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****************** Recommended Reading: Manufacturing Engineering and Technology Marks' Standard Handbook for Mechanical Engineers ******************
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1.5.a) SLAB MILL The milling described in section 1.4. would be carried out with a slab mill if the cut were wide. Slab mills cut only on the periphery, they are held on an arbour and usually used on an horizontal mill (sec. 3.2.a) to produce flat, plane surfaces. The teeth are usually helical (Fig 1.5.1.) to allow more than one tooth to cut at the same time, preventing intermittent cutting and helping to eliminate chatter; helical teeth also reduce the effects of the teeth exiting the work at maximum depth of cut when up-cut milling, as described in section 1.4. Fig 1.5.1.
The effect of introducing a helix angle to the cutting edge now results in oblique cutting, introducing a side thrust to the cutter. This helix angle can be considered the equivalent of the plan angle of approach in a turning tool. The side thrust must be directed toward the column of the milling machine.
1.5.b) SIDE AND FACE CUTTER Side and face cutters are much narrower and usually a larger diameter than slab mills. They are designed also to cut on the side of the cutter and employ a side clearance Fig 1.5.2.. Fig 1.5.2.
Click the image for full size picture Side and face cutters are used to machine vertical slots and faces (when used on a horizontal mill). Side and face cutters may also have a helix angle, however it can be seen from Fig1.5.2.b) that the trailing edge of the tooth has a negative rake, this is usually removed to prevent chipping; for this reason and also to prevent flexing of the cutter the helix angle is usually alternated. For cutting very narrow slots (approx. 0.1 – 5.0mm) a similar cutter called a slitting saw is used, these have no side clearance and cut only on the periphery, the rubbing of the sides helps to prevent wander. The sides may however be slightly concave or ‘hollow ground’ to reduce friction and overheating.
1.5.c) END MILL An end mill, like a side and face cutter can perform both peripheral and face milling. Fig 1.5.3.
Fig 1.5.3. shows how an end mill may be considered a side and face cutter of relatively small diameter with a high width to diameter ratio. Due to the shank the end mill may only cut on one end face. The end teeth of an end mill do not cut across the full diameter of the face therefore an end mill should not be fed axially into the workpiece. End mills can be used to mill flats and steps, also to generate profiles. If fed radially into the workpiece end mills may also produce slots and open sided pockets. 1.5.d) SLOT DRILL A slot drill is similar to an end mill in that it cuts on its face and periphery. Fig 1.5.4.
Click the image for full size picture Fig 1.5.4. illustrates a slot drill. Note that there are only two cutting edges, this is necessary for manufacturing purposes to allow the end cutting edges, to meet in the centre, this allows the cutter to be fed axially into the work before lateral, or peripheral, cutting commences, hence slot drill. With reference to Fig 1.3.2. it can be seen that only two cutting edges will result in an inferior surface finish unless the lateral feed rate is reduced. This is also necessary to prevent the tool breaking; by comparing Figs 1.5.4. and 1.5.3. it can be seen that there is substantially less strength in the slot drill due to the large flutes that must be ground. Three fluted slot drills are available, in this case one face cutting edge is slightly longer to allow cutting past the centre line. These slot drills have more strength than those with two flutes and improve surface finish but are more costly to manufacture. Slot drills are used only when it is impractical to use an end mill such as blind slots and pockets. They are generally available in sizes from 2mm to 50mm.
1.5.e) SHELL MILL Shell mills (often referred to as face mills) cut predominately on the end, or face. They resemble a very large diameter end mill with a very low length to diameter ratio. Fig 1.5.5.
Fig 1.5.5.a) shows a shell mill generating a horizontal face on a workpiece. Fig 1.5.5.b) shows a sectioned view of a shell mill, it can be seen that there is a far smaller area of contact with the generated finish than there is when using a slab mill; consequently a very flat surface is generated even when the cutter becomes worn, unlike a slab mill. Fig 1.5.5.b) also shows alternative cutting edges that may be employed to give the cutting edge extra strength. A shell mill uses approximately 30% less power than a slab mill and provides a better finish. Shell mills are usually very rigid due to their very short overhang – normally being directly attached to the taper of the tool holder, Fig 1.5.6.. Fig 1.5.6
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