Blade Considerations for an Under-Powered Bandmill
When working with large logs on a bandsaw mill with a relatively low-power motor, fewer teeth per inch and more lubrication might be help prevent overheating. April 18, 2015
Question (WOODWEB Member) :
We have a custom built bandmill with a 72" throat. Unfortunately it's a bit undersized (only 2" wide band) and under-powered (20HP electric) for milling wide slabs. So we will need to feed very slowly. Is there any guidance on the minimum tooth bite that we need to take without heating up the blade? Should we increase the tooth pitch to increase the bite? Are there any high efficiency lubricants that might help?
Forum Responses
(Sawing and Drying Forum)
From Gene Wengert, forum technical advisor:
A classic bite per tooth is 0.11, but with your low HP you will have difficulty with a thick cut and a conventional blade. In case a reader is not clear a band with 10" cut might have four teeth in the cut at one time, with each tooth taking 0.05" bite. If you increased the tooth spacing, or pitch, to have only two teeth in the cut then you could increase the bite to 0.10 with the same HP and feed rate. So, increasing the bite can be done by changing the pitch. As you know, changing the feed rate requires more power, so at 20 HP without a blade pitch change it is not going to be a good option. If you increase the bite per tooth there will be more sawdust, so we are also concerned about the gullet area. Lubricants - everyone seems to have their own concoction and they are very important. Often with lube you will see the rpm’s increase immediately.
From the original questioner:
Thanks this is helpful. We are in the process of putting in quite nice automation with feedback so that we can control the feed and pressure on the blade quite well. Thus I don't care so much about feed rate because wide slabs are high margin products and I am not using labor to guide the blade through the log. I think at some point I'm going so slow that I create a lot of heat on the blade? So I'm wondering how slow I can go if the pressure and speed is well controlled. Also, is there any downside to using a large tooth pitch as long as the sawdust is adequately cleared?
From Gene Wengert, forum technical advisor:
The advantage of larger pitch is that the sawdust is more of a chip and so it stays in the gullet rather than spilling out and rubbing the blade. So, your concern about very slow feed, heating, wandering, etc. is well founded. So, a skip tooth blade (half the number of teeth, or the pitch is double) should run cooler, especially on thick or deep cuts. I am just guessing, but I would think that monitoring rpm (or maybe current draw to the motor) would be a really great indicator and control feature. In a circular mill, we did that with the idea we could use an automatic feed - never completed the project. What seemed to be even better was monitoring the blade movement. I saw a large bandmill in SC or GA that did this. They, as a result, knew about feeding, sharpness, tension, etc. I am not sure if this deflection measurement has advanced and is commercially used. In the old days, we just took a handful of sawdust and looked at the chip size and character - pretty rough, but still not a bad idea.
From the original questioner:
Yes we will be monitoring current draw on the blade motor and adjusting frequency to the drive motor to keep current draw constant for a given species and width of cut. We will also monitor feed speed as an indication of blade sharpness or anything else that may affect the speed. We are a small startup (two-man operation) so we are limited to the resources we can apply. There’s no way that we know of to monitor wander although this is a big concern on wide cuts.
From Contributor B:
I operate a saw with a 72" cutting capability as well, but with a blade of only 1 1/2" depth. I've had the best luck with Timberking doublehard blades, and we cut as slow as it takes to keep the sawdust coming out of the gullets as best as possible. We have set up a sprayer with two heads using air pressure to blow the lube on the band, and plenty of it; a little too much, really, as the dust gets wet but it keeps the blade nice and cool. Often times we cut at about 1 inch per 1 1/2 to 2 seconds, and the widest we have cut so far was 68" of burled "big leaf" maple. We stop using a blade when it gets to wandering too much, maybe a quarter inch or so from the set point. As far as I can tell, we haven't overheated a blade yet after the cooling lube system was installed. Hope this helps.
From the original questioner:
Actually, our current blade is 1 1/2" as well. We were thinking to change to 2" because we can without modification to the mill. The lubrication system sounds great. I think we will try something like this. What HP motor are you running? Is it electric or gas/diesel?
From Contributor B:
Honda gas engine, two cylinder with 24 HP. It’s never even close to bogging it yet.
From the original questioner:
Is your mill home-built or a commercial model? Our mill head (band wheels, guides, frame, etc.) are custom built by gentleman in Oregon who used to own Linn Lumber Sawmills but is just doing custom work now. As I said we have a 20 HP electric so we should be ok by your experience if we go slow enough but I am concerned about the band wander. I would definitely like to keep it to 1/8 or less. Do you get this when you have a new blade? What species are you milling? We will be doing claro walnut for sure but likely monterey cypress and redwood. We will do others as well but not at full width.
From Contributor B:
Our mill is a Linn Lumber mill, absolutely modified; it is a 36" "kit", expanded to fit the travel trailer frame we put it on with strength modifications of its own. New blades are good, nicely re-sharpened ones seem to work a bit better. Wander becomes more of a problem with highly figured woods, as would be expected. We cut mostly big leaf maple and black walnut. Ours is a weekend warrior thing, I'm a slave to my 8-5 job but dreams are hard to ignore. It is an absolute blast to slice up an old, ugly burled up snag to see the wonders hidden inside. We have about 1500 bf of 2-3" slabs in our kiln right now, average down to 10% - getting real close. I didn't look, are you in the Northwest?
From the original questioner:
On the California central coast.
