Efficiency and Productivity Improvement of Milling Machines and Processes

A TREATISE ON MILLING AND MILLING MACHINES
COPYRIGHT, 1916, BY
THE CINCINNATI MILLING MACHINE COMPANY
https://   archive.org  /stream/atreatiseonmill01compgoog/atreatiseonmill01compgoog_djvu.txt


The past few years have seen an unusually rapid development in the art of milling. Experiments in cutter design, cutter and work cooling, and other branches of the art, have led to marked improvements in those elements of milling and  in the Milling Machine itself number of modifications have been done. Their compilation in complete form, as found in this book will make
them of much more general use to those interested in, and responsible for, efficient production from Milling Machines. A more complete knowledge of the action of milling cutters, the effect that
action has on production, a familiarity with the different constructions and types of milling fixtures and holding devices, the cause of unsatisfactory Milling Machine performance and the basic principles of cutter sharpening, are all necessary for the intelligent application of the modem Milling Machine.

CONTENTS

CHAPTER I  The Construction and Use of Milling Machines 7

CHAPTER II Erection, Care and Adjustment of Milling Machines 41

CHAPTER III  Toolroom Millers — The Dividing Head, etc 55

CHAPTER IV  Setting up the Machine 77

CHAPTER V An Analysis of the Process of Milling 91

CHAPTER VI Milling Machine Feeds 101

CHAPTER VII Speeds of Milling Cutters 104

CHAPTER VIII Stream Lubrication — Cutter and Work-Cooling System . . . 125

CHAPTER IX Milling Cutters —Notes on the Design and Efficiency of Modem Cutters 140

CHAPTER X Cutter Sharpening 172

CHAPTER XI Power Required to do Milling 182

CHAPTER XII Various Methods of Milling 186

CHAPTER XIII Milling Jigs and Fixtures 197

CHAPTER XIV The Sizing and Cutting of Spur Gears 247

CHAPTER XV Shop Trigonometry — Bevel Gears and their Calculation — Instructions for Cutting 267

CHAPTER XVI Spiral Gear Cutting — Calculations, Formulas, Tables, etc. 291

CHAPTER XVII Worm Gearing — Calculations and Methods of Cutting 310

CHAPTER XVIII Continued Fractions and their Application to Shop Problems — Angular Indexing 319

CHAPTER XIX Change Gears for Cutting Spirals 332

CHAPTER XX Cams — Tables for Setting the Milling Machine for Milling Spiral Cams 345

CHAPTER XXI Tables of Natural Trigonometric Functions 385

In this book we will confine ourselves to the Column and Knee Type Milling Machines and the
smaller sizes of Manufacturing Millers in most general use.

The Selection of a Milling Machine. The selection of the type of Miller best adapted for the economical production of a given class of work can not be given too careful consideration. The quantity and quality of work that the machine will produce must justify the investment.

We have gone far towards helping our customers in the solution of their milling problems and have thus gained a wider knowledge of the economic field of milling than can be obtained from the limited
experience of one shop on one class of work.

We are prepared to make complete time studies of all the milling operations on any piece of work, suggest methods, fixtures, etc., and furnish the complete equipment for doing it.

Our wide experience in this work and the great variety of milling machines made by us, enable us to recommend and furnish that size, style and type of machine which will prove most economical in view of all the conditions attendant upon its installation and use.

The most important factors:

Whether it should be a Cone-Driven or a High-Power Single Pulley type machine depends on —

The quantities in which parts are made.

The kind of work to be milled.

Power required.

Method of transmission used, whether by line shaft, group drive or individual motor drive.

Whether it should be Plain, Universal or Vertical depends on —

Whether it will be one of many machines or the only Milling Machine in the department.

The amount of time it will be used for spiral cutting.

Whether it will be used for jobbing or manufacturing.

Whether for machining flat surfaces, die sinking or gang work.

Whether it should be an Automatic depends on the quantities in which the parts are made.

The suggestions contained in the illustrations of machines in operation will be helpful in the selection of the proper machine.

The Speeds. A choice of one of 24 spindle speeds is offered, and the gears which furnish this
one speed, when reversed, will also give an additional speed in that same series. The three
series of speeds and the gear arrangements are shown in a Table

CHAPTER V
AN ANALYSIS OF THE PROCESS OF MILLING

Milling is the removal of metal by means of a tool which rotates while the work is advancing or feeding in a direction at some angle with the axis of the tool.

Classification of Milling Cutters

The tools used for milling are called milling cutters. Milling cutters as we know them have a number of teeth, but it is not absolutely necessary that they should have a large number; in fact, some milling
cutters have only one tooth. Such cutters are called fly cutters.

When a body of revolution (cylinder) is provided with cutting teeth, it becomes a Milling Cutter. When the teeth are on the outside of the cylinder,  it is called a Spiral Mill. When the teeth are on the base of the cylinder,  it is called a Face Mill. When a face mill is of small diameter and of relatively great length, it is called an End Mill. When the teeth are cut on a truncated cone,  it is called an Angular Mill; and when it is neither a cylinder nor a cone, but has an irregular outline, it is called a
Form Cutter.

From these five fundamental forms of cutters a great variety of shapes and styles of cutters for different purposes has been developed.

CHAPTER VI

MILLING MACHINE FEEDS 

The feed of the Milling Machine is the movement of the table which advances the work against the cutter. On knee and column type machines there are three possible movements of the table;
namely, lengthwise of the table (longitudinal feed), crosswise of the table and vertical. In some machines all three feeds are automatic; that is, they are power-driven, but in quite a large number
only one power feed is provided, namely, the longitudinal feed.

Two Systems in Use. There are two well-known feed systems in general use — feed in thousandths of an inch per revolution of spindle, and feed in inches per minute. With the first system the feed is driven from the spindle, so that when the spindle speed is increased, the amount of feed per minute will be increased in proportion, but the ratio between the advance of the table and the feed of the cutter will remain the same. The distance between revolution marks will therefore remain the same. With the second system, the feed is arranged in such a way that for any given position of the feed lever there is a fixed amount of feed per minute, regardless of how fast the spindle runs. A change in spindle speed will not affect the quantity of output unless the feed rate is also changed
at the same time.

Cincinnati Cone-Driven Millers are provided with 16 feed changes, ranging in the smaller machines from .006 to .250 of an inch per revolution, and in the larger sizes from .007 to .300 of an inch per
revolution of spindle.

Influence of Feed on Production. The rate of production depends directly on the rate at which the work passes under the cutter. It follows, therefore, that the feed used should be as fast as practical. There are certain conditions which frequently arise in practice, which limit the rate of feed that can be used. Quite often the piece is of such a nature that it can not be held rigidly in the
holding fixture. In still other cases the piece itself may be too frail
to stand the pressure due to a heavy feed. In such cases there are
only two things possible; either reduce the feed (table travel) and
do the work slower, or if the machine is cone-driven, reduce the
feed per revolution and increase the speed. On a High-Power
Machine this latter result is accomplished by simply increasing the
speed of the cutter, because this automatically reduces the feed per
revolution, therefore, producing smaller chips and consequently
less pressure against the work. However, high speeds have a
tendency to burn out the cutter, and therefore, if we want to increase
production by increasing speeds, we must do something to keep the
cutter from burning. This will be discussed more fully in the
chapter on Stream Lubrication.

Roughing and Finishing Cuts. Some work is milled with only
one cut to produce the desired surface. Other work requires two
cuts. In the latter case the roughing cut may be taken without
regard to the finish produced, and the only elements to be con-
sidered are: the strength of the piece itself, the power of the machine,
its ability to stand the strains and the condition of cutter, arbor
and fixture. If only one cut is taken, then the finish must also be
considered. Using spiral mills, end mills or formed mills, a very
satisfactory commercial finish is produced with from .035 to .050"
per revolution. Such a feed, and often even higher feeds may be
used for surfaces which are bolted together and which are not re-
quired to be oiltight, but for a great variety of work, a finer feed is
necessary. Work which must be scraped or which is finish ground
will easily stand .030", whereas work which must have a high finish
and does not get any subsequent operation may require a feed as
low as .020" per revolution. When very small end mills are used for
such work as die sinking, and rounding out the ends of keyways,
and various other delicate operations, a finer feed must be used,
not because of the finish, but because of the frailty of the cutter.

The relation of feed to speed on a great variety of cuts in cast
iron and steel is given in the diagrams in the following chapter on
Speeds of Milling Cutters.

CHAPTER VII

SPEEDS OF MILLING CUTTERS 

To obtain the very best results we should employ all three of these methods; that is, we should have the cutter made of some material which will retain its temper even at a high temperature;
it should be constructed in such a way that it does as little unnecessary work as possible, and there should be means of carrying off the heat as fast as it is generated. Under these conditions we can
get the highest possible speeds.


We can run at almost any speed if we are willing to regrind the cutter every five minutes, but this would not be economical. It is also possible to regrind the cutter only once every six months,
but we would have to run so slow that again this would not be economical. There is a point where we get the highest efficiency and when a shop has to mill a great number of pieces of one kind,
a few figures should be put on paper to determine which is the most economical speed at which to run the cutter.

In our own practice parts are made in comparatively small lots— several hundred at a time — and we aim to use such a combination of feed and speed as will enable the cutter to stand up for
one complete lot of pieces without resharpening.

Practical Cutting Speeds. The diagrams, Figs. 97 to 109, were developed from our own practice.

We make parts in comparatively small lots and plan our feeds
and speeds so that a cutter will mill a complete lot without resharp-
ening. The life of the cutter is therefore a factor entering into
these curves. They are applicable to modern machines equipped
with the latest design cutters and ample lubrication, where lubricant
is used. They do not show the maximum feeds and speeds that can
be used, but are a safe guide for those who are responsible for pro-
duction. It is entirely practical to very greatly exceed these feeds
and speeds on some work, but if the equipment consists of the usual
form of standard cutter as found in stock, it is necessary to reduce
the results shown by these speed curves a very substantial amount
before they can be applied.

Roughing Cast Iron with Spiral Mills. The diagram in Fig.
97 shows cutting speeds and feeds when milling cast iron at different
depths of cut with a 3' diameter cutter. The variables are the depth
of cut, the feed in inches per minute, and the cutting speed. That
part of the curves shown to the right of the heavy vertical line
drawn at 70 feet per minute cutting speed, represents good practice.
The use of these curves will be evident from the following:

Finish Milling Cast Iron Using Spiral Mills. Fig. 102
shows curves based on good practice for finishing cuts -ss" and ^'
deep, but this again does not take into consideration the influence
of diameter of cutter. These curves are a good guide for general
practice. For more exact results, refer to Figs. 103 and 104, which
are based on the use of cutters of different diameters.

Speeds and Feeds for Shell End Mills. The diagram. Fig.
105, shows curves based on good practice when using end mills,
taking cuts from ^i" to ■^'' deep in cast iron. In all of these curves
the depth of cut remains constant, the variables being as before,
the feed in inches per minute, the cutting speed and diameter of
the cutter, and there is also the additional variable, width of cut.

Safe Practical Speeds. In general practice the following cutting speeds can be safely used with modem cutters, and an ample supply of coolant for the cutter on that work which requires
coolant:

Cast Iron

Spiral Mills

Rough milling 65 to 75 feet

Finish milling 80 to 120 feet

Face Mills

Rough milling 65 feet

Finish milling 80 to 110 feet

Machine Steel

Spiral Mills

Rough milling 70 to 75 feet

Finish milling. 100 to 140 feet


Face Mills

Rough milling 60 to 85 feet

Finish milling 90 to 110 feet

Tool Steel — Annealed

Spiral Mills

Rough milling 50 feet

Finish milling 70 to 80 feet

Chrome Nickel Steel (.30 to .40 carbon drop Forgings)

Rough milling 45 feet

Tobin Bronze

Spiral Mills with Lubricant

Rough milling 90 feet

Finish milling 125 to 150 feet

Brass 200 feet

Aluminum 600 to 1,000 ft.

Other Factors Which Determine the Life of the Gutter.

The failure of the cutter is not always due to excessive speed. When
the metal is gritty a grinding action takes place, which by and by
dulls the cutter. This is especially true when cutting cast iron.
For that reason special attention must be paid to the clearance
angle of cutters, which will be taken up more in detail in the chapter
on construction of Milling Cutters.

Then there is also the legitimate wear on the cutter caused by
the edge rubbing over the work and the pressure of the chip against
the teeth. This wear is not greater with fast than with slow speeds,
but, if with slow speeds the cutter will be dulled in two days, with
twice the speed it may become dulled in one day. The amount
of work performed by the cutter will be the same for the same amount
of wear, but the time required for doing it with fast speeds will be
very much less than when slow speeds are used.