Explosive forming

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Explosive forming is a metalworking technique in which an explosive charge is used instead of a punch or press. It can be used on materials for which a press setup would be prohibitively large or require an unreasonably high pressure, and is generally much cheaper than building a large enough and sufficiently high-pressure press; on the other hand, it is unavoidably an individual job production process, producing one product at a time and with a long setup time. There are various approaches; one is to place metal plate over a die, with the intervening space evacuated by a vacuum pump, place the whole assembly underwater, and detonate a charge at an appropriate distance from the plate. For complicated shapes, a segmented die can be used to produce in a single operation a shape that would require many manufacturing steps, or to be manufactured in parts and welded together with an accompanying loss of strength at the welds. There is often some degree of work hardening from the explosive-forming process, particularly in mild steel.

Contents

Tooling

Tooling can be made out of fiberglass for short-run applications, out of concrete for large parts at medium pressures, or out of ductile iron for high-pressure work; ideally the tooling should have higher yield strength than the material that is being formed, which is a problem since the technique is usually only considered for material which is itself very hard to work.

History

The first commercial industrial application of explosive forming in the United States began in 1950 and was used into the 1970s by The Moore Company in Marceline, Missouri. Purpose was to form proprietary shaped metal cylinders for use as the central structure of industrial axial vane fans. This is detailed in a 1967 N.A.S.A. publication "High-Velocity Metalworking - a survey" at pages 73, 82 & 83. This article misstates the name of company founder Robert David Moore Sr. as "E. R. Moore". Moore ultimately did hold some patents for involved processes. [1]

Explosive forming was used in the 1960s for aerospace applications, such as the chine plates of the SR-71 reconnaissance plane and various Soviet rocket parts; it continued to be developed in Russia, and the organising committees of such events as EPNM tend to contain many members from the former Soviet Union. It proved particularly useful for making high-strength corrugated parts which would otherwise have to be milled out of ingots much larger than the finished product. An example would be a yacht constructor who produced boat hulls by making a concrete "swimming pool" into which sheet-metal was placed, and when water filled and explosively fired, produced a complete hull-form. [2]

Other uses of explosives for manufacturing take advantage of the shaped charge effect, putting the explosive directly in contact with the metal to be worked; this was used for engraving of thick iron plates as early as the 1890s. See also explosively formed projectiles for a variety of military applications of the same kind of technology.

Explosive forming of vacuum tube anode (plate) materials

In the late 1950s, the General Electric company developed an application for five-layer sheet metal composites that had been created using the explosive forming process. GE engineers used this innovative composite material to produce multi-layer vacuum tube anodes (aka "plates") with superior heat transfer characteristics. This characteristic allowed GE to build significantly higher power vacuum tubes from existing designs without expensive engineering, design, and tooling changes, providing a substantial competitive market advantage to GE in the burgeoning Hi-Fi amplifier market.

In January 1960 it was reported in contemporary GE technical literature [3] that this five-layer material was the design breakthrough which made possible the new 6L6GC. The 6L6GC was a 6L6 variant able to dissipate 26% more power compared to the otherwise identically constructed 6L6GB. According to General Electric engineer R.E. Moe, then Manager of Engineering at G.E,'s Owensboro Kentucky facility, [4] these increases were made possible by the application of the improved multi-layer plate material.

GE sourced this material from a Texas-based firm (Texas Instruments [5] ) which is reported to be the source of the explosively forged five-layer raw material specified by General Electric engineers. This manufacturer used explosive sheet metal forging processes previously developed for another customer (possibly the U.S. Navy?) The explosively formed dissimilar materials had substantially improved evenness of heat transfer thanks to the copper center layer.

GE engineers quickly saw the potential for improved heat transfer characteristics in several already popular pentode and beam tetrode vacuum tube designs, including the 6L6GB, the 7189, and eventually the 6550. The application of the five-layer (Al-Fe-Cu-Fe-Al) material to anode manufacture solved the problem of irregular heat buildup at high power levels in the anode plates of power pentodes, tetrodes, and triodes. This irregular heat buildup leads to physical distortion of the tube's plate. if allowed to continue, this spot overheating eventually results in warpage which allows physical contact and subsequent short circuits between the plate, grids, and beam formers in the tube. Such contact shorts destroy the tube.

General Electric's novel application of this innovative composite led to the creation of the 7189A variant, released in late 1959, along with the 6L6GC and other variants. By 1969, the 6550A variant had also been developed to take advantage of explosively forged composites. GE's application allowed for improved power levels in a number of already popular tube designs, an innovation which helped pave the way for substantially higher power vacuum tube stereo and musical instrument amplifiers in the 1960s and early 1970s.

Related Research Articles

Triode Single-grid amplifying vacuum tube having three active electrodes

A triode is an electronic amplifying vacuum tube consisting of three electrodes inside an evacuated glass envelope: a heated filament or cathode, a grid, and a plate (anode). Developed from Lee De Forest's 1906 Audion, a partial vacuum tube that added a grid electrode to the thermionic diode, the triode was the first practical electronic amplifier and the ancestor of other types of vacuum tubes such as the tetrode and pentode. Its invention founded the electronics age, making possible amplified radio technology and long-distance telephony. Triodes were widely used in consumer electronics devices such as radios and televisions until the 1970s, when transistors replaced them. Today, their main remaining use is in high-power RF amplifiers in radio transmitters and industrial RF heating devices. In recent years there has been a resurgence in demand for low power triodes due to renewed interest in tube-type audio systems by audiophiles who prefer the pleasantly (warm) distorted sound of tube-based electronics.

Vacuum tube Device that controls electric current between electrodes in an evacuated container

A vacuum tube, electron tube, valve, or tube, is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied.

A tetrode is a vacuum tube having four active electrodes. The four electrodes in order from the centre are: a thermionic cathode, first and second grids and a plate. There are several varieties of tetrodes, the most common being the screen-grid tube and the beam tetrode. In screen-grid tubes and beam tetrodes, the first grid is the control grid and the second grid is the screen grid. In other tetrodes one of the grids is a control grid, while the other may have a variety of functions.

Secondary emission

Secondary emission in physics is a phenomenon where primary incident particles of sufficient energy, when hitting a surface or passing through some material, induce the emission of secondary particles. The term often refers to the emission of electrons when charged particles like electrons or ions in a vacuum tube strike a metal surface; these are called secondary electrons. In this case, the number of secondary electrons emitted per incident particle is called secondary emission yield. If the secondary particles are ions, the effect is termed secondary ion emission. Secondary electron emission is used in photomultiplier tubes and image intensifier tubes to amplify the small number of photoelectrons produced by photoemission, making the tube more sensitive. It also occurs as an undesirable side effect in electronic vacuum tubes when electrons from the cathode strike the anode, and can cause parasitic oscillation.

Control grid

The control grid is an electrode used in amplifying thermionic valves such as the triode, tetrode and pentode, used to control the flow of electrons from the cathode to the anode (plate) electrode. The control grid usually consists of a cylindrical screen or helix of fine wire surrounding the cathode, and is surrounded in turn by the anode. The control grid was invented by Lee De Forest, who in 1906 added a grid to the Fleming valve to create the first amplifying vacuum tube, the Audion (triode).

A suppressor grid is a wire screen (grid) used in a thermionic valve to suppress secondary emission. It is also called the antidynatron grid, as it reduces or prevents dynatron oscillations. It is located between the screen grid and the plate electrode (anode). The suppressor grid is used in the pentode vacuum tube, so called because it has five concentric electrodes: cathode, control grid, screen grid, suppressor grid, and plate, and also in other tubes with more grids, such as the hexode. The suppressor grid and pentode tube were invented in 1926 by Gilles Holst and Bernard D. H. Tellegen at Phillips Electronics.

Dynatron oscillator Vacuum tube electronic oscillator circuit

In electronics, the dynatron oscillator, invented in 1918 by Albert Hull at General Electric, is an obsolete vacuum tube electronic oscillator circuit which uses a negative resistance characteristic in early tetrode vacuum tubes, caused by a process called secondary emission. It was the first negative resistance vacuum tube oscillator. The dynatron oscillator circuit was used to a limited extent as beat frequency oscillators (BFOs), and local oscillators in vacuum tube radio receivers as well as in scientific and test equipment from the 1920s to the 1940s but became obsolete around World War 2 due to the variability of secondary emission in tubes.

KT66

KT66 is the designator for a beam power tube introduced by Marconi-Osram Valve Co. Ltd. (M-OV) of Britain in 1937 and marketed for application as a power amplifier for audio frequencies and driver for radio frequencies. The KT66 is a beam tetrode that utilizes partially collimated electron beams to form a low potential space charge region between the anode and screen grid to return anode secondary emission electrons to the anode and offers significant performance improvements over comparable power pentodes. In the 21st century, the KT66 is manufactured and used in some high fidelity audio amplifiers and musical instrument amplifiers.

6L6 Vacuum tube

6L6 is the designator for a beam power tube introduced by Radio Corporation of America in April 1936 and marketed for application as a power amplifier for audio frequencies. The 6L6 is a beam tetrode that utilizes formation of a low potential space charge region between the anode and screen grid to return anode secondary emission electrons to the anode and offers significant performance improvements over power pentodes. The 6L6 was the first successful beam power tube marketed. In the 21st century, variants of the 6L6 are manufactured and used in some high fidelity audio amplifiers and musical instrument amplifiers.

Plate electrode

A plate, usually called anode in Britain, is a type of electrode that forms part of a vacuum tube. It is usually made of sheet metal, connected to a wire which passes through the glass envelope of the tube to a terminal in the base of the tube, where it is connected to the external circuit. The plate is given a positive potential, and its function is to attract and capture the electrons emitted by the cathode. Although it is sometimes a flat plate, it is more often in the shape of a cylinder or flat open-ended box surrounding the other electrodes.

Beam tetrode

A beam tetrode, sometimes called a beam power tube, is a type of vacuum tube or thermionic valve that has two grids and forms the electron stream from the cathode into multiple partially collimated beams to produce a low potential space charge region between the anode and screen grid to return anode secondary emission electrons to the anode when the anode potential is less than that of the screen grid. Beam tetrodes are usually used for power amplification, from audio frequency to radio frequency. The beam tetrode produces greater output power than a triode or pentode with the same anode supply voltage. The first beam tetrode marketed was the Marconi N40, introduced in 1935. Beam tetrodes manufactured and used in the 21st century include the 4CX250B, KT66 and variants of the 6L6.

Pentode

A pentode is an electronic device having five active electrodes. The term most commonly applies to a three-grid amplifying vacuum tube, which was invented by Gilles Holst and Bernhard D.H. Tellegen in 1926. The pentode consists of an evacuated glass envelope containing five electrodes in this order: a filament for indirectly heating a cathode, a control grid, a screen grid, a suppressor grid, and a plate (anode). The pentode was developed from the tetrode tube by the addition of a third grid, the suppressor grid. This served to prevent secondary emission electrons emitted by the plate from reaching the screen grid, which caused instability and parasitic oscillations in the tetrode. The pentode is closely related to the beam tetrode. Pentodes were widely used in industrial and consumer electronic equipment such as radios and televisions until the 1960s, when they were replaced by transistors. Their main use now is in high power industrial applications such as radio transmitters. The obsolete consumer tubes are still used in a few legacy and specialty vacuum tube audio devices.

EL34 Vacuum tube (valve)

The EL34 is a thermionic vacuum tube of the power pentode type, first made in 1949. It has an international octal base and is found mainly in the final output stages of audio amplification circuits; it was also designed to be suitable as a series regulator by virtue of its high permissible voltage between heater and cathode and other parameters. The American RETMA tube designation number for this tube is 6CA7. The USSR analog was 6P27S.

Vacuum tubes produced in the former Soviet Union and in present-day Russia carry their own unique designations. Some confusion has been created in "translating" these designations, as they use Cyrillic rather than Latin characters.

The 6AQ5 is a miniature 7-pin (B7G) audio power output beam tetrode vacuum tube with ratings virtually identical to the 6V6 at 250 V. It was commonly used as an output audio amplifier in tube TVs and radios. There are versions of this tube with extended ratings for industrial application which are designated as 6AQ5A, and 6AQ5W/6005 or 6005W.

The KT88 is a beam tetrode/kinkless tetrode vacuum tube for audio amplification.

Ultra-linear electronic circuits are those used to couple a tetrode or pentode vacuum-tube to a load.

807 (vacuum tube)

The 807 is a beam tetrode vacuum tube, widely used in audio- and radio-frequency power amplifier applications.

JJ Electronic, s.r.o is a Slovak electronic component manufacturer, and one of the world's remaining producers of vacuum tubes. They are based in Čadca, in the Kysuce region of Slovakia. Most of the products that JJ offers are audio receiving tubes. These vacuum tubes are mainly used for guitar and hi-fi amplifiers. In technical terms, JJ produces triodes, beam tetrodes and power pentodes. Double diode vacuum tubes for full wave AC-to-DC rectifiers are also produced. JJ also produces electrolytic capacitors for higher-voltage purposes, generally for use in audio amplifiers. JJ also manufactures their own line of high-end audio amplifiers and guitar amplifiers. In 2015, the company sales amounted to EUR 8.5 million and net income came to EUR 3.8 million. Most production is exported to the United States.

References

  1. ↑ Michael C. Noland; Howard M. Gadberry; John B. Loser; Eldon C. Sneegas (1967). High-velocity Metalworking: A Survey. Technology Utilization Division, National Aeronautics and Space Administration. pp. 73, 82, 83.
  2. ↑ "Explosive forming of boats - ABC Beyond 2000 - YouTube". www.youtube.com. Archived from the original on 2021-12-12. Retrieved 2020-12-25.
  3. ↑ http://n4trb.com/AmateurRadio/GE_HamNews/issues/GE%20Ham%20News%20Vol%2015%20No%201.pdf [ bare URL PDF ]
  4. ↑ http://n4trb.com/AmateurRadio/GE_HamNews/issues/GE%20Ham%20News%20Vol%2015%20No%201.pdf [ bare URL PDF ]
  5. ↑ "Tubes Asylum".

GE Ham News, Vol 15, No. 1, Jan-Feb 1960, pp 1, pp 7, P.E. Hatfield, R.E. Moe

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