Ann Axelsson

Effect of planing on warp in Scots Pine (Pinus sylvestris)

Abstract If a sawn board or plank that is warped after drying is being planed, the feed rollers and the pressure elements will more or less straighten the wood during planing. However, when the pressure is released, some degree of warp will recur since the wood will spring back. With a large amount of straightening, only the cross-sectional dimensions of the wood should be affected by the planing operation, leaving warp unchanged, while a small amount of straightening should have a larger impact on warp. The objective of this study was to evaluate how warp is affected by planing in an industrial planer with standard configuration. A total of 20 pine planks with the dry target dimension 50 mm×150 mm were selected, of which half were severely warped. The worst twist, crook and bow per two metres and maximum cup were measured both before and after planing. The planer in the experiment had different impacts on the different warp types. For the individual planks, twist was reduced by 25% and crook was reduced by about 20% on average. Although bow decreased for half of the planks, the total average change for individual planks was a slight increase. Cupping practically vanished.

Rotational position of curved saw logs and warp of the sawn timber

Abstract With the development of scanning technology in sawmills, it is possible to optimise log rotational position when sawing. However, choosing a different rotational position than horns down might be detrimental for the board shape after drying, especially for curved logs. Thus, there is a need to investigate at what level of log curve it is possible to freely rotate logs without causing board warp. This study was carried out through a test sawing that was conducted at a sawmill situated in the middle of Sweden. The tests were made on 177 Norway spruce logs, with varying amount of curve. Half of the logs were sawn in the horns-down position, half were sawn rotated perpendicular to horns down. Log shape and warp of the dried boards were measured. The results indicated a relationship between board spring, log curve and choice of rotational position. Furthermore, board bow was related to log curve but not rotational position. It can be concluded that for straight logs, with a bow height of less than 15 mm, an unconventional rotational position does not cause excess spring in the boards. Bow and twist are not affected by the rotational position at all.

Rectangularity of planed Scots pine (Pinus sylvestris) planks

Abstract This study deals with how warp affects the cross-sectional shape of planed planks. A total of 20 planks with dry target cross-sectional dimensions of 50×150 mm were planed to 45×145 mm. The rectangularity of five cross sections of every plank was measured before and after planing. The cutting depths were measured in 10 positions in the cross sections, and the angles between the planks and the cutters were calculated. Also, the warp, that is, twist, bow, crook, and cup, was measured before and after planing. All the studied properties pointed in the same direction. In terms of both rectangularity and angles of cut, the problems were larger in the top and butt ends of the investigated planks than in the intermediate parts, and the main reason for deviations from the desired result after planing was twist.

Potential for increasing volume yield by reducing planing allowance

ABSTRACT About half the volume of sawlogs ends up as sawn and planed timber. The rest is lost due to drying shrinkage or is turned into by products. As the raw material is a major expense for a sawmill, it is important to reduce waste. To investigate how much the volume yield in the production of sawn and planed timber could be increased by reducing the target dimensions in the sawing stage in a sawmill, two groups of sawn timber were planed under similar conditions. One group consisted of sawn Scots pine timber with a large variation in twist. The other group consisted of sawn Norway spruce timber planed under different pressure settings. Using X-ray images, the minimum dimension for avoiding planer misses was calculated for each board, to find the smallest green target dimension. This was compared to actual measured dimensions. It was found that most sawn timber had unnecessarily large dimensions, and it was also found that a reduction in the target dimensions could increase the volume yield for sawn and planed timber by more than 3 percentage points. Boards with large twist would however need a higher planing allowance. The effect of the planer pressure setting was negligible.