I've got a helical cutter head thicknesser, and just decided that it was time to rotate the blades. I followed the instructions (given the below outcome, probably not 100%), and rotated all of the cutting blades 90 degrees (for the first time).
I then put a test piece of pine through just to see if I'd done it properly. As you can see below, I clearly didn't...
Could somebody give me advice on what I'm seeing, and how I can fix it?
Thanks!
It is possible that the tear-out you have experienced is due to the installation of the thicknesser cutting heads, but I use a planer with knives so I am inexperienced with what can go wrong for your machine. However, if you have rotated the heads according to the manual, I suspect the thicknesser should be working correctly and the problem may be with the wood. More tests with different species may shed more light on the issue. Assuming that the equipment is not the culprit, I chose to address the wood as the problem in this answer. Everyone experiences tear-out from time to time, and there are several things you can examine when prepping a board that may be causing a problem here and can be reduced by studying the grain and adjusting your approach accordingly.
First, lets clarify a few terms. Regardless of the species, the board will generally have one of three grain patterns depending upon its position in the tree . Consider a tree that is cut into boards like slices of bread cut longways. This is the most common approach used in milling lumber and can be is referred to as flatsawn. The grain pattern of the boards will vary as you select boards from the outside to center cuts. The grain pattern for the early and late cut boards (outside cuts) can be described as a cathedral pattern with the typical arched pattern we are all familiar with. The grain pattern is widely spaced arches proceeding up the board length with the arches usually pointing up the tree. The grain pattern of the boards from the center cuts consist of tightly spaced parallel grain lines because they grain direction has gone from mostly parallel to perpendicular to the growth rings. These center cut boards are actually quartersawn which refers to a different approach for milling a tree that maximizes the number of boards with this perpendicular grain pattern. The boards cut between these outside or center boards will display grain directions in their thickness that are more of a 45 degree angle than parallel or perpendicular. These can be referred to as rift-sawn. The board in your picture with no clear arches and angled grain directions appears to be rift-sawn. You can confirm this be examining the thickness grain pattern. If it is at a significant angle rather than parallel or perpendicular, it is most probably rift-sawn.
As any of these boards moves past the thicknesser blades the wood is cut and pulled out of the board. Practically speaking the machine always cuts and tears out wood fibers. We want to orient the board so that the grain direction moves toward the machine bed as the board moves through. This means that as the blades cut and pull the wood fibers out of the board the fibers are also oriented out so that the fibers will be part of the future waste rather than reaching deeper into the board where the torn ends will be exposed in the end product. To do so we generally want to orient flat-sawn boards with the arches pointing away from the feed direction. For quarter-sawn lumber the direction will not matter as much. Rift-sawn lumber will present the biggest risks for tear-out because the grain direction is harder to identify and orienting the board for best results is more problematic. Looking at your board I would guess that the board orientation in its tree was such that the top of the tree (and top of the grain arches) was in the direction you fed the board into the thicknesser. resulting in tear-out reaching deeper into the board than the cutter blades. Try feeding the board into the machine in the opposite direction to see if you get better results.
Having presented some basic considerations above I can almost guarantee that they will work some of the time. The orientation of wood grain is consistently inconsistent and I am frequently fooled. This is especially true with fine fancy pattern woods such as curly or plaid grained lumbers. My approach for those is to take many very shallow passes to reduce the depth of any tear-out. Whenever tear-out happens in a desirable board, I resort using more sanding to get the finished surface.
