Correlation between cutting forces and tool wear when thread tapping AISI P20 hardened steel.

Benga, Gabriel ; Ciupitu, Ion ; Stanimir, Alexandru 等

1. INTRODUCTION

Tapping is a chip cutting method with continuous cut where the material removal is performed via a cascaded order of radially cutting teeth. This process is done by feeding the cutting tool into the hole until the desired thread depth is achieved, then the tap is reversed back to take it out of the hole and removed from the workpiece material. Tapping is usually one of the last operations performed in a production work flow and this is the reason the tap breakage could be considered very expensive taking into account the whole amount of time, labour and energy involved in the process (Armarego & Chen, 2002). Moreover thread cutting with taps it comes mostly as the final form of machining and its accuracy and precision determines the quality of the threaded product. Unlike other machining operations, tapping has only one cutting parameter that can be chosen, i.e. cutting speed (Zeus, 2005). This is the reason that all other parameters such as cutting tool material or cutting tool geometry are incorporated in the tool itself. The tapping process raises even more challenges when hardened steel workpiece material is used. A material is considered hard when it reaches 36 HRC. This HRC number seems to divide soft from hard materials. According to Hazelton (Hazelton, 2007) a material with 36 HRC is considered hard because it elongates less than 10% at this hardness. Taps are a kind of cutting tools having a very complex cutting geometry. The tool geometry is very close related with the workpiece material hardness. For hard materials a high number of flutes are required in order to have an even distribution of the high cutting forces and wear rates. Not only the number of flutes but also the rake angle plays a significant role in machining thread in hardened steels. Reducing the rake angle results in increasing the stability of the cutting edge and allows a better chip formation. On the other hand diminishing the rake angle results in increasing of cutting forces and the tap is prone to a higher wear rate. Anyway the tapping of hardened steels requires a significant power and therefore the torque needed to tap a hole is also high.

2. EXPERIMENTAL SETUP

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 150 mm x 150 mm x 30 mm and the hardness of each block was 35+2 HRC.

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

The cutting regime employed was as follows:

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

--feed rate: 1.587 mm/rev;

--length of thread: 10 mm.

The cutting forces were measured using a dynamometer and software for data acquisition Labview.

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.

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

3. RESULTS AND DISCUSSIONS

In the following figures are presented the cutting forces for each type of tool used.

Figure 1 presents a classic tap with four flutes and the chips' shape obtained during the tapping process. The cutting forces analyzed were actually the thrust force, the torque and the resultant side force. These are usually the forces analyzed by other researchers as well (Armarego & Chen, 2002; Araujo et al., 2006). Analyzing figures 2 and 3 it can be observed that cutting forces have increased from around 60 N as an average to 340 N when the tool has reached 0.3 VB wear rate, which means an almost six time higher thrust cutting force. The wear patterns have been presented in a previous paper (Benga & Ciupitu, 2008). The breakage of the tool did not occur due abrasive wear but due to cutting load developed during the tapping process. The tool life was assessed in terms of the numbers of holes tapped, which in this case were 27.

The cutting forces and the torque are presented in figure 2 for a new tool and in figure 3 for a worn tool. Figure 3 present the values of cutting forces when one land of the tap was broke due to the high load applied

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Figure 4 shows a pointed spiral tap and the chips resulted when the tap cut the thread. It can be observed that the chips are bigger that those obtained with the classic tap. Figures 5 and 6 present the cutting forces obtained for this tap shape. Initially when the tool was new the average thrust cutting force was around 30 N and when the tool has reached 0.3 VB wear, right before the tool breakage, the cutting force measured was almost 100 N. This means a three time higher cutting force than initially.

The level of cutting forces was not as high as when normal tap was used. This can be explained by the shape of the pointed spiral tap which is designed especially to decrease the cutting forces and the load applied on the tap. The tool life for the pointed spiral tap was not very different from the one obtained with normal tap (32 holes). It should be pointed out that all taps were coated with TiAlN in the same conditions so only the geometry was responsible for the differences in terms of tool wear and cutting forces. As was mentioned in a previous paper (Benga & Ciupitu, 2008) the geometry with 3 flutes for pointed spiral tap offers only 3 channels for chip evacuation comparing with the 4 flutes and therefore 4 channels when normal tap is employed. In this way the evacuation of the chips is even lower and the tap tends to become clogged and then the cutting force is increasing dramatically and the tap breaks. This can explain somehow the lack of improvement in tool life despite the fact of having a lower cutting force load for pointed spiral tap during the cutting process.

In figure 7 a spiral tap and the chips obtained are presented. In this case the chips are longer than the chips obtained with the normal tap and the pointed spiral tap. From this point of view, tapping using normal tap provides the smallest chips.

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The thrust cutting force recorded for the spiral tap was initially for the new tool in the range of 20 N and when the tool has reached VB wear 0.28 mm the cutting force has increased up to 100 N while the average was around 50 N. While the cutting forces values were comparable with those obtained for pointed spiral taps, the tool life was significantly increased (380 holes).

4. CONCLUSIONS

I. Analyzing the cutting force values and torque it appears that tool geometry has a significant influence. When new cutting tools were employed the lower values for cutting force has been recorded for the spiral tap, followed by the pointed spiral tap and then the normal tap. This was actually expected taking into account that the pointed spiral tap and spiral tap are designed lower cutting forces.

II. The torque presented a slight variation with the wear rate irrespective to the tool geometry, unlike the thrust force which has increased dramatically right before the tool breakage. Anyway the tool geometry has an important influence on the torque value since the highest value of the torque has been observed for the spiral tap (60-70 N) comparing with normal tap (25-30 N) and pointed spiral tap (30 N).

5. REFERENCES

Araujo, A.C., et al. (2006). A model for thread milling cutting forces, Int. Journal of Machine Tools ^Manufacture, vol.46, February 2006, pp 2057-2065, ISSN 0890-6955

Armarego, E.J.A. & Chen M.N.P (2002). Predictive cutting models for the forces and torque in machine tapping with straight flute taps, CIRP Annals- Manufacturing Technology, vol. 51, pp 75-78, 2002

Benga, G. & Ciupitu, I. (2008). The influence of coating and tool geometry on a tool life in a thread cutting process, Proceedings of the 19th International DAAAM symposium, B. Katalinic (Ed), pp 91-92, ISBN 978-3-901509--68-1, Trnava Slovakia, 22-25 October, 2008

Hazelton, J.L. (2007). Tapping the hard stuff, Cutting Tool Engineering Magazine, vol. 59, No. 3, March, pp 1-4, 2007

Zeus, T. (2005). High performance tapping and thread forming, Proceedings of the Int. Conf. "Smart Solutions for metal cutting", EMUGE, pp 1-15, Aachen Germany, February, 2005

Tab.1. Chemical composition of the AISI P20 mold steel

Chemica C Si Mn Cr Ni Mo S
l element
wt.%

Composition 0.31 0.4 0.75 1.2 0.8 0.41 0.008