A shuffling machine is a machine for randomly shuffling packs of playing cards.
Because standard shuffling techniques are seen as weak, and in order to avoid "inside jobs" where employees collaborate with gamblers by performing inadequate shuffles, many casinos employ automatic shuffling machines to shuffle the cards before dealing. These machines are also used to reduce repetitive motion stress injuries to a dealer.
Shuffling machines have to be carefully designed, as they can generate biased shuffles otherwise: the most recent shuffling machines are computer-controlled. The randomness or otherwise of cards produced from automatic shuffling machines is the subject of considerable interest to both gamblers and casinos.
Shuffling machines come in two main varieties: continuous shuffling machines (CSMs), which shuffle one or more packs continuously, and batch shufflers or automatic shuffling machines (ASMs), which shuffle an entire single pack in a single operation. Batch shufflers are more expensive, but can avoid the problems associated with some continuous shufflers, whereby the shuffling operation only slowly changes the state of the deck, and new cards may be taken before shuffling has sufficiently randomized the pack, allowing some players to "shuffle track" cards through the shuffling process.
A widely reported, but unpublished, study by Persi Diaconis and Susan Holmes in 2000 resulted in the redesign of many shuffling machines. SIAM News later published a reasonably detailed discussion of its results.
Early mechanical card shufflers
Patents regarding card shuffling devices started to appear in the United States around the end of the 19th century. It is unclear whether these devices were converted to commercial products or were discarded. These machines were often complex with many mechanical parts to achieve card retrieval, shuffling and distribution with pseudo-randomness.
In 1878, Henry Ash proposed an apparatus to shuffle cards. His device was a box with an open top where the operator would place the deck. The operator would then slightly shake the box to make the cards fall through a comb at the bottom of the box. About half of the cards would fall into the lower compartment while the rest were still in the upper compartment. The operator would take these upper cards, pack them together and do the same with the lower cards. The two packs would be placed upon each other to form a new deck and the operation could be repeated for better shuffling.
In 1887, Silvanus Tingley and Charles Stetson patented their "card shuffling apparatus". The device was composed of two card-holding boxes where the packs were held by springs. The device simulated a riffle shuffling by extracting the cards through a slot at the bottom of each box and placing them in a pile in the middle. The operator would turn a crank which was connected to gears and finally disks covered with rubber that were in contact with cards. This feeding mechanism ensured that the final stack was composed of cards "randomly" coming from the left or right chamber. The main difference with the next machines is that only one card would be ejected from a box during one turn.
In 1892, William H. Ranney filed a patent for a "card shuffling and dealing mechanism". The apparatus was basically an inclined box which was fixed to the border of the playing table. The cards were inserted from the top of the case and were trapped inside a receptacle hold by a lever. The operator would turn a crank which would slowly lower the bottom wall of the inclined receptacle. At this point, the device relied upon the friction force between the cards to achieve some kind of randomness. One card would start to slide as a result of the steepness and would attract a few cards with it. The number of cards being released at each turn would typically vary between one and five cards. The cards fell into another receptacle and the operator would turn the whole device to distribute the cards to another player. This rotation activated a roller which would distribute the bottom card out of the box. One year later, William Ranney proposed another version of his device where the original deck was split in half and cards would fall from one or both halves at once.