Calculation of the Dimensions of Chip-Breaking Ledges on Drills for Deep Drilling
Abstract Generated abstract
This paper examines how to determine the width and depth of chip-breaking ledges on drills used for deep drilling, where effective chip fragmentation is essential for productive operation. Using face turning of steel bushings to model different sections of a drill cutting edge, the study measures temperature, chip shrinkage, and chip form under varied cutting speeds, feeds, ridge dimensions, point angles, and wear conditions. The results indicate that suitable ledge dimensions depend chiefly on chip shrinkage, which varies along the cutting edge through the effects of speed, feed, and temperature, while the tested point angle and moderate flank wear have little influence. The work verifies Igoshin’s chip-breaking calculation method under deep-drilling conditions and proposes formulas for selecting ledge dimensions that account for shrinkage variation along the cutting edge.
Full Text
Calculation of the Dimensions of Chip-Breaking Ridges in Drills for Deep Drilling
A. V. Vodopyanov
(Presented by the joint scientific seminar of the Departments of Machine Tools and Metal Cutting and Mechanical-Engineering Technology)
One of the main factors determining the possibility of carrying out the process of deep drilling with high productivity is the shape of the chip that is formed. To obtain the most favorable chip shape, in the form of small pieces, chip-breaking ridges are ground on the rake face of the drill cutting edge.
The existing analytical dependences for determining the width and depth of the ridge, as a rule, do not take into account the operating conditions of the various sections of the drill cutting edge and are not connected with the characteristics of the cutting process.
The present work sets forth the main results of a study of the chip-breaking process in deep drilling, based on an investigation of the regularities of the cutting process at each section of the external cutting edge of the drill.
The experiments were carried out by modeling the process of deep drilling by face turning of bushings with outside diameters of 35, 29, 23, and 17 mm, using sulfofrezol cutting fluid.
The cutting conditions were as follows: cutting speed on the bushing with an outside diameter of 35 mm, \(V = 88 \div 137\) m/min; in this case the rotational speed varied from 800 to 1250 rpm, feed \(S = 0.097;\ 0.15\) mm, depth of cut \(t = 3.6\) mm.
The geometry of the drill cutting part was as follows: drill point angle \(\varphi = 0,\ 10,\ 20^\circ\), rake angle \(\gamma = 0^\circ\), clearance angle \(\alpha = 10^\circ\), ridge width \(a = 1.5 \div 2.1\) mm, ridge depth \(h = 0.65 \div 0.9\) mm.
Steel 45 was selected as the work material. The investigation was carried out in two stages. At the first stage, the temperature and chip shrinkage at various sections of the drill cutting edge were determined during operation without chip breakers.
The results obtained showed that the temperature and shrinkage are not the same at different sections of the cutting edge.
The regularities of variation of these characteristics of the cutting process as functions of cutting speed and feed are analogous to the turning process.
Cutting speed and feed affect shrinkage, since they affect temperature.
At temperatures below 600°, shrinkage increases with increasing cutting speed and feed. In the temperature range above 600°, shrinkage decreases when the cutting conditions are increased.
At the second stage, the chip-breaking process was studied. The selection of the ridge dimensions for each section of the drill cutting edge was carried out as follows. A ridge of constant dimensions along the length of the edge was ground on the rake face of the cutting edge. Then end-facing of bushings of various diameters was performed. In each experiment, the chip shape and temperature were recorded, and chip shrinkage was determined. On the basis of the experiments performed, ridge dimensions were selected that ensured normal chip breaking.
As the results of the investigation showed, the ridge dimensions depend not only on the feed, but also on the cutting speed. The influence of cutting speed on the chip-breaking process at constant feed is manifested through chip shrinkage.
Comparing the nature of the change in shrinkage obtained in experiments without chip breakers and the dimensions of the ridges along the length of the cutting edge, a definite relationship between them can be noted.
At constant feed, the greater the shrinkage, the greater the ridge width at which chip breaking occurs. Thus, at a feed
\(S = 0.15\ \frac{mm}{rev}\)
and a ridge width \(h = 1.9 \div 2.1\ mm\), normal chip breaking occurred when facing a bushing 17 mm in diameter, and the formation of a ribbon chip occurred at a diameter of 35 mm; in this case the shrinkage was respectively equal to 3 and 2.3.
When the ridge width was reduced to \(\alpha = 1.6\ mm\), the character of chip breaking changed. At a diameter of 35 mm the chip was obtained in the form of small pieces, while at a diameter of 17 mm it had the form of a ribbon with slight shrinkage.
Thus, the magnitude of shrinkage at constant feed determines the ridge dimensions at which normal chip breaking occurs. Any deviation from these dimensions leads to disruption of the chip-breaking process.
The influence of the point angle on the chip-breaking process was investigated by varying \(\varphi\) from 0 to 20°. The investigations showed that, within this range of variation, the angle \(\varphi\) does not affect the chip-breaking process.
These results are natural, since both the temperature and the chip shrinkage do not depend on the angle \(\varphi\).
The influence of wear on the chip-breaking process was studied during end-facing of bushings with a drill having an amount of flank-face wear equal to \(0.2 \div 0.3\ mm\). As the experiments showed, the amount of wear does not affect the temperature, shrinkage, or chip shape during chip breaking.
The investigations carried out on the influence of shrinkage on the chip-breaking process were confirmed in the work of V. V. Igoshin, performed in turning without the use of cutting fluid.
On the basis of studying the process of chip breaking, the author came to the conclusion that this process takes place when two conditions are fulfilled:
- When a sufficient amount of primary bending deformation is produced; the latter depends on the physical and mechanical properties of the chip heated to a certain temperature.
- As the secondary deformation increases, the bending is of the opposite sign with respect to the primary deformation. The magnitude of the secondary deformation depends on the width of the land.
Igoshin expressed the influence of the first factor through the chip-fragmentation criterion \(q\), which is equal to 0.07 for most structural steels, and through chip shrinkage; the latter fully agrees with our conclusions.
The procedure for calculating the dimensions of the land according to Igoshin is as follows:
- For a specified feed and angle in the plane, the thickness of the cut layer is determined:
\[ a_1 = s \sin \varphi . \]
- From the shrinkage, the chip thickness is found:
\[ a'_1 = \zeta a, \]
where \(\zeta\) is the chip shrinkage.
- The optimal radius of chip curling \(R_{\mathrm{opt}}\) is determined. For the mode of chip breaking in machining steel 45,
\[ R_{\mathrm{opt}} = 6.5 a'_1 . \]
- The depth of the land is assigned:
\[ h > a'_1 . \]
- The width of the land is determined:
\[ a = \sqrt{h \cdot (2 R_{\mathrm{opt}} - h)} . \]
The above procedure was verified under conditions of deep drilling.
The dimensions of the lands calculated by Igoshin’s formulas fully coincided with the experimental ones.
Thus, on the basis of proof of the identity of the cutting process in deep drilling and turning, calculation formulas have for the first time been found for determining the dimensions of chip-breaking lands in drills for deep drilling, taking into account changes in chip shrinkage along the length of the cutting edge.
LITERATURE
- V. V. Igoshin. Investigation of the Conditions for Rational Chip Removal in Turning of Metals with Cutters Having Chip-Breaking Elements. Author’s abstract of dissertation, 1967.