Å. S. Lundberg, B. O. M. Axelsson.
Luleå University , Department of Wood Technology,
Skellefteå Campus, S-931 87 SKELLEFTEÅ, SWEDEN.
Abstract
Sawing during the winter has always been a problem for the sawmills in the northern parts of the world.
Present work is an introductory study of the phenomena of frozen sawdust attached on the sawn surfaces (sawdust gluing ).
It is also a study of the cutting forces at and near a cutting edge when cutting frozen wood.
List of symbols and definitions
The following symbols are used in this paper:
Fh = Main cutting force (N)
Fn = Feed force (N)
GFI = Gullet feed index
T = Temperature (°C)
r = Edge radius (um)
Definition of edge sharpness:
1 Introduction
Sawing during the winter has always been a problem for the sawmills in the northern parts of the world. The problems can mainly be divided into four groups :
- sawdust gluing (frozen sawdust attached to the sawn surfaces)
- tool breakdown
- poor barking
- knot rupture
Sawdust gluing means that the sawdust freezes on the sawn surfaces, according to Sandqvist (1986). The dust causes environmental problems in succeeding production sectors. One concrete example is the automatic inspection of wooden surfaces by video technique. Saw dust on the surfaces may mislead the equipment, which implies bad measurement accuracy.
Tool breakdown occur mainly as broken saw teeth. This leads to poor surfaces and bad measurement accuracy. The conventional way to reduce tool breakdown in the winter is to reduce the feed speed. This, however, is not satisfactory because the productivity decreases and hence the production costs per m3 sawn timber increases.
Poor barking is a large problem during the winter season. This problem is focused especially on spruce timber. Poor barking leads to decreased chip quality, which implies lower chip value. Another result of poor barking is increased tool wear. The problem is reduced by increasing the pressure of the barking tool. This leads however to increased wear of the barking equipment and its tools and a lot of lost fibres.
Knot rupture arises mainly when using plane reducers. The problem can be reduced by letting conventional saw teeth generate the surface, which unfortunately implies sawdust production instead of the more valuable chip production. When profile reducers are used, the acceptance of some knot rupture damages is unavoidable. This however affects the timber value negatively.
Present work is a introductory study of the sawdust gluing phenomena and also a study of the cutting forces when cutting green frozen wood.
2 Experimental procedure
In order to determine the effect of sawdust gluing, a number of experimental tests were performed using a circular sawing machine with automatic feeding. Two different circular saw blades where used, according to the cutting parameters in Appendix I.
The specimens used in the tests were green frozen(T=-15°C) Scots Pine (Pinus sylvestris ) boards. The dimension of the specimens was 1000 mm long, 70 mm high and 60 mm wide. The share of sapwood in the specimens varied from 0 to 100 percent.
The amount of sawdust that was glued on the sawn surfaces in each run was collected and its mass was determined.
The specimens used in the experimental measurements of the cutting forces were Scots Pine pieces, prepared according to the wood parameters in Appendix II. The dimensions of the specimens are 150 mm long, 70 mm high and 70 mm wide.
The tools used in the cutting force measurements were prepared according to the cutting parameters in Appendix II.
The experimental investigations and the data acquisition were performed on the test equipment according to the experimental set-up in Appendix II.
3 Results and discussion
Saw dust gluing tests
The first thing noted in the experiments was that the heartwood/sapwood ratio was a determining factor for the amount of sawdust glued on the sawn surfaces. When cutting pure sapwood or pure heartwood
no or very little sawdust were stuck. When cutting specimens having 50% share of sapwood, the sawdust occurred more frequently on the sawn surfaces. The greater part of the sawdust that was glued on the surfaces was concentrated to the heartwood domain of the specimens. The sawdust had a tendency to attach the board in the end of each sawing course, according to Fig. 1.
The following discussion only deals with specimens holding 50% sapwood/heartwood ratio.
In Fig. 2 we see the mass of the attached saw dust produced during the test versus chip thickness/gullet feed index for saw blade A.
We note from Fig. 2 that a maximum occurs at approximately 0.1 mm chip thickness.
In order to increase the gullet feed index and chip thickness, the number of teeth was reduced from 32 to 16 on the saw blade used in the previous test sequence. In Fig. 3 we see the mass versus chip thickness/gullet feed index for this test sequence.
Fig. 3 shows that the amount of attached frozen chip decreases with the chip thickness. It should be noted, however, that the smallest chip thickness in this run was 0.1 mm.
Fig. 4 is a composition of the results obtained from the both previous test sequences.
In order to provoke the attaching of sawdust, a number of tests were performed, where the effect of decreasing gullet area was studied. The circular saw blade used in this set of tests was blade B according to Appendix I. This blade had half the gullet area used in previous test.
Fig. 5, where the average mass versus chip thickness/gullet feed index is plotted, is a composition of the results obtained from three different runs with blade B.
We notice an increasing amount of attached sawdust compared to previous tests and that the mass optimum is displaced towards higher gullet feed index/chip thickness.
A smaller gullet area and hence higher gullet feed index, should increase the amount of sawdust disposed for attaching on the sawn surfaces. This could partly explain the increasing amount of sawdust for saw blade B.
On the other hand an increasing chip thickness leads to a rougher structure of the sawdust probably less disposed to attaching.
As in previous tests, the amount of attached sawdust does not increase with increasing gullet feed index after the mass optimum has occurred.
The reason for the displacement of the mass optimum is at this time not fully investigated. The displacement is probably caused by a change in the structure of the sawdust due to different cutting properties of the two saw blades.
The gullet feed index / chip thickness ratio has to be expanded in future investigations to examine this phenomenon.
Cutting force measurements
In order to study phenomena related to tool breakdown, experimental cutting force measurements were performed. These studies show that the heartwood/sapwood ratio is an important factor for the influence of the temperature on the cutting forces when cutting green wood. The reason for this can be found in the great density/moisture gradient between the heartwood and the sapwood domains.
The grey scale images in Fig. 6, evaluated by means of a technique reported by Axelsson et al (1991), show distinct domains, holding the darker low-density (low moisture content) heartwood and the brighter high-density (high moisture content) sapwood respectively. The heartwood domain holds a average moisture content of about 30%(i.e. fibre saturation), and the sapwood domains about 95 %.
The higher moisture content in the sapwood domain results in a brighter region for the main cutting force as can be noted, both for frozen and non-frozen wood , thus corresponding to higher main cutting forces in that area.
For the feed force, the higher moisture content in the sapwood domain results in a darker region corresponding to a lower force both for frozen and non-frozen wood, as can be noted in Fig. 6.
The reason for the increasing main cutting force can probably be explained by effects caused by the inertia of the free water in the cell cavities. The decreasing feed force can partly be explained by the dependency between the main cutting force and the feed force as will be discussed later. Another possible explanation to the decreasing feed force is the lubricating property of the water, which may change the friction between tool and workpiece.
As can be noted in Fig. 6 , the cutting forces for the frozen specimen show higher values for the main cutting force and lower values for the feed force. The higher main cutting force can be explained by the fact that the compound strength of wood and water increases as the water freezes.
The decreasing feed force can be explained as a spring-back effect, caused by the deformation of the material by the tool. This effect is probably greater for the non-frozen wood than for the frozen wood.
It should also be noted that the feed force in the frozen sapwood shows a negative value, i.e. a tendency to pull the workpiece towards the tool.
A number of cutting force measurements where then performed on specimens having 100% sapwood.
The average and standard deviation of the cutting forces versus the temperature when cutting green pure sapwood with a tool having r= 5 um is plotted in Fig. 7. A list of the statistical data can be found in Appendix III.
The average and standard deviation of the cutting forces versus the temperature , when cutting green pure sapwood with a tool having r= 20 um is plotted in Fig. 8 and 9. A list of the statistical data can be found in Appendix III.
The spread in Fig. 8 is relatively large. This can be explained by the fact that cutting conditions are less well-defined with tools having r= 20 um than with tools having r=5um. This leads to a more diffuse cutting behaviour which gives a greater spread.
Summarising these plots, we note that
- cutting of frozen wood implies higher average main cutting force.
- cutting of frozen wood implies lower average feed force.
A hypothesis test was then performed on the data in Appendix III.
From this test it can be stated, with 95 % probability, that a difference exists on the average main cutting force when cutting frozen and non-frozen wood with a 5 um tool . Thus the statistical analysis confirm the difference in Fig. 7.
Concerning the main cutting force for a tool having r=20 um, no statistically significant effect could be observed from these experiments, as also can be seen from Fig. 8 above.
The sampled values of the main cutting force when cutting green frozen pure sapwood with the 5 um tool, were then utilised into a linear regression model. This resulted in the following equation:
Fn=14.796-0.285 Fh
The R-squared, which is an indication of how well a model explains the variation of the dependent variable, stops at 0.9 for the model above.
We notice from the model that the feed force decreases as the main cutting force increases. In Fig. 10 we see a plot of the linear regression model.We note that the feed force has a negative sign, i.e. the feed force tends to pull the workpiece towards the tool.
5 Conclusions
Saw dust gluing tests
- The heartwood/sapwood ratio is a determining factor for the amount of sawdust glued on the sawn surfaces.
- The amount of attached sawdust as a function of chip thickness/ gullet feed index tends to show a maximum value.
- The amount of attached sawdust does not increase with increasing gullet feed index after the mass maximum has occurred.
Cutting force measurements
- The heartwood/sapwood ratio is an important factor for the influence of the temperature on the cutting forces when cutting green wood.
- The temperature has a significant impact on the main cutting force when cutting pure green sapwood with a tool having r=5um.
- The main cutting force for the 5um tool increases as the temperature decreases when cutting pure green sapwood.
- The feed force for the 5um tool changes sign from a positive to a negative value as the temperature decreases when cutting pure green sapwood. This means that the feed force shows a tendency to pull the workpiece towards the tool.
- The main cutting force for the tool having r=20 um, as a function of the temperature, does not show a significant dependency of the temperature, when cutting pure green sapwood.
- The feed force for the 20 um tool decreases the temperature decreases when cutting pure green sapwood.
To sum up, the investigations about cutting forces shows a dependency between the main cutting force and the temperature, when cutting with tools having r=5 um. However, this influence is presumably too small to explain the problem with tool breakdown at the sawmills.
6 Future work
In order to further investigate the problem with tool breakdown, the interaction between the tool and workpiece has to be examined, by for example sampling the power consumption. This will give a possibility to take the dependency between the cutting forces , the amount of sawdust glued on the surfaces and the gullet feed index into consideration.
In order to simulate a more realistic industrial situation, the length of the specimens, the chip thickness and gullet feed index should be expanded.
The cutting force measurements have been performed on clear wood. The impact of the temperature on the cutting process when cutting through
fibre and density disturbances like for example frozen knots, should in future works be investigated.
Further, the friction between saw blade and specimen is a factor which may have importance for the entire behaviour of the sawdust gluing.
7 References
Sandqvist I.: Winter Sawing. Part 2. Experiences from the Sawmill Industry - Survey. (Träteknikcentrum, Report I 8605035 (1986)).
Axelsson B O M, Grundberg S A, Grönlund J A.: The use of gray scale images when evaluating disturbances in cutting force due to changes in wood structure and tool shape (Holz als Roh- und Werkstoff 49 (1991)).
Appendix I
Saw dust gluing tests.
Cutting parameters.
Saw blade A:Radius 200 mm, number of teeth 32 and 16, gullet area 120 mm2
Cutting speed:44.7 m/s
Feed speed range:4.1-20.2 m/min
Chip thickness range:0.05-0.49 mm
Gullet feed index range:0.035-0.34
Rake angle:25°
Clearance angle:10°
Side clearance angle:1°
Cutting width:3.8 mm
Profile of the gullet area of conventional type
Saw blade B:Radius 192 mm, number of teeth 16, gullet area 85 mm2
Cutting speed:42.9 m/s
Feed speed range:3.0-20.9 m/min
Chip thickness range:0.08-0.58 mm
Gullet feed index range:0.08-0.58
Otherwise similar tool data as for blade A.
Statistical data, figure 5.
Click here for statistical data.
Appendix II
Cutting force measurements.
Experimental set-up.
The principal parts of the equipment are (see figure below):
1. A rotating arm at the end of which the wooden sample is fixed. The wooden sample is moved in a circular path with a diameter of 1 m. The cutting speed can be varied from 10 m/s to 100 m/s.
2. A stand for the machine.
3. A moveable slide in which the cutting tool is fixed. The speed of the slide, and thereby also the chip-thickness can be varied within wide limits. The cutting tool is mounted onto the slide with 3 piezoelectric force gauges, measuring in three dimensions.
4. A charge amplifier. When the piezoelectric gauges are subjected to a change in force the gauges releases an electrical charge which is proportional to the change in force. These charges are converted into voltage levels by three charge amplifiers.
5. An A-D converter which samples and digitalizes the output voltage from the amplifiers. The simultaneous sampling frequency on the three channels can be varied in the interval 0.1-50 kHz.
6. A measurement computer which processes and stores the test values from the A-D converter.
Click here for a schematical view of the measuring equipment.
Wood parameters.
- Scots pine (Pinus sylvestris)
- Density(U=8%) (400-600 kg/m3)
- Moisture content (30-140%)
- Heartwood/Sapwood
- Temperature(-15, 20)°C
- Rip saw - milling angle (90°)
Cutting parameters.
- Cutting speed (15 m/s)
- Chip thickness (0.3 mm)
- Rake angle (20°)
- Clearance angle (10°)
- Edge sharpness: 5 and 20 um
- Cutting width (4.25 mm)
Appendix III
Statistical data from the cutting force measurements on pure green sapwood.
Click here for statistical data.
