Sawing square timbers
Your logs are probably moving on you while you're sawing them. Taking too much wood from one side tends to relieve the pressure on that side and not the other so it bows a little. Here's what I do if I need timbers that are held to tight dimensions. Start by cutting oversized cants or beams or whatever, go over by at least a 1/4" - 3/8" if not more if the log has a lot of spring to it. Once it's down close then you can cut the extra 1/4" or so off. If this does not work, check your mill alignment.
I have a woodmizer HD-40. Check the actual distance from bed rail to sawblade to see if it is as the scale reads. Check it on all bed rails and close to post and then away. If you don't have a manual from woodmizer get one to find out the necessary corrections. It's not easy, but very straight forward. Also, I find that a dull blade or too fast speed will pull the blade up or down.
Yes, first try to level your saw head to a bed rest/bunk. If that is okay, make sure the log is not rolling on you and the saw is not running up or down in the cut. As others have pointed out, feed rate may well be a big issue here. Too little can cause you problems as well as too much.
I was having the same problem and found my mill to be out of spec--log stops, blade level, etc. I now make it a point to check it every week or month. Don't assume your mill is perfect from week to week. Stress in the wood will also play its part as the guys have pointed out.
Wait and see what will happen as they dry.
Try turning a cut beam end for end on the mill, same fresh cut side up. Lower your band to the cut surface--if the band is parallel to the cut surface then your problem lies in your uprights (not being 90 degrees). If the cut is different, the band is not parallel to the bed. 1/16 in ten inches--that's what they make jointers for.
I have had that problem. Don't take too much wood off at one time and turn the cant more often.
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