Civil Rights
Movements, leaders, victories and the continuing fight for equality.
q2K BHP
Black History Portal
THE BHP WIRE —
HIDDEN TRUTHS
What's New!
Explore the people, places, events, achievements, struggles and stories that shaped our journey.
Movements, leaders, victories and the continuing fight for equality.
Innovation, patents, science, technology and world-changing contributions.
Pioneers, champions, Negro Leagues, records, activism and excellence.
Meet the people whose lives, choices and achievements shaped the journey.
Black towns, communities, institutions and places where history happened.
Moments that changed communities, movements, institutions and the nation.
In August 1908, a white mob attacked Springfield, Illinois’s Black community, destroying homes and businesses and lynching two Black men. National outrage over the violence helped spur the movement that created the NAACP the following year.
MORE →Reflects the personal views, recollections, and perspective of the author, Mike Davis.
This is a personal recollection on the Move fire on May 13, 1985
A rotary stage is a component of a motion system used to restrict an object to a single axis of rotation.[1] The terms rotary table or rotation stage are often used interchangeably with rotary stage. All rotary stages consist of a platform and a base, joined by some form of guide in such a way that the platform is restricted to rotation about a single axis with respect to the base. In common usage, the term rotary stage may or may not also include the mechanism by which the angular position of the platform is controlled relative to the base.
In three-dimensional space, an object may either rotate about, or translate along, any of three axes. Thus, the object is said to have six degrees of freedom (3 rotational and 3 translational). A rotary stage exhibits only one degree of freedom (rotation about one axis). In other words, rotary stages operate by physically restricting 3 axes of translation and 2 axes of rotation.
Rotary stages consist of a platform that moves relative to a base. The platform and base are joined by some form of bearing which restricts motion of the platform to rotation about a single axis. A variety of different styles of bearings are used, each with benefits and drawbacks, making them more appropriate for some applications than for others.
A plain bearing is simply two surfaces sliding against each other. Typically, a circular step on the platform mates snugly with a circular depression in the base allowing free rotation while minimizing side to side motion. A rotary stage built with this type of bearing is usually only used for coarse positioning and is adjusted manually simply by turning the platform. Index marks on either the base or the platform are often provided, allowing for somewhat repeatable positioning of the platform relative to the base.
This type of rotary stage includes ball bearing stages, crossed roller bearing stages, and possibly others. Any of a number of different rolling-element bearings may be employed. Typically, a pair of bearings is used and they are preloaded to take up any slack which could result in the stage platform lifting relative to the base.
Some rotary stages are operated simply by turning the platform by hand. The platform may have index marks for setting different angular positions relative to the base. A locking mechanism may be provided to fix the platform to the base at the desired position.
For more precise position control, a worm drive may be used. A worm wheel is fixed to the rotating platform and meshes with a worm in the base. Rotation of the worm via a manual control knob causes the platform to rotate with respect to the base. Index marks on both the control knob and the platform can be used to locate the platform very precisely and repeatably with respect to the base.
Replacing the manual control knob in the above manual worm drive scenario a stepper motor allows positioning of the rotary stage to be automated. A stepper motor rotates in fixed increments or steps. The number of steps moved is controlled by the stepper motor controller. In this sense, the stepper motor behaves much like an indexed control knob.
A DC motor may also be used in place of a manual control knob. A DC motor does not move in fixed increments. Therefore, an alternate means is required to determine stage position. An encoder may be attached to the DC motor and used to report the angular position of the motor to the motor controller, allowing a motion controller to reliably and repeatably move the stage to set positions.
When precise angular positioning over only a small total angle is required, a linear actuator (either manual, or motorized) may be used. Typically, the range of motion possible is only 10° to 20° of rotation. The linear actuator presses against a contact surface fixed to the stage platform such that extension or retraction of the actuator causes the platform to rotate. The stage platform is sprung against the actuator tip so that the contact surface stays in contact with the actuator tip when the actuator retracts.
Source: Wikipedia. Article content is retrieved live through the MediaWiki API.
A rotary table is a precision work positioning device used in metalworking. It enables the operator to drill or cut work at exact intervals around a fixed (usually horizontal or vertical) axis. Some rotary tables allow the use of index plates for indexing operations, and some can also be fitted with dividing plates that enable regular work positioning at divisions for which indexing plates are not available. A rotary fixture used in this fashion is more appropriately called a dividing head (indexing head).
A rotary table is a mechanical device on a drilling rig that provides clockwise (as viewed from above) rotational force to the drill string to facilitate the process of drilling a borehole. Rotary speed is the number of times the rotary table makes one full revolution in one minute (rpm).
A rotary stage is a component of a motion system used to restrict an object to a single axis of rotation. The terms rotary table or rotation stage are often used interchangeably with rotary stage. All rotary stages consist of a platform and a base, joined by some form of guide in such a way that the platform is restricted to rotation about a single axis with respect to the base. In common usage, the term rotary stage may or may not also include the mechanism by which the angular position of the platform is controlled relative to the base.
An indexing head, also known as a dividing head or spiral head, is a specialized tool that allows a workpiece to be circularly indexed; that is, easily and precisely rotated to preset angles or circular divisions. Indexing heads are usually used on the tables of milling machines, but may be used on many other machine tools including drill presses, grinders, and boring machines. Common jobs for a dividing head include machining the flutes of a milling cutter, cutting the teeth of a gear, milling curved slots, or drilling a bolt hole circle around the circumference of a part. The tool is similar to a rotary table except that it is designed to be tilted as well as rotated and often allows positive locking at finer gradations of rotation, including through differential indexing. Most adjustable designs allow the head to be tilted from 10° below horizontal to 90° vertical, at which point the head is parallel with the machine table. The workpiece is held in the indexing head in the same manner as a metalworking lathe. This is most commonly a chuck but can include a collet fitted directly into the spindle on the indexing head, faceplate, or between centers. If the part is long then it may be supported with the help of an accompanying tailstock.
Before the 1921 destruction of Tulsa’s Greenwood District, Black residents had created a remarkable center of business and community life. The district included stores, professional offices, entertainment venues and homes owned by Black citizens. Understanding Greenwood means learning what was built—not only what was burned.
MORE →Shirley Chisholm, elected in 1968.