PRECISE TOOL MEASUREMENT USING DIGITAL PHOTOGRAMMETRY 
Husen, B., Benter,U.* 
Institute for Photogrammetry and Engineering Surveys 
University of Hannover, Germany 
*Volkswagen AG, Wolfsburg, Germany 
ISPRS-Commission V 
Abstract: 
Tool measurement with an accuracy of a few microns is an important, but also time consuming and expensive element 
in industry. Up to now the geometry of the cutting edges of tools is checked by mechanical devices. This paper presents 
a new approach using computer vision techniques. Basic components of the system are two CCD-cameras which allow 
to get the three-dimensional position of the inserts of the tools. In order to reach the required accuracy a precise reseau 
grid is used as a control point field for the orientation of the cameras. Because of the small angular aperture of the 
CCD-cameras beam splitters have been attached in front of the cameras so that the images of the control point field 
and of the cutting edges can be evaluated one after the other. Investigations with end measures have shown that an 
accuracy of 3 um for the position of cutting edges can be reached. 
Key Words: Accuracy, Industrial, Machine Vision, 3-D 
1. Introduction 
With the increase of untended automated manufacturing 
systems, there is also a tendency to automated measu- 
ring systems, which do not only check the product quality 
of workpieces, but perform also the inspection of tools. 
This is very important for economical manufacturing, be- 
cause it helps to avoid reworking and rejecting. Due to 
the progress in computer-technology and the development 
of optical sensors opto-electronical measuring systems are 
developed, which reduce the influence of operators and 
which contribute to accelerate the measurements [Pfeifer 
et al.,1982]. 
This paper presents a new method to check the geome- 
try of drilling and milling tools, which are used for stock- 
removal production in computer controlled manufacturing 
systems. Fig.1 shows an example of a drilling tool with 
several triangle shaped cutting edges. 
  
Fig.1: drilling tool with several inserts 
The essential parts of the tools are the cutting edges and 
before a tool is used for production the position of the 
cutting edges must be controlled to guarantee the quality 
of the manufactured workpieces. Up to now this is done 
by profile projectors or coordinate measuring machines. 
The new method described here makes use of computer vi- 
sion techniques to evaluate the data of a pair of two CCD- 
cameras, which are pointed convergently on a tool. Thus 
they allow to get three-dimensional information about the 
cutting edges of the tools. 
A precise reseau grid is used as a control point field for 
the orientation of the cameras. À special approach for the 
orientation of the cameras has been developed, because of 
the small size of the available CCD-arrays, which makes it 
impossible to get control points and the unknown object 
in one image. Otherwise the image scale would be two 
small to reach the required accuracy of a few microns. 
With a scale of about 1 : 2.5 a triangle shaped insert can 
be projected on one image. At this scale the pixel size in 
the object space is about 20 microns, so that with an edge 
detection accuracy of 0.1 pixel an accuracy of 2 microns 
is possible. As no control points can be seen in the images 
at this scale, beam-splitters have been attached in front 
of the objectives so that parts of the reseau grid can be 
seen too, depending on whether the reseau or the tool is 
illuminated. Therefore a calibration of the beam-splitters 
is also required. 
2. Hardware Configuration 
The main part of the system consists of a fixed pair of stan- 
dard CCD-cameras with macro-lenses and beam-splitters. 
They can be shifted parallel to two reseau-grids which are 
placed on each side of the tool which is to be measured. 
The cameras are connected to a 386-PC with an image 
processing board inside which does above all the A/D con- 
version and stores the digital image data. 
The reseau grids with a size of 400 x 80 mm? have a mesh 
width of 2 mm. Each single cross is placed with an relative 
accuracy of 1 jm and an absolute accuracy of 2.5 jum over 
a length of 400 mm. According to the manufacturer the 
flatness of the glass varies only for 5 ym over 80 mm.