Chapter XI: Drilling and Boring in the Lathe (3)

FOR WROUGHT IRON.
+---------+--------+-----------+-------------+-----------+------------+
| Work |Roughing| Roughing |Feed as lathe| Finishing | Finishing |
|diameter.| cuts. | cuts. | revolutions |cuts. Lathe| cuts. Lathe|
| Inches. |Feet per| Lathe | per inch of |revolutions| revolutions|
| | minute.|revolutions| tool travel.|per minute.| per inch |
| | |per minute.| | |tool travel.|
|---------+--------+-----------+-------------+-----------+------------|
| 1/2 | 40 | 305 | 30 | 305 | 60 |
| 1 | 35 | 133 | 30 | 133 | 60 |
| 1-1/2 | 30 | 76 | 30 | 76 | 60 |
| 2 | 28 | 53 | 25 | 53 | 60 |
| 2-1/2 | 28 | 42 | 25 | 42 | 50 |
| 3 | 28 | 35 | 25 | 35 | 50 |
| 3-1/2 | 26 | 28 | 25 | 30 | 50 |
| 4 | 26 | 24 | 20 | 26 | 50 |
| 5 | 25 | 18 | 20 | 21 | 50 |
| 6 | 25 | 15 | 20 | 16 | 50 |
| CAST IRON. |
| 1 | 45 | 163 | 30 | 163 | 40 |
| 1-1/2 | 45 | 135 | 25 | 135 | 30 |
| 2 | 40 | 76 | 25 | 76 | 25 |
| 2-1/2 | 40 | 61 | 20 | 61 | 20 |
| 3 | 35 | 44 | 20 | 50 | 16 |
| 3-1/2 | 35 | 38 | 18 | 43 | 16 |
| 4 | 35 | 33 | 18 | 38 | 16 |
| 4-1/2 | 30 | 25 | 16 | 28 | 14 |
| 5 | 30 | 22 | 16 | 26 | 14 |
| 5-1/2 | 30 | 20 | 14 | 24 | 12 |
| 6 | 30 | 19 | 14 | 22 | 12 |
| BRASS. |
| 1/2 | 120 | 910 | 25 | 910 | 40 |
| 3/4 | 110 | 556 | 25 | 556 | 40 |
| 1 | 100 | 382 | 25 | 382 | 40 |
| 1-1/4 | 90 | 275 | 25 | 275 | 40 |
| 1-1/2 | 80 | 203 | 25 | 203 | 40 |
| 1-3/4 | 80 | 174 | 25 | 174 | 40 |
| 2 | 75 | 143 | 25 | 143 | 40 |
| 2-1/2 | 75 | 114 | 25 | 114 | 40 |
| 3 | 70 | 89 | 25 | 89 | 40 |
| 3-1/2 | 70 | 76 | 25 | 76 | 40 |
| 4 | 70 | 66 | 25 | 66 | 40 |
| 4-1/2 | 65 | 55 | 25 | 55 | 40 |
| 5 | 65 | 50 | 25 | 50 | 40 |
| 5-1/2 | 65 | 45 | 25 | 45 | 40 |
| 6 | 65 | 41 | 25 | 41 | 40 |
| TOOL STEEL. |
| 3/8 | 24 | 245 | 60 | 245 | 60 |
| 1/2 | 24 | 184 | 60 | 184 | 60 |
| 5/8 | 24 | 147 | 50 | 147 | 60 |
| 3/4 | 24 | 122 | 40 | 122 | 60 |
| 7/8 | 20 | 87 | 30 | 87 | 60 |
| 1 | 20 | 76 | 30 | 76 | 60 |
| 1-1/4 | 20 | 61 | 25 | 61 | 50 |
| 1-1/2 | 18 | 45 | 25 | 45 | 50 |
| 2 | 18 | 34 | 25 | 34 | 50 |
| 2-1/2 | 18 | 27 | 25 | 27 | 50 |
| 3 | 18 | 22 | 25 | 22 | 40 |
| 3-1/2 | 18 | 19 | 25 | 19 | 40 |
| 4 | 18 | 17 | 25 | 17 | 40 |
| 4-1/2 | 18 | 15 | 25 | 15 | 40 |
+---------+--------+-----------+-------------+-----------+------------+

These cutting speeds and feeds are not given as the very highest that can be attained under average conditions, but those that can be readily obtained, and that are to be found used by skilful workmen. It will be observed that the speeds are higher as the work is smaller, which is practicable not only on account of the less amount of work surface in a given length as the diameter decreases, but also because with an equal depth of cut the tool endures less strain in small work, because there is less power required to bend the cutting, as has been already explained.

When it is required to remove metal it is better to take it off at a single cut, even though this may render it necessary to reduce the cutting speed to enable the tool to stand an increase of feed better than excessive speed. Suppose, for example, that a pulley requires 1/4 inch taken off its face, whose circumference is 5 feet and width 8 inches. Now the tool will carry across a cut reducing the diameter 1/8 inch, at a cutting speed of 40 feet per minute, or 10 lathe revolutions per minute; but if the speed be reduced to about 35 feet per minute, the tool would be able to stand the full depth of cut required, that is, 1/8 inch deep, reducing the diameter of the pulley 1/4 inch. Now with the fast speed two cuts would be required, while with the slow one a single cut would serve; the difference is therefore two to one in favor of the deep cut, so far as depth of cut is concerned.

The loss of time due to the reduced rotative speed of work would of course be in proportion to that reduction, or in the ratio of 35 to 50. It is apparent then that the tool should, for roughing cuts, be set to take off all the surplus metal at one cut, whenever the lathe has power enough to drive the cut, and that the cutting speed should be as fast as the depth of cut will allow.

Concerning the rate of feed, it is advisable in all cases, both for roughing and finishing cuts, to let it be as coarse as the conditions will permit, the rates given in the table being in close approximation of those employed in the practice of expert lathe hands.

It is to be observed, however, that under equal conditions, so far as the lathe and the work is concerned, it is not unusual to find as much difference as 30 per cent. in the rate of cutting speed or lathe rotation, and on small work 50 per cent. in the rate of tool traverse employed by different workmen, and here it is that the difference is between an indifferent and a very expert workman.

An English authority (Mr. Wilson Hartnell), who made some observations (in different workshops and with different workmen) on this subject, stated that taking the square feet of work surface _tooled_ over in a given time, he had often found as much as from 100 to 200 per cent. difference, and that he had found the rate of _tooling_ small fly-wheels vary from 2 to 8 square feet per hour without any sufficient reason. The author has himself observed a difference of as much as 20 feet of work rotation per minute on work of 18 and less inches in diameter, and as much as 50 per cent. in the rate of tool traverse per lathe revolution.

It is only by keeping the speed rotation at the greatest consistent with the depth of cut, and by exercising a fine discretion in regulating the rotations of feed and cutting speed, that a maximum of duty can under any given conditions be obtained.

It has hitherto been assumed that the workman's attention is confined to running one lathe, but cases are found in practice where the lathes, having automatic feed and stop motions, one man can attend to several lathes, and in this case the feeds and speeds may be considerably reduced, so as to give the operator time to attend to a greater number of lathes. As an example, in the use of automatic lathes, several of which are run by one man, the following details of the practice in the Pratt and Whitney Company's tap and die department are given.

Lathe Number 1.--Lathe turning tool steel 3/8 inch in diameter and 1-1/4 long, reducing the diameter of the work 1/8 inch. Revolutions of work per minute 125. Feed one inch of tool travel to 200 lathe revolutions.

Lathe Number 2.--Turning tool steel 2 inches long and 1/2 inch diameter, reducing diameter 1/8 inch. Revolutions of work 100 per minute. Feed 200 lathe revolutions per inch of tool travel.

Lathe Number 3.--Turning tool steel 4 inches long and 7/8 inch in diameter, reducing the diameter 1/8 inch. Revolutions of work 40 per minute. Feed 200 lathe revolutions per inch of tool travel.

Lathe Number 4.--Turning tool steel 6 to 8 inches long and 1-3/16 diameter, reducing work 1/8 inch in diameter. Revolutions of work 35 per minute. Feed 200 lathe revolutions per inch of tool travel.

Lathe Number 5.--Turning tool steel 8 to 10 inches long, and 2 inches in diameter, reducing diameter 1/8 inch. Lathe revolutions 30 per minute. Feed 200 lathe revolutions per inch of tool travel.

Lathe Number 6.--Turning tool steel 5 inches long and 3-1/2 inches diameter, reducing diameter 3/16. Lathe revolutions 19 per minute. Feed 200 lathe revolutions per inch of tool travel.

The power required to drive the work under a given depth of cut varies greatly with the following elements:--

1st. The diameter of the work, all other conditions being equal.

2nd. The degree of hardness of the metal, all other conditions being equal.

3rd. Upon the shape of the cutting tool; and--

4th. Upon the quality of the steel composing the cutting tool, and the degree of its hardness.

That the diameter of the work is an important element in small work may be shown as follows:--

In Fig. 1153 let W represent a piece of work having a cut taken off it, and the line of detachment of the metal from the body of the work will be represented by the part of the dotted line passing through the depth of the cut (denoted by C). Let Fig. 1154 represent a similar tool with the same depth of cut on a piece of work of larger diameter, and it will be observed that the dotted line of severance is much longer, involving the expenditure of more power.

In boring these effects are magnified: thus in Fig. 1155 let W represent a washer to be bored with the tool T, and let the same depth of cut be taken by the tool, the diameter of the work being simply increased. It is manifest that the cutting would require to be bent considerably more in the case of the small diameter of work than in that of the large, and would thus require more power for an equal depth of cut.

Again, from a reference to Figs. 950 and 952, it will be observed that the height of the tool will make a difference in the power required to drive a given depth of cut, the shaving being bent more when the tool is above the centre in the case of boring tools, and below the centre in the case of outside tools. But when the diameter of the work exceeds about 6 inches, it has little effect in the respects here enumerated.

The following, however, are the general rules applicable when considering the power required for the cutting of metal with lathe or planer tools. The harder the metal, the more power required to cut off a given weight of metal. The deeper the cut the less power required to cut off a given weight of metal. The quicker the feed the less power required to cut off a given weight of metal. The smaller the diameter of outside work, and the larger the diameter of inside or bored work, the less power required.

Copper requires less power than brass; yellow, and other brass containing zinc, less than brass containing a greater proportion of tin. Brass containing lead requires less power than that not containing it. Cast iron requires more power than brass, but less than wrought iron; steel requires more power than wrought iron.