Continuous chip formation in drilling

doi:10.1016/j.ijmachtools.2005.03.011

International Journal of Machine Tools and Manufacture

Volume 45, Issue 15, December 2005, Pages 1652-1658

https://doi.org/10.1016/j.ijmachtools.2005.03.011 Get rights and content

Abstract

The formation of coil continous chips in drilling often leads to chip disposal problems. This paper investigates the drill chip formation process of continuous chips (spiral chips and string chips). Chip removing motions and forces are analyzed. Two models are developed to predict the spiral and string chip formation, respectively. Based on the level of bending due to the chip generation at the cutting edge and deflection by the flute, these models qualitatively investigate relationship of the point angle and flute helix angle on the average chip length. Drilling experiments validate the chip length for spiral and string chip predicted by both models.

Introduction

The success of a drilling operation primarily depends on the ability of producing chips that can be readily ejected from the drilled hole. Long chips are usually not desirable because the chips can tangle along the drill body which have to be removed manually.

Chip formation and chip breaking mechanism have been studied for many years. Starting with the Merchant chip formation model in 1945, many other studies have contributed to the understanding of this area. Although the level of understanding has been adequate, the results of past studies were based on simple cutting tools, which have the same tool geometry and cutting parameters along the cutting edge. There has been less research on complicated cutting tools, such as drills, whose geometry and cutting parameters differ along the cutting edge.

To simplify the drill cutting process, the drill cutting edge is conventionally treated as a summation of several small segments with each segment having a homogenous geometry and cutting parameters [1]. Very few studies have investigated the drill chip formation from the unique features of the drilling process.

Among the few works done for drilling chip formation, Kahng and Koegler [2] suggested that torque applied in the same direction as the chip rotation would break the chip easily. They also stated that the resulting friction torque opposed the rotation of the chip caused it to unfold. The chip would break until the strain produced reached the fracture strain. They further argued that the chip first generated was longer than the others because it encountered less resistance due to friction than subsequent chips, and the chips generated later tended to have interference with the slowed chips which were previously generated and thus were more susceptible to breakage.

Another work on ejecting helicoidal chips was reported by Sakurai et al. [3] who investigated the breaking mechanism of the continuous cone-shaped spiral chips produced during intermittently decelerated feed drilling. From this work, it was found that chip breakage occurred when the resulting friction torque between the hole wall and chip exceed the breaking torque of the chips.

This research focuses on the differences of chip formation in drilling as compared with the conventional cutting processes. The differences are:

A. Chip formation is not completed when chips leave the cutting edge. Chips will be further deformed as a result of the interaction of the chips with the drill flute and hole-wall.

B. Chip flow direction is restricted by the cutting speed difference along the cutting edge. Since the cutting speed is much slower at the point close to the drill center (inner cutting edge) than at the point close to the drill peripheral (outer cutting edge), initial chips are cone shaped and tend to flow to the drill center.

C. The chip–flute interaction is a combined effect of the drill point geometry (point angle) and the drill flute geometry (helix angle). Furthermore, this interaction will change the chip deformation after it leaves the cutting edge and result in different chip lengths and shapes.

Although initial drill chips are all spiral cones, they will change into various shapes when drilling deeper due to the interactions of the drill flute and hole wall. In this paper, analytical chip formation models have been proposed based on the chip–flute and chip–wall interactions for spiral and string chips. These models reveal that the average chip lengths of spiral chips and string chips are related to the drill point angle and flute helix angle.

Section snippets

Chip formation in drilling

Chip formation has been studied in great depth for turning and milling processes. These cutting studies have also been extended to the drilling process. One of the widely applied methods in studying the drilling process is to divide the cutting edge into several small elements and treat each element as a simple cutting tool. Although this method provides a simple solution, it neglects some unique features of drill chip formation.

Chip moving forces of spiral chips

The two major motions in the formation of spiral chips are the rotation motion on the chip axis and axial motion along the drill flute. Fig. 2 shows the forces of the spiral chip movement in the drill flute. These forces include the force resulting from the chip generation, the friction force from the flute F_f and the friction force from the hole wall F_w. The force from chip generation helps the chip to move up along the drill flute and rotate by itself. This force also tends to unfold the chip

Chip moving forces for string chips

The effect of active forces F_wf and F_wn is determined by the flute helix angle and chip moving speed v_cf. When flute helix angle is constant, the increase of chip speed v_cf will decrease F_wf and increase F_wn. In spiral ejection, because v_cf is small, for most cases F_wf can help the chip moving upwards. However, in string chip removal process, because chips unwind, the chip speed v_cf is much faster than that of spiral chips. The speed v_cf of string chips approximated by the chip forming speed at

Influence of drill geometry on spiral chip formation

Spiral chips are structurally determined to rotate on their own axis while moving upwards. This motion is only free when the drilled hole is shallow. As drilling progresses, chips come in contact with the drill flute and hole wall and more friction force will prevent the chip from moving. This is the case especially when a chip is bent by the drill flute. When the impediment forces reach a critical value, the chip will stop spinning and break. Thus, spiral chip length depends on the smoothness

Experimental verification for the spiral chip length

Experiments were conducted to verify the assumptions that the length of spiral chips will be shorter when |θ−β| becomes large since the interaction forces increase as the angle difference |θ−β| increases. In the experiments, 6.35 mm HSS drills with 1.56 mm web thickness were used to drill the workpiece of AISI 1048. Fig. 5 shows the chip flow angle with different point angles based on Eq. (9).

In order to create the angle difference of |θ−β|, 6 drills with helix angles of 24, 30, 38° and point

Influence of drill geometry on string chip formation

Spiral chip length modeling was based on the difference between the natural flow angle and the actual chip flow angle (flute helix angle). The same concept will be applied to string chip length modeling.

The difference between string chip modeling and spiral chip modeling is the definition of the natural exit angle. In spiral chip formation, a natural flow angle exists when the chip formed freely. However, because string chips are not a natural form, the natural flow angle does not exist during

Experimental verification of string chip length

In order to compare the effect of different θ−ψ, three drills were used in tests. As listed in Table 2, drills 1 and 2 have the same helix angles, 39°, and different point angles. Drills 1 and 3 have the same point angles but different helix angles. All of the drills are 6.35 mm HSS with web thickness of 1.56 mm. In the three tests, the workpiece (AISI 1040) was drilled to the depth of 35 mm with rotation speed of 1000 rpm and feed speed of 75 mm/min. Each test was repeated three times. The chips

Summary

This paper examined the chip formation mechanisms of spiral chips and string chips. In this study, the interactions from drill flute and workpiece on chip forming and evacuation were analyzed. Also, this study proposed chip length models for both spiral chips and string chips based on the difference between the natural chip exit angle and the actual chip exit angle. Based on these models the chip forming smoothness can be revealed and the average chip length can be predicted qualitatively.

References (4)

A.R. Watson
Drilling model for cutting lip and chisel edge and comparison of experimental and predicted results—I. Initial chip lip model
Int. J. Mach. Tool Des. Res.
(1985)
C.H. Kahng et al.
A study of chip breaking during twist drilling
SME Trans. NAMRC
(1977)

There are more references available in the full text version of this article.

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