Troubleshooting a Wandering Blade Problem
A portable band-saw mill owner gets help figuring out what makes his blade "dive" into cuts. June 22, 2005
Question
I run a woodmizer LT-70 electric, and prior to that I had an older LT-40. I have cut several hundred thousand board feet in the last few years and do my own blade maintenance. I have modified the rubber band wheels into steel wheels and removed the bottom ceramic guides from the guide wheels. I have also been involved with a high production mill, in which we only used stellite tipped bands on all of the bandsaws. These bands are 2 inches wide with a 1 1/2 inch tooth spacing and welded on a stellite tip. I have also used these bands on my LT-70.
My problem is that I am having trouble with diving blades. This problem seems to be consistent with all blades, new or re-sharpened. I have tried many different brands, and have also tried different set and hook angles. Everything on the mill is perfectly aligned and nothing is worn. This problem is very distinguished in very dry western red cedar. I can cut slowly, however when I try to speed up the machine, it wants to wander.
I have spoken to the woodmizer dealer in and they have told me that they are having the same problem with dry pine. Matter of fact, they have a customer who had an older LT-40 and when this customer bought the new super LT-40, he could no longer cut straight. He proved this to them by bringing the woodmizer folks some of his dry pine logs, and they also could not cut them straight on their super LT-40. However, when they tried cutting these logs on their regular LT-40, they had no problem.
Now, the only difference between the mills is blade speed. The LT-40 has a speed of 4500 rpm’s and the LT-40 super and the LT-70 have a speed of 5500 rpm’s. They also tried cutting one of the dry pine logs on a LT-70 diesel mill and idled the mill down to a blade speed of 4500 rpm’s, but this also did not solve the problem
.
Now what I need to find out is how I can cut straight and speed up my mill. Should I try to slow down my blade speed, or maybe even speed it up? Any suggestions would be great.
Forum Responses
(Sawing and Drying Forum)
From contributor L:
I have an LT-40. My solution to this problem would be to lubricate the tension rods with a graphite spray.
Also, what could be happening is that:
1. You may be getting a false reading on the tension gauge. (To check this, tap the back of the wheel aliment.) I use a small block of wood. If your tension gauge drops, then that could be your problem.
2. The tension on the drive belts isn’t set correctly.
3. It could be that you need to use more water when sawing.
4. There could be sap on the band blade – make sure that doesn’t happen.
From contributor S:
I've had diving problems myself when re-sawing dry WRC timbers for local contractors. The stuff heats up the blade so much that I have to have my helper stay on the tensioner throughout the whole cut.
I believe this would be compounded by a wider blade. The dry fibers spring back so much after the cut that they squeeze the back of the blade – the more back on the blade, the more squeeze that occurs. I usually switch to re-sharped (1 1/8") 3/4 pitch 10 degree blades when I saw WRC or other dry wood.
From the original questioner:
Keep in mind that I am using the LT-70, which has an air bladder tensioner that is supposed to be accurate.
From Gene Wengert, forum technical advisor:
To the original questioner: Does it wander with thick cants only? Can you cut 4" and 6" pieces straight, but not thicker cants? If so, I would think that the gullet design of the tooth is too small, the gullet fills to capacity when cutting fast, and the extra sawdust is spilling out of the gullet onto the sides, and this is causing the blade to get pushed to one side.
The problem of spilling is much worse with drier wood. You might also want to try putting a water hose close to the cut at the top of the blade. The water will cool the blade, wet the sawdust a bit, and also help lubricate. If the water works, then a larger gullet is also indicated, as spilling also heats the blade and causes it to wander.
From contributor I:
To the original questioner: Have you ever tried bending the teeth? Instead of 1 down, 1 straight, 1 up, try 2 down, 2 straight, 2 up.
From the original questioner:
Gene, I thought that the gullet was the problem. However, the stellite tipped blades have a huge gullet and 1 ½ tooth spacing and a deep gullet, probably at least 5/8 deep. I will try more water, but I have in the past, as this mill is equipped with lubemizer. I really want to change blade speed. I checked on a band-speed calculator and my speed is 5725 rpm’s and by dropping my drive pulley down to a 4" instead of a 5", it will change to 4580 rpm’s.
From contributor E:
To the original questioner: I had problems sawing pine myself, and after all I went though trying to figure it out, I found it was the tension. Even though my mill tensioner is spring tension, that was the problem. If you drop your blade speed that much, you may lose a lot of your sawing speed.
From Gene Wengert, forum technical advisor:
To the original questioner: Sometimes a blade has too many teeth. For the power available, this means that each tooth has to take a small bite. A small bite means lots of dust, and it is hard to hold the dust in the gullet. I have seen that by spacing the teeth further apart. Each tooth takes a bigger bite, the chips are larger, and they stay in the gullet.
One way to reduce the number of teeth is to wedge and sharpen every other tooth. The unused teeth are shortened so they do not contact the wood. If this does not work, the cost is only one blade. (Note: Feed speed is the same as with twice as many teeth.)
From contributor A:
I notice the same problems a few years back when cutting dead standing southern yellow pine. It comes from the drywood clamping down on the blade and heating it up. More lubrication helps some, but it gums up the gullets with sawdust and water, which causes more heat. I went to a 13 degree (which has more set and bigger gullet then the 10 degree I was using), and the problem went away. I tied a 13 degree 0.055 on my mill, and it cut great, but it really loaded my little mill and the life of the blade was about 3,000 board feet.
If you can add a bit more set, it may help a lot. So much sawdust has to spill over the blade to keep the board from clamping the blade back to the cant. It should, however, be loose and not hard-packed.
From contributor S:
I ran into a similar problem a few years ago when milling old growth WRC salvage logs. (These were really ugly). I started with 2 inch blades that had 1 inch pitch, and ended up going to 1 1/2 width and 7/8 pitch and about an 8 degree hook angle( the 8 on the hook angle because of the knots I was encountering).
I found that with these logs I had to open up the set on the blades to .024+ (on each side), otherwise the blades would overheat. The down side of tuning a blade in this manner is that you have to cut slower, and your blades dull faster. I was able to make 26 and 28 inch wide cuts perfectly using this approach, but found the wider the cut, the more set I needed to apply.
In a 28 inch cut, I was using about a .027 set. My machine has a 42 hp turbo Kubota diesel and will fit a 36 inch log in the throat. My blade speed at that time was slightly upwards of 4200 square feet/minute. Another idea is that your 1 1/2 tooth spacing may be too aggressive. I would suggest trying a blade with less tooth spacing and see if it helps.
From contributor W:
When cutting dry incense cedar, I have to keep an eye on the tension gauge because the blade heats up so rapidly that the tension drops right off. If I haven’t checked the tension gauge and I look over and see that the tension has dropped, I throw the water to it and it usually pops right back up unless I have let it go for too long. Sometimes the water is on full-bore to keep the blade cool enough.
From contributor T:
We run a LT-70 diesel, use the .055 13 pitch blade, and have the ceramic guides in place and adjust it to .008" clearance. We saw a lot of old western red cedar and only have a problem when the blade starts to get dull and hit knots, which is usually around 400-500 board feet, as western red cedar is a very abrasive wood. We also run the water setting on about 4, which is not a lot. We have the mill hooked up to a dust exhaust system so dust is not a problem.
The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).
Comment from contributor C:
I've been in saw milling almost 50 years but only have been band saw milling a short time. I had the same problem not getting a straight cut and dipping. I found out real quick I was sawing too fast for the spacing of the teeth. I slowed down the blade speed and it took care of the problem.
Comment from contributor J:
I've been sawing about 15 years with bandmills and have encountered my fair share of problems too. I mostly cut red cedar on the west coast Queen Charlotte Islands, Can. If you use Stellite on a Woodmizer you must find a band that is as thin as possible, .035 work awesome, lots to clear the sawdust and keep the blade cool. When using spring set you must be diligent about keeping the set above 24 thousand, especially if they are bands over .040. I hate spring set, although they do last long but you have to set all the time. I have an LT 40 SH and a Cutting Edge and use 1 1/4 wide 1"TS Stellite tipped .035 and love them. They are as floppy as a whey noodle when you put them on but are the best. Two blades last me sometimes six weeks of cutting. That’s about 30,000 board feet. They are well worth the $180 they cost. They cut like glass. The key is that you must grind the gullets every four hours even if the blade is still sharp.
