DESIGN OF MILLING CUTTERS

Profile-Sharpened Milling Cutters

• Profile-sharpened milling cutters are sharpened by grinding a narrow land, mainly on the tooth flanks.

• The advantages of these cutters are their longer life.

• Comparative investigations have shown that

1. The life of these cutters is from 1.5 to 3

times that of form-relieved cutters,

2. Simple manufacture (except for form

cutters) and

3. Their capacity for producing a better

surface finish.

(a) Trapezoidal

(b) Parabolic

(c) Double-back shape

Profile-Sharpened Plain Milling Cutters

• Straight-flute plain milling cutters are very seldom used at the present time because of their non-uniform cutting action.

• Only cutters of narrow width, such as side milling cutters, may have straight teeth.

• Plain milling cutters from 60 to 90 mm in diameter are used mainly for a depth of cut up to 5 mm,

• From 90 to 110 mm for a depth of cut up to 8 mm, and

• From 110 to 150 mm for a depth of cut up to 12 mm.

• The face width of the cutter should be somewhat larger (from 2 to 5 mm) than the width of the workpiece.

• The cutter diameter D is an important constructional element.

• Dependent upon the cutter diameter are such factors as

1. Heat removal,

2. chip thickness,

3. number of teeth,

4. shape of the teeth and the

5. Diameter of the mounting hole.

A Larger cutter diameter is more advantageous.

1. This enables a larger and more rigid arbor to be used,

2. The teeth and flutes can be arranged more expediently and

3. The number of teeth can be increased,4. Heat removal is improved and the5. Feed per minute can be increased.On the other hand, the amount of material

required for the cutter and its cost are increased,

• Denoting the diameter of the mounting hole by d. the thickness of the body by m and the height of the tooth by H the cutter diameter can be calculated by the formula

D=d+2m+2H

Cutter and Arbor Diameters, mm

Depending upon their purpose, cutters are classified as:

1. coarse-tooth, i.e. cutters with a large pitch and a small number of teeth and

2. fine-tooth, i.e. cutters with a small pitch and a large number of teeth.

The teeth are stronger in cutters with a coarse pitch.

►A large tooth carries heat away from the cutting edge more efficiently,

►permits a greater number of sharpenings and►the flutes provide more chip space.

A disadvantage of coarse-tooth straight-flute cutters is that they do not operate smoothly.

For these reasons, coarse-tooth cutters are used for roughing and fine-tooth cutters only for finishing.

Usually z = m.√D

• Fine-tooth plain milling cutters are commonly used for finishing and semi-finishing.

• They are unsuited for roughing since the tooth pitch is small and chip space is inadequate.

• These cutters have a helix angle ω=30° to 35°;

• They have from 10 to 12 teeth.

• Coarse-tooth plain milling cutters have fewer teeth (6 to 18) and a helix angle ω=40°.

• Since the helix angle ω ranges from 30° to 45°, provisions should be made for taking up the axial force which may be of considerable magnitude.

• The effect of the axial force can be excluded by using interlocking cutters.

Interlocking plain milling cutters can only be used in a set.

• One cutter has right-hand and the other left-hand flutes.

• Axial forces are counter balanced since they are opposite in direction.

• The set consists of two cutters: right-hand and left-hand;

• The interlocking joint is designed so that the cutting edges of one cutter over-lap the cutting edges of the other.

• This is done by making projections and recesses at the end faces of the mating cutters.

• The projections of one cutter enter the recesses of the other.

• In order to have the projections opposite the corresponding recesses, it is necessary that angle ε1

be maintained.

• This is the angle between one of the teeth and the keyway.

• Such cutters have a small number of teeth (4 to 6)

• Beginning with a diameter of 100 mm and larger, plain milling cutters are usually of inserted-blade construction.

• This saves expensive high-speed steel, lengthens the service life of the cutter body and simplifies heat treatment.

Inserted-blade interlocking plain milling cutters with a 45o helix angle

• The cutter body has tapered slots with longitudinal serrations on one wall.

• Parallel blades with a serrated back are inserted into the slots.

• The blades are clamped by plain wedges with a 3° taper.

• Before sharpening, the blades can be raised one or more serrations in the slots to restore the initial diameter.

Plain milling cutter with brazed helical tips of carbide

• In comparison with high-speed steel cutters the use of cutters with brazed helical tips of cemented carbide enables the output to be increased by three to five times and cutter life is enhanced from two to five times.

Shell end mill with tapered serrated blades

• Body of the cutter, made of structural steel, has blade slots tapered to an angle of 5.

• The blades are driven into the slots where they are held by friction.

• After sharpening the cutters several times the blades can be raised by one serration in the slots to restore the initial diameter of the cutter.

Setting over blades that are tapered in two directions

• The cutter consists of a body and blades, but the blades and the slots in the body are tapered in two directions:

5° lengthwise and

2°30' crosswise. • This means that the

slot is wider toward the bottom.

• The second taper angle (2°30') provides dual adjustment.

• As the blade is raised in the slot to the next ser ration, it also advances in the body, increasing the overhang at the face of the cutter

• As a matter of fact, if a blade with only a single 5o taper is set over one serration it will occupy position // and its overhang from the body will not be changed.

• If the blade and slot are tapered in two directions, (5° and 2°30'), the blade will occupy position /// when it is raised one serration and the over- hang from the body will be increased.

Face milling cutter with the blades sharpened separately

• The blade 2 is set in body 1 in reference to the precisely finished walls of the slots.

• A fixed amount of overhang of the blades is maintained with the aid of adjusting screw 3 and setting ring 4.

Shell end mill having inserted blades with mechanically clamped throw-away pentagonal

carbide inserts

• The cutter has a pentagonal cemented-carbide insert 1 which is fitted on pin 2 secured in blade 3.

• Wedge 4 and socket screw 5 hold blade 3 in cutter body 6.

Slot milling cutter

Half-side milling cutter

Staggered-tooth side milling cutter

Interlocking side milling cutter

Inserted-blade half-side milling cutter

Inserted-blade side milling cutter

Form-Relieved Milling Cutters

• As a rule, form-relieved teeth are designed on cutters intended for milling contour surfaces.

• The principal feature of a form-relieved tooth is that it is sharpened by grinding only the tooth face, thus preserving the profile throughout the life of the cutter.

Principles of cutter relieving and relief angles

• As the workpiece (cutter being relieved) rotates uniformly, the relieving tool advances uniformly toward its centre.

• The amount of relief, or relief value, characterizes the advance of the relieving tool.

• From the curvilinear triangle A’B’C’,

A’B’ = (π.Dc)/z and

B’C’ = K.• K = A’B’. tan α

π.Dc

• K = -------- tan α

z

• This is the main formula for determining the relief value K.

• Let us now determine the relief angles of a relieved tooth at various points along the profile.

• The shape of the profile is of prime importance in selecting the relief angle.

• If we consider a portion of AB of the cutter profile to the cutter axis, its relief angle will be constant and equal to

K.z

α = arctan -------

π.Dc

Since K, Dc and z don’t

vary

• The portion BC is inclined at an angle φ.

• The relief angle measured in a plane perpendicular to the cutter axis will vary along this portion because diameter Dc gradually decreases to Ds.

• Since Dc > Ds,the relief angle at point C will be larger than at point B.

• The relief angle αx in the normal section I-I can also be determined.

• For this purpose, we construct triangle MNL with side K1

and hypotenuse K.

• If the radius of the point being considered is taken into account, then the relief angle in a section perpendicular to the cutting edge for any point of the cutter profile will be determined by

Where R = outside radius of the cutter

rx = radius at the point being considered

Angular relieving

• In angular relieving, the relieving tool travels, not perpendicular to the cutter axis, but at some angle to it.

• In triangle D1CD, side K1 represents the displacement of point D in a plane perpendicular to the cutter axis, and side K2 is the displacement in a plane parallel to the axis.

• Then the angle of inclination τ of the tool slide in reference to a plane perpendicular to the cutter axis is determined from the equation

K2

tan τ = ----

K1

• The relief value required for selecting the cam is

• The calculated value of K can be rounded off to the nearest one that can be obtained by an available cam in the set.

• Relief value K can be determined if either K 1 or K 2 and angle τ are known. Thus

Elements of a form-relieved cutter

• Ray I is the initial position of the tool at the beginning of the relieving motion,

• Ray II is the moment the tool contacts the cutter tooth,

• Ray III is the moment the tool leaves the relieved surface,

• Ray IV is the beginning of tool retraction, and

• Ray V is the end of tool retraction and the beginning of the relieving motion for the next tooth.

• Denoting the angles between the rays by φ1; φ2;φ 3; φ4 we can find angle φ5 corresponding to the forward travel of the tool. Thus

φ5 = φ1+φ2+φ 3

Angles ε and φ4 being known, and assigning angles φ1 and φ 3 we can find angle φ2

Thus,

Φ2 = Φ2 - φ4 - φ1 - φ 3

• On the basis of angle Φ2 the tooth base thickness C1 can be determined by:

C1 = [(D/2) + (K.Φ2 / ε ) - h]. sin Φ2

• To determine the position of point M it is necessary to calculate diameter D2 by

D2 = D – [(2.K. Φ2 )/ε] -2h