Pretty sure they aren't too tight, but not sure what is going on. I have had 3 Munk blades break on me now before I even had the chance to dull them the first time. Not breaking at the weld either. I tighten them just enough so that they don't flutter while in a cut. Two of them have broken while just sitting running and not even cutting anything. Grrrrrr!
Have you checked your saw alignment, band wheels and guides to make sure they are correct?
What make of saw are you running?
Could be that the returning blade is "fluttering" up in the housing where you can't see it. If you get the tension just wrong, the natural frequency of the blade will be close to the RPM of the drive wheel. This can cause the blade to fatigue fail real quick. Wood-Mizers have an adjustable set screw in the upper housing that doesn't quite touch the blade. If the returning blade starts to flutter, it'll contact the set screw and the vibration gets damped out. I don't know what other mills use to deal with this problem.
If this is your problem and you've got nothing to restrain the returning blade, increasing the tension by 30% to 50% might kill the vibration.
Munks blades run best under high tension. Crank up the tensioner. I wood also check all the alignments before I went any further. We don't break Munks blades. They's GOOD Blades. ;D ;D :D
It's a home built mill, but very professionally done. I will take off the top guard and watch to see if it's getting flutter going into the housing. The alignment seems to be ok. They aren't coming off and then breaking, they are breaking then coming off. I did notice a bit of slack in the drive belt. The blade rides on top of it, so it might be the culprit if flutter is a factor.
Since your mill is home made, how much vibration do you have in the band wheels. Is you bands riding the crown of your band wheels, or are they "hunting for the high spot" and whipping back and forth?
If you are using ballbearings for the saw guides, make sure they turn freely.
Another thing to check if you are using auto tires for band wheels is that the tires are round. If they are not, it puts a lot of stress on the bands.
19 inch band wheels and, by the naked eye, they seem to run very true and straight with hardly any visible vibration. One thing I was wondering about was if 19 inches was two small of a wheel to use with the Munk blades type of steel.
WM uses 19" wheels, we use 24" dia. taars. Makes a BIG difference. Munks blades are stiffer than WM. Not sure about the Double hard WM blades. I believe the teeth are harder but not the blade itself.???