Tutorials/Mob grinder

Mob grinders are the last part of a mob farm and are used to kill mobs en masse while also collecting all dropped items at a convenient location. They are a mechanism and can be considered a special type of  trap.

Mob grinders are sometimes specific to a certain size of mob. Tall mobs usually refer to tall hostile mobs, namely skeletons, zombies, and creepers, but can also refer to cows, sheep and zombie pigmen. Small/short mobs are only 1 block tall and are spiders, pigs, wolves, and chickens.

Octopi and Ghasts have unique behaviour, spawning conditions, and/or size making them difficult to grind. Therefore, they are often omitted when someone says 'all mobs'.

Records are a creeper drop that will not be seen in mob grinders. See the record page for more information about the item.

Common Styles
The following are the most common styles of mob grinders. They are efficient and relatively small.

Lava Retrieval Traps


Lava tends to destroy drops, but this can be avoided by suspending the end of a lava stream on a ladder or forcing mobs to the termination point of a lava stream. Ladders and signs are considered solid blocks by liquids but empty blocks by mobs.

The canals from the mob farm send mobs into this final water channel. The mobs are forced into the chest high lava blade. The same water that drags the mobs into the lava also pushed the drops into a collection point. Combine with a final perpendicular-flow water current to deliver all items to a single tile, on which a wooden pressure plate may be placed and attached to redstone torches to indicate the availability of items.

Piston Smasher Trap
This trap consists of a piston which pushes a mob under a sticky piston. The sticky piston simultaneously pushes a block on the mobs head to suffocate it. This setup is connected to a repeater to allow new mobs to enter the smashing area. Water can be used to transport items from the smashing area to a collection point. The trap damages about one heart per second. This setup only works for tall mobs, but the principle can be used for smaller mobs as well.

Drowning
Drowning grinders work by sending mobs into a small area without an air pocket. Items drop in water and can be easily collected from below. This video tutorial shows step by step how to attach a simple drowning trap to a vertical mob elevator.

Uncommon
The following grinders can be effective in the right situation but are less common due to size, inconvenience, or other disadvantage.

Fall Damage Mob Killer
Space permitting, fall damage can be used to dispatch mobs. This style of mob grinders doesn't work for chickens since they glide rather than fall. Spiders should be sorted out before they are dropped as they can climb the inside of the drop-tube and potentially clog it. Mobs must drop from at least 23 blocks to recieve fatal damage. Use a water column full of source blocks to float mobs up at least 23 blocks. At the top of this column of water, have another stream push the mobs into an empty tube (the drop-tube). This tube helps prevent skeletons from firing arrows at you while they fall. If desired, a control lever can be built that will collect mobs at the top. This can either toggle a door or piston, both designed to artificially clog the macine until released.

Sunlight Death Chamber
Zombies and Skeletons burn in sunlight which can be an effective way of dealing damage and separating creepers. A sunlight death chamber is simply a room with a glass ceiling. This is a very simple mob grinder to make but has several downsides:
 * Daytime. Only the sun can burn these mobs, making this useless at night and when overcast.
 * Unblocked. No blocks can be directly above the trap (with the exception of glass).
 * Dry. Water extinguishes fire and since the mobs burn, no water can be present in the chamber. This can be simplified by having a valve which can be shut off during the day.
 * Limited Selection. This only works for two of the dozen mobs in the game.

Mob arena
Strictly speaking, this is not a mob grinder but does have a similar outcome and is a way of using the mobs spawned from a mob farm. The mobs are collected in chambers and slowly released into the arena to fight against players.

The concept is simple, 2 tall buildings, with about 15 rooms inside, that have holes at all corners, and water around the sides to push anything that goes to the side into the hole at the corner. Then once the mobs fall down, they are trapped in a simple holding area until a lever is pulled to let pistons open so that the mobs can fall out.

Video explanation and showing the concept at work: Youtube