3 sentences

1. 他不小心把头在门框上磕了一下。

   tā bù xiǎo​xīn bǎ tóu zài mén​kuàng shàng kē le yī​xià.

   He accidentally bumped his head on the door frame.

   Source: Dong Chinese Dictionary

2. 这把菜刀不小心磕在石头上，崩了一个口子。

   zhè bǎ cài​dāo bù xiǎo​xīn kē zài shí​tou shàng, bēng le yī gè kǒu​zi.

   This kitchen knife was accidentally struck against a stone, and a chip was taken out of the blade.

   Source: Dong Chinese Dictionary

3. 他把烟斗里的烟灰磕了出来。

   tā bǎ yān​dǒu lǐ de yān​huī kē le chū​lái.

   He tapped the ashes out of his pipe.

   Source: Dong Chinese Dictionary