The influence of coating and tool geometry on the tool life in a thread cutting process.

Benga, Gabriel ; Ciupitu, Ion

1. INTRODUCTION

Internal thread cutting is a very demanding operation taking into account the possibility of damaging both the tool and the workpiece material. Usually the thread cutting is performed in the late stage of the manufacturing process and therefore a major investment in labor cost, energy and material is already done. Thus a breakage of the tap can significantly decrease the productivity of the process (Veldhuis et al, 2007). Recent improvements regarding the tool life of cutting tools have been achieved by the development of titanium aluminum nitride TiAIN coatings. Coatings as TiAIN display a very interesting combination of properties, including high hardness at elevated temperature together with thermal and chemical stability, as well as low thermal conductivity (Fox-Rabinovich et al, 2004; Benga, 2008). TiAlN offers superior performance for a range of metal machining and fabrication applications. The reason for this lies in the addition of aluminium to TiN or specifically the formation of aluminium oxide on the surface of the tool which increases the operational temperature range of the coating to 800[degrees]C compared with 500[degrees]C for TiN. As the TiAlN coating is heated in air a thin layer of amorphous aluminium oxide forms on the surface of the coating, which protects the coating from further oxidation. This results in better hot hardness than most other coatings (Iwai et al, 2001).

2. EXPERIMENTAL PROCEDURE

The work piece material used was AISI P20 hardened mold steel with a chemical composition presented in table 1. The dimension of the work piece material was 150mm x 150mm x 30mm and the hardness of each block was 35 [+ or -] 2HRC.

The tests were performed on an Okuma Cadet Mate CNC vertical milling center and the tool wear was measured using a Mitutoyo optical microscope. As a wear criterion was chosen the [V.sub.B] flank wear [V.sub.B] = 0.3 mm.

The cutting regime employed was as follows:

--cutting speed: 8 m/min approximately 260rpm;

--feed rate: 1.587 mm/rev;

--length of thread: 10 mm.

The holes tapped were through holes in order to facilitate the chip evacuation.

Three different tap designs were employed for the tests as follows: normal taps, spiral taps, pointed spiral taps.

The taps are presented in figure 1.

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The cutting tools were made of hss and then coated using PVD technique with TiAlN. The diameter of the taps was 3/8" which approximately means 9.5 mm.

3. RESULTS AND DISCUSSIONS

The tests were performed initially using both coated and uncoated taps of each type and the wear was measured after 4,5 holes tapped. At the beginning the tool was unclamped after each hole in order to measure the wear but it was observed that the wear does not advance so fast. The following figures present the wear patterns for each tap.

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Figures 2 and 3 present the wear patterns when normal tapes were used. In the case of the HSS tap it is obvious that breakage occurred on one flute of the tap after tapping 20 holes. A possible explanation in this case is that the wear did not occur mainly due to the friction between the tool and the work piece material but due to the cutting load developed during the internal thread process which caused breakage of the tool. The wear pattern when the TiAlN coated tap was employed is different and seems to be the result of the abrasive wear.

This happens due to the TiAlN coating which does not exhibit edge brittleness and can be used for interrupted cuts without chipping. The use of TiAlN has increased the tool life by 1.35 times taking into considerations the number of holes tapped.

Figures 4 and 5 show the wear pattern for the spiral pointed taps. In the case of the uncoated tap (fig.4) the dominant wear mechanism seems to be chipping of the edge. There is no breakage like happened with the uncoated normal tap but chipping of the edge is present. This can be due to the vibrations occurred during the thread cutting process combined with the cutting load.

The TiAlN spiral pointed coated tap develops a regular wear on the clearance face. Ag ain the presence of TiAlN coating improves the toughness of the tool and the wear is produced mainly because of the abrasion between the tool and the work piece material and the delaminating of the coating layer. The use of TiAlN in this case results in a tool life improving of about 100%. Anyway comparing with the performance of the normal tap, the tool life foe spiral pointed tap is not very good, being even lower in the case of the uncoated tap. A possible explanation could be the fact that the number of flutes is lower (3 flutes for pointed spiral tap) than for normal tap (4 flutes for normal tap) and therefore the number of channels for chip evacuation is lower as well. As a consequence the chip evacuation is worse and this may cause chips to be blocked and even tool breakage. There is no significant difference between the tool life of the TiAlN normal tap coated and the TiAlN spiral pointed tap coated. This can be attributed to its nano layers and the high hardness of the TiAlN coating (Santos et al, 2007).

In figures 6 and 7 the tool wear of the spiral taps is presented. This taps were by far the best in terms of tool life. Regarding the TiAlN coated spiral tap, the tool life exhibited was 14 times higher than for coated normal tap and almost 12 times higher than for coated pointed spiral tap. This tool life can be attributed on one hand on the improving of chip evacuation and on the other hand on the very good tribological behavior offered by the TiAlN coating. The spiral flute taps are especially designed for machine tapping of blind holes. The right hand spiral flutes direct the chips back out of the drilled hole, minimizing clogging and tap damage at the cutting chamfer. In the following figures are presented several tool life graphics for the cutting tools employed.

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4. CONCLUSIONS

I. Irrespective to the tool geometry it was obvious that cutting tools coated with TiAlN have showed higher tool life than the HSS tools. The high hardness of this coating does not allow the appearance of an irregular wear. The abrasion wear and the delaminating of the coating layer appear to be the dominant wear mechanism of these cutting tools. Abrasion wear is caused by the friction generated between the tool flank and work piece material.

II. The tool geometry has a significant influence on the tool life. While there is no big difference of using normal taps or pointed spiral taps, the use of spiral fluted taps has dramatically improved the tool life. The spiral taps allow the chips to be directed out of the hole much more easily minimizing clogging and tap damage at the cutting edge.

5. REFERENCES

Benga, G. (2008). The influence of different PVD coating techniques on cutting forces in a drilling process, Proceedings of the 6th International Conference DAAAM Baltic Industrial Engineering, R Kyttner (Ed), pp. 407-412, ISBN 978-9985-59-783-5, Tallinn, Estonia, April 2008

Fox-Rabinovich, G.; Weatherly, G.C. Dodonov, et al (2004) Nano-crystalline filtered arc deposited (FAD) TiAlN PVD coatings for high-speed machining applications, Surface and Coatings Technology, Volumes 177-178, January 2004, pp. 800-811, ISSN 0257-8972

Iwai, Y.; Honda, T.; Yamada, et al. (2001) Evaluation of wear resistance of thin hard coatings by a new solid particle test, Wear, Volume 251, 2001, pp. 861-867, ISSN 0043-1648

Santos, J.A.B.O, Sales, V.F. et al (2007) Tribological evaluation of TiN and TiAlN coated PM-HSS gear cutter when machining 19MnCr5 steel, The International Journal of Advanced Manufacturing Technology, Volume 31, Numbers 7-8/ January, 2007, pp. 629-637, ISSN 0268-3768

Veldhuis, S.C., Dosbaeva, G.K., Benga, G. (2007) Application of ultra-thin fluorine-content lubricating films to reduce tool/workpiece adhesive interaction during thread-cutting operations Int. Journal of Mach. & Manufact., vol. 47, 2007, pp. 521-528, ISSN 0890-6955

Table 1. Chemical composition of the AISI P20 mold steel

Chemical C Si Mn Cr Ni Mo S
element
wt.%

Composition 0.31 0.4 0.75 1.2 0.8 0.41 0.008