Abstract:
A method and apparatus is provided for making impressions of edges and profiling the edges using a profiling machine. The apparatus allows the impressionable material used for forming the impression to be held in a fixed position while the impression of the edge is formed, and then allows the impression to be withdrawn in a manner that leaves the impression substantially unchanged and substantially true to the profile of the edge. The apparatus also comprises fittings to make the apparatus compatible with a profiling machine and allows the impression to be positioned in the profiling machine in substantially the same orientation every time an impression is taken.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     Mechanical parts designed for use in severe environments are often subject to extreme mechanical stress. In critical environments such as in aircraft engines, these parts must have a degree of reliability built into them that allows them to operate with the lowest possibility of failure. Often such mechanical parts are subject to failure due to the stress at particularly highly stressed locations within the parts. Anything that tends to concentrate stress in one location or another can give rise to failure.  
         [0002]     In particular, sharp breaks such as edges or corners formed when a tool cuts through a piece of material, sharp bends in a material, or surface irregularities can all cause stress to concentrate causing susceptibility to failure. Edges created in the machining of parts can act as concentrators of stress if the edges are not properly finished. Edges are particularly vulnerable to undesirable stress concentration when the edge occurs in an area where high stress is already present.  
         [0003]     One way to minimize stress concentrators, also known as stress risers, is to carefully finish surfaces eliminating burrs and scratches. In the example of an edge, finishing the at risk edge very smoothly and replacing the sharp break of the edge with a radiused edge, multi-axis machined edge, or other edge treatment having known and specifically chosen characteristics can serve to retard the concentration of stress. Once a surface has been appropriately treated, it is critical to measure the results of the treatment and to critique the treated surface for adherence to engineering specifications.  
         [0004]     Historically, edges have been measured by a variety of methods. Prominent among these is a method called wax-and-trace. This method relies on an operator to apply an impressionable material such as a heated wax to the desired part feature by hand, allow the wax to cool, and remove the wax, all the time positioning and holding the wax by hand. All this is to be done without distorting the profile of the impression. The removed impression in the wax is then visually aligned in a profiling machine often using a clay mount and trying to ensure that the direction at which the tracing element of the profiling machine approaches the impression is normal to contour centerlines. In addition to contact or tracing methods there are numerous other methods for profiling an impression among which are optical methods, interferometric methods, acoustic methods, or other methods that may be used in profiling.  
         [0005]     The above procedure can lead to some inaccuracies. Shortfalls include: inability to reliably locate the feature to be measured; impressionable material deformation during application, cooling, and removal; and inability to accurately align the impressionable material to the profiling machine sensors.  
         [0006]     Accordingly, a need exists to be able to make impressions of the features of surfaces that do not suffer from the limitations of holding the impressionable material by hand and that allow repeatable and accurate placement in a profiling machine for accurate measurement of the features of surfaces.  
       BRIEF DESCRIPTION OF THE INVENTION  
       [0007]     The invention relates to an apparatus for measuring surface features in machined parts comprising: 
        a) a fixture to hold an impressionable material in fixed position against a surface feature for the period needed for the impressionable material to conform to the surface feature and register the shape of the surface feature;     b) said fixture removable such that the profile created in the impressionable material remains substantially unchanged by removal; and     c) said fixture mountable in a profiling device repeatably from measurement to measurement.        
 
         [0011]     In another embodiment the invention relates to an apparatus for measuring edges in machined parts comprising: 
        a) a fixture to hold an impressionable material in fixed position against an edge for the period needed for the impressionable material to conform to the edge and register the shape of the edge;     b) said fixture having a positioning element and a measuring element;     c) said measuring element removably fitted to said positioning element in a manner to hold said measuring element in alignment with said positioning element;     d) said measuring element removable from said positioning element such that the profile created in the impressionable material remains substantially unchanged during removal; and     e) said measuring element configured to be mountable in a profiling device repeatably from measurement to measurement.        
 
         [0017]     In one aspect, the invention relates to a method for measuring surface features in machined parts comprising: 
        a) holding an impressionable material with a fixture in fixed position against a surface feature for a period of time sufficient for the impressionable material to conform to the surface feature and register the shape of the surface feature;     b) removing the fixture from the surface feature while maintaining the profile of the surface feature substantially unchanged by removal; and     c) mounting the fixture in a profiling device repeatably from measurement to measurement.        
 
         [0021]     In another aspect, the invention relates to a method for measuring edges in machined parts comprising: 
        a) holding an impressionable material in fixed position against an edge using a fixture having a positioning element and a measuring element;     b) holding the impressionable material in fixed position for a period of time sufficient for the impressionable material to conform to the edge and register the shape of the edge;     c) holding the measuring element in alignment with the positioning element, the measuring element being removably fitted to the positioning element;     d) removing the measuring element from the positioning element while maintaining the profile created in the impressionable material substantially unchanged during removal; and     e) configuring the measuring element to be mountable in a profiling device repeatably from measurement to measurement       
 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]      FIG. 1  is a view of the apparatus according to one embodiment of the invention;  
         [0028]      FIG. 2  is a part registration plate in position on a part to be measured;  
         [0029]      FIG. 3  is a spring loaded part registration plate mounted on a part to be measured; and  
         [0030]      FIG. 4  is a wax impression mounted on a profiling machine being profiled.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0031]     The invention is applicable to a range of industrial processes where it is desirable to form impressions of surface features. The invention comprises an apparatus for forming such impressions using impressionable material and removing the impression of the feature so that it remains substantially unchanged. The fixture for doing this can also be configured to mount on a profiling machine in a manner that ensures accurate positioning from measurement to measurement.  
         [0032]     The invention is an improvement in the art by which many features such as surface irregularities, sharp breaks, and edges are measured. While edge measurements are a good example of this improvement in the art, they are only one example of many. The method and apparatus are applicable to any surface feature of a part.  
         [0033]     The apparatus comprises a fixture which can be removed once an impression of the desired surface is formed. The fixture can then be placed in a profiling machine to measure the characteristics of the edge profile impressed in the impressionable material.  
         [0034]     In another embodiment, the apparatus can be comprised of a plurality of elements, each accurately registered to the others to maintain the integrity of the impression taken, with at least one element removable for profiling the surface feature in a profiling machine.  
         [0035]     Referring to the drawings, FIG. 1  shows a view of one embodiment of the apparatus  20  comprising: a measuring element  22  and a positioning element  24 .  
         [0036]     The measuring element  22  is comprised of a “V” block  26  for holding a cylinder of impressionable wax  66  with a hold down  28  for maintaining the position of the wax cylinder  66  for the measurement.  
         [0037]     The alignment slide  30  is under “V” block  26  which has a keyway  32  machined in a “U” shape to closely fit the alignment insert  34  and maintain a substantially constant orientation between the positioning element  24  and the measuring element  22  when they are placed together. The alignment slide  30  also has a hand grip  40  to provide a hand hold for the withdrawal of the wax impression from the surface feature being measured. At the end of the measuring element  22  opposite the hold down  28  is profiling plate  36  for supporting the measuring element  22  on a profiling machine. The profiling plate  36  further comprises a profiling key  38  for mounting the measuring element  22  on a profiling machine utilizing the profiling plate  36  and the profiling key  38  to position the measuring element  22  in substantially the same position, repeatably, every time the measuring element  22  is mounted on a profiling machine. Because the location on the profiling machine is repeatable from measurement to measurement, the measuring element  22  is said to be repeatably mounted on the profiling machine.  
         [0038]     At the end opposite the hold down  28  of the assembled apparatus  20 , mounted on the alignment insert  34  is a stepped gage block  52 . When the apparatus  20  is assembled, the stepped gage block  52  engages the profiling plate  36  to regulate the closeness of approach of the hold down  28  end of the “V” block  26  to the part being measured. The stepped gage block  52  adjusts the depth of the impression.  
         [0039]     The positioning element  24  comprises the alignment insert  34  fitted for holding the positioning element  24  and the measuring element  22  in alignment when they are placed together. The positioning element  24  further comprises a positioning base plate  42  which supports the alignment insert  34 . The positioning base plate  42  is mounted on a part registration plate  44 .  
         [0040]     Referring to  FIG. 2 , the part registration plate  44  is specifically fitted to maintain the position of the assembled apparatus  20  by referencing to specific well defined and measured locations on the part being measured. Each well defined and measured location is called a datum  46 . The datums  46  allow reproducibility of apparatus  20  location from measurement to measurement and from part to part.  
         [0041]      FIG. 2  shows the part registration plate  44  of  FIG. 1  formed to contact the datums  46  identified on the dovetails  48  of an aircraft engine turbine disk  64 . In this embodiment the area of the aircraft engine turbine disk  64  between the dovetails  48  called a bottom  50  is subject to high stress and the edges of the aircraft engine turbine disk  64  there must be controlled. Impressions of the edges of the aircraft engine turbine disk  64  are made in the bottoms  50  and are profiled for edge control. The part registration plate  44  is fitted so that it contacts the datums  46  of the part such that the part registration plate  44  is always in the same position with respect to the datums  46  of the dovetails  48  when it is moved among the dovetails  48 .  
         [0042]      FIG. 3  shows another embodiment in which the part registration plate  44  of  FIG. 1  and  FIG. 2  is modified to incorporate springs  54 . The springs  54  position this spring loaded part registration plate  68  of  FIG. 3  against the datums  46 . In this way the apparatus  20  is held steady while the impression is made.  
         [0043]      FIG. 4  shows an impression  56  being measured by a profiling machine head  58 . The profiling head  58  contacts the impression  56  to create a profile of the impression  56 , and the measuring element  22  is positioned by the key  38  in the profile machine keyway  60  and by the stop block  62  which positions the measuring element  22  at the same point along the profiling machine keyway  60  every time a new impression  56  is profiled.  
         [0044]     In using one embodiment of the apparatus  20  of  FIG. 1 , the positioning element  24  is first positioned on the part with the part registration plate  44  aligned by the datums  46 . The measuring element  22  is positioned on the positioning element  24  with the profiling plate  36  resting on a step of the stepped gage block  52  that is about  1  centimeter above the alignment insert  34 . The wax cylinder  66  is then slid down the “V” block  26  until it contacts the surface feature to be measured, and the hold down  28  is used to secure the wax cylinder  66  in place. The measuring element  22  is removed and the end of the wax cylinder  66  is heated to plasticity. While the wax is still plastic, the measuring element  22  is placed back on the positioning element, and the stepped gage block  52  is disengaged allowing the surface feature to penetrate into the soft wax by about 1 centimeter ensuring a consistent amount of penetration from measurement to measurement. The apparatus  20  then remains in place until the softened wax has hardened becoming solid. The measuring element  22  is then carefully withdrawn along the alignment insert  34  keeping everything in alignment as the solidified impression  56  departs the feature maintaining the impression substantially unchanged. The measuring element  22  is then placed on the profile machine with the profiling key  38  in the profile machine keyway  60  and the stop block  62  aligning the measuring element  22  and the impression  56  under the profiling head  58 . The profiling head can then measure the profile of the impression  56 .  
         [0045]     This invention has been explained with respect to the details, arrangements of components, and certain specific embodiments shown in the accompanying drawings. These embodiments can be modified by those skilled in the art without departing from the spirit and scope of this invention. The appended claims are intended to be interpreted to cover apparatus and methods that do not depart from the spirit and scope of this invention.