Metal twist drills are – as the name suggests – drills that can be used to drill in steel, iron, stainless steel, aluminium, brass and copper. Depending on the material you want to work with, you would have to use different drills. Here are 3 facts you should know about metal twist drills.

Description: HSS E twist drills are ground twist drills with a 135 degree point angle and cross-ground from 3 mm. They have a 5 % Co content.

The clearance angle is created by the rear of the cutting surfaces (main clearance surface) of the drill. Care must be taken to ensure that the relief on the cutting edge of the drill is not too large, otherwise the cutting edge can break out more easily.

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Twist drills have different leads at the drill tip for different materials. The rule is a taper shell grinding with an angle of 118 degrees. It makes the drill insensitive to impact and lateral forces because the main cutting edges are robust.

Type W has a side rake angle in the range 27° – 45° and is used for soft or long chipping metals such as aluminium and copper.

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The rake angle of the drill cutting edge is determined by the side rake angle of the drill helix. It has a decisive influence on chip formation and chip removal. It therefore depends on the properties of the material. The three most important rake angles are designated by the letters N, H, W.

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The cross cutting edge can have a cross-grinding (form C) – also called split point or a pointed cross cutting edge (form A). The main differences can be seen in the diagram.

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The main cutting edge takes over the actual drilling process. They are formed by the flute or rake face as well as the undercut faces. The twist drill has two main cutting edges. They are connected by a transverse cutting edge.

For harder materials a larger point angle is useful. Typical here is a point angle of 135 degrees and a cross-grinding. It reduces the contact pressure because of the smaller cutting edge length.

In the middle of the tip of the drill is the transverse cutting edge. This is also the contact line between the two relief surfaces. The size of the transverse cutting edge can be reduced by certain grinding processes. With larger drills this is done by so-called pointing or by cross grinding. This reduces the friction of the drill on the workpiece and facilitates the feed.