Abstract:
A method for determining the wall thickness of a metal tube is described. This method has the following features: (a) preparation of a metal tube of a determined length; (b) arrangement of two clamp contacts on the metal tube with an exactly defined distance of separation; (c) connection of the ends of the metal tube with a power source; and (d) measurement of the voltage drop in the metal tube between the clamp contacts.

Description:
RELATED APPLICATIONS  
       [0001]     This application is related to and claims the benefit of priority from German Patent Application No. 10 2004 053 021.1, filed on Nov. 3, 2004, the entirety of which is incorporated herein by reference.  
       FIELD OF THE INVENTION  
       [0002]     The invention concerns a method for determining the wall thickness of a metal tube.  
       BACKGROUND  
       [0003]     DE 35 42 681 A1 describes a method for producing high-quality drawn tubes, in which a metal strip is shaped into a tube with a longitudinal slit, and the longitudinal slit is welded. In a final operation, the welded metal tube is reduced in its outside diameter and its wall thickness with a drawing ring and a floating mandrel in a continuously operating tube-drawing machine.  
         [0004]     When the tube-drawing machine is being set up, especially with respect to the selection of the drawing tools (mandrel, die), the wall thickness of the reduced metal tube must be measured before production can begin. In many cases, this can significantly delay the beginning of production.  
         [0005]     In the continuous production of metal tubes with an outside diameter of less than 5 mm and a wall thickness of less than 0.1 mm, the wall thickness has been measured by optical means up until now. In this measuring operation, a short piece of the finished tube is embedded in casting resin, and after the casting resin has cured, a microsection of the surface is prepared, which can then be viewed under a microscope. A photograph of the transverse microsection is taken, and the tube is measured.  
         [0006]     A high-precision microscope with a camera adapter and suitable measuring software are necessary for the extremely thin metal tubes under discussion here. This apparatus is very expensive, and the operation is very time-consuming, i.e., it can take several hours. During this period of time, production cannot be started.  
       OBJECTS AND SUMMARY  
       [0007]     The objective of the present invention is to specify a method for determining the wall thickness of longitudinally welded metal tubes in which the measurement result is available in a much shorter time, and the measurement can be performed in the production facility. The measurement result should be reproducible with a maximum deviation of 1%.  
         [0008]     The chief advantage of the invention is that, especially with the use of a reference tube whose outside diameter is the same as the outside diameter of the tube to be produced but whose wall thickness is greater than the wall thickness of the tube to be produced, the accuracy of the measuring method can be increased, since both tubes have the same temperature and the same specific resistance. The resistance of the metal tubes (finished tube with the smaller wall thickness and reference tube with the larger wall thickness) is inversely proportional to the wall thickness. Since the outside diameter of the finished tube and the reference tube is the same, the wall thickness can be determined very quickly and, if necessary, can be corrected by replacing or reworking the mandrel. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     The invention is explained in greater detail below with reference to the specific embodiments schematically illustrated in  FIGS. 1 and 2 .  
         [0010]      FIG. 1  shows a side view of a machine of the type described, for example, in DE 101 51 827 A1 for the continuous production of longitudinally welded metal tubes.  
         [0011]      FIG. 2  is a perspective view of the device for measuring the thickness of a metal tube according to one embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0012]      FIG. 1  shows a side view of a machine of the type described, for example, in DE 101 51 827 A1 for the continuous production of longitudinally welded metal tubes.  
         [0013]     A metal strip  2 , which consists, for example, of austenitic steel, is drawn from a supply reel  1  and fed to a set of shaping rollers  3 , in which the metal strip  2  is gradually shaped into a tube  4  with a longitudinal slit. The longitudinal slit is sealed with a welding device  5 , preferably a laser welding device. After the welding device  5 , the welded tube  6  is grasped by a collet feed,  7 , which conveys the metal strip  2 , the slit tube  4  and the welded tube  6  through the production unit. The collet feed is described in German Auslegeschrift (Late Disclosure) 11 64 355.  
         [0014]     The welded tube  6  is then wound onto a cable drum  8 .  
         [0015]     The method described above is limited to tubes with an outside diameter of at least 2 mm.  
         [0016]     There is a need for metal tubes with an outside diameter of less than 2 mm and a wall thickness of less than 0.1 mm. Metal tubes of this description are used in the production of hypodermic needles.  
         [0017]     To be able to produce metal tubes with these dimensions, the longitudinally welded tubes produced by the method described above must be reduced to the desired dimensions by drawing in one or more steps.  
         [0018]     To make the longitudinally welded tubes suitable for the subsequent drawing operations, it is necessary to provide the inner surface of the metal tube  6  with a lubricant film.  
         [0019]     For this purpose, lubricant from a reservoir  10  is introduced by a lance  9  into the slit tube  4  or the welded tube  6 .  
         [0020]     The outside diameter of the welded metal tube  6  can be reduced by a drawing die  11 . If a mandrel (not shown) is positioned in the welded metal tube  6 , for example, a so-called floating mandrel, the wall thickness of the welded metal tube  6  can be reduced at the same time.  
         [0021]     If several drawing operations are to be carried out with the illustrated machine, either several drawing dies  11  and feeding devices  7  must be arranged in succession, or the cable drum  8  filled with the welded metal tubing  6  is placed in the position of the supply reel  1 . In this case, the set of shaping rollers  3  and the welding device  5  are not operated.  
         [0022]     The invention will be explained in greater detail with reference to  FIG. 2 .  
         [0023]     To determine the wall thickness of the metal tube, a length of tube  12  is prepared and clamped into the clamping devices  13  and  14 . The clamping devices  13  and  14  are made of an electrically conductive material, e.g., steel, and have a design similar to that of a cutting contact. The clamping devices  13  and  14  are mounted a well-defined distance apart on a foundation that is not specified in detail but consists of an electrically insulating material. The length of the tube  12  is greater than the distance that separates the clamping devices  13  and  14 . The ends of the metal tube  12  are connected to a power source, so that a current flows through the metal tube  12 . The voltage drop along the metal tube  12  is picked up and measured at the clamping devices  13  and  14 . In this regard, the contact resistance between the metal tube  12  and the clamping devices  13  and  14  is not measured as a measuring error.  
         [0024]     The wall thickness can be determined from the outside diameter of the metal tube  12 , which can be accurately measured, and the measured voltage drop, together with the intensity of the measuring current, the voltage, and the material properties of the tube.  
       S   =       D   2     -           D   2     4     -       ρ   ⁢           ⁢   lI       U   ⁢           ⁢   π                 
 
         [0025]     The wall thickness can be directly calculated from the formula. The symbols in the formula have the following meanings: 
        S=wall thickness     D=outside diameter of the measured tube     ρ=specific resistance of the tube material     l=distance between clamping devices  13  and  14      I=current intensity     U=measured voltage drop        
 
         [0032]     The accuracy of the measuring method can be further increased if a reference tube is first reduced only in its outside diameter without a mandrel. The resistance of this reference tube is determined. The resistance value of a metal tube that was drawn with a mandrel, i.e., whose outside diameter and wall thickness were reduced, then varies inversely as the wall thickness of the reference tube, so that the wall thickness can be determined quickly and with great accuracy.