Part 4 (1/2)
The back slope of a tool is measured from a line _A--B_ which is parallel to the shank, and the clearance angle, from a line _A--C_ at right angles to line _A--B_. These lines do not, however, always occupy this position with relation to the tool shank when the tool is in use.
As shown to the left in Fig. 12, the base line _A--B_ for a turning tool in use intersects with the point of the tool and center of the work, while the line _A--C_ remains at right angles to the first. It will be seen, then, that by raising the tool, as shown to the right, the _effective_ clearance angle [alpha] will be diminished, whereas lowering it, as shown by the dotted lines, will have the opposite effect.
A turning tool for bra.s.s or other soft metal, particularly where considerable hand manipulation is required, could advantageously have a clearance of twelve or fourteen degrees, as it would then be easier to feed the tool into the metal; but, generally speaking, the clearance for turning tools should be just enough to permit them to cut freely.
Excessive clearance weakens the cutting edge and may cause it to crumble under the pressure of the cut.
[Ill.u.s.tration: Fig. 12. Ill.u.s.trations showing how Effective Angles of Slope and Clearance change as Tool is raised or lowered]
=Angle of Tool-point and Amount of Top Slope.=--The lip angle or the angle of keenness [delta] (Fig. 10) is another important consideration in connection with tool grinding, for it is upon this angle that the efficiency of the tool largely depends. By referring to the ill.u.s.tration it will be seen that this angle is governed by the clearance and the slope [beta], and as the clearance remains practically the same, it is the slope which is varied to meet different conditions. Now, the amount of slope a tool should have depends on the work for which it is intended. If, for example, a turning tool is to be used for roughing medium or soft steel, it should have a back slope of about eight degrees and a side slope ranging from fourteen to twenty degrees, while a tool for cutting very hard steel should have a back slope of about five degrees and a side slope of nine degrees.
[Ill.u.s.tration: Fig. 13. (A) Blunt Tool for Turning Hard Steel. (B) Tool-point Ground to give Keenness]
The reason for decreasing the slope and thus increasing the lip angle for harder metals is to give the necessary increased strength to the cutting edge to prevent it from crumbling under the pressure of the cut.
The tool ill.u.s.trated at _A_, Fig. 13, is much stronger than it would be if ground as shown at _B_, as the former is more blunt. If a tool ground as at _A_, however, were used for cutting very soft steel, there would be a greater chip pressure on the top and, consequently, a greater resistance to cutting, than if a keener tool had been employed; furthermore the cutting speed would have to be lower, which is of even greater importance than the chip pressure; therefore, the lip angle, as a general rule, should be as small as possible without weakening the tool so that it cannot do the required work. In order to secure a strong and well-supported cutting edge, tools used for turning very hard metal, such as chilled rolls, etc., are ground with practically no slope and with very little clearance. Bra.s.s tools, while given considerable clearance, as previously stated, are ground flat on top or without slope; this is not done, however, to give strength to the cutting edge, but rather to prevent the tool from gouging into the work, which it is likely to do if the part being turned is at all flexible and the tool has top slope.
Experiments conducted by Mr. F. W. Taylor to determine the most efficient form for lathe roughing tools showed that the nearer the lip angle approached sixty-one degrees, the higher the cutting speed. This, however, does not apply to tools for turning cast iron, as the latter will work more efficiently with a lip angle of about sixty-eight degrees. This is doubtless because the chip pressure, when turning cast iron, comes closer to the cutting edge which should, therefore, be more blunt to withstand the abrasive action and heat. Of course, the foregoing remarks concerning lip angles apply more particularly to tools used for roughing.
[Ill.u.s.tration: Fig. 14. Grinding the Top and Flank of a Turning Tool]
=Grinding a Lathe Tool.=--The way a turning tool is held while the top surface is being ground is shown to the left in Fig. 14. By inclining the tool with the wheel face, it will be seen that both the back and side slopes may be ground at the same time. When grinding the flank of the tool it should be held on the tool-rest of the emery wheel or grindstone, as shown by the view to the right. In order to form a curved cutting edge, the tool is turned about the face of the stone while it is being ground. This rotary movement can be effected by supporting the inner end of the tool with one hand while the shank is moved to and fro with the other.
Often a tool which has been ground properly in the first place is greatly misshapen after it has been sharpened a few times. This is usually the result of attempts on the part of the workman to re-sharpen it hurriedly; for example, it is easier to secure a sharp edge on the turning tool shown to the left in Fig. 12, by grinding the flank as indicated by the dotted line, than by grinding the entire flank. The clearance is, however, reduced and the lip angle changed.
There is great danger when grinding a tool of burning it or drawing the temper from the fine cutting edge, and, aside from the actual shape of the cutting end, this is the most important point in connection with tool grinding. If a tool is pressed hard against an emery or other abrasive wheel, even though the latter has a copious supply of water, the temper will sometimes be drawn. When grinding a flat surface, to avoid burning, the tool should frequently be withdrawn from the stone so that the cooling water (a copious supply of which should be provided) can reach the surface being ground. A moderate pressure should also be applied, as it is better to spend an extra minute or two in grinding than to ruin the tool by burning, in an attempt to sharpen it quickly.
Of course, what has been said about burning applies more particularly to carbon steel, but even self-hardening steels are not improved by being over-heated at the stone. In some shops, tools are ground to the theoretically correct shape in special machines instead of by hand. The sharpened tools are then kept in the tool-room and are given out as they are needed.
=Cutting Speeds and Feeds.=--The term cutting speed as applied to turning operations is the speed in feet per minute of the surface being turned, or, practically speaking, it is equivalent to the length of a chip, in feet, which would be turned in one minute. The term cutting speed should not be confused with revolutions per minute, because the cutting speed depends not only upon the speed of the work but also upon its diameter. The feed of a tool is the amount it moves across the surface being turned for each revolution; that is, when turning a cylindrical piece, the feed is the amount that the tool moves sidewise for each revolution of the work. Evidently the time required for turning is governed largely by the cutting speed, the feed, and the depth of the cut; therefore, these elements should be carefully considered.
Cutting Speeds and Feeds for Turning Tools[1]
+---------------------------------++---------------------------------+ | Steel--Standard 7/8-inch Tool ||Cast Iron--Standard 7/8-inch Tool| +-----+-----+---------------------++-----+-----+---------------------+ | | | Speed in Feet per || | | Speed in Feet per | |Depth|Feed | Minute for a Tool ||Depth|Feed | Minute for a Tool | | of | in | which is to last || of | in | which is to last | | Cut | In- | 1-1/2 Hour before || Cut | In- | 1-1/2 Hour before | | in |ches | Re-grinding || in |ches | Re-grinding | | In- | +------+------+-------++ In- | +-------+------+------+ |ches | | Soft |Medium| Hard ||ches | | Soft |Medium| Hard | | | |Steel |Steel |Steel || | | Cast | Cast | Cast | | | | | | || | | Iron | Iron | Iron | +-----+-----+------+------+-------++-----+-----+-------+------+------+ | | 1/64| 476 | 238 | 108 || | 1/16| 122 | 61.2 | 35.7 | |3/32 | 1/32| 325 | 162 | 73.8 ||3/32 | 1/8 | 86.4 | 43.2 | 25.2 | | | 1/16| 222 | 111 | 50.4 || | 3/16| 70.1 | 35.1 | 20.5 | | | 3/32| 177 | 88.4| 40.2 ++-----+-----+-------+------+------+ +-----+-----+------+------+-------++ | 1/32| 156 | 77.8 | 45.4 | | | 1/64| 420 | 210 | 95.5 || 1/8 | 1/16| 112 | 56.2 | 32.8 | | 1/8 | 1/32| 286 | 143 | 65.0 || | 1/8 | 79.3 | 39.7 | 23.2 | | | 1/16| 195 | 97.6| 44.4 || | 3/16| 64.3 | 32.2 | 18.8 | | | 1/8 | 133 | 66.4| 30.2 ++-----+-----+-------+------+------+ +-----+-----+------+------+-------++ | 1/32| 137 | 68.6 | 40.1 | | | 1/64| 352 | 176 | 80.0 ||3/16 | 1/16| 99.4 | 49.7 | 29.0 | |3/16 | 1/32| 240 | 120 | 54.5 || | 1/8 | 70.1 | 35.0 | 20.5 | | | 1/16| 164 | 82 | 37.3 || | 3/16| 56.8 | 28.4 | 16.6 | | | 1/8 | 112 | 56 | 25.5 ++-----+-----+-------+------+------+ +-----+-----+------+------+-------++ | 1/32| 126 | 62.9 | 36.7 | | | 1/64| 312 | 156 | 70.9 || 1/4 | 1/16| 90.8 | 45.4 | 26.5 | | 1/4 | 1/32| 213 | 107 | 48.4 || | 1/8 | 64.1 | 32.0 | 18.7 | | | 1/16| 145 | 72.6| 33.0 || | 3/16| 52 | 26.0 | 15.2 | | | 3/32| 116 | 58.1| 26.4 ++-----+-----+-------+------+------+ +-----+-----+------+------+-------++ | 1/32| 111 | 55.4 | 32.3 | | | 1/64| 264 | 132 | 60.0 || 3/8 | 1/16| 80 | 40.0 | 23.4 | | 3/8 | 1/32| 180 | 90.2| 41.0 || | 1/8 | 56.4 | 28.2 | 16.5 | | | 1/16| 122 | 61.1| 27.8 ++-----+-----+-------+------+------+ +-----+-----+------+------+-------++ | 1/32| 104 | 52.1 | 30.4 | | 1/2 | 1/64| 237 | 118 | 53.8 || 1/2 | 1/16| 75.2 | 37.6 | 22.0 | | | 1/32| 162 | 80.8| 36.7 || | 1/8 | 43.1 | 21.6 | 12.6 | +-----+-----+------+------+-------++-----+-----+-------+------+------+ |Steel--Standard 5/8-inch Tool ||Cast Iron--Standard 5/8-inch Tool| +-----+-----+------+------+-------++-----+-----+-------+------+------+ |Depth|Feed | Soft |Medium| Hard ||Depth|Feed | Soft |Medium| Hard | | of | |Steel |Steel |Steel || of | | Cast | Cast | Cast | | Cut | | | | || Cut | | Iron | Iron | Iron | +-----+-----+------+------+-------++-----+-----+-------+------+------+ | | 1/64| 548 | 274 | 125 || | 1/32| 160 | 80.0 | 46.6 | |1/16 | 1/32| 358 | 179 | 81.6 ||3/32 | 1/16| 110 | 55.0 | 32.2 | | | 1/16| 235 | 117 | 53.3 || | 1/8 | 75.4 | 37.7 | 22.0 | +-----+-----+------+------+-------++-----+-----+-------+------+------+ | | 1/64| 467 | 234 | 106 || | 1/32| 148 | 74.0 | 43.3 | |3/32 | 1/32| 306 | 153 | 69.5 || 1/8 | 1/16| 104 | 51.8 | 32.0 | | | 1/16| 200 | 100 | 45.5 || | 1/8 | 69.6 | 34.8 | 20.3 | | | 3/32| 156 | 78 | 35.5 ++-----+-----+-------+------+------+ +-----+-----+------+------+-------++ | 1/64| 183 | 91.6 | 68.0 | | | 1/64| 417 | 209 | 94.8 ||3/16 | 1/32| 135 | 67.5 | 39.4 | | 1/8 | 1/32| 273 | 136 | 62.0 || | 1/16| 94 | 47.0 | 27.4 | | | 1/16| 179 | 89.3| 40.6 || | 1/8 | 64.3 | 32.2 | 18.8 | | | 3/32| 140 | 69.8| 31.7 ++-----+-----+-------+------+------+ +-----+-----+------+------+-------++ | 1/64| 171 | 85.7 | 50.1 | | | 1/64| 362 | 181 | 82.2 || 1/4 | 1/32| 126 | 63.2 | 36.9 | |3/16 | 1/32| 236 | 118 | 53.8 || | 1/16| 87.8 | 43.9 | 25.6 | | | 1/16| 155 | 77.4| 35.2 || | 3/32| 70.4 | 35.2 | 20.6 | +-----+-----+------+------+-------++-----+-----+-------+------+------+ | 1/4 | 1/64| 328 | 164 | 74.5 || 3/8 | 1/64| 156 | 77.8 | 45.4 | | | 1/32| 215 | 107 | 48.8 || | 1/32| 116 | 57.8 | 33.8 | +-----+-----+------+------+-------++ | 1/16| 79.7 | 39.9 | 23.3 | | 3/8 | 1/64| 286 | 143 | 65.0 || | | | | | +-----+-----+------+------+-------++-----+-----+-------+------+------+
[1] Cutting speeds for tools of a good grade of high-speed steel, properly ground and heat-treated.--From MACHINERY'S HANDBOOK.
=Average Cutting Speeds for Turning.=--The cutting speed is governed princ.i.p.ally by the hardness of the metal to be turned; the kind of steel of which the turning tool is made; the shape of the tool and its heat-treatment; the feed and depth of cut; whether or not a cooling lubricant is used on the tool; the power of the lathe and also its construction; hence it is impossible to give any definite rule for determining either the speed, feed, or depth of cut, because these must be varied to suit existing conditions. A general idea of the speeds used in ordinary machine shop practice may be obtained from the following figures:
Ordinary machine steel is generally turned at a speed varying between 45 and 65 feet per minute. For ordinary gray cast iron, the speed usually varies from 40 to 50 feet per minute; for annealed tool steel, from 25 to 35 feet per minute; for soft yellow bra.s.s, from 150 to 200 feet per minute; for hard bronze, from 35 to 80 feet per minute, the speed depending upon the composition of the alloy. While these speeds correspond closely to general practice, they can be exceeded for many machining operations.
The most economical speeds for a given feed and depth of cut, as determined by the experiments conducted by Mr. F. W. Taylor, are given in the table, ”Cutting Speeds and Feeds for Turning Tools.” The speeds given in this table represent results obtained with tools made of a good grade of high-speed steel properly heat-treated and correctly ground. It will be noted that the cutting speed is much slower for cast iron than for steel. Cast iron is cut with less pressure or resistance than soft steel, but the slower speed required for cast iron is probably due to the fact that the pressure of the chip is concentrated closer to the cutting edge, combined with the fact that cast iron wears the tool faster than steel. The speeds given are higher than those ordinarily used, and, in many cases, a slower rate would be necessary to prevent chattering or because of some other limiting condition.
=Factors which limit the Cutting Speed.=--It is the durability of the turning tool or the length of time that it will turn effectively without grinding, that limits the cutting speed; and the hardness of the metal being turned combined with the quality of the tool are the two factors which largely govern the time that a tool can be used before grinding is necessary. The cutting speed for very soft steel or cast iron can be three or four times faster than the speed for hard steel or hard castings, but whether the material is hard or soft, the kind and quality of the tool used must also be considered, as the speed for a tool made of ordinary carbon steel will have to be much slower than for a tool made of modern ”high-speed” steel.
When the cutting speed is too high, even though high-speed steel is used, the point of the tool is softened to such an extent by the heat resulting from the pressure and friction of the chip, that the cutting edge is ruined in too short a time. On the other hand, when the speed is too slow, the heat generated is so slight as to have little effect and the tool point is dulled by being slowly worn or ground away by the action of the chip. While a tool operating at such a low speed can be used a comparatively long time without re-sharpening, this advantage is more than offset by the fact that too much time is required for removing a given amount of metal when the work is revolving so slowly.
Generally speaking, the speed should be such that a fair amount of work can be done before the tool requires re-grinding. Evidently, it would not pay to grind a tool every few minutes in order to maintain a high cutting speed; neither would it be economical to use a very slow speed and waste considerable time in turning, just to save the few minutes required for grinding. For example, if a number of roughing cuts had to be taken over a heavy rod or shaft, time might be saved by running at such a speed that the tool would have to be sharpened (or be replaced by a tool previously sharpened) when it had traversed half-way across the work; that is, the time required for sharpening or changing the tool would be short as compared with the gain effected by the higher work speed. On the other hand, it might be more economical to run a little slower and take a continuous cut across the work with one tool.
The experiments of Mr. Taylor led to the conclusion that, as a rule, it is not economical to use roughing tools at a speed so slow as to cause them to last more than 1-1/2 hour without being re-ground; hence the speeds given in the table previously referred to are based upon this length of time between grindings. Sometimes the work speed cannot be as high as the tool will permit, because of the chattering that often results when the lathe is old and not ma.s.sive enough to absorb the vibrations, or when there is unnecessary play in the working parts. The shape of the tool used also affects the work speed, and as there are so many things to be considered, the proper cutting speed is best determined by experiment.
=Rules for Calculating Cutting Speeds.=--The number of revolutions required to give any desired cutting speed can be found by multiplying the cutting speed, in feet per minute, by 12 and dividing the product by the circ.u.mference of the work in inches. Expressing this as a formula we have