaniu~ Machining. 5. Kaczmarek, J. Principles of Machining by Cutting, Abrasion, and Erosion, Wydawnictwa Naukowo - Techniczne, Warsaw, 1976.