Trent E.M., Wright P.K. Metal Cutting

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LUBRICANTS 323

tic silicates by including them inside the work material (as described in Chapter 9) is the only

practical method of getting a lubricant to the seized part of the interface.

10.3.2 Lubrication at the periphery of contact or during intermittent cutting

To understand the value of cutting lubricants, the emphasis must now be shifted to consider-

ation of i) those areas of the tool/work interface which are not seized, and ii) of those conditions

of cutting where seizure is reduced to a minimum or eliminated.

Seizure may be avoided when the cutting speed is very low, as near the center of a twist drill.

Seizure may also be avoided when cutting tooth engagement times are very short. Examples

include many milling operations - or, the multi-toothed hobs used for gear-cutting - or, when the

feed and depth of cut are very small, so that no position on the interface is more than a few tenths

of a millimeter from the periphery of the contact area.

Under such conditions metal-to-metal contact may be very localized. Consequently, the tool

and work surfaces may be largely separated by a very thin layer of the lubricant which acts to

restrict the enlargement of the areas of contact. In other words, what are known as “boundary

lubrication” conditions may exist. In this situation, a good lubricant, especially one with the

extreme pressure additives chlorine and sulfur, will reduce cutting forces, reduce heat genera-

tion, and greatly improve surface finish.

Under conditions of cutting where seizure does takes place, there is a peripheral zone around

the seized area where contact is partial and intermittent. This is shown diagrammatically in Fig-

ure 3.17 for a simple turning tool, operating under conditions of seizure but without a built-up

edge. In the peripheral region, the compressive stress forcing the two surface together is lower

than in the region of seizure.

10.3.3 Experiments in controlled atmospheres: an overview

The action of lubricants in this peripheral region, and the character of effective lubrication are

shown in experiments on metal cutting in controlled atmosphere carried out by Rowe and oth-

ers.

10,11

When cutting iron, steel, aluminium or copper in a good vacuum (10

-3

mbar) or in dry

nitrogen, both the cutting force (F

) and the feed force (F

) were much higher than when cutting

in air. The chip remained seized to the tool over a longer path and was much thicker. The admis-

sion into the vacuum chamber of oxygen, even at a very low pressure, resulted in reduction of

the contact area and tool forces to those found when cutting in air.

Therefore, the oxygen in the air surrounding the tool under normal cutting conditions acts to

restrict the spreading of small areas of metallic contact in the peripheral region into large scale

seizure. Unless the cutting speed is very low, oxygen in the surrounding atmosphere cannot com-

pletely prevent seizure in the region near the cutting edge; however, it can restrict the area of sei-

zure. The freshly generated metal surfaces on the underside of the chip and on the workpiece are

very active chemically and are readily re-welded to the tool even after initial separation.

The role of the oxygen in air, or the chlorine and the sulfur in a lubricant, is to combine with

the new metal surfaces and reduce their activity and their affinity for the tool.

324 COOLANTS AND LUBRICANTS

10.3.4 Importance of “natural air” as a lubricant

The freshly generated surface of the work material is very clean and chemically active. How-

ever, the surfaces of steel or carbide tools, before cutting starts, are contaminated with oxide,

with adsorbed organic layers and other accidental oils. When cutting starts, the strength of the

bond formed between tool and work material at first depends on the cleanliness of the tool sur-

face. However, the unidirectional flow of work material across the tool surface tends to remove

contaminated layers. This action requires a finite time which varies greatly with the tool, work

material and cutting conditions.

When turning nickel, steel and titanium alloys a strongly

bonded interface is established very rapidly. However, in interrupted cutting operations, such as

milling, continuous contact times are often much shorter than one second. Then the area of

bonding may be considerably reduced or eliminated by the periodic exposure of the tool surface

to the action of air and active lubricants. This is particularly true when extreme pressure lubri-

cants are used with sulfur and chlorine additives.

Thus air itself acts to some extent as a cutting lubricant.

If cutting were carried out in oxy-

gen-free outer space, problems of high cutting forces and extensive seizure would be encoun-

tered. The action of air modifies the flow of the chip at its outer edge when cutting steel. In this

region, oxygen from the air can penetrate some distance from the chip edge and act to prevent

seizure locally, i.e., at the position H-E in the diagram, Figure 3.17.

Figure 3.3 shows scanning electron micrographs of a steel chip cut at 49 m min

-1

(150 ft/min)

- note the segmented character of the outer edge. Sections through this outer edge show a typical

“slip-stick” action, Figure 10.8. The steel first sticks to the tool. However, the presence of oxy-

gen in this region then restricts the bonding between tool and work material to small localized

areas. Then, the feed force becomes strong enough to break the local bonds, and a segment of the

chip slides away across the tool surface. The process is then repeated - successive segments first

stick and then slide away to form the segmented outer edge of the chip. Oxygen is able to pene-

trate for only a short distance - e.g., 0.25 mm (0.010 in) - from the outer edge of the chip at this

cutting speed. Further inside the chip body, seizure is continuous. Figure 10.9 is a section

through the same chip at a distance of 0.50 mm (0.020 in) from the edge and shows a flow pat-

tern typical of seizure.

10.3.5 Importance of nitrogen in protecting the tool surfaces

In summary, oxygen plays a very important role in creating oxides on the tool surface and

freshly cut work material surfaces.

These oxides “in the right places on the rake face” reduce friction and tool wear. However, “in

the wrong places” oxides accelerate notch wear.

Deep grooves are often worn in the tool at the positions where the chip edge moves over the

tool with this slip-stick action (Figures 6.23 and 7.26). This “grooving” or “notching” wear is

associated with chemical interaction between the tool and work material surfaces and atmo-

spheric oxygen. The rate of grooving wear is strongly influenced by jets of gas directed at this

position. When cutting steel with high speed steel or cemented carbide tools, wear at this posi-

tion is greatly accelerated by a jet of oxygen and retarded or eliminated by jets of nitrogen or

argon. This same effect is observed when cutting with a CVD-coated tool where the coating is

TiC. However, there is usually no grooving on tools coated with alumina, as long as this coating

LUBRICANTS 325

remains intact. Grooving is greatly reduced with TiN coated tools,

once again showing their

benefits.

This illustrates an important issue. Namely that the nitrogen in air plays an important role in

reducing oxidation of the tool when cutting steel and other metals at high speeds. This can be

demonstrated by a simple experiment.

On those surfaces of the tool which exceed 400°C dur-

ing cutting, colored oxide films are formed - the familiar “temper colors”. If tools used for high

speed cutting of steel are closely examined, two areas of the surface are seen to be completely

free from the oxide films even though they were at high temperature during cutting. These

regions, marked X in Figure 10.10, are on the clearance face just below the flank wear land, and

on the rake face just beyond the worn area. The freedom from oxide films is not because these

areas were cold during cutting - they were in fact closer to the heat source, and at a higher tem-

perature than the adjacent oxidized surfaces.

FIGURE 10.8 Section through steel chip near outer edge showing slip-stick action at interface in sliding

wear region

326 COOLANTS AND LUBRICANTS

FIGURE 10.9 Section through same steel chip as Figure 10.8, 0.5 mm from outer edge showing flow-

zone characteristic of seizure at interface

The explanation is that, during cutting, these areas of the tool surface form are part of a very

fine crevice. This is the region just below G in Figure 3.17 on the clearance face, and just

beyond B-B

´ on the rake face. The opposing face of this crevice is a freshly generated metal sur-

face at high temperature. It is free from oxide or other contaminating layers and highly active

chemically. It combines with and eliminates all the oxygen available in this narrow space, leav-

ing a pocket of nitrogen which acts to protect the tool surface from oxidation. Further down the

clearance face and at the edges of the chip more oxygen has access. Thus the tool is coated with

oxide films, although the temperature is lower. This is shown diagrammatically in Figure 10.10.

Also, Figure 10.11 is a photomicrograph of the clearance face of a carbide tool used to cut steel

at high speed. However, the protective action of atmospheric nitrogen is eliminated if a jet of

oxygen is directed into the clearance crevice. The tool is then very heavily oxidized in the region

which was previously free from oxide films (compare Figures 10.11 and 10.12).

LUBRICANTS 327

FIGURE 10.10 Occurrence of oxide films on tools used to cut steel at high speed

FIGURE 10.11 Clearance face of carbide tool used to cut steel at high speeds in air

328 COOLANTS AND LUBRICANTS

FIGURE 10.12 As Figure 10.11, but with jet of oxygen directed at clearance face

10.3.6 Summarizing the role of air in cutting and implications for dry cutting

In summary, there are two counterbalancing effects of the 80% nitrogen and 20% oxygen in

air. The oxygen in the air acts as a “lubricant”, reducing the area of seizure. Simultaneously, the

nitrogen in the air modifies this action and largely prevents a serious problem of oxidation of the

tools. This is especially important when cutting high melting-point metals at high speed, since

carbide tools are oxidized very rapidly when exposed to air at temperatures of the order of

900°C.

10.3.7 Extreme pressure lubricants

The active elements in extreme pressure lubricants operate in a similar manner to oxygen, but

are much more effective. Chlorine forms chlorides by reaction with both tool and work materi-

als. The chloride acts effectively only below the temperature at which it decomposes - about

350°C. Sulfides, formed by reaction at the interface, are effective up to about 750°C. Sulfur

additives are thus more effective at high speeds and feed.

Although the cutting lubricants are applied as liquids, in most cases they must act in the gas-

eous state at the interface because of the high temperature in this region. A flood of lubricant is

more effective than air because it allows penetration of the active elements into the interfacial

crevice. This further restricts the area of seizure. In this sense, water acts as a lubricant as well as

a coolant, penetrating between the tool and work surfaces and oxidizing them to restrict seizure.

One of the most effective cutting lubricants at medium to high speeds is carbon tetrachloride

(CCl

). The authors and colleagues have used it with care in their laboratories. However,

because of its toxic effects, CCl

cannot be used in industrial cutting. Figure 10.13 shows the

influence of CCl

and water on the tool forces when machining steel over a range of cutting

speeds.

Because of their action in reducing contact area, active lubricants effectively reduce

the tool forces. They are most efficient at low cutting speeds and have a relatively small effect at

speeds over 30 m min

-1

(100 ft/min). By reducing forces, power consumption is reduced and

temperatures may be lowered. As a final comment, carbon tetrachloride is not considered as a

lubricant at all under sliding contact situations. With the transparent sapphire tools at very low

LUBRICANTS 329

speeds, CCl

caused extreme bonding at the rear of contact - termed as zone 2 adhesion by

Doyle et al. and Wright.

This was most noticeable when cutting aluminum and lead.

FIGURE 10.13 Influence of CCl

and water as lubricants on tool forces in relation to cutting speed. (After

Rowe and Smart

)

10.3.8 Surface finish

Improved surface finish is a major objective of cutting lubricants. In this respect they are par-

ticularly effective at rather low cutting speeds and feed rates in the presence of a built-up edge.

As an example, Figure 10.14 shows the surface finish traces made using a “surface profilome-

ter”. Experiments were on turned low carbon steel surfaces, produced by cutting at 8 m min

-1

(25 ft/min) at a feed of 0.2 mm (0.008 in) per rev., both dry and using CCl

as a cutting fluid.

The very great improvement in surface finish is caused by reduction in size of the built-up edge.

Often, when cutting in air under such conditions, the built-up edge is very large compared with

the feed, as shown diagrammatically in Figure 10.15a and in the photomicrograph Figure 3.21.

Soluble oils act to reduce the built-up edge to a size commensurate with the feed, Figure 10.15b.

330 COOLANTS AND LUBRICANTS

FIGURE 10.14 Surface profilometer traces of steel surfaces cut dry and with CCl

as cutting lubricant

Experiments have shown that the same result can be achieved by the use of distilled water or

of a jet of oxygen gas. This suggests that the stability of a large built-up edge, which consists of

many layers of work material (Figure 3.21), is at least partially dependent on a protective pocket

of nitrogen when cutting in air. When air is replaced by oxygen, water vapor, or some chemically

active gas they combine with fresh metal surfaces. Adhesion is then reduced between the layers

of the built-up edge. This stable built-up edge is then much smaller.

The surface finish is also improved by a reduction in size of the fragments sheared from the

built-up edge and remaining on the work surface. This appears to be achieved by the action of

the active lubricant vapor on the path of the fracture which forms the new surface when a built-

up edge is present. Compare Figure 3.21 (cutting in air) with Figure 10.16 (cutting with a chlori-

nated mineral oil).

10.3.9 Influence of cutting speed and tool material type

The influence of active lubricants on the rate of tool wear is very complex. In some conditions

the rate of wear may be unaffected by the lubricating (as opposed to cooling) action of lubri-

cants. In other cutting conditions, it may be greatly decreased or increased. There has been far

too little study of the mechanisms by which the lubricants affect tool wear, and few general rules

can be stated. Four examples are given to illustrate some effects of water-based (as opposed to

oil-based) lubricants:

10.3.9.1 Cutting steels at low speed and feed using carbide tools

When cutting steels at low speed and feed using carbide tools, the active lubricants which reduce

the size of the built-up edge, also increase the rate of wear and change its character.

The rate of

flank wear is much increased and a shallow crater is formed close to the cutting edge, as shown

diagrammatically in Figure 10.15b.

LUBRICANTS 331

FIGURE 10.15 (a) Built-up edge when cutting dry; (b) built-up edge when cutting with soluble oil

lubricant

Figure 10.17 shows the rake face wear on four tools after cutting a) in air, b) with a mineral

oil, c) with distilled water, and d) with a jet of oxygen. The accelerated wear on the two latter

tools is the result of interaction of the tool and work materials with an active gas or liquid envi-

ronment to form. A small crater is formed where the chip contacts the tool after passing over the

built-up edge (Figure 10.15b).

The mineral oil does not reduce the size of the built-up edge but does slightly reduce the small

amount of wear on the rake face (Figure 10.17a and b). The craters formed with water (or solu-

ble oil emulsion in water) are smaller and closer to the cutting edge. They have the same general

shape as high-speed craters. They are not, however, formed by the same diffusion mechanism as

the craters when cutting steel at high speed with WC-Co tools (Figure 7.10). These low-speed

craters (Figure 10.17c and d) occur only at low speed and feed - e.g. 30 m min

-1

and 0.1 mm/rev

feed. Increasing cutting speed or feed changes the flow pattern around the cutting edge and elim-

inates crater wear.

332 COOLANTS AND LUBRICANTS

FIGURE 10.16 Section through quick-stop showing built-up edge after cutting 0.15% C steel at low

speed using chlorinated mineral oil lubricant

In summary, there are some unusual issues that occur with lubricants. The interactions between

the lubricant and the tool are exceptionally dependent on cutting speed and tooling type. For

example, at high speeds, the use of oil emulsion lubricant slightly reduces the rate of crater wear

by diffusion on carbide tools. However, it causes another type of crater wear at low speed and

feed. A large increase in wear rate also occurs when a soluble oil cutting fluid is used during cut-

ting of cast iron with carbide tools at medium and low speeds. This emphasizes the importance

of understanding the mechanisms of tool wear for control of tool life.

10.3.9.2 Cutting steels at low speed and feed with high speed steel tools

In continuous turning of steel with high speed steel tools the rate of flank wear may be greatly

increased by use of water or a water-based cutting lubricant when compared with dry cutting.

This has been observed by a number of research workers.

6,15,16

Under some test conditions, the

rate of flank wear was accelerated by a factor of five or more

when lubricant was used, com-

pared with dry cutting. This particularly occurred at low feed, and despite the fact that tool tem-

peratures were lowered by the use of water-based fluid. Water-based emulsions accelerated the