BACKGROUND 
     1. Technical Field 
     The present disclosure relates to test devices, and particularly, to a device for testing the distance between centers of two through holes. 
     2. Description of Related Art 
     In machining, a lot of through holes are created in workpieces. To ensure that the distance between two adjacent through holes meets a specification, a caliper is generally used for testing the distance. However, due to the complicated operation of the caliper and human error, the reliability of the test is reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Many aspects of the present embodiments can be better understood with reference to the following drawings. The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the present embodiments. Moreover, in the drawings, all the views are schematic, and like reference numerals designate corresponding parts throughout the several views. 
         FIG. 1  is an exploded, isometric view of an exemplary embodiment of a test device. 
         FIG. 2  is an assembled, isometric view of the test device of  FIG. 1 . 
         FIG. 3  is an exploded, isometric view of the test device of  FIG. 2  and a test article. 
         FIG. 4  is a sectional view of  FIG. 3 , taken along the line of IV-IV, showing the test device in use. 
         FIGS. 5-7  are plan views of  FIG. 3 , but showing three different states. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure, including the accompanying drawings, is illustrated by way of examples and not by way of limitation. It should be noted that references to “an” or “one” embodiment in this disclosure are not necessarily to the same embodiment, and such references mean at least one. 
     Referring to  FIGS. 1-3 , an exemplary embodiment of a test device  100  tests whether the distance “W” between the center O 1  of a first through hole  312  and the center O 2  of a second through hole  314  defined in an article  300  meets specifications. The specification distance between the center O 1  and the center O 2  is “L”, a permissible error is “a”. The center O 1  of the first through hole  312  and the center O 2  of the second through hole  314  must meet a determined specification that the distance between the centers O 1  and O 2  is within the range of “W=L±a”. The test device  100  includes a supporting member  20 , two elastic members  30 , a benchmark member  40 , a connecting plate  50 , a test member  60 , a first sliding member  70 , a second sliding member  80 , and two circlips (clips  90 ). In this embodiment, the article  300  may be a part of a computer. 
     The supporting member  20  includes a substantially rectangular support plate  22 , a positioning block  24  perpendicularly extending down from the bottom surface of the support plate  22 , and an extension block  26  extending up from a first end of the support plate  22  and at a side of the support plate  22 . A guide hole  222  is defined in the other end of the support plate  22 , away from the extension block  26 . An elongate first guide slot  224  is defined in the first end of, and adjacent to the extension block  26 . The first guide slot  224  is parallel to an elongated direction of the support plate  22 . The positioning block  24  is substantially rectangular, and is located between the guide hole  222  and the first guide slot  224 . The bottom surface of the positioning block  24  is parallel to the top surface of the support plate  22 . A rectangular second guide slot  264  is defined in the distal end of the extension block  26 , opposite to the first guide slot  224 . The extending direction of the second guide slot  264  is parallel to that of the first guide slot  224 . 
     The benchmark member  40  includes a cylindrical main body  42 , and a tapered positioning portion  44  extending from a first end of the main body  42 . An annular groove  46  is defined in a circumference of the main body  42 , adjacent to the other end of the main body  42 . The diameter of the main body  42  is substantially equal to the diameter of the guide hole  222 . The diameter of the larger end of the positioning portion  44  adjacent to the main body  42  is greater than the diameter of the guide hole  222 . 
     The connecting plate  50  is substantially rectangular. A through hole  56  is defined in the middle of the connecting plate  50 . Two screw holes  58  are defined in the connecting plate  50 , flanking the through hole  56 . 
     The test member  60  is similar to the benchmark member  40 , and includes a cylindrical main body  62 , and a tapered test head  64  extending from a first end of the main body  62 . An annular groove  66  is defined in the circumference of the main body  62 , adjacent to the other end of the main body  62 . The diameter of the main body  62  is substantially equal to the diameter of the through hole  56  of the connecting plate  50 . The diameter of the larger end of the test member  64  is greater than the diameter of the through hole  56 . 
     The first sliding member  70  includes a rectangular sliding block  72 , and two flanges  76  extending out from opposite sides of the top of the sliding block  72 . A guide hole  722  is defined in the middle of the top of the sliding block  72 . Two stepped holes  724  are defined in the sliding block  72 , flanking the guide hole  722 . 
     The second sliding member  80  includes a substantially rectangular sliding portion  82 , a circular washer  87 , and a screw  89 . Two substantially rectangular protrusions  84  extend forward from the front side of the sliding portion  82 , each topped with a latch or claw (projection  842 ). The sliding portion  82  and the protrusions  84  cooperatively form a receiving slot  86 . The width of the receiving slot  86  is equal to the diameter of the main body  62  plus twice the permissible error “a”. A screw hole  824  is defined in the rear surface of the sliding portion  82 , opposite to the receiving slot  86 . The external diameter of the washer  87  is greater than the width of the second guide slot  264 . 
     Each clip  90  is a circlip, to fit in the grooves  46  or  66  of the main bodies  42  and  62 . 
     In this embodiment, the elastic members  30  are coil springs. 
     Referring to  FIG. 2 , in assembly, one elastic member  30  fits about the main body  42  of the benchmark member  40 . A first end of the elastic member  30  resists against the positioning portion  44 . The main body  42  extends through the guide hole  222  from the bottom surface of the support member  20 , with the second end of the main body  42  extending from the top surface of the support plate  22 . One of the clips  90  is engaged in the groove  46  through the opening  92 , and resists against the top surface of the support plate  22 . A second end of the elastic member  30  resists against the bottom surface of the support plate  22 . The benchmark member  40  is slidably engaged in the guide hole  222 . 
     The sliding block  72  is inserted into the first guide slot  224  from the top surface of the support member  20 . The flanges  76  are supported on the top surface of the support plate  22 . Two screws  69  extend through the corresponding stepped holes  724 , to engage with the connecting plate  50 . The connecting plate  50  is fixed to the bottom surface of the first sliding member  70 . The two opposite sides of the connecting plate  50  are slidably engaged with the bottom surface of the support plate  22 . 
     In a similar fashion, the other elastic member  30  fits about the main body  62  of the test member  60 . A first end of the elastic member  30  resists against the test head  64 . The main body  62  of the test member  60  extends through the through hole  56  of the connecting plate  50  and the guide hole  722  of the first sliding member  70 , from the bottom surface of the support member  20 . The second end of the main body  62  extends from the top surface of the first sliding member  70 . A clip  90  is engaged in the groove  66  of the test member  60  through the opening  92 , and resists against the top surface of the first sliding member  70 . The other end of the elastic member  30  resists against the bottom surface of the connecting plate  50 . The test member  60  is slidably engaged in the guide hole  722 . 
     The sliding portion  82  of the second sliding member  80  is inserted into the second guide slot  264  from the front side of the extension block  26 . The projections  842  are blocked by the front surface of the extension block  26 , to prevent the disengagement of the second sliding member  80  from the extension block  26 . The screw  89  extends through the washer  87 , to screw into the screw hole  824  of the sliding portion  82 . 
     The first sliding member  70 , the connecting plate  50 , and the test member  60  can move together along the first guide slot  224 , relative to the benchmark member  40 . The second sliding member  80  can move in the second guide slot  264 , between the washer  87  and the projections  842 . The positioning portion  44  and the test head  64  are lower than the bottom surface of the positioning block  24 . 
     Referring to  FIG. 3 , the distance between the axis “X 1 ” of the benchmark member  40  and the central line “S” of the second sliding member  80  is equal to the specification distance “L”. The distance between each of the opposite ends of the receiving slot  86  and the central line “S” is equal to the radius of the main body  62  plus the permissible error “a”. Therefore, the distance “W” between the first through hole  312  and the second through hole  314  is L−a≦W≦L+a. 
     Referring to  FIGS. 4 to 7 , in use, the article  300  is placed under the test device  100 . The axis X 1  of the benchmark member  40  is aligned with the first through hole  312 . The test member  60  is moved along the first guide slot  224  to seek the alignment of the axis X 2  with the second through hole  314 . The positioning portion  44  of the benchmark member  40  is inserted into the first through hole  312 , and the test head  64  of the test member  60  is inserted into the second through hole  314 . The positioning block  24  is supported on the article  300 , deforming the elastic members  30 . Pushing the screw  89 , the second sliding member  80  is moved forward. If the main body  62  of the test member  60  is fully received in the receiving slot  86 , it means that L−a≦W≦L+a, and the article  300  passes this test (shown in  FIG. 4  or  FIG. 5 ). If the main body  62  of the test member  60  is not received in the receiving slot  86 , it means that W&lt;L−a or W&gt;L+a, and the article  300  fails the test (shown in  FIG. 6  and  FIG. 7 ). 
     Even though numerous characteristics and advantages of the embodiments have been set forth in the foregoing description, together with details of the structure and function of the embodiments, the present disclosure is illustrative only, and changes may be made in details, especially in the matters of shape, size, and arrangement of parts within the principles of the embodiments to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.
A test device for testing whether the distance between centers meets specifications includes a support member, a benchmark member set on a first end of the support member, to be positioned at the first through hole, a test member set on a second end of the support member, to be positioned at the second through hole, and a sliding member defining a receiving slot. The test member is also slidable in a direction perpendicular to the slidable direction of the test member. If the test member can enter the receiving slot, the first article is qualified, if test member cannot enter the receiving slot, the second article does not meet the required standard.