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Wood machining equipment that can be deployed universally with limited modifications

Vertongen Houtbewerkingsmachines bvba in Bornem develops and produces tenoners and profile moulders for the machining of wood mainly used to produce wooden windows and doors. The core of such a machine consists of a cutter set that machines the extremities (tenoner) or sides (profile moulder) of the wooden beams. To work with even more flexibility without having to convert the cutter sets, the cutting system was expanded from two to three cutter sets per spindle. The company asked Sirris to analyse the risks with regard to strength and vibrations and, if required, to propose solutions.

 

The cutter sets, which can be up to more than 100 mm high each, are mounted on a vertical spindle "in cantilever".  The extra cutter set means that additional mass is added on a longer spindle. This increases the risk that the sag will be too extensive due to the milling forces and that vibrations will occur that may have an impact on the quality of the machined material or even the stability of the equipment.

 

Depending on the product and the beam thickness, the milling unit (i.e. the milling spindle and its drive) is set to the correct height and depth and therefore the position on the load-bearing frame varies. This in combination with the diversity in cutter sets and possibly leaving a pack out, leads to very different dynamic conditions. This meant that it could not be excluded that the machine would actually work at a critical speed. 

Modified design

In the study that Sirris carried out for Vertongen, the following aspects were dealt with: a comparison of a short and long spindle, critical speeds, gauging and modelling of the static and dynamic behaviour without and with added imbalance and the sagging and fatigue resistance of the spindle including when operating at a critical speed with imbalance.

 

Modifications were implemented to the milling spindle and bearing based on the study. For the profile moulder that operates at a higher speed than the tenoner, additional support was provided for the column on which the milling unit is positioned. Recommendations regarding the required balancing class for the cutter sets and the option of including a warning system that is triggered when the vibration levels are too high, for example, when a cutter is damaged, were also provided. To conclude, they determined the consequences of a smaller spindle diameter for the mounting part in cantilever of the spindle. This ensures that the variant with three cutting sets can also be marketed in regions were milling with smaller mounting diameters is common practice.

 

Thanks to the limited but targeted interventions in the design, market introduction followed very quickly. Vertongen has been successfully marketing its machines with the longer cutting spindle since mid 2017.