Abstract:
A radio frequency cover and complementary shaped hole that securely fit together to provide electro magnetic and electrostatic shielding of adjustment access holes. The cover preferably is cold headed and the complementary hole is either stamped, machined or cast for inexpensive mass production. Each cover preferably has a socket or slit on its top surface to receive and be guided by a tool, which also secures the cover in an access hole within a second access hole in radio frequency and/or electronic systems. The covers include self tapping thread formers to further reduce manufacturing time and costs.

Description:
FIELD OF INVENTION 
     The invention relates generally to radio frequency (rf) plugs that are inserted into access holes in walls, covers and chassis of radio frequency electrical or electronic equipment to prevent each access hole from acting as an antenna and radiating rf energy to nearby equipment, and more specifically to rf plugs that fit flush, seal in rf energy and are easily installed. 
     BACKGROUND OF INVENTION 
     The need for access holes in the walls, covers and chassis of enclosed electronic equipment is well known. These holes allow access for adjusting and/or testing internal components of an assembly while enclosed in radio frequency shielding. Once the adjusting and/or testing is completed, the access holes must be covered or plugged. That is because discontinuities, such as access holes, act as antennae for the electromagnetic fields from the electrical circuitry adjusted or tested by means of such holes, According to well known electromagnetic field theory, the amount and frequency of the electromagnetic energy that is spuriosly emitted depends upon the frequency and power level of the enclosed electrical circuitry. The enclosed circuitry may be for digital logic, computers, radios or telephone equipment. All of these use radio frequency signals that can become spurious emissions if not appropriately contained. 
     The known cover  100  for access holes , as seen in FIG. 1, has a domed or button shaped body  102  and has curved, springy appendages  104  coming out from the bottom circumference of the body. These appendages  104  are crafted such that when the right sized cover  100  is placed over a hole (not shown) and pressed in, the appendages  104  are initially forced inward by the sides of the hole and then as the bottom of the button nears the surface in which the hole is located their springiness and their shapes allow the appendages to expand outward. Also, because of the shape of the appendages  104 , this expanding action against the edges of the hole actually draws the cover  100  down toward the surface and locks it in place. To be effective, the known cover  100  must be bigger than the hole it is in—otherwise the spring action would pull the body portion  102  into the hole. This size difference between the cover  100  and the hole it goes into prevents covering a hole inside of another a hole unless the holes and their respective covers are made progressively and substantially larger towards the outside of the enclosure. Covers within covers are also hard to install. Furthermore, the appendages must be shaped for a range of hole depths and material thicknesses. If the hole is too deep, the pulling action and the locking do not occur. If the hole is very shallow, the pulling action and the locking will not occur either and the cover  100  will be loose and less effective. Furthermore, the appendages  104  as they extend into the enclosure can and will begin to act as antennas for emissions within the enclosure if the effective size of the appendages  104  is approximately a quarter wavelength. This often leads to unintended coupling of circuits when a cover  100  is installed. 
     Thus, it is an object of the present invention to provide a cover or plug that mounts flush with the top and bottom surfaces of the hole it is inserted into. 
     It is another object of the invention to provide a cover or plug that can be readily passed through a first hole to cover an interior access/test hole. 
     It is a further object of the present invention to provide a locking on almost any thickness of material. 
     SUMMARY OF INVENTION 
     Briefly stated in accordance with one aspect of the invention, the aforementioned problems are addressed by providing an apparatus for enclosing an electrical circuit. The apparatus includes a conductive enclosure that has an interior surface and an exterior surface. This conductive enclosure has a hole formed therein. The hole is defined in a first portion by a first cylindrical surface that has a first radius. This first cylindrical surface is connected at a first end thereof to the interior surface of the enclosure. The hole also being defined in a second portion by a second cylindrical surface that has a second radius. This second cylindrical surface has multiple semi-cylindrical voids spaced equally around its perimeter. Each of the radii of the semi-cylindrical voids is smaller than the smaller of the first and second radii of the first and second cylinders. The second cylindrical surface is connected at a first end thereof to the exterior surface of the enclosure and extends such that a second end of the second cylindrical surface is coplanar with the second end of the first cylindrical surface;. Also defining the hole is a ring shaped surface that is located between the second ends of the first and second cylinders and located in the common plane of those second ends. A cover is adapted to fit in the hole and engage the surfaces of the conductive enclosure sufficiently to prevent emission of electrical and electromagnetic fields through the hole. 
     In accordance with another aspect of the invention, the aforementioned problems are addressed by providing an apparatus for enclosing an electric circuit includes a conductive enclosure that has an interior surface and an exterior surface. This conductive enclosure has a hole formed therein. The hole is defined in a first portion by a first cylindrical surface that has a first radius. This first cylindrical surface is connected at a first end thereof to the interior surface of the enclosure. The hole also being defined in a second portion by a second cylindrical surface that has a second radius. This second cylindrical surface has multiple semi-cylindrical voids spaced equally around its perimeter. Each of the radii of the semi-cylindrical voids is smaller than the smaller of the first and second radii of the first and second cylindrical surfaces. The second cylindrical surface is connected at a first end thereof to the exterior surface of the enclosure and extends such that a second end of the second cylindrical surface is coplanar with the second end of the first cylindrical surface. Also defining the hole is a ring shaped surface that is located between the second ends of the first and second cylindrical surfaces and located in the common plane of those second ends. A cover is adapted to fit in the hole and engage the surfaces of the conductive enclosure sufficiently to prevent emission of electrical and electromagnetic fields through the hole. The cover has a first cylindrical portion to fit in said first cylindrical portion of the hole and a second cylindrical portion to fit within the second portion of the hole. The second cylindrical portion of the cover has multiple protrusions adapted to fit into the multiple semi-circular voids. Additionally, the cover has a recess therein for receiving a tool to rotate the cover into sufficient engagement with the surfaces of the enclosure in order to hold the cover in place. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a side view of a prior art cover. 
     FIG. 2 is partially cut away exploded view of an enclosure and a cover according to the present invention and the access hole that the cover it fits into. 
     FIG. 3 is an elevational view of a partially cut away enclosure with a hole therein to receive a cover according to the invention. 
     FIG. 4 is an elevational view of a cover. 
     FIG. 5 is an elevational view of a cover and a partially cut away enclosure according to the present invention. 
     FIG. 6 is an elevational view of FIG. 5, except the cover has been rotated within the hole. 
     FIG. 7 is an elevational view of a cover and a partially cut away enclosure according to another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring now to FIG. 2, an enclosure  200  is shown for enclosing electronic circuitry (not shown) and the inherent electromagnetic fields (not shown) that attend the presence of electronic circuitry. An example of a hole or void  202  is formed in enclosure  200 . The purpose of hole  202  is to provide access for adjustment to variable components, such as variable resistors, potentiometers, variable inductors, variable capacitors, switches, etc. (not shown), located within the enclosure  200 . After any and all adjustments have been made, hole  202  is covered with a cover  210 . Cover  210  when installed in hole  202  acts electro-magnetically and electrostatically as part of enclosure  200  to shield the enclosed electronic circuits from electromagnetic fields and electric fields from outside of the enclosure  200 . In a like manner, cover  210  when installed in hole  202  also acts as part of the enclosure  200  to keep electromagnetic fields and electric fields that are located within the enclosure  200  from escaping from the enclosure  200  by covering the hole  202 . The cover  210  when inserted into its receiving hole  202  makes the enclosure appear electro-magnetically and electrostatically continuous, just as if the hole  202  was not there. 
     The hole  202  has a special shape for receiving the cover  210 . Each hole  202  has a circular lower portion  204  defined by cylindrical surface  205  and a mainly circular upper portion  206 . The lower portion  204  is smaller in diameter than the mostly circular upper portion  206 . This relationship causes a surface  207  that is recessed from the surface of the rest of the outer surface of enclosure  200  and generally parallel thereto. The mainly circular upper portion is defined by surface  207  and generally cylindrical surface  208 . Generally cylindrical surface  208  would be cylindrical but for three approximately semicircular cutouts  209 . This recessed surface  207  and the cutouts  209  are better seen in FIG.  3 . The enclosure  200  is typically a soft, electrical conducting material such as aluminum or brass. Since the material of enclosure  200  is relatively soft, one or more holes  202  may be formed by casting, stamping or drilling and milling procedures. 
     Each hole  202  is defined by two cylindrical surfaces  205 ,  206  that are located one adjoining the other. In one embodiment of the invention, the two surfaces  205  and  206  and the two hole portions they define are co-axial, i.e. located along a common axis. In a second embodiment, the mainly circular surface  206  decreases in radius in the direction that the cover  210  is to be rotated for installation. Thus, for one example for a clockwise rotation, a radius of surface  206  changes from 0.150 inches immediately clockwise from one semicircular cutout  209 , to a radius of 0.148 inches approximately half way to the next semicircular cutout  209  in the clockwise direction and to a radius of 0.145 inches immediately before the next cutout  209  in the clockwise direction. This slight reduction in the radius of the mainly circular surface  206  increases the interference fit between a cover  210  and a surface  206  in the direction that the cover  210  is to be rotated in order to secure it tightly, which in turn helps keep the cover  210  fitting tightly in hole  202  after numerous insertions and removals. Those in the art will recognize that if the cover was to be secured by rotating it in the counterclockwise direction, then the increase in the interference fit would be in the counterclockwise direction. 
     Referring now to FIGS. 2 and 4, one embodiment of the cover  210  will be described. The cover  210  is adapted to fit in the hole  202 . Cover  210  has a first cylindrical portion  214  and a second generally cylindrical portion  216 . Since hole  202  has three cutouts or voids,  209 , the second cylindrical portion  216  has three protrusions  219  that are matched in size and spaced relationship with cutouts  209 . In the top of the cover  210 , a recess  220  is formed. Recess  220  is provided so the cover can be inserted onto the end of a tool shaft, placed in a hole  202  and then rotated up to approximately 30 degrees by means of that tool (not shown). The recess  220  may be adapted to receive different tools. In FIGS. 1-5 a hexagonal recess to accept an alien wrench is shown. In FIG. 6, a recess for use with a torx wrench is shown. Recesses that accept standard screwdrivers and cross-indexed screwdrivers are also contemplated but not shown. For ease of use, cover  210  may be made of a magnetic material, such as steel. If cover  210  is made of magnetic material, then a magnetized tool will help keep a cover  210  on the tool until the cover is properly positioned in hole  202  and locked in place by rotating the cover in the hole  202 . Also, if two different level of enclosures are used, successively larger covers can be inserted on the various level of enclosure to seal access holes to more than one level of enclosure without the covers falling off the tool. 
     Recess  220  also allows the user of the cover to rotate the cover  210  approximately 30 degrees while it is in hole  202 . This rotation of a hard cover  210  in a hole  202  surrounded by a relatively soft material causes zero helix partial threads to be formed by the protrusions  219  in the surface  206 . This forming rotation leads to a high amount of friction between protrusions  219  and the surface  206  that locks the cover  210  in place. The protrusions  219  typically will compress the sides of the threads in surface  206  in their immediate vicinity leading to elastic recovery in the threads the protrusions  219  turned through. This elastic recovery of the threads behind the protrusions  219  aid the locking action. The cover  210  remains in place until it is removed for access or adjustment or both. FIG. 6 shows a cover  210  locked in place. 
     FIG. 7 shows an embodiment of the invention wherein the recess is formed to receive a torx head tool instead of the hexagonal socket shown in FIG.  6 . Because of torque characteristics, the torx head embodiment is considered to be the preferred embodiment. 
     Thus, it will now be understood that there has been described an apparatus for providing an enclosure with a sealable covers for adjustment holes. While the invention has been particularly illustrated and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form, details, and applications may be made therein. For example, the cutouts may be other shapes other than semicircular as long as the protrusions can fit therein in order to form the securing threads. It is accordingly intended that the appended claims shall cover all such changes in form, details and applications which do not depart from the true spirit and scope of the invention.