In this article we will discuss about the important factors that require due consideration in the milling work: 1. Selection of Milling Machine 2. Selection of Proper Tool for the Job 3. Gang Milling 4. Design of Work-Piece for the Milling Operation 5. The Milling Process 6. Roughening 7. Finishing Cuts.
1. Selection of Milling Machine:
There are many factors to be considered in selecting a milling machine before deciding to use a vertical, a horizontal or a universal machine. In particular the best way of clamping the job should always be studied carefully. Workpieces of U-shaped cross-section which have to be machined inside are usually best machined on a horizontal miller (Fig. 16.70) so that the coolant and chips flow clear from work.
If however it is essential that machining process be under constant observation, as for instance in the milling of dies, then the work is best machined on a vertical miller (Fig. 16.71). Another important point worthy of close study is the selection of most suitable milling cutter for the job, for instance plain milling cutters, end mills, side milling cutters etc. Plain milling cutters, of course, are used with horizontal or plain milling machines.
It should be ascertained that the inside taper in the spindle nose is clean and undamaged. Before inserting the cutter arbor into the spindle taper, take a clean rag around round a piece of wood and clean the inside taper carefully. Also clean the taper of the cutter arbor.
Other points which justify careful examination can best be given in the form of questions as follows:
i. Is the milling spindle property adjusted and has proper play?
ii. Is there excessive play in the nut for table, or is there too mush play between worm and rack in the table?
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iii. Is the coolant-supply properly working?
iv. Is the machine properly prepared for the job?
v. Is the knee properly clamped to the column?
vi. Is the saddle properly clamped to the guide ways?
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vii. Is the overarm properly clamped?
viii. Are the arbor supports mounted and securely clamped?
All these parts of the machine must be properly tightened up and clamped, otherwise the milling cutter arbor does not function properly and the work-piece will not be correct to size. Find out whether the dogs are correctly adjusted.
2. Selection of Proper Tool for the Job:
Plain milling cutters and similar tools cutting at the cylindrical surface for instance end mills, shell end mills etc., should have widely spaced teeth. Also the teeth should be sturdy and cut on a spiral angle of at least 35 degrees.
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When using spiral toothed cylindrical cutters on the older types of milling machines, attention has to be paid to the fact that the end thrust must always be towards the spindle end.
Therefore, always use right hand milling cutters with left hand spiral and vice versa (Fig. 16.72).
Hole type plain milling cutters and end mills should have a diameter as small as possible and a bore as large as possible. Select a cutter with an axial pitch and at least equal to the width of cut to be taken.
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For better results the axial pitch may be twice, thrice or even four times the width of cut. It is a mistake to believe that a milling cutter will work uniformly, that is, without fluctuations in the power required to remove the metal; if more than one tooth is cutting at a time.
Hole type end mills are not recommended for ordinary face milling, but for milling shoulders (Fig. 16.73) they may be used with arbor yoke.
Face milling cutters are recommended for cuts similar to Fig. 16.74.
Spiral end mills are recommended for jobs similar to Fig. 16.75. They should not be used for milling slots, grooves etc. (see Fig. 16.77). For the latter named jobs it is better to use slotting end mills with only a few but sturdy teeth (see Fig. 16.78).
3. Gang Milling:
(In gang milling, two or more cutters are mounted on the arbor and all of them remove the metal simultaneously). Balance the cutters using some cutters with right hand spiral and some with left hand. The flutes should form an angle in order to allow the chips to flow off laterally and prevent them from sticking (Fig. 16.78).
If gang milling has to be carried out on an old type of machine attention must be paid to reduce the axial thrust to the very minimum and to entirely eliminate it, if possible. If there is any axial thrust it must be towards the spindle nose only.
Has the milling cutter proper cutting angles?
Cutting edges with no rake (i.e., when the faces of the teeth are ground radially) should be avoided entirely for regular milling cutters and if possible on form cutters too.
The following angles show the best results:
The angle given must be measured in direction at right angles to the face of the teeth.
Is the milling cutter in good condition?
Dullness:
Dull tools give rough finished surfaces and need more power in operation. Cutting edges of cutter must not be allowed to become red hot during sharpening.
Out of Truth of Cutter:
Incorrect clearance angle gives a poor cut and an uneven finished surface, whilst at the same time the durability of the tool is impaired.
Other points of importance are:
(i) Are the rakes and clearance angles correct for the material which is being machined ?
(ii) When using form cutters, does an incorrect rake cause alterations in profile of work-piece?
(iii) Regrinding of cutters at the right time is of primary importance. Hard-alloy cutting edges should be lapped.
(iv) Is the tool properly clamped?
(v) Is the cutter arbor clean and in good condition?
(vi) Is the cutter arbor rigid enough? It is out of truth?
(vii) Are the sides of the collets and of the milling cutter dead parallel to the axis of the cutter arbor?
(viii) Are the key ways in the cutter, in the collets and the key itself parallel to the axis?
The above conditions must be carefully observed or else arbor of the milling cutter will be bent when the clamping nut is tightened.
Clamping of Plain Milling Cutters:
The cutter arbor should be as thick as possible and the distance between cutter and spindle end or arbor should be as short as possible. Therefore mount the milling cutter as near as possible to the spindle end of arbor.
The cutter arbor specially on heavy machines, should have a sleeve of ample diameter. Also arbor yoke and overarm brace should be used.
Clamping of Shell End Mill:
The cutter arbor should be as short as possible and drive by key should be at right angles to the axis of the cutter arbor (Fig. 16.79). This method of transmission is superior to keyways especially for milling cutters of small diameter, because such cutters have very little material between flutes and bore and consequently are liable to breakage.
4. Design of Work-Piece for the Milling Operation:
Is the work-piece rigid enough for the milling operation? Does it withstand the pressure of the cutter or does it yield? These are the important questions one should put.
Investigate whether it is feasible to strengthen the work-piece by providing ribs, thicker walls etc., in order to make it more sturdy and thereby reduce the machining time. Is it possible to make the work-piece of a different material, which would be just as suitable for the job, which could be machined faster and to better advantage?
Investigate also whether the job is designed with due regard to facilitating economic machining operations.
Changes in the recess radius at the ends of flutes, keyways etc., are often the means effecting considerable reduction in machining times. Sometimes by providing flutes, keyway etc. that go right through the whole length of the work-piece instead of using short ones running only part of the length, it is possible to use sturdy side milling cutters instead of small end mills [Figs. 16.80 (a) and (b)].
Is it possible to reduce machining time by making two parts of the work-piece instead of one?
Is the work-piece properly clamped?
Is it provided with proper pads, margins on other surfaces where it can be clamped to advantage?
If not, investigate whether it is possible to provide auxiliary surfaces for clamping, which can be removed after machining. The questions of clamping surfaces merits careful study and proper provision in design will often avoid the use of expensive clamping fixtures.
As a general principle, the work-piece should be mounted upon the table in such a position that the milling cutter is allowed to work as near to the table and column as possible. In this connection refer to Figs. 16.81 and 16.82. If necessary change over to another type of machine to a vertical, if a horizontal miller proves impracticable or vice versa. The height of the clamping fixture should be kept as small as possible.
5. The Milling Process:
Machining time, quantity of chips per unit of time, power absorbed by the machine, durability of the cutter and last but not least the smoothness of the milled surface are all influenced by the cutting speed and by the amount of feed and depth of cut chosen for the job.
6. Roughening:
On lathes, the rates of feed depend on the number of revolutions of the spindle consequently on the cutting speed. On modern milling machines, however, the rates of feed are independent of the spindle speed, so that an increase in the r.p.m. of cutter the spindle does not influence the machining time.
Moreover, it is not practicable to increase the cutting speed too much, as milling cutters lose their sharpness much more quickly under high speeds than under lower speeds, and soon begin to vibrate. The durability of a milling cutter decreases much more quickly than in direct proportion to the increase in cutting speed. The machining time on milling machines, therefore, can be reduced only by increasing the amount of feed.
Sometime however in certain jobs, the maximum permissible feed need not have to be used. This may be found necessary particularly in case of jobs which are not sufficiently rigid (because of thin walls or other reasons) or when the milling cutter has teeth which are not even sturdy as is often the case with thin end mills, thin straddle milling cutters, circular saws etc.
In such cases it is of course permissible to increase the cutting speed a little above the figure given in the table on next page. However it is not advisable to increase the cutting speed in proportion to the reduction of feed rate, as such an increase would result in the cutter becoming dull too soon.
Under conditions otherwise it will be found that cases will sometimes arise where the cutting speed has to be reduced so that the vibrations of the cutter are not of the same frequency as the inherent vibration of the machine.
Vibrating milling cutters can often be made steady by reducing the cutting speed.
The main object of roughing, apart from reducing the total machining time is to remove the greatest possible amount of chips per unit time, as is practicable. The quantity of chips removed depends upon the feed as well as upon the depth of cut.
Experiments have shown that the power absorbed by a milling machine is greater when milling with deep cuts and a small amount of feed than with light cuts and a great amount of feed. Moreover, in the latter case the forces on the cutter and consequently the stresses upon the cutter arbor are smaller.
To summarise the foregoing, it may be said, that better output can be obtained with a high feed rate and a small depth of cut, (upto about 5 mm), the cutting speed being of secondary importance. However in some cases it may be necessary to depart from this rule, i.e., by using high feed rate with small depth of cut.
If for instance one operator is attending to several machines it may be more advantageous to take deep cuts (and consequently less cuts) with a low feed rate. In such a case the time per cut is increased, but the total machining time is reduced, especially where the cut is short and the time needed for setting the depth of cut represents a considerable percentage of the total time.
The above figures apply where the work-piece is sufficiently sturdy and the milling cutters are rigid. The cutting speed has to be lowered when the chips section is increased. In the case of materials which are not homogeneous and also in the case of the hardest material in each category the lowest figures in the above table should be used. For soft materials use high figures.
7. Finishing Cuts:
The main effect of finishing is to secure a smooth surface. The amount of metal removed is of less importance. Every revolution of the cutter (not every cut of each tooth as one may be inclined to believe) produces a wave-like marking on the milled surface, and the surface will be smoother, if the distance between two markings is shorter.
Consequently it may seem feasible to increase the cutting speed for a given rate of feed—but this could only be done to the detriment of the durability of the tool. Therefore, the correct course to adopt is to reduce the feed rate and simultaneously increase the cutting speed within reasonable limits.
By a proper selection of both factors it is possible to obtain a smooth machined surface even when using coarse pitch cutters. Only a few materials—for instance special alloy steels—require special milling cutters.