Experimental Study of Methods for Increasing the Durability of Cutting Tools

A. N. Dobrovidov
Professor, Doctor of Technical Sciences

A. M. Rozenberg
Professor, Doctor of Technical Sciences

A. N. Eremin
Docent, Candidate of Technical Sciences

I. Objectives of the Study

The principal aim of the present work was to test various methods for increasing the durability of drills. Among such methods, first of all, is the method of chemical etching proposed by engineer Vasiliev, which required verification for the reasons set forth below; and also the method of electrolytic etching.

In addition, according to the research program that had been drawn up, it was necessary to test changes in the geometry of drills that lead to increased durability. These include: grinding of the front face of the drill in order to reduce the rake angle at the drill lip as it approaches the periphery, grinding (relieving) of the land, and double sharpening.

Grinding of the front face of the drill, i.e., of that surface of the helical flute along which the chip flows, is strongly recommended by Prof. I. I. Semenchenko (Semenchenko, Cutting Tool, Part 1, pp. 241–243, ONTI, 1938) on the following grounds expressed by him. The rake angle on the principal lips of a drill is a variable quantity, increasing from the center toward the periphery. When measured in a plane normal to the cutting edge of the drill, this angle at the periphery reaches a considerable value, on the order of 30–35° (depending on the helix angle of the drill). Such a value of the rake angle, as applied to a cutting tool intended for machining mild steels, may be considered acceptable; for machining cast irons of all hardnesses and steels of high hardness, however, such a high value of the rake angle must lead to a reduction in the durability of the cutting tool. Prof. Semenchenko suggests that, by grinding the front face in such a way that, on half of the drill lip (from the middle of the radius to the periphery), we form the front face as a plane inclined at some optimum rake angle, we shall obtain an increase in the durability of the drill. Moreover, according to his same considerations, such grinding gives additional advantages by creating a constant rake angle on part of the lip. Thus, grinding of the front face should lead to a reduction of the rake angle.

In production, such grinding is used and gives a positive effect when drilling especially hard and tough steels of the Hadfield type,

in application to ordinary medium-hardness and hard machine-construction steels, Prof. Semenchenko’s proposal on increasing durability in sharpening had not been tested by anyone and therefore required experimental verification.

The sharpening of the drill land (bevel) in that part of it which takes direct part in the cutting process was proposed and tested by Patka (Werkstattstechnik, 1928), then was checked by Maslin (Mashinostroitel’, No. 2 for 1941) and gave a positive result in the sense of increasing durability when drilling medium-hardness steels. Therefore it was desirable to test this method of increasing durability in application to the drilling of steels above medium hardness, and also in combination with etching, if the latter gives a positive result. The advisability of sharpening the drill lands (bevels) consists in a considerable reduction of their width and, as a result, in a reduction of friction and in the creation of a clearance angle on that part of the land which takes direct part in the cutting process.

The advisability of double sharpening with respect to increasing drill durability must be considered sufficiently established, and testing this method made sense in combination with etching, if the latter gives a positive result. In addition, we do not know how far this method has been tested in application to the drilling of high-hardness steels. When drilling high-hardness steels, double sharpening, for theoretical reasons, may also fail to increase durability. The positive effect of double sharpening is based on the fact that double sharpening, by lengthening the cutting edge, makes the chip thinner and thereby facilitates the work of the peripheral parts of the cutting edges. But, in parallel with this, double sharpening, by reducing the point angle of the drill, increases the rake angle, thereby worsening the removal of heat from the cutting edges, which in drilling high-hardness steels may produce a negative effect. Therefore, when drilling high-hardness steels, double sharpening may fail to increase durability. This circumstance required verification.

Such were the objectives of the investigation.

II. Research Program

According to the original program, the tests were to be carried out with one drill size for each material being machined. The durability of drills that were not etched, etched, with a sharpened front face, etc., was to be compared by comparing experimentally obtained graphs \(vT\) (speed, durability). The \(vT\) dependences were to be determined experimentally for one material being machined, for one drill diameter, at three feeds and at four speeds for each feed.

Owing to a number of circumstances, the research program was somewhat changed. These circumstances were as follows. The batch of drills for the experiments was to be supplied by the Tomsk Tool Plant. These drills were supplied with a delay of three months.

To carry out experiments by the method indicated above, it was necessary to have a large quantity of completely homogeneous metal. Since it was not possible to obtain metals directly for experiments under wartime conditions, we concluded an agreement with one of the plants, according to which we manufactured simple parts for the plant from its metal, and the experimental drilling operations were to be carried out in manufacturing them. By the beginning of the work a large batch of metal—steel 45—had been delivered by the plant, which was to serve as the experimental material. Since the drills for the experiments

were supplied with a great delay, and it was impossible to postpone the manufacture of parts; thus a situation arose in which, by the time the experimental drills were received, most of the metal had already been used up, and subsequently the metal began to arrive in small batches and with different strength characteristics, which did not permit the planned experimental procedure to be carried out.

Therefore, in experimenting with drills we had, in a number of cases, to abandon the construction of \(vT\) curves and to conduct experiments by comparing the lives of drills under one and the same cutting condition. At the same time, instead of carrying out experiments on only one grade of steel (for which steel 45 had originally been selected), we carried out experiments on steels \(H_B = 180\text{–}200\) (steel 45), steel \(H_B = 250\), steel \(H_B = 270\), and steel 40Kh. Instead of carrying out experiments with only one drill size, we found it possible to conduct them with three different diameters.

In addition, the time during which we did not have experimental drills was spent on conducting a broad series of systematic experiments to determine the effectiveness of using etching of cutters when machining steel 45.

This was of substantial interest, since, on the one hand, Eng. Vasil’ev’s conclusions regarding the effectiveness of etching cutters had been made by him on the basis of experiments in machining especially hard steels (\(H_B = 300\text{–}400\)) and had not been verified on steels of ordinary hardness; on the other hand, any results obtained by us in experiments with cutters, if confirmed by experiments with drills, would acquire particular validity.

Thus the research program was considerably expanded. First of all, we turn to the description of the experiments with lathe cutters.

III. Experiments with Lathe Cutters

Engineer Vasil’ev proposed a method for increasing the life of a cutting tool by chemically treating its faces (Stanki i instrument, 1939, No. 12; 1940, Nos. 3, 4, 5, 6, and 11–12).

According to Vasil’ev’s data, if a tool that has been finally sharpened is etched before being put into operation with a special reagent that removes from the faces of the tool the layer damaged by grinding, then the life of such a tool increases by a factor of two or more.

In the table presented by Vasil’ev and borrowed by us from his article (Stanki i instrument, 1940, No. 4–5), there are, however, apparent contradictions (see Table 1).

Judging from the table, the ratios of the lives of etched and unetched cutters fluctuate over a wide range, from 1.24 to 4.1. It is not possible to discern any regularity in the change of this ratio with a change in the hardness of the material being machined or in the cutting condition. In the table cited, the following contradictions are observed. When machining material of one and the same hardness, the lives obtained by Vasil’ev do not at all conform to any regularity. Thus, if we compare rows 2–3 and 4 of the table, we see that when machining one and the same material with constant depth of cut and feed, but with different cutting speeds, the lives obtained are greater for the higher cutting speeds, which is completely absurd. Rows 5 and 6 give the same inconsistency. If we compare rows 9 and 10, we see that when machining a softer material at a lower cutting speed and with a smaller depth of cut, a shorter life was obtained.

Table No. 1

Despite the presence of such contradictions, the question of increasing tool life by chemical treatment deserves attention, since here, by means of the very simple operation of etching, it is possible to obtain a considerable saving of scarce cutting steels if Vasil’ev’s data on the increase in life are justified. Therefore, in the laboratories of metal cutting and metallurgy of the Tomsk Polytechnic Institute, a broad investigation was undertaken with the aim of checking Vasil’ev’s proposal. Carrying out such an investigation was also of interest because engineer Vasil’ev’s experiments had been performed on steels mainly of very high hardness \((H_B = 300—400)\), which are comparatively rarely machined with steel cutting tools. Engineer Roginskii’s work also concerns such steels (Stanki i instrument, 1940, No. 11—12). It was therefore of interest to clarify this question as applied to the machining of steels of the hardnesses most commonly used in machine building. In addition to Vasil’ev’s proposed method of chemical etching, a method of electrolytic etching was also tried. Our entire investigation was conducted as applied to lathe tools. The tools were made of RF1 steel and of EI-184 steel. The tools of RF1 steel were solid; those of EI-184 steel had brazed-on plates. The brazing of the plates to the shank was carried out in an electric furnace with the aid of a welding powder of the following composition:

Ferromanganese \((73.0\% \ \mathrm{Mn},\ 0.40\% \ \mathrm{C})\) . . . . 75%

Copper filings . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25%

The brazing process was as follows. A plate of high-speed steel EI-184 is carefully fitted to the holder. Powder of calcined borax is poured into the seat for the plate in the holder, and the holder is heated in a furnace at \(1100—1200^\circ\). The holder is then removed from the furnace and the seat is again sprinkled with borax. The borax melts, and the seat is cleaned with a steel brush. Onto the seat is poured welding powder mixed with borax (1 part by weight of borax and 3 parts by weight of powder), and the EI-184 steel plate is laid on it. The thickness of the powder layer is 3 mm. The plate is sprinkled with borax on top. The holder is then heated in the furnace at \(1190^\circ\) for 2–3 min. After this time has elapsed, the holder is removed from the furnace and the plate is pressed down with a rod. After this operation the holder is transferred to a furnace \((t \sim 800^\circ)\) and cooled together with the furnace.

The quality of the weld overlay obtained was irreproachable.
The heat-treatment regime for cutters made of RF-1 steel was as follows:

Hardening at 1280°, cooling in oil.
Tempering three times for 1 hour at 550°.

The heat-treatment regime for cutters made of EI-184 was found by us through preliminary experiment, and the best heat treatment was obtained under the following regime:

Hardening temperature 1200°, holding 6 min.
1st tempering 540°, holding 1 hour.
2nd tempering 560° ” 1 hour.
3rd tempering 560° ” 1 hour.

The Rockwell hardness of the cutters obtained was:

Cutters made of RF-1 steel . . . . . \(R_c = 65—67\)
” ” EI-184 . . . . \(R_c = 63—65\)

The geometry of the cutters adopted was as follows:

Front angle . . . . . . . . . . . 18°
Back clearance angle on the main and auxiliary cutting edge . . . . . 10°
Principal cutting-edge angle in plan . . . . . 80°
Auxiliary angle in plan . . . . . 10°
Radius of nose rounding . . . . 1 mm
Cross-section of cutter body . . . . 18 × 13 mm
Size of EI-184 steel plates . . . . . . . 8 × 15 × 20 mm

Sharpening and resharpening of the cutters was carried out on a Gisholt machine with abundant cooling. When resharpening cutters dulled during the experiment, the entire affected dulled layer was removed.

Chemical Treatment of Tools

To remove the surface layer damaged by grinding on the tool blade, engineer Vasil’ev recommends using the following reagent: 100 cu. cm of sulfuric acid, 50 cu. cm of nitric acid, 50 g of copper sulfate, and 850 cu. cm of water. After the tool is immersed in the liquid, copper begins to be deposited on it according to the reaction \(\mathrm{CuSO_4 + Fe \to FeSO_4 + Cu}\); after 5 min. the copper deposit cracks, and after another 10 min. it peels off. The normal duration of etching for ordinary cutters made of high-speed steel is considered to be 15 minutes.

The author considers the addition of copper sulfate and the consequent formation of a copper film on the cutter to be necessary. In his opinion, the copper layer equalizes the etching action of the acids on the hard and soft constituents of the steel, owing to the changing concentration of acids under this layer.

This explanation is incorrect. The process of dissolving metal in acid is an electrolytic process. High-speed steel is a three-phase and even a multiphase system. For simplicity we shall consider it three-phase: one phase—complex carbides, another—martensite, and the third—austenite. The phase most resistant to acid (carbides) will be the positive (not dissolving in acid) electrode of the galvanic microelement; martensite and austenite will give the complex negative (dissolving in acid) electrode. But first of all the more electronegative martensite will dissolve in the acid, and then the austenite. After etching, the surface of the tool blade will be nonuniform; it will consist of projections (carbides) and depressions (martensite and austenite).

In order that all the constituents of the structure be etched more or less equally, it is necessary to create an artificial galvanic element in which—

the electrode would be very strongly electropositive with respect to all three phases. The presence of a copper deposit predetermines the existence of such an electrode, since copper is an electropositive metal. Therefore, properly speaking, there is no need to use copper sulfate in the recipe proposed by Engineer Vasil’ev. One may make do with any soluble copper salt or with a salt of any electropositive metal whatever (for example, nickel sulfate).

The only question is which salt is currently cheaper and less scarce.

We tried replacing copper sulfate with cuprous and cupric chloride and obtained the same results and a completely identical course of the process.

It is obvious that the more electropositive the metal deposited on the blade surface during etching, the more strongly the potential difference between the individual phases of the alloy dissolving in the electrolyte (carbide, austenite, and martensite) is smoothed out, and the more uniform their dissolution will be. Uniform dissolution will give a good, smooth surface of the tool after etching. But metals more electropositive than copper, unfortunately, belong to the category of noble metals (gold, platinum), and the use of their salts in the present case is, of course, excluded.

To some extent it is possible to equalize the potential difference of carbide, austenite, and martensite during electrolytic etching in a suitable electrolyte. As is known from the theory of metal corrosion, electropositive metals (in a pair with other metals) sometimes become electronegative when the composition of the electrolyte is changed. A solution of sulfuric acid with the addition of some inhibitor is suitable as the electrolyte. We used a 5% solution of sulfuric acid (and also a 10% solution of potassium or sodium sulfate) with the addition of 0.25% dextrin. Dextrin may be replaced by starch or gelatin in the same quantities.

A steel plate serves as the anode, and the tool to be etched as the cathode. The current density is about 0.2 A per $\text{cm}^2$ of the etching surface.

In ordinary and electrolytic etching, the seam between the high-speed steel plate and the holder must be protected from etching by covering it with a layer of paraffin. In order to use the reagent more economically, the holder should likewise be protected from etching in the same way.

Before the seam is coated with paraffin, the tool is degreased. The uniformity of etching depends on the thoroughness with which this operation is carried out.

We carried out degreasing by washing the tool in a solution:

Water ........ 1000 cu. cm.
Soap .......... 20 g.
Caustic soda .. 50 g.

In this solution the tools were washed with vigorous rubbing using a stiff bristle brush. A heated solution ($\sim 50^\circ$) works better.

Comparative tests of tools

Comparative tests of tools subjected to chemical treatment and of ordinary tools in their initial state were carried out on Udmurt and MT-30 lathes. The tests were mainly carried out on steel 45 with hardness $H_B = 180\text{--}200$. In a number of experiments the tests were also carried out on other steels, as will be mentioned below. Work with the tools was performed on blanks that had been preliminarily turned, and in all experiments a constant depth of cut $t = 2$ mm was used. Such a comparatively small

A greater depth of cut was adopted in order to save metal. At the same time, 2 mm may be regarded as a cutting depth intermediate between finishing and roughing chips. The tests were carried out at a number of feeds: 0.1 mm/rev, 0.19 mm/rev, 0.36 mm/rev.

In the tests the cutters were run until dulling, the criterion of dulling being the appearance of a shiny strip on the cutting surface, which corresponded quite precisely to the moment of failure of the blade. The greatest number of tests was carried out with EI-184 cutters. Dulling of these cutters occurred through the formation of a depression on the rake face, gradually reaching the cutting edge and destroying it, and through wear of the auxiliary flank face at the nose of the cutter.

As a result of the tests performed, straight lines \(vT\) (speed, life) were obtained in logarithmic coordinates, which make it possible

Fig. 1

Fig. 2

to judge with complete certainty the comparative life of etched and unetched cutters. Figure 1 presents the results of tests on steel 45 (\(H_B = 180—200\)) with feed \(S = 0.1\) mm/rev, without cooling. Shown here are the results of tests both with unetched cutters and with cutters that underwent etching for different durations, from 10 to 30 min. As can be seen from Fig. 1, in accordance with the symbols explained in the figure, etching of cutters when working with feed \(S = 0.1\) mm/rev produces no effect, giving practically the same life as that of unetched cutters, irrespective of the duration of etching.

In Fig. 2 are presented the results of tests with a feed of \(S = 0.19\ \text{mm/rev}\). Included here are tests carried out with cutters subjected to both chemical and electrolytic etching for various durations, from 10 to 30 min. Also presented here are tests carried out with cooling by a 5% emulsol solution in water. As the figure makes quite clear, at a feed of \(S = 0.19\ \text{mm/rev}\), neither chemical nor electrolytic etching gives any increase in durability. The same result is obtained when working with cooling, where the points of the tests with etched cutters lie exactly on the durability straight line of unetched cutters.

The cutters in our work were sharpened on a special Gisholt machine. Sharpening was carried out very carefully, with abundant cooling. It was suggested that rougher sharpening of the cutters should give lower durability and then, perhaps, etching would give an increase in durability. Although after sharpening on the Gisholt machine the cutters did not have polished faces, and on these latter the strokes from the grains of the grinding wheel were quite clearly visible, it was nevertheless decided to try working with more roughly sharpened cutters. To do this, the cutters were sharpened by hand on a coarse emery stone with strong pressure; at the same time the aim was only not to produce burns on the cutter faces. The results of tests with these cutters are shown in Fig. 2 by shaded circles; moreover, as can be seen, these tests fell on the general durability straight line.

Fig. 3.
Inscriptions in the figure: \(T\) minutes; Steel 45; cutter \(30 \times 18\); \(S = 0.366\ \text{mm/rev}\); \(V\), m/min.

Fig. 4.
Inscriptions in the figure: \(T\) minutes; Steel 45; cutter RF-1; \(S = 0.19\ \text{mm/rev}\); \(V\), m/min.

In Fig. 3 are presented tests with a feed of \(S = 0.366\ \text{mm/rev}\); here too the test with the etched cutter lay exactly on the durability straight line, giving no effect whatsoever.

In Fig. 4 are presented the results with cutters made of RF-1 steel; here the circles mark tests with unetched cutters, and the crosses—with cutters that had undergone chemical etching. Fig. 4 shows that for cutters made of RF-1 steel as well, etching gives no increase in durability.

On the same figure a tool-life line is plotted, obtained for cutters made of EI-184 steel. It may be concluded that these cutters, obtained with the heat treatment we used, are of very high quality. Steel RF-1, as is known, is one of the best high-speed steels, possessing high cutting properties. As Fig. 4 shows, cutters made of EI-184 steel give a reduction in cutting speed, in comparison with cutters made of RF-1 steel, of only 4–8%, which under production operating conditions is quite unnoticeable.

It should be noted that such high quality of cutters made from the substitute EI-184 can be obtained only with careful heat treatment, under strict control of the hardening and tempering temperatures.

In addition to the experiments on steel 45, we carried out experiments comparing the life of unetched and etched cutters made of EI-184 steel on U7 steel with hardness \(H_B = 250\). The results of these experiments are presented in Table 2.

Table 2

It is clear from the table that, in machining this harder metal as well, etching of the cutters gives no increase in life. On the contrary, in this case the lives of the etched cutters proved lower than those of the unetched ones.

IV. Experiments with Drills

The main part of the experiments was carried out with drills 22 mm in diameter, made of EI-184 steel. These drills were manufactured by the Tomsk Tool Plant as a special batch for the experiments.

In addition to this size, drills of grades “R” and “RO” with diameters of 19.5 mm and 39 mm were used in the experiments.

The principal geometrical characteristics of the drills are presented in Table 3.

Table 3

The drills were sharpened on a special drill-grinding machine of the “Oliver” type. Sharpening was carried out with abundant cooling. The accuracy of sharpening was checked with an indicator, and no difference was permitted in the position of the two main cutting lips of the drill of more than 0.03 mm.

Regrinding after the drill had become blunt was carried out so that the entire layer of drill metal damaged during operation or by blunting was removed.

Thus, for example, if in the preceding test the drill land (ribbon) had received damage over some length, then during regrinding this entire length was removed from the drill.

The geometry of the heel face, which in sharpening according to the “Oliver” type is a helical surface, is characterized by the clearance angles presented in Table 4.

Table 4

The clearance angles were measured on a special device. The table gives the clearance angles in the cylindrical section of the drill.

The tests with drills were carried out on machines 2135 and 2150 of the Lenin Plant. For these machines, which initially had only 6 speeds, interchangeable gears were made, by means of which the number of speeds was increased. Thus, on machine 2135 it was possible to obtain the following spindle speeds: 53, 84, 95, 131, 151, 172, 200, 236, 272, 310, 320, 360, 415, 492, 500, 576, 648, 766, 908, 1050, 1070, etc.

During operation the number of revolutions was checked, with a deviation of no more than 2% being permitted.

When installing drills on the machine, the drill was obligatorily checked with an indicator in two mutually perpendicular planes, with runout of no more than 0.05 mm being permitted.

The work was carried out with abundant cooling by a soda emulsion supplied by a pump.

The work material on which the drill tests were carried out was as follows:

1. Carbon steel ........ \(H_{\mathrm{B}} = 170—200\).
2. Carbon steel ........ \(H_{\mathrm{B}} = 250\).
3. Carbon steel ........ \(H_{\mathrm{B}} = 270\).
4. Chromium steel 40Kh ........ \(H_{\mathrm{B}} = 180\).

The work material was received in bars, cut into blanks of the required length, and put into the tests. Hardness control was performed on each blank, with a hardness deviation of no more than \(\pm 10\) Brinell units being permitted. In addition, in each group of experiments intended to reveal the influence of any factors on life, the tests were carried out on metal not only of one hardness, but also of one profile (for the most part even from one bar).

Thus we could be sure that comparative tests of the drills were being carried out on quite homogeneous metal.

The drill tests were conducted on blind holes, with the drilling depth ranging from 45 to 50 mm. Of course, in each series of tests there was a constant drilling depth, since by itself it affects drill life.

The dulling process and the dulling criterion

Although many thousands of experiments have been carried out with drills, up to the present time the literature has no clear conception of the course of drill wear in various operating cases and of the criterion of dulling.

dulling of the drill. In one of the latest sources (NKTM, Bureau of Technical Standards, Standard Norms of Wear and Tool Life of Cutting Tools. Oborongiz, 1941, p. 22) the following is printed on this subject: “When machining steels with high-speed drills, the wear value on the flank face \(\mu = 1.0—1.2\) mm, with accompanying wear along the chamfer and the front face, is adopted as the criterion of dulling. When the stated amount of wear on the flank face is reached, the corners become rounded.”

Experiments show that in a number of cases such dulling is altogether impracticable, and the drill will be broken before the amount of wear on the flank face reaches \(1.0—1.2\) mm.

The question of drill dulling when drilling steels should be considered depending on the operating regime at which the drill works.

1. At low cutting speeds and very long tool lives, considerably exceeding economic tool lives, the dulling process is characterized by mechanical abrasion of the cutting edges. A hollow is formed on the front face, and on the flank face—a wear chamfer, which gradually widens. At the same time scratches appear on the drill margins, and the corners also gradually become rounded. The wear chamfers

Fig. 5

Fig. 6

on the back (flank face) of the drill are usually of unequal width, widening from the chisel edge toward the periphery of the drill (see Fig. 5). In this case, after prolonged work of the drill, the width of the wear chamfer on the flank face may reach \(1.0—1.2\) mm. But not infrequently, even in this case, before the dulling chamfer on the flank face reaches the indicated value, the corners and margins become so severely worn that the work has to be stopped. Fig. 6 shows the course of wear during tests as a function of the drilling depth traversed after reaching the total drilling depth \(L = 7250\) mm, which corresponded to a tool life, in minutes, \(T = 159\) min. The dulling chamfer on the flank

flank reached 1, 1–1.2 mm. In Fig. 7 a case is presented where, at a life \(T = 47.5\) min. (which is twice the economical value for a diameter \(d = 22\) mm), the width of the wear land on the back face reached only 0.6 mm; the work, however, had to be stopped, since the wear of the corner at the transition from the main cutting edge to the margin reached 1.3 mm.

1. At high cutting speeds and lives close to the economical ones, blunting of the drill quite naturally occurs in the areas of greatest heat generation. Such areas are the corners of transition from the main cutting edges to the margins of the drill, since here, on the one hand, the cutting speed is greatest, and on the other hand, the process of chip separation is most difficult, being simultaneously under the action of both the main and the auxiliary cutting edge (the margin). In addition, the heat of friction of the margin, which has no clearance angle, against the wall of the hole is released here as well. All this, taken together, leads to the fact that at high speeds and economical conditions the main cutting edges wear extremely little, while the corners wear very intensively.

Fig. 7

Fig. 8

In Fig. 8 the appearance of a blunted drill is shown. In Fig. 9 the increase in wear of the back face and of the corners is shown. The latter figure shows that at \(T = 24\) min. the wear on the back face reaches a value of 0.45 mm; the wear of the corners is 0.65 mm. It should be noted that wear of the corners in this case usually proceeds very rapidly in the final moments of operation of the drill. The worn corners give the drill such a shape that it can no longer penetrate into the metal. This moment is accompanied by a sharp squeal, which is a very good indicator of the moment of final blunting of the drill. If at this moment the feed is not quickly switched off, breakage of the drill inevitably occurs. Thus, a convenient criterion of drill blunting for experiments is the sharp squeal, which indicates the moment of final blunting of the drill. This is the criterion we used in the majority of our experiments.

It should be added to this that if a land (margin) had previously been ground on the drill, then rounding and wear of the corner are usually not accompanied by screeching; the corner may be worn very severely without danger to the integrity of the drill, and here it is necessary to establish some limiting amount of wear, which is to be taken as the criterion of dulling. In this case we brought the wear of the corner to such a value that the wear extended over the entire width of the drill land.

Here it is also necessary to point out a quite peculiar type of wear which we observed and which, so far as we know, has not been described anywhere up to the present. It consists in the following.

Fig. 9

After some time following the beginning of work, a mark (a narrow groove) appears on one or, simultaneously, on both lands, located from the corner at a distance approximately equal to half the feed (Fig. 10). This groove forms on the drill corner a peculiar whisker. During operation of the drill the groove deepens and widens somewhat, while the whisker narrows. During operation the whisker is usually protected on the front side by a build-up of metal, which protects it from wear. The part of the land lying above the whisker, and the heel face of the drill, are worn away considerably; deep scratches appear on them, which on both sides as it were undercut the base of the whisker from the side of the land and from the side of the main cutting edge. When the base of the whisker becomes badly damaged, it breaks off; the corner of the drill land at once becomes dulled, which is usually characterized by a sharp screeching.

Fig. 10

It is remarkable that the appearance of whiskers is always accompanied by a twofold or greater increase in drill life as compared with the life with the usual type of dulling under the same conditions. We observed this in drilling all steels. Formation of whiskers was observed by us in 20–30% of all tests. We shall exclude these cases from comparison with others, since this must be regarded as a favorable, though not usual, case of wear and dulling of the drill. In what follows we shall indicate how it is poss—

but to make use of the observed phenomenon for increasing drill life.

In choosing the experimental cutting conditions, in most cases we sought to obtain drill lives lying near or below the economical ones. Most of the tests gave lives from 5 to 25 min. This aim was dictated both by considerations of saving time and metal, and by the consideration that any methods of increasing drill life should be tested precisely under conditions with economical lives. A considerable number of tests was also carried out by us with conditions giving longer lives, in order to make sure how the methods for increasing life would act in this case.

Although in most cases we worked with short-life conditions, over the entire course of the work we drilled more than 5000 experimental holes, with a total length of more than 250 meters.

A. Influence of relieving the rake face of a drill on its life

Relieving of the rake faces was carried out on drills of 19.5 mm and 39 mm diameter. The relieving was performed with a grinding device, in such a way that the rake angle on the relieved part was 18° and that there was a completely smooth transition from the relieving plane to the surface of the spiral flute of the drill (Fig. 11). The symmetry of the cutting edges obtained after relieving the rake face was carefully checked by means of an indicator.

Determination of the influence of relieving the rake face on drill life was carried out by us mainly when drilling hard steels \(H_B = 240\text{—}255\), but

Fig. 11

Fig. 12

some experiments were also carried out on steel \(H_B = 180\). Drills of 19.5 mm diameter worked in solid metal; drills of 39 mm diameter worked in a 19.5 mm hole, i.e., here reaming took place. In all cases the comparison of life was made on one and the same drill, i.e., a drill tested under some condition without relieving ...

of the front face, then received secondary sharpening. By this we excluded the possible influence of [[unclear: letter/symbol]].

Fig. 12 presents the results of tests of drills without secondary sharpening and with secondary sharpening of the front face in steel [[unclear: grade]]. The drill operated with a feed \(S = 0.195\) mm/rev. Fig. 12 shows that secondary sharpening of the front face did not increase the life of the drill, and that both the experimental points with secondary sharpening and the points from tests without secondary sharpening of the front face lay well on one and the same straight line \(V—T\).

The results of tests of drills \(d = 19.5\) mm with a sharpened and an unsharpened front face in steel \(H_B = 240—255\) are presented in Table 5.

Table 5

Thus, drills of diameter 19.5 also did not increase their life with secondary sharpening of the front face. Experiments with secondary sharpening of the front face in steel of hardness \(H_B = 180\) gave exactly the same results in principle.

To explain the results obtained, let us turn to Figs. 13, 14, and 15. Figure 13 shows the process of blunting of the main cutting edges and of the land corner for a drill \(d = 39\) mm with a sharpened front face. It follows from the figure that blunting of the main cutting edges proceeds very slowly, while at the same time the land corner (chamfer) becomes rounded very rapidly, and the drill fails precisely for this reason. Figures 14 and 15 show the process of blunting of a drill \(d = 19.5\) mm with an unsharpened and a sharpened front face. From a comparison of these two figures it follows that secondary sharpening of the front face changes nothing essential in the process of drill blunting. In both cases the moment of final blunting is characterized by rapid wear of the land corner (chamfer) and by the resulting squealing of the drill, which makes it necessary to stop work, since otherwise the drill will break.

Thus, by changing the form of the front face of the drill and the magnitude of its rake angle by means of secondary sharpening of the front face, we do not change the blunting process of that element of the drill (the land corner) which limits its life in operation; moreover, we have observed that a drill with a sharpened front face produces a corrugated chip (Fig. 16). This is apparently explained by the fact that the direction of the sharpened portion of the front face no longer coincides with the direction of the helical flute of the drill; the chip, moving along this portion of the front face and acquiring its direction, bears against the opposite wall of the helical flute, which impedes its separation and causes it to become corrugated. This is associated with additional work and additional heat generation, and must adversely affect the life of the drill.

Thus, it may be stated that secondary sharpening of the front face of a drill for the purpose of increasing the cutting angle when drilling steels with hardness up to \(H_B = 250\) gives no positive effect in terms of increasing life and cannot be recommended for use.

Fig. 13

Inscriptions in the figure:

• Drill \(d = 39\) mm; \(P0\)
• Steel \(H_B = 250\)
• Conditions: \(S = 0.195\) mm/rev; \(n = 200\) rpm; \(v = 24.5\) m/min
• Life \(T = 29.6\) min
• Legend:
• dashed line with triangles — from the corner of the land
• solid line with circles — main cutting edge
• Drilling through a predrilled hole \(\varnothing 19.5\) mm
• Annotation: land width
• Vertical axis: wear \(\delta\), mm
• Horizontal axis: drilling depth \(b\), mm

Fig. 14

Inscriptions in the figure:

• Drill \(d = 19.5\) mm; \(P2\)
• Steel \(H_B = 250\)
• Conditions: \(S = 0.275\) mm/rev; \(n = 360\) rpm; \(v = 22\) m/min
• Life \(T = 15.5\) min
• Legend:
• dashed line with triangles — from the corner of the land
• solid line with circles — main cutting edge
• Vertical axis: wear \(\delta\), mm
• Horizontal axis: drilling depth \(b\), mm

Fig. 15

Inscriptions in the figure:

• Drill \(d = 19.5\) mm; \(P2\)
• Operation: [[unclear: handwritten note on working mode]]
• Steel \(H_B = 250\)
• Conditions: \(S = [[unclear: feed value]]\) mm/rev; \(n = 180\) rpm; \(v = 22\) m/min
• Life \(T = [[unclear: value]]\) min
• Legend:
• dashed line with triangles — from the corner of the land
• solid line with circles — main cutting edge
• Vertical axis: wear \(\delta\), mm
• Horizontal axis: drilling depth \(b\), mm

Fig. 16

B. Effect of Etching Drills on Their Durability

In conducting experiments with cutters it was found that, when machining ordinary machine-building steels of medium and above-medium hardness, etching does not increase cutter durability. Moreover, it turned out that both chemical and electrolytic etching gave the same negative results.

When experimenting with drills, we considered ourselves entitled to assume that here, too, chemical and electrolytic etching would give the same results; at the same time, however, we believed that the negative results of work with etched cutters did not at all necessarily predetermine the obtaining of the same results when working with drills, since drills have such an element as the margin, which cutters lack, and the working conditions of this element, as a result of the absence of the back angle, are quite distinctive. Therefore we considered it necessary to conduct experiments with etched drills, irrespective of the results obtained in etching cutters.

In the figure: drills \(d = 22\), \(3\omega = 80\); steel \(H_B = 180\); \(S = 0.575\).
○ Not etched
× Etched
Speed \(V\), m/min

Fig. 17

First of all let us turn to the results of experiments with unetched and etched drills on steel \(H_B = 170\text{--}190\). In the experiments, drills \(d = 22\) mm made of high-speed-steel substitute EI-184, manufactured by the Tomsk Tool Plant, were used. The drills worked with a feed of \(0.575\) mm/rev.

The results of these experiments are presented in Fig. 17, where circles denote the results of experiments with unetched drills, and crosses—with etched drills.

with drills. Fig. 17 shows that etching the drills, when working on steel of hardness \(H_b=180\), gives no effect, for the tests with etched as well as with unetched drills lay, relative to one and the same mean straight line, in exactly the same way.

Fig. 18 presents the course of the wear process of two drills, one of which had undergone the etching operation. Here a long-life regime was chosen, in order to show most clearly how etching affects the wear of the cutting elements of the drill during operation.

Fig. 18

Labels in the figure:

• Drill \(d=22\), EI-184
• Steel \(H_b=180\)
• Regime: \(S=0.575\) mm/rev; \(n=360\) rev/min; \(v=24.9\) m/min
• Legend: dashed line — etching; solid line — unetched
• Vertical axis: wear \(b\)
• Horizontal axis: drilling depth \(L\), mm
• Note: “On different lips”
• Note at upper right: “Wear of the corners of the lands \(>0.3\) mm”

Fig. 18 shows that in both drills the process of wear of the clearance face behind the cutting edge proceeds approximately identically, and in the etched drill it is even somewhat more intensive. Thus this figure also indicates the absence of any influence of drill etching on its life when drilling steel \(H_b=180\).

We also carried out a number of analogous tests on chromium steel 40Kh of hardness \(H_b=180\), and here exactly the same results were obtained, i.e., etching did not increase drill life.

Turning to harder steels, we carried out tests on the drilling of carbon steels of hardness \(H_b=250\) and \(H_b=270\). The tests showed that when drilling these steels, chemical treatment of the drills (etching) already gives an increase in drill life.

Table 6 gives a comparison of the life of etched and unetched drills when machining steel of hardness \(H_b=250\).

Table 6 shows that here we have a more than twofold increase in drill life as a result of their etching.

In Fig. 19 the results of similar tests on steel \(H_b=270\) are represented graphically. The figure shows that here too etching gives an increase in life of more than two times.

Thus it may be considered that etching drills increases their life only when drilling steels of hardness \(H_b=250\) and higher; for ordinary machine-building steels, however, it gives no result.

Table 6

Figure labels: Tool life \(T\), min.; Speed \(V\), m/min. Drill \(d = 22\) mm; EI-184; steel \(H_B = 270\); \(S = 0.275\). Legend: not etched; etched.

Fig. 19

B. Influence of chamfer grinding on drill life

The influence of grinding the chamfer (the back of the margin) was first revealed by Tatka, and then, in work on steel with \(H_B = 180\), confirmed by Maslennyi. Our task was not to repeat the investigations mentioned, but to determine the influence of chamfer grinding in combination with etching of drills, i.e., to determine the influence on tool life of the combined action of etching and chamfer grinding. Since etching affected tool life only when machining steels of high hardness (\(H_B = 250\) and \(H_B = 270\)), the influence of chamfer grinding also had to be checked in drilling these steels.

The form of relieving the back of the chamfer is shown in Fig. 20. The relieving was done by hand, in such a way that the width of the remaining cylindrical surface of the chamfer was about 0.3–0.4 mm, and the length of the relieved part was 2–3 mm. It should be noted that attempts to leave a narrower cylindrical surface of the chamfer, down to 0.15–0.18 mm, led to a reduction in drill life.

Fig. 20

The results of experiments with simultaneous etching and relieving of the chamfer are presented in Fig. 21 for steel \(H_B=250\), and in Fig. 22 for steel \(H_B=270\).

Both figures are quite indicative. It follows from them that, when drilling steels with \(H_B=270\) and \(H_B=250\), the combined action of etching and chamfer relief makes it possible to increase the life of the drill on average by a factor of 5.

We also carried out several experiments with relieving the chamfer on those steels for which etching of the drill did not increase its life. In experiments on steels with \(H_B=180\) and 40X, as a result of relieving the chamfer we invariably obtained an increase in life of more than twofold.

Fig. 21

Fig. 22

We also tested, on steel with \(H_B=180\), the combined action of etching and relieving of the chamfer. It turned out that the combined action is equal to the action of chamfer relief alone, i.e., the effect of etching was not manifested here either.

Thus, the use of chamfer relief is always expedient, giving, when drilling steels of all hardnesses, an increase in life. When drilling especially hard steels with \(H_B=250\) and higher, the simultaneous use of chamfer relief together with etching of the drill gives a fivefold increase in drill life.

d. The Effect of Double Sharpening on Drill Life

As was indicated above, testing double sharpening made logical sense only in conjunction with etching, i.e., on those steels for which etching the drills produced an increase in life. Therefore, we tested double sharpening in conjunction with etching when drilling steel \(H_B = 250\).

Double sharpening was carried out in such a way that the length of the secondary-sharpening cutting edges was equal to 0.2 of the drill diameter, and the included angle of these edges was \(70^\circ\) (Fig. 23). Double sharpening was performed by hand, owing to the absence of a machine on which it would have been possible to sharpen the drill with a variable point angle \(\psi\). In doing so, the length of the secondary-sharpening cutting edges, the symmetry of their arrangement, and the value of the point angle were carefully checked. The results of the experiments are presented in Fig. 24.

Fig. 23

Fig. 24

Fig. 24 shows that, in the experiments with double sharpening, a considerable scatter of results was obtained. Some of the experiments gave a certain increase in life in comparison with etching without double sharpening; others, on the contrary, gave a decrease in life. On average, however, as Fig. 24 shows, double sharpening did not provide an addition to the life obtained by etching. It may be assumed that, when drilling steels of high hardness, double sharpening no longer gives such positive results as when drilling steels of ordinary hardness, since double sharpening increases the rake angle, which, when drilling steels of high hardness, already produces a negative result. This consideration is also supported by the nature of the dulling of the drill with double sharpening, which we obtained in experiments when drilling steel \(H_B = 250\).

In this case the dulling was no longer concentrated at the chamfer corner, as had always occurred with normal sharpening; instead, the main cutting edge of the secondary sharpening was subject to destruction, as shown in Fig. 25.

Fig. 25

D. A special type of drill blunting and its use for the purpose of increasing tool life

Above, in the chapter “The blunting process and the blunting criterion,” we have already mentioned the peculiar wear of drills that we observed, when on the land (facet) at the corner of the main cutting edge a “whisker” is formed (see Fig. 10). In those cases where this type of wear occurs, the drill, as a rule, gives increased tool life.

To illustrate this proposition, Table 7 collects cases of drill operation with the formation of a whisker, and the tool lives obtained in this case are compared with the tool lives under the normal blunting process.

Table 7

Table 7 shows that, when there is wear of the drill accompanied by the formation of whiskers, the tool life of the drill usually increases by more than two times.

On the basis of these observations, the idea arose that if a whisker were artificially created on the drill, the tool life of such a drill should be increased. A small number of experiments was carried out to verify this supposition. The artificial whisker on the drill was created in the following manner.

The drill was clamped in the chuck of a lathe. On the carriage there was mounted a tool-post grinding attachment with a fine-grained grinding—

with a wheel dressed in the same way as for grinding a triangular thread. By slowly rotating the machine spindle with the drill clamped in the chuck and bringing up the carriage with the rotating wheel, it is not difficult to make on the chamfers of the drill a groove 0.5 mm deep and 0.5 mm wide, located at a distance of 0.5 mm from the corner of the chamfer.

Table 8 presents the results of tests with drills without lands and with artificial lands.

Table 8

As Table 8 shows, the formation of artificial lands increased tool life almost twofold. Thus, by the simple operation of making a groove on the drill chamfer, for which a simple fixture can be made, it is possible to increase drill life substantially.

General Conclusions and Findings

On the basis of the experimental work carried out, the following conclusions may be drawn:

1. Etching of lathe cutters made of RF-1 and EI-184 steel does not increase tool life when working steels with hardness up to $H_B = 250$ and cannot be recommended for use in production.

2. Resharpening the front face of drills when drilling steels with hardness $H_B = 250$ and $H_B = 270$ does not increase tool life.

3. Etching of drills when drilling steels with hardness up to $H_B = 220$ does not increase tool life.

4. Etching of drills when drilling steels with hardness $H_B = 250$–270$ gives a twofold increase in tool life.

5. The combined action of etching and resharpening the chamfer of drills when drilling steels $H_B = 250$–270$ is expressed in a fivefold increase in tool life in comparison with ordinary drills.

6. Since etching of drills increases their life only when drilling steels of high hardness, which are used comparatively rarely in production, there is no point in carrying out this etching in the manufacture of drills at tool plants; it is more expedient to perform it at the production facility carrying out drilling operations on these high-hardness steels.

7. The artificial formation of a land makes it possible to obtain an additional increase in drill life.