23 februari 2018
An ordinary human being is incapable of achieving the feed forces used, for example, in a column drilling tool, except when small bit sizes are used. In hand drilling it is therefore necessary to reduce the feed force needed by selecting the correct bit and the application of a drilling technique.
The speed at which the cutting edges penetrate through the material is called the cutting speed. It is zero in the center of the hole and highest in the periphery, which is also where it is measured. It is specified in meters per minute (m/min). Different types of material require different cutting speeds.
The free speed of a drilling tool is specified in number of revolutions per minute (rpm) and is stated on the rating plate of the tool.
There are three points to keep in mind about the drill bit : the drill bit material, the drill bit surface treatment and the drill bit type.
High speed steel is the most common material used in the manufacture of bits. Cemented-carbide bits are essential for highly abrasive materials, such as composites with reinforcement of fiber-glass, carbon fiber or kevlar fiber, to assure that they can be used for a reasonable lengh of time between resharpenings.
By the other hand, the steam-tempered bits have less friction due to their smoother surface, so, the drilled material doesn’t stick so easily and also protects against corrosion.
There are three types of drills and the rule is that they ground in different ways: jobber drill, quick spiral drill and slow spiral drill. What varies here is the angle and the material they are recommended for.
When it comes to handheld drilling operations, it is required to have speed and precision. The machines also must be light and strong in order to avoid operator fatigue. Pneumatic drills are compact and lightweight so they are easy to hold and adapted to the hand and arm anatomy.
There are basically 4 different types of drills: pistol grip drills, straight drills, angle drills, and screw-feed drills.
Each specific type of drill is appropriate for the working environment and it influences directly the ergonomics for the operator. Uncomfortable working postures are common in handheld drilling operations, but this problem can often be avoid by using the right type of tool.
Some examples below:
When drilling in a high position, use a straight drilling tool
When drilling in a low position, use a pistol grip drilling tool