|Working with Presses - Course: Mechanical woodworking techniques.Trainees' handbook of lessons (Institut für Berufliche Entwicklung, 16 p.)|
|3. Technological Application of the Press|
The gluing in sheets of different solid wood thicknesses is especially necessary in frame furniture production. If skillfully manufactured the produced pieces of work are of an especially high stability. The shrinkage and swelling behaviour can be adjusted to the corresponding purpose of application.
Figure 4 - Block of single pieces of work with triangle marking and numbering
When the glued blocks are put together the shrinkage and swelling behaviour of the separate pieces of work should be taken into consideration. If only two pieces of work are placed together their left faces are put one on to the other. These faces become hollow when loosing moisture. A splitting up of the joint edges cannot occur with the glued piece of work. With more than two pieces of work always the left sides and the right sides are joined.
In the middle sheets you may also place shorter pieces of work with a cross joint. Because during the bending stress of the ready-made pieces of work mainly the edge zones are subjected to stress a cross joint has little effect on the stability.
To manufacture a glued block also other timber materials can be used as basic material.
The used glue should have a high viscosity but it should not be soaked up by the material or dry out when the glue is put on and the piece of work being fastened in the press. The features of the various glues are different.
Dispersion glues do not need a hardener for binding. With little extraction of water these glues loose their viscosity quickly. When placing the forced feed the glue cannot spread uniformly in the joint which results in failure gluings. Because of these features dispersion glue should be applied very thin and in large amounts.
The period of time between putting the glue on and clamping the piece of work should be very short and that is why only a few pieces of work should be clamped together. When the forced feed is starting the glue must be still liquid!
The application of glue on urea-formaldehyd basis allows the clamping of a larger amount of pieces of work because this glue alters its viscosity only little after the glue has been put on. But it is subject to dry quickly.
That is why after the glue had been put on the joint should be closed immediately by joining both the joint faces. Urea-formaldehyd glues require a special hardener for binding. By mixing it with a 2 % to 3 % binding agent it is made ready to use. The processing period starting now should not be exceeded. The processing period depends on the type of the binding agent and on the room temperature. To avoid losses of time the working process prior to the clamping of the piece of work must be well prepared.
Before using the glue for the first time you should study the processing instructions carefully.
The stability of a glued joint is increased if the glue layer between the pieces of work is thin. That is why the joint faces should be clean and even. If too much binding agent is added to the glue its solidity is reduced!
The pieces of work should be arranged on the pressing table always symmetrically to the press spindle.
Figure 5 - Example for the distribution of the pieces of work on the pressing table
Why must always the left sides of the pieces of work form a
joint when two wooden pieces are glued to a
Why can the pieces of work in the middle be short and lengthwise
arranged when gluing a
Why should only a few pieces of work be pressed together at the
same time when applying dispersion
Wall-to-wall elements or inside doors are often manufactured in lightweight construction. Here thin plates are glued on to a wooden frame. With this kind of planking the stability of the frame element is increased many times over. The edge joint of the frame is subject to stress only during pressing. That is why a simple and saving joining can be applied.
Figure 6 - Putting glue on frame woods
The glue should not be put up to the inner edge of the frame. The excess glue emerging from the frame edges features a great shrinkage measure when drying out. The resulting forces would damage the plates inwards the frame. Then the outlines of the frame woods would be visible on the outside faces of the plates.
Figure 7 - Gluing
(1) Gluing the full frame width, marks on the board's face
(2) Right gluing, no marks