Laser Etching of Randomly Oriented Parts:

Definition of problem:

Process/operation: laser guidance
Product: metal parts
Production issues:

Consequences: Poorly placed characters on part surface. Characters may be ill-formed or missing.  

Solution:

General overview:
Note: this setup was successfully demonstrated on a Trumpf www.trumpf-laser.com VectorMark system at the IMTS www.imts.com show in 2002. 

A laser etching system requires an input string or graphic to print and a target location on which to print it. If the laser’s beam is aimed down onto a horizontal table, the largest component of placement error is typically in the plane of the table – the X-Y plane. The beam is focused at some working distance, Z. It is steered around the part by a pair of galvanometer mirrors. Therefore, the galvo head is typically directly above the part – perpendicular to the X-Y plane.

The fixturing of the part, for whatever reason, may be too loose to position the part accurately so a vision system can be employed to precisely locate the part in the X-Y plane and also report it’s angle of rotation T, from its expected position.

There is a difficulty in placing the camera when the galvo head is already directly above the part. The camera must occupy a place beside the galvo head at some angle from perpendicular to the X-Y plane. This skewed view of the part creates perspective distortion in the camera’s image. The net effect of this distortion is that squares appear to become trapezoids.

Fixturing & Lighting:
The lighting system may change, depending upon the surface features of the part being located. For instance, a part with definite perimeter features should be back lit or presented against a sharply contrasting background color. Parts with defined interior features may be front lit to accentuate that geometry.

Vision Hardware:
A Cognex Insight 5403 cameras with a resolution of 1600x1200 pixels will provide 0.0025”/pixel resolution on a spacious 4 x 3” field of view. This equates to placement accuracy of about +/-0.001”

Programming:
Calibration for a trapezoidal field of view is accomplished within the Cognex In-Sight programming architecture. In short, the camera corrects for the trapezoidal appearance of the image and provides offsets to the laser software in the expected rectangular coordinates. The laser software, (Trumpf VectorMark, in this case) communicates with the camera via serial or Ethernet protocols. The laser accepts the X,Y, Angle offsets calculated within the camera and corrects the laser beam paths to correctly mark the part.

Results for the customer:

The etching placement is greatly improved.
Fixturing is loosened and thus allows easier part loading
Etching quality may be inspected after the fact to ensure proper setup and laser power.