To obtain best results in drilling operations, following points should be observed carefully:
(i) Machine tool must be rigid enough otherwise parts may be deformed by the cutting forces generated. Drill feeding arrangement must be directionally stable.
(ii) Accurately ground drill should be used so that uniform chips are produced by both cutting edges, otherwise unbalanced forces may deflect the tool during machining.
(iii) Axis of spindle, adapter and tool must coincide, otherwise errors and inaccuracies as described below may occur.
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(iv) Workpiece must be held very rigidly. The various types of errors or imperfections that may occur due to non- observance of above conditions are described below:
(a) Error in shape—i.e., hole diameter may not be uniform throughout the depth of hole; e.g., hole may be bell mouth (tapered) or bell shaped (convex), concave or inclined.
(b) Burrs may be formed on the circumference of drilled hole and at the entrance to and exit from the workpiece.
(c) Error in hole location which may be eccentric, or hole may not be perfect, irregular or bigger or smaller than the desired size.
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Sometimes special twist drill geometries are used for special applications.
These are described below in brief:
1. For drilling very hard materials, double point angles may be ground on the point to reduce chipping of corners of the tips. This can be done by first grinding the larger included angle and then regrinding the smaller one on the corners giving the effect of a chamfer.
2. Chip breakers may be built into drill design to prevent long stringy chips.
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3. Sometimes to obtain more accurate holes, a second pair of margins, located at the rear of the lands, which acts as steadying element, is provided.
4. Sometimes a radial lip twist drill having curved outer lips is used to produce a smoother finish in the hole and for reducing the burrs at the bottom of hole. Curved outer lips allow stress to be uniformly distributed over the entire cutting area.
5. For production of accurate hole, a drill in which the point is reduced from a chisel edge to a spiral point, can be used as it provides a better self-centering effect.
6. Drills with higher helix (fast spiral) angle, which assist in clearing chips from holes, are used for drilling deep holes in aluminium, magnesium, copper and die-cast metals.
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For drilling plastics and brass, drills with low helix (slow spiral) angles are used.
7. Core (three and four-flute) drills are used for enlarging cored, punched or drilled holes. These cannot cut holes in solid materials as cutting edges do not extend to the centre of drill. These usually incorporate a pilot to assume accuracy of hole alignment. These produce more and better finished hole than those cut by ordinary two fluted drills if all cutting edges are sharp and of same length and point angle.
These are extremely rigid and can be used at higher feed rates because of greater number of flutes.
8. Sometimes steps drill having two or more diameters produced on diameters of drill is used for obtaining more diameter holes in one pass. Similarly multiland drills, which are combination tools, are also used for doing two operations simultaneously.
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As soon as drills get worn out to a certain extent, these should be resharpened. Cutting ability of drill is reduced when it is worn; cutting forces are increased. Temperature also increases during cutting; poor surface finish is produced. If resharpening is delayed then more material would be required to be ground.
In a drill, wear occurs mainly on the flank surface and is predominant at the outer corner and chisel edge. Drill wear can be measured on a tool maker’s microscope. Permissible values of flank wear are 0.32, 0.57, 0.7, 0.85, 1.00 mm for 10, 20, 30, 40, and 50 mm diameters drills respectively.
9. Drill point of a drill should be properly shaped to obtain maximum life and efficiency. A drill point can have several forms which directly affect the production rate, accuracy of drilling and number of holes that can be drilled between succession grinds. Drills have to be suitably adapted for the desired application by providing proper point angle, relief angle and chisel edge angle depending on the material to be drilled.
General purpose drill has point angle of 118° and angle at the periphery of the drill is 8 to 12°. Flat-pointed drills with point angle of 140° are generally used to drill stainless steel and similar hard and tough materials as they permit greater feed pressure. For drilling softer materials like wood, plastics etc., long point with point angle of 90° is used.
The Recommended drill angles for different materials:
