Hello, everyone. I am new at milling and new at using the forum. I live in Guatemala and I bought a home made band saw mill and put a VW engine on it. My boards are coming out thinner at both ends, and thicker in the middle. What can be causing that and how can I fix it? Any ideas? Thanks in advance. Matt C.
Could be log stress. How often do you turn your log? What type of logs? Does it do it in every cut? Does it vary under different feed rates?
Do you start in slow, speed up through the middle and then slow down before exiting the log? The increased pressure from the higher speed can make the blade dive down.
After posting this I went to "extras" and then to "removing stress in a cant." I see that the problem might be the wood and the operator and probably not the machinery itself. That is good news. Cedar Eater I will also keep an eye on my speed when cutting to see if that is it. Ron Wenrich, I have to check, but it seemed to be happening frequently. There are varieties of trees here that dont grow up north and the names seem to vary from country to country. I was just cutting some "Volador" and "Cola Iguana". I have no idea if they have other names up north. Any suggestions as to how to minimize the stress problem? Thanks a lot. This is the steep part of the learning curve for me. Matt
After you check the mill itself for straightness, get down and look along the log as you are sawing it. If the wood contains tension wood, as you saw off one side the remaining wood will pull the cant up into an arch. Then of course you cut a straight line through that, and the board is thicker in the middle, and the cant bows up a little more.
What can you do? Keep flipping the cant so you take a couple of boards off each side, minimising the way it's arching up.
Or, consider quarter sawing the log so you take vertical grain boards. This is how a lot of high tension woods are best handled. The individual boards may tend to bow a little, but that can be edged out later once the boards are dry.
Welcome to the Forestry Forum, Matthew C.
Welcome to the Forestry Forum Matthew C. Regarding the sawmill, you can stretch a string from end to end to verify that it is not bowed. After that, immediately rotate your cant when you see either end lift up from the sawmill bed while sawing.
I would start with the basics. After all, we can't have a straight house with a crooked foundation, right? Be sure that your mill's frame is straight, level and plumb all the way around first. Then you can rule this out. If your mill is sagging in the middle it would cause this as well. Then follow what the other guys were saying, they're smart. When I first set up I had to make some adjustments because the weight of the mill and vibration would cause it to settle a bit.
...and make sure your track isn't sagging as you move the carriage from one end to the other.
Sounds like, possibly each end of the mill isn't being probably supported. Check your jacks/legs. I've had this problem with my mill when the ground softens up and the end settles.
Some logs or stress full logs have to be cut one board on one face at a time, then rotate 180° and cut one board. Cutting too deep can cause bending.
Jim Rogers
Matthew C; as others have said welcome to the forum.
I do not disagree with any of what the others have said, but I wish to add the following.
For example: If the log is 20' long and the bedrails do not support 6" at each end then as you saw the ends will droop and as they do your board ends will get thinner ends and the last board will have very thick ends.
Also, I have had some logs that once you remove a slab on the opening face then it causes the log to bow such that when you rotate 180* it only touches the 2 center bed rails, but as you saw it it goes down towards the end bed rails. This will also get you thinner end(s) than the middle.
I hope you can find the root cause and solve your problem.
Gerald
If successive pieces have thin ends, then we suspect tension or seers in the log...growth stress. Frequent turning 180 degrees is advised to keep stresses balanced. If one piece is thin, but the next piece is OK or a bit thick, then we suspect dry ends and saw tension
Quote from: GeneWengert-WoodDoc on May 30, 2016, 03:08:03 PM
If successive pieces have thin ends, then we suspect tension or seers in the log...growth stress. Frequent turning 180 degrees is advised to keep stresses balanced. If one piece is thin, but the next piece is OK or a bit thick, then we suspect dry ends and saw tension
Is there a book with this info, or is this learned.
So much Great info.
Terry