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When the job calls for drilling materials like concrete or stone , a normal rotary drill can't handle it. The simple solution is the impact drill.

The Impact drill has a shaft that can move back and forth about an eighth of an inch. The back of the shaft has a corrugated plate attached to it which rests on a similar stationary plate.
When the shaft rotates and pressure is applied on the drill , the cam action of the rotating plate against the stationary one causes a rapid hammering effect which allows the bit to penetrate hard materials.

The drilling efficiency of impact drills is very low and the lifetime of the cam plates is very short if used to drill harder materials.They are best at light jobs like drilling soft brick or plaster.

I have had some experience repairing and selling these tools and I find that under heavy use , the lifetime of the cam-plates are not more than 5 to 6 months. Unfortunately, in most models , the plates are not replacable. However , the flattening out of the cam plates does not harm the drill much and the drill will still be useful for metal and wood drilling chores which do not need the impact feature.
The force required to drill is quite high and since there are 8 or 12 corrugations on the hammer plates , the frequency of the impacts for a 2000 RPM drill is about 16000 - 24000. The resulting noise is extremely annoying and the vibrations are hand numbing.

Note that impact drills are different from rotary hammers (also called electro-pneumatic or hammer drills) which work on a different principle and can easily bore large holes in solid rock.

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