Turn-milling of tree discs to evaluate the texture of the annual growth rings

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Jocelin Anthony
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1 Universität Stuttgart Institut für Werkzeugmaschinen Prof. Dr.-Ing. Dr. h. c. mult. U. Heisel Turn-milling of tree discs to evaluate the texture of the annual growth rings Authors Prof. Dr.-Ing. Dr. h.c. mult. Uwe Heisel; Prof. Dr.-Ing. habil. Johannes Tröger; Dr.-Ing. Sergey Martynenko; Dipl.-Ing. Kai Schumacher Speaker Dipl.-Ing. Kai Schumacher Universität Stuttgart Institute for Machine Tools Holzgartenstrasse 17 D Stuttgart, Germany Abstract The analysis of the pattern of annual growth rings is done by a lot of different disciplines: Phytology, wood biology, climatology, pollution control, geography, ecology, archaeology, architecture, science of forestry etc. But the machining of cross-grained wood is one of the most difficult grinding tasks in woodworking. Up to now, such tree discs are prepared with a special machine, which cuts the surface with a sharp blade. This solution is very expensive, because for every test specimen a new blade is required. Furthermore it is just possible to prepare a small stripe instead of the whole disc. And sanding of the surface has the disadvantage, that the cells in the wood would be filled with dust. With the machining process of turn-milling it is possible to finish such tree discs in an adequate quality and in a short cutting time. 1 Introduction During the last decades the climate and atmospheric conditions on earth rapidly changed. Those changing environmental influences are visible directly in the annual rings of trees. The analysis of such annual rings gives important information about earlier environmental impacts. To carry out such studies it is necessary to prepare the tree discs in an adequate way. Up to now, this was not very easy to handle, because wood machining with a regular milling cutter could cause a crushing of the wood cells, or the wood could bunk as a result of the disadvantageous angle between the directions of feed and the wood fibres. So far the preparation of such wood samples was usually done with a device called Mikrotom, on which the samples are planed with a very sharp blade. But with this equipment it is

2 not possible to prepare a complete tree disc, just a small stripe of the wood can be finished. In addition, this method is very expensive, because for each preparation a fresh blade is required. And the finishing by sanding is not appropriate, since the dust will fill up the wood cells, which complicates the analysis of the structure of the annual rings. The subject of this essay is a new machining operation to produce complete tree discs economically and in a constant good surface quality over the whole cross sectional area. 2 Machining strategy Turn-milling is a combination of turning and milling in the area of shape cutting chipping technology. The kinematics of the feed movement during turn-milling results from the interaction between the rotary motion of the work piece and the linear motion of the tool in orthogonal direction to the rotation axis of the work part. To obtain an good surface quality, a combination of several machining methods is suggestive. For machining the method of cone-face-milling shall be used. Secondary, the tool shall have bossed cutting edges. With those two conditions it is possible to get an excellent surface quality [3], [9]. Finally the work piece will rotate by its own axis, so the cutting direction is always at right angle to the annual rings during the milling operation. This will be realised with a turntable. During cone-face-milling, the penetration of the tool and the work part is described with conic sections. This is shown in Fig. 1. Fig. 1: Penetration of tool and work piece during cone-face-milling [3] Due to the fact that during cone-face-milling the setting angle κ is very small, there will be produced relatively long and thin chips. The thickness of such chips is very slight in the area where quality is build up. As a result, the achievable surface quality is very high in regard to planarity of the surface and the quality of the borders [3], [9].

2 Turntable Like in chapter 2 already mentioned, the tangential feed shall be done by a rotary table. This device is a turntable named NC 150-TAA delivered by the company Weiss (Fig. 3). Fig.

3 3 Trial Equipment 3.1 Milling machine For the tests, a CNC-controlled router machine with 5 axes is available. That means, the milling aggregate is able to move in all three room directions and has two additional axes of rotation (Fig. 2). Fig. 2: Milling machine type PE 170 [7] 3.2 Turntable Like in chapter 2 already mentioned, the tangential feed shall be done by a rotary table. This device is a turntable named NC 150-TAA delivered by the company Weiss (Fig. 3). Fig. 3: Rotary table type NC 150-TAA [10] with work piece This device is able to position heavy loads dynamically and exactly in free selectable angle positions. On top of the turntable is a round board with an appliance to fix the tree discs on it. The work piece has first to be centred on the table and then it can be fixed with the clamping jaws.

3.3 Milling tool The processing tool is a cone milling cutter with a diameter of 140 mm (Fig. 4). This tool has a constant setting angle κ and just one exchangeable insert.

4: Milling tool To change the chip angle, a hexagonal nut and a headless screw have to be loosed.

4 3.3 Milling tool The processing tool is a cone milling cutter with a diameter of 140 mm (Fig. 4). This tool has a constant setting angle κ and just one exchangeable insert. The cutting edge is bossed formed like demanded with a radius of 50 mm. The special feature of the tool is the possibility to adjust different angles of inclination and variable chip angles. Fig. 4: Milling tool To change the chip angle, a hexagonal nut and a headless screw have to be loosed. After this, the tooling insert has to be adjusted to the demanded position and finally to be fixed again. For each angle of inclination exists a separate tooling insert. 3.4 Work pieces In the experiments are different sorts of wood to examine, like softwood (for example spruce) and hardwood (for example oak). Before the tests, the wood samples have to be prepared in a special way. First the wood discs are sawed off the log. Then they have to be dried in a warm room over several weeks. When the discs are dry, they must be rough machined on both sides to get a plane bearing and an acceptable surface for the finish. After that, the plan milled slices are around 6 cm thick and ready for the experiments (Fig. 5). Fig. 5: Slice of spruce with diameter 25 cm The diameter of the tree discs is limited to 32 cm, because bigger parts can not be mounted on the turntable with its clamping device. And the slices have to be at least 3 cm thick to avoid a collision of the machining tool with the clamping jaws.

3.5 Measurement technique The rawness of the machined tree discs will be measured with a laser analyser [4]. This method is quick and simple to handle.

5 3.5 Measurement technique The rawness of the machined tree discs will be measured with a laser analyser [4]. This method is quick and simple to handle. Each sample will be scanned five times parallel and five times in right angle to the annual rings. The measuring track is 12,5 mm long. In addition to the measurement with a laser beam, a visual analysis of the cross section is planned. Therefore, a light-optical microscope is available. 4 Milling experiments 4.1 Designation of the process parameters To ensure that the cutting direction is always in right angle to the annual rings, the tree disc has to rotate around its own axis during the machining process. The milling tool is moving in radial direction from the edge of the disc to the centre. This principle is demonstrated in Fig. 6: Fig. 6: Kinematics of the feed movement during turn-milling [6] The cutting speed v c results from the rotation of the tool n WZ, the rotation speed of the work part n WS and the diameter of the tool d WZ. v c = ω r = π r d (4.1) Another important parameter is the feed speed v f, which consists out of two components in turn-milling. The first part is the radial feed speed v frad. It is orientated orthogonal to the annual rings and points to the centre of the tree disc. The second component is the tangential feed speed v, which is parallel to the annual rings. All summarised, the feed speed can be expressed by following equation: f 2 frad 2 v = v + v (4.2) The radial component can be disregarded in this case, because it is very small. If both components would be considered, the calculation would be around 0,03% more exactly. So the tangential feed speed can be equated with the absolute feed speed.

6 v f r v = nws 2 π (4.3) 1000 The feed per cutting edge f z is associated directly with the feed speed, so the radial component can be disregarded too. f z f ztan = v 1000 ( n z) WZ z is the number of cutting edges per tool. The feed per cutting edge has a big influence on the surface quality of the work piece. To keep a constant surface quality during the whole milling process, the feed per cutting edge should be held constant. The tangential feed speed is calculated as the product of the rotating speed of the tree disc and the radial distance between the centre and the position of the tool r. (4.4) v () t ωws = r (4.5) To achieve a steady high quality it is important to have a constant tangential feed speed during the entire machining process. Now it is to note, that the radius is variable. To establish a constant tangential feed speed, the rotation speed of the work piece has to be a function of the time too. () t () t const. v = r ω WS = (4.6) Furthermore, the feed per revolution r u shall be constant during the cutting process. r = ru = 2π rϕ = const. ϕ (4.7) Based on the two equations (4.6) and (4.7), the time-dependent process parameter v f RAD and ω WS as well as the tool position can be calculated: v frad () t = ( 2 v r ) 2 1 t v WS () t = t 0,5 ω (4.9) 2 r The rotation speed of the milling tool will be kept constant. It is calculated by the quotient of the chosen cutting speed v c and the diameter d of the tool. v n = c WZ d (4.10) Fig. 7 shows the calculated process parameter for the milling of cross-grained wood with f ztan = 0,24 mm, r U = 1,6 mm, n WZ = 8000 min -1, d = 41 mm and z = 2. In the diagram, the process parameter are illustrated in relation to the position of the tool, or in other words, to the radius of the milling track. (4.8)

7 Fig. 7: Calculated process parameters [6] This idealised, theoretical gradient of the procedural setting parameters is not possible in reality. For example, the rotation speed of the work piece must be endless in the centre of the tree disc, which no real machine can realise. Another problem is, that the machine is not infinitely variable. So there exists a deviation between the real process and the theoretical course. The three process parameters radial feed speed, rotation speed of the work piece and tangential feed speed are pictured in Fig. 8. Fig. 8: Real process run [6]

In the diagram can be seen, that the parameters are changing in forty steps. 4.2 Execution of the experiments In the experimental series were varied process parameters as well as tool characteristics.

8 In the diagram can be seen, that the parameters are changing in forty steps. 4.2 Execution of the experiments In the experimental series were varied process parameters as well as tool characteristics. A snapshot of such a machining procedure is shown in Fig. 9. At the already milled area at the edge of the tree disc, the annual rings are clearly visible. Fig. 9: Snapshot of a machining process First, the influence of the tangential feed speed v was explored. For this reason three different sequences for the turntable were programmed. Those chosen sequences demonstrated to be advantageously in preliminary tests. In addition, the action of the rotation speed of the tool n WZ was checked. The tree discs were machined with parallel feed and the parameter was changed from min -1 in steps of min -1 up to min -1. Another exploration was done by varying of the radial feed speed v frad. At the milling tool itself the parameters setting angle of the machine κ M, the angle of inclination λ and the chip angle γ were varied. 4.3 Evaluation of the tests After the milling procedure, the work pieces were scanned with the laser analyser to get the rawness of the surface. With this measuring device it is possible to measure the rawness data R max, R a, R z, R k, R pk, R vk. To evaluate the surface of the wood samples it was a reference piece available. As an example, the rawness in function of the rotation speed of the tool is demonstrated in Fig. 10:

Fig. 10: Influence of the rotation speed of the tool n WZ on the surface quality (measurement orthogonal to the annual rings) If the rotation speed is increased to 10.

A possible explanation could be, that with higher number of revolutions the vibrations of the machine will increase, and this vibrations could have a negative influence on the wood surface.

This check is more important than the measurement with the laser analyser, because it is very important for the analysis of the annual rings, that the wood cells are in an excellent condition and not

9 Fig. 10: Influence of the rotation speed of the tool n WZ on the surface quality (measurement orthogonal to the annual rings) If the rotation speed is increased to min -1, the surface quality is grading down. A possible explanation could be, that with higher number of revolutions the vibrations of the machine will increase, and this vibrations could have a negative influence on the wood surface. The best result was achieved by driving a rotation speed of min -1. Afterwards, the samples were examined with a microscope. This check is more important than the measurement with the laser analyser, because it is very important for the analysis of the annual rings, that the wood cells are in an excellent condition and not destroyed during the milling procedure. In Fig. 11 can be seen two examples. Fig. 11: Comparison between the surface of a sooner test (left) and with optimised process parameters (right) The right picture shows the cell structure of the test with the optimal parameters. The cells of early wood as well as those of late wood are very good visible. In comparison to this good result the microscopic picture of one of the first experiments is shown on the left side. Here the cell membranes are crushed and the tubes can almost not be recognized.

10 5 Summary The aim of this project was to develop and test a technology to machine cross-grained wood, called turn-milling. During the experiments the procedural influences were analysed. Following parameters were varied: rotation speeds, cutting speeds, feed speeds, and the geometries of tool and cutting edge. The experiments proofed the possibility to machine a complete tree disc economical and in a good surface quality with the method of turn-milling by constant tangential feed speed. 6 References [1] Ettelt, Bernhard, Gittel, Hans-Jürgen Sägen, Fräsen, Hobeln, Bohren (Sawing, Milling, Planing, Drilling). 3. revised and completed edition, DWR- Verlag Weinbrenner GmbH & Co.KG, 2004 [2] Gottlöber, Christian Ein Weg zur Optimierung von Spanungsprozessen am Beispiel des Umfangsplanfräsens von Holz und Holzwerkstoffen (A Way to Optimise Chipping Processes, for Example the Peripheral Face Milling of Wood and Wood Composites). TU Dresden, Degree, 2003 [3] Graef, Jürgen Kegelstirnplanfräsen (cone-face-milling). Medienverlag Köhler, Degree, 2004 [4] Keyence Corporation Technical manual, CCD-Laser-measurement-system, series LK-2000 [5] Lehmann, Thomas Orthogonales Drehfräsen (Orthogonal Turn-Milling). Carl Hanser Verlag Munich, Vienna, Degree, 1992 [6] Martynenko, Sergey Research report. Institute for Machine Tools, Universität Stuttgart, 2007 [7] Max Mayer Maschinenbau GmbH Technical manual, milling machine PE 170 technical data [8] Stühmeier, W. Fräsen von Spanplatten mit hochharten Schneidstoffen (Milling of Chipboards with Adamantine Cutting Materials). TU Braunschweig, Degree,1989 [9] Tröger, Johannes Kegel- und Stirnplanfräsen (Cone- and Front-Face- Milling). Lecture manuscript, Institute for Machine Tools, Universität Stuttgart, 2007 [10] Weiss GmbH Technical manual, turntable Type NC 150-TAA

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