From TARDIS Index File, the free Doctor Who reference.

Chumbley
Type:	Robot
Created by:	Rills
Appearances:	DW: Galaxy 4

A Chumbly was a type of robot encountered by the Doctor, Vicki and Steven on a doomed planet where the Rills and Drahvins had crashed. They were created by and under the control of the Rills.

Abilities

The Doctor deduced they were blind because they seemed to feel their way around the exterior of the TARDIS. Later, he determined that they had the ability to detect sound waves and heat. They were powered by a form of magnetism, and could be immobilized by ensnaring them in a metal mesh. They resisted gunfire by voluntary deactivation. When they switched themselves off, their bodies telescoped down, reducing their height roughly in half. The concentric rings of metal that comprised their bodes folded down into each other, thereby protecting the interior of the robot. (DW: "Four Hundred Dawns")

Their primary task was to act as a translator between the Rills and other beings. They could analyze the speech patterns of other beings and allow the thoughts of the Rills to be "spoken" through their onboard speakers in the language of the recipient. They continued to appear around the Drahvins' ship because the Rills were merely trying to communicate.

The Chumbleys assisted the Rills in a variety of other ways. Because the Rills could not metabolize oxygen, the Chumbleys were the only way in which the Rills could experience oxygen-rich planets, such as the one on which they had crashed. They were thus a kind of probe, sending a variety of data back to their masters. (DW: "Air Lock")

Offensive capabilities

Although principally tools of exploration and communication, the Chumbley's also had offensive capabilities. They were equipped with variety of weapons. They had a flame thrower (DW: "Four Hundred Dawns"), and could also fire projectiles akin to bullets and ammonia bombs. (DW: "The Exploding Planet")

Name

Their true, Rill name was unknown. The name "Chumbly" was in fact a nickname Vicki gave to them, based upon the "wobbly" sound they made when moving. When she told the Rills she had named the robots "Chumblies", they did not object. Indeed, they were quite happy to use the term themselves. (DW: "Air Lock")

Behind the scenes

According to Jeremy Bentham, the Chumblies were designed by Richard Hunt. Like Daleks, they required a performer to operate the machinery from inside. However, their small, three-foot stature meant that those performers all had to be similarly diminutive. One of them was Angelo Muscat, who would later play the butler in The Prisoner. The action of the Chumbley's telescoping down into a "closed" position when deactivated made the prop too small for any performer. Thus, a "performer-less" prop was constructed for filming the "down" position.