Part I – Roughness
The surface quality of a sanded wood-based material depends on several factors:
Figure 1: Factors for Total Surface Quality
In Part I of «Surface quality of a sanded wood-based material», we explain the influence of the stock removal and grit sequence more closely.
Sanding is a material-removing process based on the use of a geometrically non-specific cutting tool. The grain is never of uniform shape and the cutting angle therefore never constant – entirely in contrast to sawing or planing. In these processes, the cutting angle is constant, meaning that as feed rate increases, cutting capacity increases (almost) proportionally. For example, if you plane a wood panel down half-a-millimetre, at a rate of 10 m/min., increasing the feed rate to 20 m/min. will likewise result in removal of half-a-millimetre. In other words, increasing feed rate initially has no influence on stock removal. This is not so with sanding. If the feed rate is increased in a sanding process, cutting capacity does not increase proportionally. Sanding a panel of 0.5 mm at 10 m/min. generates cutting energy X. While subsequently increasing the feed rate to 20 m/min. does increase the power consumption of the sanding motor, the panel nevertheless will be thicker at the end of the machine than the one sanded at 10 m/min. Therefore, the cutting capacity is not equal to two times X.
2 illustrates this fact.
Figure 2: Microscopic view of the path of a sanding grain through the sanding process
Figure 2 shows the path of a sanding grain through the sanding process. It is evident that the process can be divided into several phases:
- In Phase I, the grain penetrates the surface of the workpiece. At that moment, energy is released in the form of thermal energy, but primarily deformation energy. In Phase I, the substrate goes through elastic deformation, above all towards the centre of the panel.
- In Phase II, the material additionally goes through plastic deformation, by rising up to the left and right of the grain.
- Not until Phase III is the desired effect achieved, namely stock removal.
Deformation is an unwanted process. It leads to what is known as the «spring back effect», where material pushed towards the centre of the panel springs back up after the grain comes back out of the substrate. Different grit sizes are available to minimize this effect as much as possible. The larger the grit size of the sanding belt, the lower the elastic deformation. The spring-back effect increases if an attempt is made to achieve high stock removal with a small grit size and high feed rate. In this case, the grain is forced entirely into the substrate, leaving no more chip space around the grain. The material therefore cannot be transported/removed. If so the material is only compressed, i.e. elastically deformed. In severe cases, this can lead to burn stripes on the sanding belt.
Good to know: Deformation can be measured. For this purpose, take a sample measuring 50 x 50 mm from a sanded panel. First measure the thickness of the panel. Then wet the surface with water, wait about 30 seconds and measure the thickness of the sample again. The difference in thickness is a measure of the swelling behaviour of the panel surface. The more material that is deformed during sanding, the more pronounced the swelling behaviour.
To minimize elastic deformation as far as possible, the manufacturer of the wood-based material must consider how many heads and which grit sequence he wants to use to sand his panels. Generally speaking, the more sanding heads he uses, the greater the size difference can be between the first grit size used and the last. High stock removal and a high surface quality are achieved in this way, because the marks made by the coarse belt in the first unit can be smoothed out by the finer belts in the subsequent units. Although investment costs rise with the number of heads on a sanding machine, sanding belt consumption is a factor that should not be left out of the calculation (Figure 3).
Figure 3: Consumption of sanding belts per year as a function of the number and type of sanding heads
3 clearly shows that sanding belt consumption decreases dramatically as the number of sanding heads increases. This diagram shows annual belt consumption at a mean feed rate of 45 m/min. and mean removal of 0.6 mm. The decrease in belt consumption can be attributed to the fact that the necessary sanding force can be distributed among several belts as the number of sanding heads increases. The load per belt decreases and service life increases.
Stock removal versus surface quality
As mentioned, a coarse grain is needed in order to remove material. In contrast, a fine grain is required to achieve the desired level of surface roughness. In other words, a compromise has to be made between stock removal and surface quality.
Figure 4: sanding marks from last & second last sanding units
If, in selecting the grit sequence, the jump in grit size is too large, the sanding marks made by the coarse grain cannot be smoothed out, resulting in localised depressions in a panel.
In figure 4 sanding marks in different directions can be seen (white and yellow arrows). They result from the previous and penultimate pair of sanding head and are oriented accordingly. The marks are evidence of the typical Steinemann cross-sanding.
If marks from the penultimate head are still present, this is an indication that the grinding pressure of the last pair of N-heads must be increased or that the grain sequence was selected incorrectly. The “finish” of the last abrasive belt grain was not copied to 100%. This often leads to customer complaints. Steinemann therefore recommends not to skip more than one grit size. In addition, the current consumption of the fine grinding heads should be observed and continuously increased over time. This prevents unnecessarily deep scratches.
One of the most important rules to follow in sanding is to strictly separate calibration from fine sanding. In calibration sanding, the panels are sanded to size. About 80 to 90% of the total material to be removed should be sanded off in the calibration units. Another important task of calibration sanding is to ensure a uniform panel thickness. If a panel is thicker or thinner in the middle than on the sides after leaving the calibration units, either the machine settings need to be checked or the drums evaluated. If a panel is parallel and roughly 0.1 mm thicker than the target value after calibration sanding, then it is ready for fine sanding.
Fine sanding is exclusively intended to remove the sanding marks caused by calibration sanding, i.e. chatter marks and deep grit marks. A sanding thickness of 0.1 mm should be sufficient to remove these marks. If the power consumption of the fine sanding heads is too high (the current to the fine sanding heads should be about 7.5 – 10 amps for new belts), it is a sign that either the panel thickness after calibration is too high, or the grit marks to be smoothed out are too deep. In the first case, the solution lies in checking the basic machine settings and the condition of the drums. In the second, the focus should be on the selected grit sequence.
Figure 5: Effect of contact pressure on various parameters
Figure 3 shows, among other things, how roughness depends on the contact pressure of the sanding platens. As soon as the grit marks from calibration sanding are removed, increasing sanding pressure no longer improves roughness. Consequently, roughness is not improved by contact pressure.
The sanding process is highly complex. It is not always easy finding the right mix between coarse grain in calibration sanding, and fine grain in fine sanding. We therefore invite our customers to ship unsanded panels to Steinemann in Switzerland. If we have the specifications for the targeted surface quality (ideally based on a reference panel) and the required feed rate, we can give you tips and ideas for optimising your sanding process. Our assessment naturally is based on the type of machine and panel you use.