Advanced Drilling

Flute length

Flute length greatly affects hole accuracy and drill tool life. The depth of the hole and the regrinding allowance determines the optimum flute length. As the flute length increases, the drill becomes longer, thus lowering the rigidity of the drill. A low rigidity drill is prone to wander when entering the workpiece and this can result in a reduction of hole accuracy and tool life. Therefore, it is essential to select a drill with a flute length as short as possible.

Point angle

The size of the point angle will affect the chip thickness.

As shown below, if the cutting feed, fr, is fixed, a small point angle will generate a chip thickness of h. When the point angle increases so too does the chip thickness, h'. With an increase in chip thickness, chip breaking can be improved. So if problems such as chips gathering around the tool are experienced, changing to a drill with a larger point angle can solve the problem.

Web thickness & web taper

Both web thickness and web taper are important factors that decide the drills cross-sectional geometry, together with the flute width ratio.

For example, if the web thickness is increased, then the drills rigidity will also be increased. However when increasing the web thickness the flute area will become narrower leading to poor chip disposal. Additionally when increasing the web thickness, the thrust will increase, affecting the entire drilling process. High-speed steel drills, generally have a web thickness of 10%-20% of the drill diameter. For small diameter high-speed steel drills, a larger ratio will be usedand for larger diameters the ratio will be decreased. Taking into account the high toughness of high-speed steel drills, the design tends to prioritize on chip disposal. Meanwhile, for solid carbide drills used with high-rigidity, high-horsepower machining centres, the trend is towards increased web thickness, 20%-30% of the drill diameter. This is aimed to give drills more rigidity to be able to withstand the high feed rates that are required to break the chips into short lengths. Increasing the web thickness will lengthen the chisel edge leading to an increase in thrust. To reduce this thrust carbidedrills employ web thinning. Web thinning is a process used to shorten the chisel edge with the objective being to reducing the thrust exerted on the drill.

Web tapering increases a drills rigidity and is used for high-speed steel and for deep hole drills. In general, the webtaper is within a 2mm increase in width for every 100mm length. If the web taper is too large then the chip disposal propertieswill deteriorate.

Web thickness and web taper are designed according to the drill tool material and application material.

Margin width

Margin width is designed to be around 6%-10% of the drill diameter considering the drills guide properties and frictional resistance. Drills for deep hole drilling have a narrower margin width to reduce the resistance on the wall of the drilled hole. Note however that the margin width can be increased to provide a burnishing effect and to increase hole roundness and improve surface roughness.

Back taper

The back taper is designed to reduce the amount of rubbing that occurs between the drill and the hole. Back taper can be referred to as an angle but it is generally shown as the reduction in diameter over a 100mm flute length. This can be seen in the image below. When drilling soft and difficult-to-cut materials, the hole diameter becomes smaller than the actual drill diameter. Therefore, to prevent torque* from increasing it is necessary to employ a larger back taper to these type of drills. If the back taper is not increased then the drilled hole can contract and tighten on the drill, possibly leading to breakage. Note however, if the back taper is too large this will decreases the guide properties of the drill and can leading to the drill wandering. The back taper is designed according to the drill tool material and the workpiece material.

Clearance angle

The flank of the cutting edge is given a clearance angle to prevent the drill from rubbing with the workpiece. As shown below, when a drill rotates once, the cutting edge moves forward in an axial direction by fr. If the clearance angle is not made larger than θ1 (an angle formed by the drill's outermost circumference and feed), then the drill's flank will come into contact with the workpiece and the thrust force will drastically increase. Therefore, the minimum clearance angle needs to be made larger than a value found by the formula below.

The clearance angle is generally between 7°~15°. A larger clearance angle lowers the thrust and flank wear. However with an increase in the clearance angle the cutting edge strength reduces, leading to the possibility of chipping and fracturing.

Rake angle

The drill's rake angle (axial direction) correlates with the helix angle. The larger the helix angle the larger the rake angle. Although the actual rake angle differs according to the actual position along the cutting edge, the helix angle and the rake angle are equal at the peripheral cutting edge.

θ2, the rake angle at the peripheral cutting edge (helix angle), can be found by using formula below.

Primary relief (depth)

The depth of the primary relief is generally around 0.2mm～0.5mm. It is given to avoid friction between the drill and the workpiece. The depth of the primary relief is designed to prevent and reduce the amount of torque generated as the margin wears. This depth also assists in ensuring that coolant is able to reach the cutting edge. For small diameter drills, this depth of clearance is closely related to drill rigidity, thus greatly affecting tool life.

Flute width

The drills flute width determines the drill rigidity and chip disposal properties. The balance is called flute width ratio and is shown by a ratio between the land width at the drill point θ1 and flute θ2. In general, the flute width ratio is 1～1.2. However drills with superior chip disposal properties will have a larger ratio, while drills with high rigidity will have a small ratio.