Productivity Science of Machining I - Industrial Engineering Research by Taylor Part 1

Lesson 2 of   Process Industrial Engineering ONLINE Course (Module)  

Productivity science is the foundation for industrial engineering.

For every basic production process we need productivity science. - Narayana Rao

Productivity science is required for sand casting, die casting, forging, welding etc. Industrial engineering research is to be carried out in each of these basic production processes. Taylor did 26 years of research in turning and boring and published "The Art of Metal Cutting" in 1907.

Lesson 45 of Industrial Engineering ONLINE Course

Lesson 2 of   Process Industrial Engineering ONLINE Course (Module)      

IE Research by Taylor - Productivity of Machining  - Part 1 - Part 2  - Part 3  - Part 4 - Part 5 

Lesson 46 - Productivity Science of Machining for Process Improvement Part 2

Paper Published in the Proceedings of IISE 2020 Annual Conference.

Frameworks for Productivity Science of Machine Effort and Human Effort

Rao, Kambhampati Venkata Satya Surya Narayana. IIE Annual Conference. Proceedings; Norcross (2020): 429-434.

https://www.proquest.com/openview/5786c4e6edff56abf808b4db26f083b3/1.pdf

Taylor is the first person who wrote about a system to improve productivity in machine shop. He contributed to productivity science, productivity engineering and productivity management. It is important to study the productivity science developed by Taylor through his paper "The Art of Metal Cutting." Number of tables were shared with participants along with the paper presented in 1906. The folder containing tables is not yet available in the web space. 

Taylor did research on productivity improvement of machining in turning process for 26 years and provided number of relations between cutting variables and productivity. An attempt is made in this series of notes to present the research results and conclusions in the order of importance given by Taylor regarding productivity improvement potential.

The numbers refer to passage numbers in the paper.

ELEMENTS AFFECTING CUTTING SPEED OF TOOLS IN THE ORDER OF THEIR RELATIVE IMPORTANCE 

278 The cutting speed of a tool is directly dependent upon the following elements. The order in which the elements are given indicates their relative effect in modifying the cutting speed, and in order to compare them, we have written in each case figures which represent, broadly speaking, the ratio between the lower and higher limits of speed as affected by each element. These limits will be met with daily in machine shop practice. 

279 (A) The quality of the metal which is to be cut; i.e., its hardness or other qualities which affect the cutting speed.

Proportion is as 1 in the case of semi-hardened steel or chilled iron to 100 in the case of very soft low carbon steel. 

280 (B) The chemical composition of the steel from which the V tool is made, and the heat treatment of the tool.

Proportion is as 1 in tools made from tempered carbon steel to 7 in the best high speed tools. 

281 (C) The thickness of the shaving; or, the thickness of the spiral strip or band of metal which is to be removed by the tool, measured while the metal retains its original density; not the thickness of the actual shaving, the metal of which has become partly disintegrated.

Proportion is as 1 with thickness of shaving 3/16 of an inch to 3.5 with thickness of shaving 1/64 of an inch.  

282 (D) The shape or contour of the cutting edge of the tool, chiefly because of the effect which it has upon the thickness of the shaving.

Proportion is as  1 in a thread tool to 6 in a broad nosed cutting tool. , 

283 (E) Whether a copious stream of water or other cooling medium is used on the tool.

Proportion is as 1 for tool running dry to 1.41 for tool cooled by a copious stream of water. 

284 (F) The depth of the cut; or, one-half of the amount by which the forging or casting is being reduced in diameter in turning.

Proportion is as 1 with 1/2 inch depth of cut to 1.36 with 1/8 inch depth of cut. 

285 (G) The duration of the cut; i. c., the time which a tool must last under pressure of the shaving without being reground.

Proportion is as 1 when tool is to be ground every 1.5 hour to 1.207 when tool is to be ground every 20 minutes.

286 (H) The lip and clearance angles of the tool.

Proportion is as 1 with lip angle of 68 degrees to 1.023 with lip angle of 61 degrees. 

287 (J) The elasticity of the work and of the tool on account of producing chatter.

Proportion is as 1 with tool chattering to 1.15 with tool running smoothly. 

288 A brief recapitulation of these elements is as follows: 

(A) quality of metal to be cut: 1 to 100; 

(B) chemical composition of tool steel: 1 to 7;

 (C) thickness of shaving: 1 to 3.5; 

(D) shape or contour of cutting edge: 1 to 6; 

(E) copious stream of water on the tool: 1 to 1.41; 

(F) depth of cut: 1 with 1/2 inch depth to 1.36 with 1/8 inch depth of cut; 

(G) duration of cut: 1 with 1.5 hour cut to 1.20 with 20-minute cut; 

(H) lip and clearance angles: 1 with lip angle 68 degrees to 1.023 with lip angle of 61 degrees; 

(J) elasticity of the work and of the tool: 1 with tool chattering to 1.15, with tool running smoothly.


(A) quality of metal to be cut: 1 to 100;

279 (A) The quality of the metal which is to be cut; i.e., its hardness or other qualities which affect the cutting speed.

Proportion is as 1 in the case of semi-hardened steel or chilled iron to 100 in the case of very soft low carbon steel.

A. QUALITY OF METAL BEING CUT
THE EFFECT OF THE QUALITY OF THE METAL BEING CUT UPON CUTTING SPEED


1129 we made great numbers of experiments upon the effect of the quality of the metal being cut upon the cutting speed.

SYSTEMATIC CLASSIFICATION OF STEEL FORGINGS AND CASTINGS ACCORDING TO THEIR CUTTING SPEEDS

1133 For practical use in the machine shop it is evident that the most important and logical method of classifying all forgings and castings which are to be machined is in accordance with their cutting

speeds.

1134  We have divided all metals into classes according to their cutting speeds, which vary from one another with the common ratio of 1.1, namely: Class No. 1 corresponds to that metal which will give us the highest cutting speed which we are likely ever to use in a machine shop; Class No. 2 represents a metal whose cutting speed is that of Class No. 1 divided by 1.1, or [Cutting speed of Class 1/1.1] and so on, the cutting speed of each class being connected with the one preceding it by the ratio of 1.1.

1135 It is of great importance to connect this numerical scale of hardness (which varies by the common rate of 1.1) directly and permanently with certain qualities of metal and with cutting tools

of definitely known cutting properties. As a basis for accomplishing this we would state that Class No. 13 upon this scale corresponds to a cutting speed of 60 feet per minute, for a standard cut of 20 minutes duration when a high speed inch tool (see Folder 5, Fig. 24) of the chemical composition of tool No. 27 (Folder 21) is used, taking a depth of cut of inch and feed of inch.

1136 Our experiments indicate also that Class No. 13 represents a speed of 99 feet (in round numbers 100 feet) for the best high speed tool (Folder 20, Tool No. 1), running under the same conditions as

stated in paragraph 744.

1137 Using this data as a basis, our scale of "hardness classes"

for metals can be connected with other shapes of tools and other qualities of tool steel, other depths of cut, and other thicknesses of feed, by reference to the various tables and formula given throughout this paper.

1138 In using this classification it will be noted that the best modern high speed inch tool, if cutting metal belonging to Class 1 would have a cutting speed of 316 feet per minute with a standard

inch depth of cut and inch feed; and such a metal as this would be much softer than any steel which is cut in a machine shop.

1139 By referring to paragraphs 545 to 747, it will be seen that for what we call a hard steel forging of about the quality of a hard locomotive tire, a cutting speed of 45 feet corresponds to Class 21 and 1/4, while a soft steel having a cutting speed of 198 feet corresponds to Class 5 and 3/4.

1140 Having a clearly defined hardness classification of this sort for metals enables us to tie all of our experimental and practical work for years past together, even although tools of different chemical

composition, and therefore of different cutting speeds, were used. This system of classification is also admirably suited for use on a slide rule.

THE EFFECT OF THE QUALITY OR HARDNESS OF STEEL FORGINGS UPON THE CUTTING SPEED
1141 There are three important elements which affect the hardness or the cutting properties of steel forgings:
a Their chemical composition.
b The thoroughness with which the metal is forged, that is, the amount that the cross-section of the ingot has been reduced in making the forging and the forging heat.
c The subsequent heat treatment which the forging receives, that is, whether it has been laid down to cool in the air, annealed, or oil hardened, and the exact temperatures of annealing and the rapidity of cooling.

1142  It may be said, however, that for steel containing 0 .40 per cent of carbon or less, the percentage of carbon is a fairly reliable guide to the hardness or cutting speed.

BEST GUIDE TO HARDNESS AS IT AFFECTS CUTTING SPEEDS LIES IN THE PHYSICAL PROPERTIES OF STEEL AS INDICATED BY THE TENSILE STRENGTH AND PERCENTAGE OF STRETCH AND THE CONTRACTION OF AREA OF STANDARD TENSILE TEST BARS CUT FROM THE BODY OF THE FORGING

1143 The physical properties of steel constitute a fairly accurate guide to its cutting speed; and these properties are best indicated by the tensile strength and percentage of stretch and contraction of area obtained from standard tensile test bars cut from such a position in the body of the forging as to represent its average quality and then broken in a testing machine.

1144 It is of course impossible in most cases for any ordinary machine shop to cut test bars from the forgings which are actually used in the shop. It is, however, entirely possible and in many

cases desiräble to purchase forgings and castings with certain guaranteed tensile strength, stretch and contraction, and thus insure both the superior quality of the metal bought, and at the same

time obtain metal practically uniform in its cutting speed. In our search for a guide to the cutting speed of metals, this has proven the only reliable index to the cutting speed.

1145 On Folder 23, Table 141, and also on Folder 12, Tables 81 and 83, we record a large number of steel forgings which were accurately standardized by us in the course of our experiments. Their

chemical composition and physical properties will be found opposite the cutting speeds for our standard j- inch tool, with standard 20-minute cut, inch depth of cut arid inch feed, the speeds in

one column corresponding to tool No. 27 (Folder 21) and in the other column to tool No. 1 (Folder 20).

1146 A study of this table, however, will show that in general the cutting speeds grow slower as the percentage of carbon in the steel to be cut grows greater. In general, also, it will be noted that the cutting speed becomes slower as the tensile strength of the metal becomes higher, and that the cutting speed grows faster as the percentage of stretch increases.

1147 We have discussed at considerable length in paragraphs 506 to 579 (theory as to why pressure has no relation to cutting), a theory explaining why the cutting speed increases with an increase

of stretch and diminishes with an increase of tensile strength.

1149 We have developed the following empirical formula which is at least a partial guide to the cutting speed of steel of good quality when the physical properties of the forging are known as represented by a standard test bar 2 X inch cut from the body of the forging and broken in a testing machine.

1150 The cutting speed as represented by this formula will be found in the column in Folder 23, Table 141, opposite the actual speeds obtained from running the tools, and an inspection of the figures

given in these two columns will indicate the degree of accuracy with which the formula represents the facts.

1151  We do not feel satisfied with this formula, and shall endeavor to find a better substitute.

1152 It is well known that in the harder grades of steel, particularly those which are harder than hard tire steel, both the tensile strength and the percentage of stretch are variable, that is, materially different results will be shown from test bars cut from the same forging. For this reason also the tensile strength and stretch, in our judgment, will never prove an accurate guide to the cutting

speed of very hard forgings. However, it is the best guide at present known, and therefore has a certain value.

1153 We have tried experiments along several different lines in our endeavor to find some quick and reliable index to the hardness (i. e., the qualities which affect the cutting speed) of forgings and

castings, all, however without satisfactory, practical results. Among these we would mention:

a the ordinary abrasive tests in which metals of known hardness are used to scratch the metal to be examined;

b indenting the metal which is to be cut by pressing a punch or knife edge with a given pressure down into the face of the metal to be tested, and then measuring the extent of the indentation.

c The use of a special drilling machine, in which standard drills are used under definite pressure, and in which the distance drilled in a given number of revolutions is measured.

1156 One of the greatest needs in the art of cutting metals is a more accurate standard by which to foretell the cutting speed of forgings and castings, and this should form a subject for future

experiments.

EFFECT OF THE QUALITY OR HARDNESS OF THE CAST IRON UPON THE
CUTTING SPEED

1157 It is much more difficult to predict the correct cutting speed for cast iron than for steel, and as yet no reliable method for doing this has come to our attention.

1158 Viewed from the standpoint of chemical analysis, the cutting speed becomes slower, the larger the amount of combined or cement carbon contained in the casting, and the cutting speed

becomes less the smaller the amount of silicon contained in the casting. The amount of combined carbon, however, depends largely upon the rate or rapidity with which the cast iron has been cooled

after being poured into the mold; so that the mixture of the metal in the cupola does not constitute an accurate guide to the hardness of castings.

1159 It is needless to call attention to the fact that thin sec tions of cast iron which are cooled rapidly are harder and must be cut at slower cutting speeds than thick sections of metal made from the same heat and cast at the same time. Therefore, a study of the hardness of castings as it affects their cutting speeds must be made in each machine shop upon the particular castings actually used in order to obtain reliable results.

THE QUALITY OF RED HARDNESS IN TOOLS PLAYS A MUCH SMALLER PART IN CUTTING CAST IRON THAN IN CUTTING STEEL THEREFORE, WITH HIGH SPEED TOOLS THERE IS A MUCH LESS PERCENTAGE OF GAIN IN CUTTING CAST IRON THAN IN CUTTING STEEL

1160 It is a constant source of surprise that the high speed tools do not make the same proportionate gain in cutting cast iron as they do in cutting steel. Exact figures will be found in the table on Folder 20, where by comparing the cutting speeds of tools No. 85 (old fashioned carbon tempered tool) and No. 65 (old style Mushet self-hardening tool) with the best high speed tool No. 1, it will be noted that in cutting cast iron the best high speed tool cuts only about three and one-third times as fast as the old fashioned carbon tool, while when cutting both very hard and medium steel, the new high speed tool cuts between six and seven times as fast as the carbon tool.

In the revision of this note, the language will be simplified. Presently, the passages from the paper, "The Art of Metal Cutting" are being given as it is.

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Lesson 46 - Productivity Science of Machining II - Industrial Engineering Research by Taylor Part 2

Updated 14.7.2022,  30.12.2021, 13.10.2021,  16 July 2021

Pub. 1 July 2020