In order to reduce factors that are not good for machining, such as uneven part surfaces or slightly used drill bits, a simple way to avoid "slipping" is to use a fixed-point drill. The tool is designed to leave a dent on the surface of the part so that the drill bit can be engaged during the drilling process and maintain its correct alignment to prevent the drill bit from deviating from the route.

Geometry

The shape of the spot drill is short and thick, the chip removal groove is very short, and the alloy centering drill with excellent rigidity has better effect. Its drill tip geometry is just enough to drill a shallow hole on the surface of the workpiece; Compared with standard drills, the centering drill has a very thin chisel edge, which can effectively cut the workpiece material.

Drilling

Before the drilling process, first use a spot drill to drill hole with a depth of 1 to 2 times the diameter to ensure the accuracy of the drilling process of ordinary drill bits, and reserve chamfers for tapping to facilitate tapping.

The drill tip angle of the centring bit shall be equal to or greater than the drill tip angle of the secondary bit. This can eliminate excessive cutting pressure on the cutting edge of the drill (this pressure will cause the tool to fail in advance and reduce the quality of the hole).
When both the centering drill and the drill bit use the same drill tip angle, the drill tip and cutting edge of the secondary drill bit will cut into the workpiece at the same time. This may cause flutter due to the same cut-in angle, which may cause secondary bit chipping.
For carbide drill bits, it is correct that the tip angle of the fixed point drill should be greater than the tip angle of the secondary drill bit. However, for high-speed steel secondary drills, we hope that the cutting edge can cut into the workpiece before the drill tip. Because the cutting edge of the high-speed steel drill bit is not as fragile as the carbide drill bit, we hope that the cutting edge will contact the workpiece before the chisel edge, and the drill tip will not participate in the cutting.
Carbide drill is just the opposite. Cemented carbide drill bits are usually designed to reduce the chisel edge. Due to the brittleness of cemented carbide, we do not want the cutting edge to cut into the workpiece first. Therefore, it is recommended to use a centering drill with a larger drill tip angle, so that the chisel edge can be cut into the workpiece before the cutting edge.

When is it not recommended?

When an irregular surface is being machined, such as the side or inclined plane of a cylinder, this point drilling may not be enough to hold the hole in the correct position. In these cases, a flat-bottom drill may be more appropriate.

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