Abstract
Dimensional accuracy of machining translates into susceptibility to defects in assembly of furniture elements. In the initial drilling phase, the tip of the drill may slip due to the properties of the workpiece, which may result in inaccurate machining. Taking this into account, it was decided to investigate this phenomenon for drilling in the side of the plywood board. Samples for testing were made of 18 mm thick, 13 layer birch-alder plywood, covered with melamine film, glued with phenol-formaldehyde glue. With the use of an industrial dowel drilling machine, 30 holes in each of three examined layer were made. All holes were made parallel to the layers – on the side of the plywood board, and their axes were located in three adjacent layers: the birch veneer layer, the adhesive layer, and the alder veneer layer. Two types of geometric accuracy of holes were analyzed: the distance between the real center of the hole and the nominal position and also the inclination of the hole axis from the nominal axis. The holes made in the adhesive layer showed approximately twice larger deviations compared with the holes made in two adjacent layers of veneer. There was no significant relationship between the deviations of the angle of holes axis and the inaccuracies in the position of their centers. Main conclusion: When drilling in the side surface of the plywood board, less accurate holes are obtained if the hole axis is located in the adhesive layer, and there is more accuracy if the hole axis is located in the veneer layer.
Download PDF
Full Article
The Accuracy of Holes Drilled in the Side Surface of Plywood
Maciej Sydor,a,b,* Tomasz Rogoziński,a Kinga Stuper-Szablewska,a and Karol Starczewski b
Dimensional accuracy of machining translates into susceptibility to defects in assembly of furniture elements. In the initial drilling phase, the tip of the drill may slip due to the properties of the workpiece, which may result in inaccurate machining. Taking this into account, it was decided to investigate this phenomenon for drilling in the side of the plywood board. Samples for testing were made of 18 mm thick, 13 layer birch-alder plywood, covered with melamine film, glued with phenol-formaldehyde glue. With the use of an industrial dowel drilling machine, 30 holes in each of three examined layer were made. All holes were made parallel to the layers – on the side of the plywood board, and their axes were located in three adjacent layers: the birch veneer layer, the adhesive layer, and the alder veneer layer. Two types of geometric accuracy of holes were analyzed: the distance between the real center of the hole and the nominal position and also the inclination of the hole axis from the nominal axis. The holes made in the adhesive layer showed approximately twice larger deviations compared with the holes made in two adjacent layers of veneer. There was no significant relationship between the deviations of the angle of holes axis and the inaccuracies in the position of their centers. Main conclusion: When drilling in the side surface of the plywood board, less accurate holes are obtained if the hole axis is located in the adhesive layer, and there is more accuracy if the hole axis is located in the veneer layer.
Keywords: Plywood; Drilling; Wandering of drill
Contact information: a: Faculty of Wood Technology, Poznań University of Life Sciences, ul. Wojska Polskiego 28, 60-637 Poznań, Poland; b: Digitouch sp. z o.o., ul. Różana 6, 62-002 Suchy Las, Poland;
* Corresponding author: maciej.sydor@up.poznan.pl
INTRODUCTION
Holes in wood and wood-based materials are usually made to attach fasteners (e.g. with screws furniture dowels and other connectors (Smardzewski 2015; Sydor 2019; Sydor and Wieloch 2009). In case of industrial furniture production, holes are made with multi-spindle drill. Among many other furniture materials, plywood, a multilayer composite made of wood veneers glued in layers, are used. The adhesive joint between the individual veneer layers affects the machinability of the plywood (Guo et al. 2018). As a result, when making deep holes (parallel to the of veneer layers) in the side of a board, there are usually problems with their geometrical accuracy. This applies in particular to drilling deep holes of small diameter, where an unfavourable wandering of drill may occur. This phenomenon may result in an inadequate hole position or one that is not parallel to its axis (tilted axis of hole). Ultimately, this may result in the improper assembly of furniture components. An example of furniture particularly sensitive to inaccuracies of preparation are modular polymorphic furniture (Starczewski 2016; Laskowska and Sydor 2018). The pin connector used in them requires small-diameter holes (typically diameter 3 to 4 mm) of relatively large depth (60 to 100 mm). These holes are usually made both perpendicular and parallel to the layers of veneers (in a face and in a side of plywood sheets; Fig. 1). The most commonly used board thickness range for cabinet furniture is from 16 to 20 mm.
Fig. 1. Furniture with pin connectors requiring long holes in plywood (photo by M.S.)
The technological drilling process involves four phases (Gong et al. 2005). The first phase lasts until the rotating drill starts touching the workpiece. The second phase begins when the tip of the drill is in contact with the workpiece. This phase involves a so-called drill slip, a random phenomenon that proceeds in a manner resultant from the properties of both material and the drill. The slip only occurs during several drill turns, after which the end of the drill stabilizes in a new position, sometimes deviated from the nominal position, and begins penetrate the workpiece. The third phase, penetration of the drill tip, begins after the slip has stopped and the drill forms the full diameter of the drilled hole. The fourth phase begins when the entire drill bit penetrates the workpiece and the side surface of the drill comes to contact with the opening wall. This phase ends when the drill reaches the full depth of the hole. The second phase phenomena are responsible for the deviation of the hole centre from the nominal position, while the phenomena occurring in the third phase mainly correspond to the deviation of the resultant axis as compared with the nominal one (Gong et al. 2005).
The accuracy of the dimensions of the pilot holes for furniture fasteners has a large impact on the susceptibility to assembly and on the strength performance of furniture joints (Branowski et al. 2018). Due to the layered structure of the plywood, the drilling process takes place in a specific way, different in case of other wood composites.
The available scientific literature on drilling conditions in plywood is given in Table 1.
Table 1. Literature on Drilling Operations Typically Employed for Plywood
The authors of the publications presented in Table 5 focused on the research of drilling processes in face of plywood boards (perpendicular to the veneer layers). The analysed literature on drilling in plywood boards showed no description of the experiment of drilling in side board surfaces (parallel to veneer layers).
Due to the internal structure of the plywood, the location of the hole in relation to the individual layers of plywood may prove important. Therefore, in order to determine the impact of the hole axis on the non-homogeneous side surface of the hardwood plywood, the drilling experiment was prepared, where holes were drilled in the side surface of such a board and the deviations of their resultant position (distance hole centres from the line being the nominal dimension) and angle deviation of holes (tilt of hole axis) were measured.
EXPERIMENTAL
Material
Three samples for testing were made of 13 layer hardwood (birch and alder) plywood covered with melamine film glued with water resistant glue, based on phenol-formaldehyde resin. They were 320 mm in length, 45 mm width, and 18 mm thick. This enabled the formation of 30 holes in each of them. The most important physical parameters of the plywood used in the experiment are summarized in Table 2.
Table 2. Important Physical Parameters of the Plywood Samples
Samples for testing were cut from one piece of the plate to obtain possibly homogeneous material properties for each of the three samples. This was because the properties of plywood, even made of the same species of wood and using the same adhesive, may differ from each other. These differences result from the heterogeneity of the raw material (veneer) and from the possible technological differences in the production of plywood (e.g. temperature, pressure or pressing time) or from various storage conditions. The holes made under the experiment were intended for mounting steel spigots in the form of 57 mm pins of 3.4 mm nominal diameter.
Methods
To make holes in the side surface of the plywood board, a multi-spindle dowel drilling machine was used (Felder FD 21 professional, Fig. 2). The machine had 21 spindles (11 with right-hand rotation and 10 with left-hand rotation). A central right-turn spindle was used to drill all holes. The spacing between the holes was 10 mm.
Fig. 2. The method of making holes (photo by M.S.)
The twist drill bit was used (with cylindrical right-handed shank of 3.8 mm nominal diameter, with 75 mm total length, 45 mm body length, and 118° point angle, made of high-speed steel (HSS)). Spindle speed was 2800 rpm. Drilling feed 42 m / min and feed per revolution 0.15 mm / rev. were applied.
Ninety holes of 10 mm in depth were made. Thirty of them were made in the birch veneer layer (in the centre of the board, on the mark made in “(A) birch veneer”), in the layer of the adhesive joint (below the centre, the “(B) glue”) mark, and in the alder veneer layer (below the joint, marked “(C) alder veneer”). Making holes in particular layers of plywood means that the nominal hole axis is located directly in this layer. The holes in “(A) birch veneer” layer were located perpendicular to the longitudinal section (tangent or radial) of the wood. The holes made in the “(C) alder veneer” layer (in the centre of the panel, i.e. above the adhesive joint), were made in the direction perpendicular to the cross-section of the wood. Figure 3 presents holes made in three layers of the plywood board.
Fig. 3. Holes made in three layers of the plywood
After the holes were made, deviations of their resultant position were measured (the centre distance of each hole from the nominal line on which the centres of all holes should be located). Exemplary deviations of the hole centre position from the nominal line are shown in Fig. 4.
Fig. 4. Exemplary deviations of the hole centre position from the nominal line (“(C) alder veneer”)
The inaccuracy of each the hole axis with respect to its nominal axis was measured in a single plane. The measuring directions and an exemplary deviation of the hole axis from the nominal axis are shown in Fig. 5.