Mob grinders are the last part of a mob farm and are used to kill mobs in massive amounts 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, chickens and ocelots.
Squid and Ghasts have unique behavior, spawning conditions, and/or size making them difficult to grind. Therefore, they are often omitted when someone says 'all mobs'.
Mob grinders will not produce Music Discs or rare drops as the mob deaths do not meet the conditions for those items. However if built in a particular way the conditions can be favorable to induce such situations, but will require the player to impart minimal input in such instances.
 Common Styles
The following are the most common styles of mob grinders. They are efficient and relatively small.
 PC/Xbox Minecraft Mob Grinders
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 or players. Also consider using water to drown mobs to ensure that no drops will be lost. You may use a simple 8X8 area with a mob spawner leading to a water drop that mobs can push each other into, making so the drops come out when mobs spawn, and makes lag a nonoccurrence.
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 or lamps to indicate the availability of items. Alternately, the stream can empty the items into a hopper connected to a chest, (or a further item sorting system) for retrieval at the players leisure. As of the second Pre-release of 1.9, Endermen do not pick up Stone or Cobblestone, so you do not have to worry about them ruining your system of spawning the mobs.
 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 mob's 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.
The downside to this trap is the slow rates of kills, since each mob needs about 10 seconds of suffocation before it dies. This problem can be fixed by having a row of pistons push together at once, suffocating multiple mobs in every smash.
Timing is very important for this trap to work efficiently: For single mob smashes, a loop of at least 30 repeaters set at maximum are needed, giving 12 seconds per loop. With this set up, turn on the loop for a little more than 10 seconds (to ensure they die) and have the loop off for the rest, giving 10 second kills and 2 second load times. For a full row of 8 pistons, the load time is approximately 6 seconds, meaning the loop needs to have 40 repeaters giving you a 16 second loop. Load time for a stream of 8 blocks long and 2 blocks wide, with pistons on both sides, is about 8 seconds. This amounts for at least 48 repeaters.
Another downside to piston smashers is the inability to handle spiders. You would need another way to sort out spiders (such as a cactus choke).
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.
This type of trap is advantageous because of the impossibility of losing items to lava/cacti as well as the possibility for compact designs.
 Fall Damage
A fall damage grinder consists on a mob farm where mobs are spawned and a platform where mobs fall to death. Different methods to can be used to make the mob farm and to push the mobs to fall from a lethal height, and also to retrieve the loot from the mobs.
The fall damage formula is: x-3=damage in 1/2 hearts taken, when x=blocks fallen, so 22 blocks will leave a normal mob (creepers, zombies, etc.) with 1/2 a heart.
 Item sorting hopper platform
Build a hopper platform under a mob farm, where mobs will fall to death. Hoppers automatically retrieve the loot and can be connected to chests to storage it in several ways. In the following video the hopper platform is connected to an item sorter mechanism:
 Fall damage with railroad retrieval system
You can make a mob farm in the air and then set a platform area where mobs will fall to death. To retrieve the resulting loot you can build a railroad system in such a way you will be automatically retrieving all the loot being away from keyboard. Falling mobs will hit you and could stop your riding, so it's important to have the smashing area covered with powered rail and the curves made out of normal rails should be out of reach from the falling mobs.
In the following video you can watch how it works, the mob farm uses water dispensers pushing mobs in cycles of time, the platform is completely covered with powered rails (that are powered with redstone torches from below) and normal rails outside the falling area.
- Firstly put water under your spawner on one side so that it will all flow into one direction.
- Where the water ends, dig a 2x2 shaft at least 20 deep (best to do more for insurance as they sometimes grab the wall and reduce damage)
- At the bottom of this chamber make a little room with a ladder or stairs to the surface or your base, etc. (Down here is where you will collect your drops from)
- Remove all lighting from near the spawner and make it completely dark, but light the bottom chamber to prevent anything spawning in your collection area.
- Note that it would be smart to dig under the spawner and look for any cave systems, light them up so nothing spawns, this will increase your overall yield.
 1x1 Fall Chamber
This is a good method of killing mobs and getting their drops. How to set up:
- Dig a 1x1 hole down 23 blocks (minimum). With fall damage mobs (or players) don't take damage for the first 3 blocks and take 1 damage (half a heart) for every block over 3. So to do 20 damage (10 hearts) the needed distance is 23. If you want them to drop XP the distance needs to be 22 so they survive with 1 health left, then a player must deal the killing blow (actually this will work so long as any player deals damage to it, whether it's the final blow or not).
- Connect the bottom to some stairs leading to your house. (You will go down here to get the drops.)
- On the top, make a piston disappearing floor using a sticky piston and a NOT gate. See the section Tutorials/Piston Uses for more information on this.
- Connect the floor to a lever in your house.
- Build fences all around the hole and connect to your mob farm.
You are done!
How to operate:
- Wait for the mob to stand on the floor.
- Pull the lever.
- Pull the lever back, then go down the stairs.
- Get the drops, and repeat.
Note: This will not work with chickens, ghasts, or blazes due to their flying ability. This will also not work for spiders as they need a 2x2 minimum to fall and often grab onto the edges of walls and reduce fall damage. Ocelots and snow golems are also immune to this because they do not take fall damage. Make sure you can see the grinder from the lever, so you know when to pull the lever. You can make more holes and make this bigger. Just repeat the process.
Another tactic is to place water on a higher area so it is flowing. Put your hole at the end of the water. I recommend doing this in a large row, and inside a completely dark building so mobs will spawn even in daytime. You will need a separate shaft connected to it in order to retrieve the dropped supplies. This is advantageous because it will work by itself 24/7 and is fairly simple to build. Once completed, all you have to do is look for stuff on occasion. You do not have to watch and wait for a mob to walk into place, and this works with any mob, even ones that can fly.
 Minecraft Pocket Edition Mob Grinders
 Minecraft PE Funnel Fall Damage Grinder
There are many different styles of mob fall traps out there, but very few of them work in Minecraft PE because of the current lack of redstone and levers. This trap design may not be as efficient as some other traps, but it works well, is safe for the player, and is easy, albeit slightly time-consuming to construct. The first thing you need to do is find a body of water. This should not be overly hard to do. It doesn't matter how big the body of water is, it just needs to contain a water source block. Walk a few steps away from the water source. 3-7 blocks is optimal, but you can go anywhere as long as you can get water from your source to flow there. Now dig straight down. This should be safe, as there are no underground lava chambers or caves in Minecraft PE. You should dig down at least 23 blocks. When you are at the bottom, make a 2x1 hallway that extends four blocks away from the shaft. This would mean that the entire length of the hallway is 5 blocks, including your shaft. At the end of your short tunnel (the fifth block), dig one block down. Now make a straight staircase back to the surface. If you were to walk from the bottom of your shaft all the way up to the surface, you should not have to turn. Another alternative is if you make a large tower up (25 blocks) and a hole for them to fall through. The spawn area should be fairly large. When you are at the surface, walk back in the direction of your shaft. When you are two blocks away from it, dig another shaft straight down. You should emerge from this shaft in the hallway you dug earlier. Remember to keep this all lit up so that you do not get monsters spawning prematurely! Go back up the stairs. Go back to your holes. You may want to temporarily block the first hole to avoid falling into it. You are now going to create your mob farm. Make a two block high wall directly around your hole. Make a 2 wide ledge around the upper layer of your wall, like this: This is a top view of your structure, the numbers represent how many blocks thick the wall is. O is the hole:
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 2 2 1 1 1 1 2 O 2 1 1 1 1 2 2 2 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
And a side view:
c c c c c c c c c c
Next, build a two high wall up from the outer ring of your ledge. Build a 2-thick ledge around the top of this wall. Continue doing this until you feel your mob farm is big enough. Cap it off with a roof. Its silhouette should end up looking something like this:
c c c c c c c c c c c c c c c c c c c c c c c c c
If you fall into it, you will not be able to get out with out placing extra blocks. If you fall down the hole, you will die. Be extremely careful, and if you do end up getting trapped, remember to remove all the blocks you used to get yourself out.Make sure that there are no sources of light in the mob trap.Go back down your staircase. Place a ladder in the block between the first and second holes. Go to the fifth block, the one where you dug one down. Place blocks over the fourth block so that you have a window you can see into the trap from where you are standing, but the monsters should not be able to get out. I used glass for this step. Place another ladder beside and above you. Here's s side view:
c c c c c c c c c c t g c l l c c c c c
C is cobblestone, or your building material. G is glass. L is ladder, and t is an optional torch. Go back up your stairs. Go to the first hole, unblock it, and let water from your nearby source block flow down it. Go back down the stairs. You should be able to see the water as a column. Soon, monsters will come crashing down the second hole. When this happens, break the ladder that is beside the water. The water will flow over to you and be stopped by the second ladder. It will bring you the monster's drops and the ladder. Put the ladder back in its place, underneath the torch. If a monster falls down while the water is flowing, it will not die. However, it will be very easy for you to kill it by striking at its feet. Creepers will not blow up and skeletons will not shoot you because they cannot see you. Your trap is complete, enjoy your spasmodic monster drops and having the living daylights scared out of you the first few times a monster falls down the hole.
 Minecraft PE Fall damage Grinder
Alternatively, it is possible to set up a standard water channel fall damage mob grinder in Minecraft PE, but the locations you can create them are more limited, and you can only create the water channels one layer tall. This is only true before version 0.7.0.
You will need to find a wide area of shallow water, preferably at least 18x18, and no deeper than 5 water blocks. Working on ice is even easier. This build relies on the fact that solid blocks can replace water source blocks.
Simply build a standard 4-channel configuration, but fill in ALL the water blocks except for the two at the end: (TOP VIEW)
X X X X X W W X X S S X X S S X X S S X X S S X X S S X X S S X X X X X X X X X X S S X X X X X X X X X X W S S S S S S S S S S S S S S W X X W S S S S S S S S S S S S S S W X X X X X X X X X X S S X X X X X X X X X X S S X X S S X X S S X X S S X X S S X X S S X X W W X X X X X
Now, dig down in the center 23 blocks down (it is safe to dig straight down because there are no lava lakes or caves, yet, in PE), then laterally 10 blocks, then build a staircase back up to the surface.
Once back up top, put a 3-high wall around the structure, and fill in all of the remaining water blocks with whatever material you have. Dig out the sand blocks shown above to start the flow of water, and put a roof over top. Litter the top of the roof and surrounding areas with torches, then go downstairs and start collecting your drops!
You can also add multiple layers with reduced efficiency for subsequent layers. Since water source blocks are only reliably present at sea level, any subsequent layer will be dry farms. Build additional 3-high walls on top of the previous structure, put a new roof on it, then dig out some of the floor blocks to expose the area beneath. The more blocks you remove, the more likely mobs will fall, but the less likely mobs will spawn. Checker patterns of 3x3 blocks or simply removing 2x blocks on the diagonal seem to work. Then remove your torches, dig out of the new layer, seal it up, and replace the torches on the roof. Repeat as materials allow.
At this time, the only drops you can get are feathers, arrows, string, and gunpowder.
Another easy way to build a fall trap is to start out with multiple (Around 7-11) stacks of 64 dirt or any type of stone. start by building a 4x4 tower. You don't need to have it be specifically 4x4 though. Any size will work.
Build up until you reach a lethal height. Once at that height expand to the sides of the tower about three blocks and build a wall and roof. Jump down respawn and collect the items you dropped! Now back at ground level make a doorway.
don't worry about light because the spawn room should be to high up for sunlight to get in and effect the spawn/ spawn rate. the next step is optional but better because you wont get mobs falling on you and hurting you when you collect the drops. (yes it happens often) all you need to do is make a river flowing to a collection pool and voila! You have an easy effective and pretty cool looking fall trap.
 Minecraft Pocket Edition Multi-Level System
With the recent addition of buckets to Minecraft Pocket Edition, many different opportunities to farm monsters have come. Here is a practical design that yields well over 400 drops per hour when properly set up. Before building the spawning chambers, a good idea is to create an infinite water source, so you can refill your buckets when the water has been used. To start the spawning tower, build a 1-block tall 22x22 block wide ring of your building material. Then add 4 7x7 rings of cobblestone 2 blocks away from the sides of the larger ring that was previously made.
Once you have made all 4 smaller rings, add water source blocks in 2 spots in the 4 corners
As mentioned, place the water in all 4 corners. Once you place all the source blocks in the corner, the water should end where the 7x7 rings of cobblestone end. At the center of the 4 7x7 rings, place 2 source blocks.
The water should flow all the way to the end of the 7x7 rings. Then fill in the 7x7 rings with your preferred solid block. The bottom part of the chamber should now be finished. ++++picture 5
The following grinders can be effective in the right situation but are less common due to size, inconvenience, or other disadvantage.
 Cactus Choke
This mob grinder is for killing off spiders, sorting out spider-size/spider-jump/spider-climbing related issues. This is a simple design, relying on the spider's 2x2 size and jumping ability. The grinder stops all spiders at a bottleneck, where the water-conveyor narrows from 2 blocks wide to 1 block. A cactus is placed on the side, on top of a sand block (make sure the cactus is out of the water so it doesn't destroy the drops).
 Alternate Cactus Choke
Similar to the above cactus choke, but more efficient and items won't get stuck behind the sand block. At the end of a stream, there is a small gap with a stream and a cactus behind that. This allows cacti to kill spiders without having them climbing to get to the cactus. This can also sort mobs from spiders similar to the above cactus choke if done right.
 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 receive fatal damage. Use a water column full of source blocks to float mobs up at least 28 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 machine until released.
Here is a how-to on a simple but effective Fall Damage Mob Trap/Grinder.
1. Construct a 20x20 platform of any solid block 28 blocks high above the ground.
2. Make walls surrounding platform six blocks high.
3. Inside the walls, make four water channels coming from the middle of each four walls to the center. This should create a 2x2 square of dry block in the middle. Cut out that square, that will be the hole the mobs fall through.
4. Make the channels 2 blocks high so mobs can't jump out of them.
5. Make four platforms between the four corners that the water channels create that are flush with the top of the water channels. (this will be the area that the mobs spawn on)
6. Make a roof over the whole thing. You should be able to walk around inside your spawner with a space 3 blocks high. A second layer of ceiling blocks can be added to prevent Endermen from spawning.
7. Remove any torches to make it dark inside.
 Fall Damage Experience Grinder
When a skeleton, zombie, or creeper (Which have 10 hearts of health) drops 22 blocks it will leave it with one half-heart of health left. Endermen have 20 hearts, so at 42 blocks, they will have 1/2 of a heart, and spiders have 8 hearts, so they need to fall 18 blocks, and the hole has to be 3x3 or bigger to prevent the spiders climbing the walls. A canal can then carry them to a hole in the wall, where the player can reach in and punch them to gain both experience and the drops. This works best when you make this in a dungeon.Firstly, make water flow to a 1x2 hole in the dungeon. Then,have water source blocks alternate with signs to the desired drop height. At this point have water flow to its end and then mine down to the place where the mobs hit. It is best to leave them with half a heart, because mobs do not disappear over time, but pickups do. Then mine a pathway to your house. Then go back to the fall point and place a block so that only the mobs' legs are showing so that they cannot hurt you. Then you have a working mob trap.
 Sunlight Death Chamber
Zombies and Skeletons burn in sunlight which can be an effective way of dealing damage and separating creepers. This can also be done with monster spawners, a tutorial is available here. A sunlight death chamber is simply a room with a glass or hollow 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 Skeletons and Zombies. (As of 1.0 this no longer works for Endermen).
 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
 Egg Spawner Grinder or "Mob Vending Machine"
With the addition of the mob spawner eggs in Minecraft 1.1, it is possible to create an egg spawner based grinder. Very useful for getting mob based items such as bone and rotten meat in adventure maps, but useless in survival because it requires spawn eggs.
You put an egg over a block which have a sign or a ladder blocking the water in the upper layer to flood down and at the same time pushing mobs up, then they go through a corridor and falls in a 1x3x5 corridor covered in the upper block with air, midblocks with cobweb (or air) and in the down block with water and with a dispenser that shoots fire arrows with a clock mechanism. It is possible to shoot the eggs with a dispenser creating a "mob item vending machine".
You only need to be careful with one thing: NEVER put creepers with other mobs in the machine because if they start to fight inside the water canal the creeper could blow up and destroy part of the machine causing lava to flow outside of the machine in 90% of the cases killing you. It is secure to put creepers alone in the machine even if you put 64 or more creepers in so they don't blow up. And the overhang in the egg placing area prevent creepers and other mobs to attack you one time that the water push the mobs up in the moment that they spawn.
To automate the entire mechanism, place a dispenser in the egg placing area, stock it with eggs, and connect it with redstone to a button. Connect a lever to a clock mechanism to shoot the arrows. From here, it's just a matter of pressing a button to spawn a mob, and then using the lever to launch arrows at it.
To retreive your drops without any chance of despawning, put a hopper in the ground/floor at the end of your water flow or wherever your items drop out. Put a double chest under the hopper, and make sure to go empty the chest before it fills up and items pile up outside. Simplest way to do it: W=Water, O=Air or Block, H=Hopper
 Minecraft PE
A simple yet effective way of making PE grinders is making the farm, then adding holes all over the floor of the farm, which lead to a lit chamber where the drops are. All you must do is wait, and the drops come straight to you.