In this article we will discuss about:- 1. Meaning of Milling Cutter 2. Material of Milling Cutter 3. Hand of Milling Cutter Rotation 4. Elements of Fluted Milling Cutter 5. General Comments on Form of the Cutting Edge of Milling Cutter 6. Number of Teeth on Milling Cutter 7. Face Milling Cutter 8. End Mills 9. Carbide Milling Cutters 10. Precautions in Use of Milling Cutters.
Contents:
- Meaning of Milling Cutter
- Material of Milling Cutter
- Hand of Milling Cutter Rotation
- Elements of Fluted Milling Cutter
- General Comments on Form of the Cutting Edge of Milling Cutter
- Number of Teeth on Milling Cutter
- Face Milling Cutter
- End Mills
- Carbide Milling Cutters
- Precautions in Use of Milling Cutters
1. Meaning of Milling Cutter:
Milling cutter is the cutting tool used in milling machines. It has a cylindrical body, rotates on its own axis, and is provided with equally spaced teeth which engage the work-piece intermittently. The cutter teeth are machined to give cutting edge on the periphery. They may be gashed either axially or spirally. The material from workpiece is removed by relative movement of workpiece and cutters.
There are a variety of cutters available depending upon the type and location of teeth, ways of holding the cutters etc. The teeth of the milling cutters can be straight or parallel to the axis of rotation or at an angle known as helix angle. They may be on the cylindrical surface or the flat surface (one side or both sides). Further the helix may be right or left-handed.
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The cutter may be of the solid type with teeth and body on one piece or of the inserted type, the body being of low carbon steel and the teeth of any kind of tool steel. In the integral tooth cutters, the teeth are formed by cutting away material from the body of the cutter; the body is cast or forged with integral projections to which blocks of some cutting material are attached by brazing or welding.
Inserted blade cutters have forged steel bodies with slots or grooves machined in the body periphery. Cutting blades are inserted in the slots and fastened in place by some mechanical means.
2. Material of Milling Cutter:
All important tool materials like carbon steel, high speed steel, cast non- ferrous cutting alloys, sintered carbide, etc. are used for milling cutters. Solid type of cutters may be made of carbon steel, or generally of HSS. Bodies of milling cutters having blade of hard cutting materials are made of carbon steel or high grade alloy steel.
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Blades for inserted-blade cutters are made of HSS or a cast non-ferrous cutting alloy like satellite, or cemented carbide. Sintered carbide in the form of tips may be brazed to the teeth of integral tooth or inserted-blade cutters. In some designs, the blades of solid cemented carbide are mechanically locked.
3. Hand of Milling Cutter Rotation:
In order to determine the hand of rotation of any milling cutter, look at it from the front or cutter end of the spindle. If the spindle rotates clockwise, the rotation is left hand rotation or left- hand cut, and if it rotates counter clockwise then it is right hand or a right-hand cut (Refer Fig. 16.29).
The hand of helix can also be judged the same way. If from front or cutting end of a cutter the helix appears to have a clock-wise contour, it is right helix and if counter clockwise then it is left helix.
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4. Elements of Fluted Milling Cutter
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The teeth of fluted cutters are designed to cut on the periphery and in many cases on the side as well. A typical milling cutter with various angles and cutter nomenclature is shown in Fig. 16.30.
i. Arbor:
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It is the shaft on which the milling cutter is mounted and driven.
ii. Shank:
It is the parallel or tapered extension along the axis of the cutter employed for holding and driving.
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iii. Cutter Body:
This is the main frame of the cutter on which the teeth are brazed or mechanically held or are integral with it. It has either a hole for mounting on an arbor or a solid shank for mounting in the spindle or collet.
iv. Periphery:
It is locus of the cutting edge of the cutter and is an imaginary cylindrical surface enveloping the tips of the cutting teeth. It determines the diameter of the cutter.
v. Cutting Edge:
Cutting edge of a milling cutter is the only portion that touches the work. It is the intersection of the tooth face and the tooth flank of back surface. The cutting edge is generally a line which may be straight, helical or some complex profile.
vi. Gash:
It is the chip space or flute between the back of one tooth and the face of the next tooth.
vii. Face:
It is that portion of the gash adjacent to the cutting edge on which the chip impinges as it is cut from the work.
viii. Fillet:
It is the curved surface at the bottom of gash which joins the face of one tooth to the back of the tooth immediately ahead.
ix. Land:
This is the narrow surface back of the cutting edge resulting from providing a clearance angle. It never touches the work and is less than 1.5 mm in width.
x. Tooth Face:
This is the surface upon which the chip is formed when the cutter is cutting. It may be either flat or curved.
xi. Back of Tooth:
The back or flank of the tooth is created by the gullet and relief angle (secondary clearance). It may be flat or curved surface.
xii. Lip Angle:
It is the inclined angle between the land and the face of the tooth. It is also equal to the angle between the tangent to the back of the cutting edge and the face of the tooth.
xiii. Clearance Angle (Primary Clearance):
This is the angle between a line through the surface of the land and a tangent to the periphery at the cutting edge. It is necessary to prevent the back of the tooth from rubbing against the work. It is always positive and should not be small so as to weaken the cutting edge of the tooth.
For most of the commercial cutters over 75 mm diameter, the clearance is 3 to 5°. Small diameter cutters have increased clearance angles to eliminate tendencies for the teeth to rub against the work.
Clearance values also depend on the various work materials e.g., for cast iron, 4° to 7° is required; whereas soft materials like magnesium, aluminium and brass are cut efficiently with clearance angles of 10° to 12°.
xiv. Relief Angle (Secondary Clearance):
A secondary clearance is generally ground at back of the land to keep the width of the land within the proper limits. It is necessary because after several sharpening of the cutter, the width of the land increases to a point where it begins to interfere with the work. It is usually 3° greater than the clearance angle (Primary clearance). Not all cutters have secondary clearance.
xv. Rake:
If the face of a milling cutter lies along a radius of the cutter, it is said to have zero rake. If the face of cutter lies along a line on either side of the radius, it has a positive or negative rake. If this line lies on the same side of the radius as tooth, it is a positive rake. If it lies on the opposite side of the radius of tooth, it is a negative rake.
For most of high speed cutters, positive radial rake angles of 10° to 15° are used. These values are satisfactory for most materials and represent a compromise between good shearing or cutting ability and strength. Milling cutters made for softer materials like aluminum and magnesium can be given much greater positive rake (20 to 25°) with improved cutting ability.
Usually only saw type and narrow plain milling cutters have straight teeth with zero axial rake. As cutter increases in width a positive axial rake angle is used to increase cutting efficiency. Positive rake is mostly used for high speed cutters, because it improves the flow of metal along the face, with resulting lower tooth temperature, lower horse power, longer tool life and a better finish.
For high-speed steel and carbide-tipped cutters, negative rake angles (both radial and axial) are generally used. The primary object of the negative rake is to protect the cutting edge of the carbide tools which are relatively brittle compared to high speed steel. The negative rake causes the cutting forces to fall within the body of the cutter and particularly the portion supporting the inserted tooth and is thus ideal for carbide-tipped cutters.
With proper handling of tool, negative rake gives better finish on steel than positive rake. Improved tool life is obtained by the use of increased tip angle and it can withstand shock loads better. Plain milling cutters with teeth on the periphery are usually given a negative rake of 5° to 10° when steel is being cut. Alloys and medium carbon steels require greater negative rake angles than soft steel. Exceptions to the use of negative rake angles for carbide cutters are made when soft non-ferrous metals are being milled.
5. General Comments on Form of the Cutting Edge of Milling Cutter
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It is proved that coarse teeth are more efficient for removing metal than fine teeth. A coarse tooth cutter takes thicker chips and has free cutting action and more clearance space for chips.
As a consequence, these cutters provide increased production and decreased power consumption for a given amount of material removal. Also, fine tooth cutters have a greater tendency to chatter than those with coarse tooth. However, they are recommended for saw cutters used in milling of thin materials.
When the teeth of the milling cutter are straight, the cutting edge engages the work along the full width of cut at the same time. The cutting pressure on the tooth rises suddenly and continues to rise to the end of the cut, where it drops suddenly. Any sudden change of teeth load results in a shock which causes chatter. Chatter reduces tool life and gives a poor finish. But straight teeth are easier and cheaper to make and sharpen.
When the cutter teeth are formed on a helix, the length of the cutting edge in contact with the work varies with the helix angle, the depth of cut, and the position of the tooth along its path. At the beginning of contact, the length of the cutting edge engaging the work is small, it increases gradually to a maximum and decreases gradually to zero. The result is better finish, a more widely distributed load on each tooth and less chatter.
6. Number of Teeth on Milling Cutter:
Usually milling cutters and milling conditions vary so widely that it is difficult to set hard and fast rules for determining the number of teeth to be provided on a milling cutter.
For fluted and relieved cutters, a reasonably proportioned tooth can be obtained by the formula Z = 2.75 √D – 5.8
(where Z = No. of teeth, D = diameter of cutter in mm)
Fairly coarse teeth are obtained for cutters over 66 mm diameter by the formula Z = (D/12) + 8
For an inserted, blade-face mill it is better to assess the number of teeth on the assumption of their being spaced a suitable distance apart on the periphery of the cutter
Thus,
Number of blades = circumference of cutter / desired blande spacing.
7. Face Milling Cutter
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Face milling cutters with multiple-tooth inserts are used for removing metal at high material removed rates. It generally consists of a large-diameter cutter body with a number of mechanically fastened inserted tools. Large volume material is removed by radially deep and axially narrows cuts. Down milling is preferred but a backlash eliminator is required between the lead screw and nut of the milling machine.
The true rake angle of cutter depends on the axial rake, radial rake and corner angle, and it directly affects the shear angle in chip removal operation. It also affects the cutting force, horse power and temperature generated during cutting. Larger value of true rake angle lowers the cutting forces, horsepower and cutting temperatures, but leads to weakened cutting edge prone to breakage and chipping.
The face milling cutter because of its large bulky body is relatively rigid. Surface finish depends on feed rate and the number of teeth.
Cutter body diameter depends on the length of the workpiece and on the clearance available on either side of the workpiece. More number of inserts are used for higher productivity consistent with available chip space to avoid chip crowding and better chip disposal.
Since a face mill with uniform spacing between teeth may result in vibration and chatter due to a regular tooth-impact frequency, a slight, uneven spacing is recommended. Vibration and chatter can also be controlled by selecting cutting speed (lower value) appropriate for the axial depth of cut (high value).
8. End Mills
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With use of numerical control machines, end mills find more usage for operations like die sinking, pocketing, and generation of sculptured surfaces. These are capable of removing material with the periphery of the tool and with the end if it has bottom cutting features.
The dimensional accuracy of an end mill operation depends primarily on the rigidity of the set up, and on the radial and axial depth of cut, the thrust force produced. Since end mill acts like a rotating cantilever, gripped by the machine tool spindle, the end deflection for a given cutter is directly proportional to the thrust and to the cube of the effective overhang. Improved surface finish is achieved by using a large diameter cutter with many teeth and milling at a small feed rate.
The end mill geometry is a compromise of tooth edge strength, chip space, chip flow, rigidity and capability to withstand impacts as each tooth engages the workpiece. The end mill geometry involves helix angle, core diameter, radial and axial rake angles, radial and axial clearance angles, corner radius, etc.
The core diameter of the end mill determines its rigidity. The number of teeth on the end mill also affects rigidity. However chip space available decreases as core diameter of cutter increases. Chip disposal also depends somewhat on the helix angle, being easier for high helix angle. But as helix angle increases, cutting edge becomes weaker and thus its value is taken around 15 to 30 degrees.
It is found that a normal rake of 10 to 15° combined with a 30″ helix angle provides the best general purpose effective rake suitable for most workpiece materials. As the radial rake face surface of an end mill tooth is subjected to the sliding action and pressures of chip curling and forming, it should be smooth and free of ridges to enhance chip flow and minimize heat and galling.
An end mill cutter should be selected with geometry and cutting edge angle compatible with the workpiece being machined so as to minimize side thrust, avoid tooth and shank breakage, minimize wear, minimize chip clogging, and maximize material removal rate.
9. Carbide Milling Cutters
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With the development of tough, shock resistant and cratering resistant carbides, these find wide applications in milling cutters too. These could be brazed, inserted or be of throwaway type inserts. Solid carbide cutters are used on smaller sizes of end mills. Brazing of multi-tooth cutters is difficult.
Re-sharpening of cutter is difficult and costly. Any wear or breakage of one tip calls for discarding/regrinding of all the tips, resulting in increased downtime. In the case of inserted type cutters, the carbides are brazed to an insert, which is mechanically held in the cutter body. It is very easy to replace the insert and impart it desired geometry.
The disadvantages of brazed tip cutters are overcome by the use of throw-away tips. Milling cutters with modular concept are available in which, on the same body, several type of cutters for different applications can be obtained by changing the insert seats.
Throwaway carbide tips are usually 12.7 mm square size for face milling. Triangular tips with 9 and 12.7 mm inscribed circle are used for end milling, slot milling and shoulder milling applications. Tips are available in both positive and negative rake versions.
The three most popular combinations of radial and axial rakes (as regards cutter geometry) commonly used are shown in Fig. 16.50.
From the point of view of economy, doubled negative rake cutters are preferred. These can be worked at higher speeds when the cutting forces are less. These are used for general purpose milling where the machine and workpiece are rigid, depth of cut is less, heavy interrupted cuts and impact loading are experienced, for harder materials and thin stock.
Double positive rake cutters are used where machine and job rigidity is less, power and speeds available are insufficient, and for heavy depths of cut and work hardening materials. Negative radial and positive axial cutters combine the advantages of both negative and positive rakes.
While positive axial rake results in smoother cutting, low cutting forces and better chip disposal, negative rake tip results in high tip strength for impact loading and high contact.
In the case of carbide cutters, it should have enough teeth so that at least one tooth is in cut at all times to prevent impact effect. Too many teeth of cutter are not desirable since these result in higher cost, higher indexing time for tip indexing, reduction of chip space and fragile cutter.
Chip load per tooth is another criterion to determine number of teeth on a cutter. Chip space deserves good attention particularly when taking deep cut or heavy feed, or when facing wide surface. If chip gullet is less, chip may jam in it and break the carbide-tip. When milling steel, larger chip space is essential to handle the continuous chips produced.
Coarse pitch cutters (with less number of teeth) have low axial run out and can be worked at higher chip loads, which is favourable from the point of view of force and power. These incur less vibrations and are recommended for roughing steels and when the available power is limited.
Fine pitch cutters are used for interrupted surfaces where at least two teeth should be in contact with the workpiece at a time. These result in higher productivity and are used on machines permitting very heavy feed rates.
They have tendency for vibrations and thus used for roughing rigid cast iron components and for finishing steel components with a small depth of cut. Medium pitch cutters are used for machining cast iron with interrupted cuts and for finishing steel.
Surface Finish:
In milling operation, the surface finish is dependent on the axial run out of the cutter (which should be less than 0.03 mm), type of tip, feed, and other machining conditions like rigidity of machine, workpiece and vibration. In order to get true axial running of the cutter, it is necessary to tighten all the tips with uniform pressure.
In order to get good surface finish (even at higher feed rates), a wiper flat is provided, on both negative and positive rake tips, perpendicular to the axis of rotation of the cutter. This also, compensates for any inaccuracies in axial setting of the tips. The wiper flat is generally far greater than the feed per tooth.
Depending on the size of cutter and feed employed, one tip with wiper flat may not be able to wipe out all the feed marks, and as such more number of wiper inserts, having the same axial positioning, uniformly distributed along the periphery of the cutter may have to be provided.
10. Precautions in Use of Milling Cutters
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i. The milling machine must be stopped before setting up or removing a workpiece, cutter, or other accessory.
ii. All the chips should be removed from the cutter. A wiping cloth should be placed on the cutter to protect the hands.
iii. If cutting teeth are dull, these should be sharpened on a tool and cutter grinder.
iv. Burrs should be removed from the milling cutter with an abrasive stone.
v. Hole type milling cutters should be stored on a rack. It should be ensured that the teeth do not hit other cutter or metal parts.
vi. The bore size, or bore diameter and the key must fit with a sliding fit.
vii. The cutter should be rotated in right direction only.
viii. Any movement of the arbor nut during cutting must be in the direction that tightens it.
ix. The workpiece and cutter should be kept as cool as possible.
x. The cutting speed, feed and cutting fluid should be most appropriate for the material, cutter and milling process.
xi. The table surface should be protected with a wiping cloth.