Abstract:
A brake drum caliper is disclosed having a beam and two jaws for measuring the inside diameter of a cylinder, with the improvement of limit stops on each jaw for accurately measuring diameter of the cylinder in at least two depths from the top of the cylinder.

Description:
BACKGROUND OF THE INVENTION 
   As cars are built today to last ten or fifteen years and be driven 100,000 to 200,000 miles, brakes usually need to be resurfaced at least once during the life of every car. Most owners do not become aware of the need for brake repair until the brakes make a sound indicating wear beyond the useful depth of braking surface. By this time, the brake drum, against which the movable shoe engages for braking action, becomes scored. Then, the drum must be machined to present a round, smooth surface for the new brake shoe surface to engage. 
   In order to machine the brake drum to present a round, smooth surface, calipers have been developed to accurately measure the inside diameter of the brake drum. It is essential to have a constant inside diameter in the refinished drum to engage the shoe surface evenly. U.S. Pat. No. 5,317,814 shows a drum caliper that can accurately measure diameter only if the points  44  and  74  of the caliper are at the same depth within the drum. If the beam is not precisely parallel to the drum, the measurement of diameter will be imprecise. 
   U.S. Pat. No. 5,465,501 was intended to correct the problem of having a skewed plane  38  ( FIG. 3 ) giving an inaccurate reading. However, the slidable pins  26  and spring detents or other locking mechanisms described in the patent become fouled in an environment where metal shavings are ubiquitous. If a pin inadvertently slides inwardly within the drum, instead of resting on the periphery of the drum, a skewed angle can result with imprecise measurements. There is a need for fixed stops for precise depth locations, rather than slidable stops that can slip off the periphery of the drum. 
   SUMMARY OF THE INVENTION 
   The present invention substitutes a precise measurement of diameter of a brake drum by locating the caliper at precisely the same depth below the surface for each measurement. Two measurements at different depths assure a constant smooth diameter for the machined drum. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a front view of the caliper in a partially opened position. 
       FIG. 2  is an isometric view of the jaws with stops in the lower position. 
       FIG. 3  is an isometric view of the jaws with stops in the upper position. 
       FIG. 4  is an isometric view of the jaws with stops in an intermediate position 
       FIG. 5  is an isometric view of the caliper in a drum with the stops in the upper position of  FIG. 3 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  shows a caliper  11  comprising a beam  12 , preferably of hardened stainless steel with inches and millimeters scribed on the beam to indicate distance. Beam  12  has at the zero end a base plate  13  from which fixed jaw  14  extends. A commercially available movable counter  16  slides on the beam  12  to provide an accurate reading of diameter inside a brake drum to two decimal places in millimeters or three decimal places in inches. Rotatable set pin  20  holds counter  16  in place once the diameter is determined, and LED window  30  displays the diameter in either inches or millimeters. The electronic readout counter  16  is secured to movable base plate  17 , which also carries movable jaw  18 . Fixed jaw  14  has a fixed point  19  to engage one side of the drum for measuring diameter. Movable jaw  18  has a corresponding point  21  to engage the opposite side of the drum at its widest diameter. 
   Fixed jaw  14  has a stop  22  that precisely locates the point  19  at a predetermined depth within the drum by resting on the annular surface of the drum. Stop  22  is movable in a vertical plane through 180 degrees of movement by rotating around pin  23  held in holder  24  secured to jaw  14 . Stop  22  is an L-shaped arm that can proceed from an upper position shown in  FIG. 1  to a lower position shown in  FIG. 2  and any intermediate position indicated in  FIG. 4 . 
   Movable jaw  18  also has a stop  26  at exactly the same distance from point  21  as the stop  22  is from the point  19  in fixed jaw  14 , so long as each of stops  22  and  26  are in the same point along the 180 degree rotation. Pin  26  has a holder and pivot pin (not shown) as in holder  24  and pivot pin  23  of jaw  14 . 
     FIG. 2  is a partial isometric view of the fixed jaw  14  and the movable jaw  18  with their respective stops  22  and  26  in the lowermost position relative to points  19  and  21 . Stop  26  has a pivot pin  27  and holder  28  that were not visible in  FIG. 1 . By having stop  22  on one side of fixed jaw  14  and stop  26  on the opposite side of movable jaw  18  permits the caliper  11  to measure the maximum diameter of the drum without regard to the thickness of jaws  14  and  18 . 
     FIG. 3  is a partial isometric view of the jaws of  FIG. 2 , but with the stops  22  and  26  in the uppermost position as in  FIG. 1 . Points  19  and  21  on jaws  14  and  18 , respectively, engage the inside of the drum (not shown) at the widest diameter. Stops  22  and  26  rest on the annular ring of the drum to precisely locate points  19  and  21  at the same depth within the drum. 
     FIG. 4  is a partial isometric view of jaws  14  and  18  with stops  22  and  26  at the same intermediate point in the 180 degree range of movement of the pins about pivot pins  23  and  27 , respectively. In operation, the caliper  11  is set in the lowermost position of  FIG. 2  so that pins  19  and  21  are set to determine the maximum diameter at that depth. Then, the stops  22  and  26  are moved towards the positions in  FIG. 4  as the caliper  11  is raised from the lowermost depth. Finally, a check of diameter at the uppermost position shown in  FIG. 3  is made. If the diameter is unchanged, the drum surface is smooth and round. No slidable pins have to be manipulated at the various positions of depth, as in the prior art. 
     FIG. 5  illustrates the combination of a drum  31  with an upper surface  32  and an inner annular ring  33  on which stops  22  and  26  may rest as they pass from lowermost position to uppermost position (shown) in the 180 degree trajectory. Drum  31  has an opening  34  for mounting the drum on a wheel bearing (not shown) and holes  36 ,  37 ,  38  and  39  for allowing the wheel to be bolted to the car. Surface  41  is the one whose diameter is being measured for uniform diameter and smoothness, as it is the surface that brake shoes bear upon for slowing the vehicle. 
   The width of the brake shoe surface and corresponding drum surface is usually between 5 cm and 15 cm. I have found that the two maximum distances between the two limits for stops  22  and  26  may be between 20 mm and 40 mm apart. For most applications, if the closest limit for stops  22  and  26  to the measurement points  19  and  21  is between 20 mm and 40 mm, then the furthest limits for stops  22  and  26  may be between 20 mm and 40 mm from the first limit point. 
   The preferred embodiment is intended to be merely illustrative of how the invention may be used by one skilled in the art. It is intended to enable one skilled in the art to make and use the invention without limiting it to the specific features shown. Also. the caliper may be adapted for measuring roundness of any circular surface, and is not limited to brake drums.