Laser predrilling for high-precision positioning and installation of fully threaded screws in glued-laminated timber (glulam)

ABSTRACT

In many applications of joining and reinforcing timber elements with self-tapping fully threaded screws, the exact positioning of the screws becomes difficult as the screws tend to “stray” from their designated axis during insertion in wood and wooden semi-products. Experimental investigations show that besides natural local discontinuities like knots or resin inclusions, the screw-to-grain angle and the screws stiffness have an influence on the insertion process and thus can be identified as factors to a deviation of fully threaded screws. Mechanical predrilling cannot solve this problem, as the drills also stray during the predrilling process. As an alternative, a pyrolytic method of predrilling based on laser radiation proved suitable for obtaining guide holes with adequate straightness. This paper includes a description of the developed drilling processes using different lasers beam sources. The resulted borehole geometries of a QCW fiber laser, as well as an ultrashort pulsed laser, show satisfying results. Furthermore, the short- and long-term bond behavior of fully-threaded screws in a glued-laminated timber matrix, installed in laser predrilled guide holes, was tested and evaluated. The results show that with the new innovative wood processing method an exact positioning of fully-threaded screws is possible, almost without loss of withdrawal resistance of the screws.

KEYWORDS:

• Fully threaded screws
• wood processing
• laser
• predrilling
• pyrolytic ablation
• bond-behavior

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The work presented in this paper was supported and financed by the German Research Foundation (DFG) [grant number 406036741].