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Tooth by Tooth: optical 3D precision measurement of saw blades.

The precise manufacturing of tools is an indispensable prerequisite for optimum machining results. Saw blades, for instance, are available with higly-optimised tooth and blade forms, which have to be precisely reproduced. Accurate measurement is necessary for production control as well as for further optimisation.

This includes shapes of individual teeth as well as their accurate sequence on the saw blade, which results in an immensely large range between the dimensions of the entire tool and the detail resolution required. The number of individual parameters to be determined is also very large. With high performance saw blades, for instance, the accurate reproduction of tooth forms in all three dimensions must be measured over the entire tooth surface areas. Only optical measuring technology can deliver the large number of measuring points required in an acceptable time. Tactile methods, in comparison, could only deliver a very limited number of parameters.

The key technology: three dimensional vision sensors

For the measurement of fine surface detail, fringe projection is a prime method of choice. The projection and acquisition of stripe patterns from different viewing angles allows for a triangulation of all surface points, similar to laser line scanning, but simultaneously for the entire surface. An important improvement with this method is the exploitation of stripe flanks for improved triangulation accuracy: digital projectors and cameras allow to generate and measure 'gray value' gradients at extreme accuracy and deliver an improvement in triangulation and distance resolution by up to two orders of magnitude.

The automat for the complete measurement of saw blades and ribbon sections presented here (figure 1) includes two sensors of this kind for the 3D measurement of single teeth. Each tooth is measured from the left and the right side (upper and lower side in the machine, figure 3), with a short timely distance to exclude crosstalk. With a measuring field of approx. 2 [cm.sup.2], distance resolutions in the sub micron range, and according accuracies, are achieved. This allows for a precise measurement not only of the lateral clearance angle, but also of the camber of the tooth flanks. Moreover, thickness and offset of teeth can be measured. The full accuracy of the sensors can be exploited for thickness measurement by measuring a caliper between runs, which allows to compensate for temperature drifts of the mechanical framework.


The measuring principle is fully scalable, from millimeters to meters

The 3D measuring head employed are newest generation vision sensors by GFM. They have built-in intelligence and can be used for most various control, measurement and inspection applications.

The underlying measuring principle is fully scalable, allowing sensors with measuring fields of about 1 mm up to over 1 m. According to layout, measuring times can range from a few seconds down to fractions of a second. Beneath precision measurement, this also allows for supervision tasks like completeness checks and general shape measurement. The sensors communicate by an Ethernet interface and are easily combined in large numbers, as they can deliver the readily calculated 3D profile and even complex form parameters, therefore freeing the control computer from a great part of workload.

Due to their functional principle, the 3D sensors are less optimally suited for the measurement of two-dimensional shapes, or silhouettes. Therefore, an additional 2D camera sensor was integrated into the machine. As usual for such sensors, it has a telecentric lens. With such a lens, object rays are not running concentric, but approximately perpendicular to the very large lens plane and to the--relatively smaller--measuring object. This allows for a most accurate measurement of the lateral tooth shapes.


The measurement requires little preparation and runs fully automatic

The measurement is prepared by inserting the saw blade into the CNC operated transport carriage. A rear bedstop provides for a proper pre-alignment of the teeth row. The saw blade is then held down on the carriage by two or three rubber armed clamping appliances.

The carriage then proceeds into the measuring chamber in the central part of the machine, and stops there, controlled by the 2D camera. The first tooth is now fine positioned and measured.

Controlled by the camera image, the saw blade is advanced tooth by tooth. A little later, the first teeth are reaching the 3D sensors (figure 3) and are measured by them simultaneously. The assembly of 2D and 3D data is performed in the central control computer, according to the exactly acquired carriage position data. The central computer also controls the operator console and display in the left part of the machine.


The measurement requires just between two and three seconds per tooth, which results in comfortable total measuring times even for larger objects.

A detailed protocol completes the measurement

For the first time ever, this allows for a complete acquisition and control of the entire, complex form of a saw blade. This proves to be a valuable, powerful and effective tool not only for quality assurance in production, but also for the ongoing optimisation of products and manufacturing processes.

The vision sensor technology presented here recommends itself for a multitude of similar measuring tasks, not only but foremost in the tooling industry.


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Title Annotation:ARTICLE
Comment:Tooth by Tooth: optical 3D precision measurement of saw blades.(ARTICLE)
Publication:British Plastics & Rubber
Date:Jul 1, 2013
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