The thrashing machine, or, in modern spelling, threshing machine, was a machine invented by Scottish mechanical engineer Andrew Meikle for use in agriculture. It was designed (c.1796) for the separation of grain from stalks and husks.

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The 1881 Household Cyclopedia said of this machine,

"Since the invention of this machine, Mr. Meikle and others have progressively introduced a variety of improvements, all tending to simplify the labor, and to augment the quantity of the work performed. When first erected, though the grain was equally well separated from the straw, yet as the whole of the straw, chaff, and grain, was indiscriminately thrown into a confused heap, the work could only with propriety be considered as half executed. By the addition of rakes, or shakers, and two pairs of fanners, all driven by the same machinery, the different processes of thrashing, shaking, and winnowing are now all at once performed, and the grain immediately prepared for the public market. When it is added, that the quantity of grain gained from the superior powers of the machine is fully equal to a twentieth part of the crop, and that, in some cases, the expense of thrashing and cleaning the grain is considerably less than what was formerly paid for cleaning it alone, the immense saving arising from the invention will at once be seen.

"The expense of horse labor, from the increased value of the animal and the charge of his keeping, being an object of great importance, it is recommended that, upon all sizable farms, that is to say, where two hundred acres [800,000 m²], or upwards, of grain are sown, the machine should be worked by wind, unless where local circumstances afford the conveniency of water. Where coals are plenty and cheap, steam may be advantageously used for working the machine."

Modern day combine harvesters (or simply combines) operate on the same principles and use the same components as the original threshing machines built in the 19th century. Combines also perform the reaping operation at the same time. The name combine is derived from the fact that the two steps are combined in a single machine. Also, they are self-powered, usually by a diesel engine, and self-propelled.

Today, as in the 19th century, the threshing begins with a cylinder and concave. The cylinder has serrated bars, and rotates at high speed (about 500 RPM), so that the bars beat against the grain. The concave is curved to match the curve of the cylinder, and serves to hold the grain as it is beaten. The beating releases the grain from the straw and chaff.

Next, the beaten grain is lifted through a set of straw walkers, which carry the large pieces of straw away allowing the grain and chaff to fall below. Below the straw walkers, a fan blows a stream of air across the falling grain, removing dust and fines and blowing them away.

The grain falls into a set of two sieves mounted on an assembly called a shoe. The sieve is shaken mechanically. The top sieve has larger openings, and serves to remove large pieces of chaff from the grain stream. The lower sieve separates clean grain, which falls through, from incompletely threshed pieces. The incompletely threshed grain is returned to the cylinder by means of a system of conveyors, where the process repeats.

Some threshing machines were equipped with a bagger, which invariably held two bags, one being filled, and the other being replaced with an empty. A worker called a sewer removed and replaced the bags, and sewed full bags shut with a needle and thread. Other threshing machines would discharge grain from a conveyor, for bagging by hand. Combines are equipped with a grain tank, which accumulates grain for deposit in a truck or wagon.

A large amount of chaff and straw would accumulate around a threshing machine, and several innovations, such as the air chaffer, were developed to deal with this. Combines generally chop and disperse straw as they move through the field, though the chopping is disabled when the straw is to be baled, and chaff collectors are sometimes used to prevent the dispersal of weed seed throughout a field.

The corn sheller was almost identical in design, with slight modifications to deal with the larger kernel size and presence of cobs. Modern-day combines can be adjusted to work with any grain crop, and many unusual seed crops.

Both the older and modern machines require a good deal of skill to operate. The concave clearance, cylinder speed, fan velocity, sieve sizes, and feeding rate must be adjusted for crop conditions.