Inconsistent timber dimensions

[Garrett]

Hi, guys.  I'm fairly new to milling, but building a timber frame carport out of local pine trees on my property here in Middle Georgia. The problem I'm having is the ends of the timbers I'm cutting are right on point as far as dimensions (6x6 posts, 6x8 beams).  However, the center of every one of the these timbers are 1/8 - 1/4" wider.  And after two cuts, I notice one end of the cant is almost always lifted up from the bunk. I've checked my bunks and they're dead straight across using a string to check and they're all the same distance away from the blade.  Is this an issue with the logs themselves?  Do they naturally "curl" or lift up as you're cutting?  If that's the case, how do you combat that?

Woodland Mills HM126T

[scsmith42]

A lot of times logs will have tension in them, which causes either the boards to lift off the cant during milling, or the cant itself to move.

If the tree was a leaner, an off-center pith is another indicator of tension in the log.

I pay close attention to each board as I mill, looking for signs of any movement.  Usually I'll circle around the cant in order to see if/what movement is on all four faces and factor that into my subsequent milling pattern.  I'd rather have a board bow up and down from end to end, versus crooking to the side.  The former can usually be addressed during installation or reduced during drying, but a crook will have to be edged out.

[TroyC]

I'm sure some more experienced sawyers will chime in, but I had the same issue with a nice big pine my neighbor gave me. I needed some 16' beams, tree looked good enuf. After getting the cant squared, it would do wierd stuff like you said. I cut the beams oversized so after they dried I could resaw them.

I think you might have tension issues. I learned a lot sawing that big pine. The more experienced guys on here can look at that tree and tell you how it will react when milling.

[terrifictimbersllc]

Yes, a property of the logs, there is stress in them, the wood reacts as it is being taken apart.  If it were a big block of cheese things would be different but, because it's wood,  the remaining wood (cant) is stiff, not flexible.

You've probaby seen already that initial boards being taken off often lift up on the front end.  So the cant is reacting opposite to that, the ends are going down  (which they cant when they're setting on the mill), so the middle goes up instead.  Whether this results in cants /timbers bigger in the center is going to depend on how much if any you are flipping the cant 180 in reaction to your seeing the stress relief. I doubt that it is really 1/8 to 1/4 thicker in both dimensions all the time.

Managing the stress relieve is an ongoing consideration in sawing, throughout every day and in most every log.  Some do just lie there like cheese but most don't.

[Southside]

SYP, especially fast growing / plantation wood, is famous for moving like that.  What was the diameter of the logs?  With SYP posts and beams you want to remove the side lumber from the cant evenly and work your way down to the center of the log so the remaining juvenile and jacket wood is balanced on all four sides, this will help prevent movement during sawing, and later on when drying.  

Don't try to get two 6x beams out of a 12" log, the will curl in opposite directions on you.  

[ladylake]

 Sure could be stress.  Are the ends of you mill supported good and not sagging when you have the head out to the ends, if so that would cause the center of a beam to be thicker. . The easy way to tell is to take a long 1" board and skim off about 1/4" then turn it over and see if it lays flat on all of the bunks.  Steve

[DaleK]

How many clamps are you using and where are you clamping them? You can manage the stresses in lots of ways but if you can't clamp the cant down tight from one end to the other it's tough to keep consistent

[Garrett]

Thanks for the replies and insight so far.  Most of these logs are around 10-15" in diameter, so I'm not trying to get any additional boards out of them.  Typically, I'll take the first cut down to where I have about 7" of flat face then rotate 90 degrees.  I'll take another pass down to where I have a continuous corner that meets the previous flat face and repeat until I have a 6" cant.  If it is stress that's causing my problem, as it seems like that might be the case, is there any way to combat it so I have consistent timbers?  Southside, you recommended taking away the sapwood evenly; should I make shallower passes and continuously rotate as I work my way down, possibly turning it more than four times?  Or is my best option to plane the belly out once the cant is cut?  Ladylake - it's an HM126T, which means the mill has an integrated trailer. There are 6 jacks evenly spaced along the roughly 18' long bed. 

[Garrett]

DaleK - there are two clamps on this mill.  As situated currently, they are about 2' in from either end of these 10' posts I'm currently cutting. 

[Southside]

Well, it depends. On a log with a centered pith I will generally saw down maybe halfway to my final cut before I flip the cant, then flip 180 to equalize what has been removed, do the same for the other two faces. Now I have an oversized cant, so I work each face down to the final cut as I go, again generally turning 180 degrees as that gives me two wider boards vs turning 90 degrees. 

If the pith is offset then I will usually work the thicker side down more until I even off the excess, then follow the above procedure, knowing to keep looking for movement because the log definitely has stress. 

[Garrett]

Thanks again, y'all. And since I have the ear of a couple of veterans it seems, how important is it to raise the narrow end of the log up on the bed for the first couple of cuts in attempt to keep the pith centered?  Am I wasting my time trying to do this?

[Southside]

For dimensional lumber raising up the small end to center the pith always produces better results. 

[Garrett]

Thanks, Southside!

[Jim_Rogers]

Any timber 5"x5" or larger should be boxed heart.

Cut a little of each side, like one 1" board or one 2" plank and rotate 180° often to evenly release stress/tension.

Jim Rogers

[Garrett]

Thank you, Jim.  I actually used your information in an earlier topic to plan out where my scarf joints are going to be in my top plate (over the braces).  I'm doing my best to get to boxed heart, but some of these logs aren't as straight or big as I would like, but it's what I got on my land.  

[Joe Hillmann]

Are you cutting the beams out with 4 cuts or are you sneaking up on the final size?  If you are trying to do it in 4 cuts I would guess it is just stress in the logs.
If I want a 6x6 beam I would cut an 8x8 and then take a one inch cut off each side to get down to the 6x6 you want.  That way the first 4 cuts relieve most of the tension and the last 4 cuts don't cause much movement.  If that doesn't do it I would take a 1/2 inch cut of each side, then do it again.  That way you have even more chance of releasing the tension before you get to your final size.

Are your blade guides putting down pressure on the blade, even when you aren't cutting?  If not that could allow the blade to lift as it gets into the cut.

Are the rails the head rides on the same height from the bunks along the entire length of the mill?

What are you making? Does it really matter if your green timbers, which are going to shrink, are perfect?   I have a homemade mill and am cutting for a log cabin.  At first I was very careful of getting perfectly even logs.  It was a ton of work with constant re-leveling and fine adjustments of the mill and as they dried they changed anyways.  I gave up on perfect and aimed for good enough and plan to use shims as need to make up for the difference in the logs.

[Joe Hillmann]

Are you propping up the small end of the log so the center of the log is level?  If not that could be part of your problem.  On the big end the center of the log is at the center of the timber.  At the small end the center of the log could be at the edge or even outside of the timber.  That would create uneven stress in the log and warp it.

You should jack up the small end of the log so the center of it is the same height as the center of the log on the big end of the log.

[Garrett]

Yea, I've been jacking up the small end of the log for every log I've ever milled.  I saw it in a video and wasn't exactly sure how necessary it was, but now I understand why.  

[WDH]

Rotate 180° not 90°.   Always "balance" the rings.  The ring pattern on one edge should look like the ring pattern on the opposite edge of a board or cant.  Doing that and rotating 180° when you see signs of stress is about all you can do except try to select straight logs with minimal sweep where the pith is in the center on both ends of the log.  Nature, however, does not always cooperate.  

Where are you located in Middle Georgia?

[stavebuyer]

As previously noted;

Remove waste in 1" or less increments and turn 180 to keep things in balanced. Cutting a little over dimension to allow for a trim cut is extra insurance. Sawing "rough" green to exact dimension is a tall order.

[Joe Hillmann]

Yea, I've been jacking up the small end of the log for every log I've ever milled.  I saw it in a video and wasn't exactly sure how necessary it was, but now I understand why.  

How long are the logs?
If the logs are too long and only supported very near both ends that could cause stress in the logs.  If the logs are long and are sagging when you jack up the narrow end you could consider driving a wedge between the log and a bunk somewhere near the center of the log to help support it.
If the logs are only 8 feet this probably isn't your issue, but if they are much longer and you are using small diameter logs it MAY contribute to your problem.

[Garrett]

WDH - Thanks for the input.  I'm in Mansfield, GA.  About 10 minutes south of I-20. 

[Garrett]

Joe - My posts (6x6") I'm cutting to 124" to give me a little leeway to get the final length down to 10' (including tenons).  The only long logs I'm cutting are 16' for the top plate beams (6x8").  I'll keep in mind the wedge idea.  

[longtime lurker]

As everyone has said logs have tension and move during sawing and it's a common cause of thickness variation.

However it pays to remember that there can be other reasons for thick and thin boards, including  the saw deflecting around hard spots like knots and saw wander due to being blunt or insufficient tension. And if it's saw wander then if you get a thick board the next board will often be thin, and you need to face (shim) cut to get back to a level surface. I've seen a lot of good log turned into bad lumber because of blunt saws making thin boards.

[Peter Drouin]

 Sawing "rough" green to exact dimension is a tall order.

If I didn't do that, I wouldn't get anything done. :D :D ;)
I do have the saw set up to cut 1/8" big for timbers.