As a designer and manufacturer of bandsaws I am often asked by my customers or other bandsaw users why their bandsaw is not cutting straight. Unsurprisingly there are several possibilities. I’ll address the most common ones.
This discussion addresses mostly high-speed bandsaws such as typically used to cut foam, insulation, plastic, and wood. Blade speeds range from 3,000 to 8,500 feet per minute for these types of machines. The blade wheels are usually crowned with a rubber tire on the rim. High-speed machines are meant to run without coolant. Low-speed bandsaws are generally used for cutting metal and have blade speeds in the range of 100-300 feet per minute and often but not always use coolant.
Related to selecting the correct blade is making sure it is sharp. Unsurprisingly a dull blade will not cut as well as a new sharp blade. You might expect that a dull blade would cut more slowly, but it is also more likely to cut crooked. As an analogy, consider the case of cutting a steak with a steak knife. If the knife is sharp then the steak will part easily and cleanly with you putting relatively little pressure on the knife to press it into the steak. If the knife is dull then you will have to press hard on the knife and saw it back and forth to get the knife through the steak. Something similar happens with a bandsaw machine.
When the bandsaw blade is sharp it does not take the operator much force to push the workpiece into the blade and get a cut. If the bandsaw blade is dull, just like with the steak, the operator will have to push the workpiece with more force to feed it into the blade. The difference between the steak and the bandsaw is where the steak knife is short and thick, the bandsaw blade is generally long and skinny. This becomes a problem because it is possible to push hard enough on the blade to cause it to twist left or right. Properly set blade guides will help prevent this, as discussed below, but will still allow some twist and crooked cuts.
The next step is to make sure the blade is installed correctly. For most blades it is obvious if the blade teeth are pointing the correct direction. On a typical wood or metal cutting blade the hook or of point of the blade should be pointing down for a vertical blade bandsaw, or for a horizontal blade saw the hook of the teeth should be pointed such that they enter the workpiece first as the blade moves. Typical bandsaw blades can be flipped “inside out” to make the teeth point the other direction. On some knife edge style blades and grit edge blades you can get a fresh sharp cutting edge from a blade this way, just by flipping it inside out. Some specialty blades, such as honeycomb style blades, are designed to run in a direction that appears backwards from typical blades. If you have one of these specialty blades you probably already know about this.
Make sure the blade is running in the correct direction. On machines powered by three phases power so it is common to get the wiring reversed and have the blade running the wrong direction. On all vertical blade bandsaws the working portion of the blade should go down. This forces the workpiece down into the table. Otherwise, the blade is trying to lift the workpiece up from the table. For horizontal bandsaws in almost every case the working portion of the blade will run towards the blade wheel connected to the motor. If the blade is running the wrong direction please call a qualified electrician to make the necessary wiring changes.
Industrial bandsaws generally have one of two blade tensioning mechanisms: screw over spring or pneumatic. Hydraulic tension mechanisms work basically the same as pneumatic systems, just with higher pressures and more messy oil leaks. Screw over spring mechanisms generally do not have a way to directly set or read the blade tension. Some do have a calibrated dial which claims to show the tension on the blade with a needle moving across a scale. I have little faith in these. For screw over spring mechanism, the best way to get an accurate blade tension is with a blade tension gauge. Lenox makes a decent one and Starrett makes a very nice, and more expensive one. If you have multiple machines, or are working with an application where correct blade tension is essential, then I strongly recommend getting a gauge. Pneumatic or hydraulic systems use a pressure regulator valve to set the blade tension. The manufacturer generally provides a chart or table showing what pressure to use to achieve a desired blade tension for various sizes of blades. Air pressure typically ranges from 10 to 120 psi. Just set the regulator to the appropriate pressure and the machine takes care of the rest.
If the blade is tensioned correctly the next thing to look at is how the blade is tracking on the wheels. High-speed machines generally have a crown or rounded surface on the blade support wheels, the same way a bicycle tire has a round cross-section. The blade should run at or near the center of the crown and not excessively close to the front or rear edge of the wheel. Being ⅛ or ever ¼ inch from the exact center of the wheel is generally not a problem for most machines. If the blade is barely on the wheel then it is probably necessary to align the bandsaw wheels. Before making any adjustments to the blade wheels check the adjustment of the blade guides. Improperly aligned guides can cause the blade to track improperly on the wheels.
A common misconception about bandsaw guides is they are what holds the blade in place. They do not, they are more of a backup. With a well set up bandsaw you can take the guides off completely and run the machine and cut material with no guides at all. The crown of the bandsaw wheels is what keeps the blade in place. The blade will always try to climb to the highest point of the wheel, the top of the crown. If the wheels are worn and do not have a distinct top to the crown then the blade will not know where to go, and may wander around. This of course can cause poor cut quality and difficulty in setting the guides. If the wheels are excessively worn you should consider resurfacing the blade wheels before trying to set the guides.
Typical bandsaw blade guides have three elements, one on each side of the blade and one to the rear of the blade. The guide elements can be wheels, sliding blocks, or a combination of both. For the side guides the ideal is a close but free running fit, about the thickness of a dollar bill. Again ideally, the blade should not touch the guides in normal operation, but if they are just kissing then they are probably OK.
The rear guide should be ⅛ to ¼ inch clear of the back of the blade. During regular cutting on a well set up bandsaw the blade will move or bow backwards a bit as you press the workpiece into it. If you press really hard then the blade will press hard against the rear guide. I often use this as my indicator of when I am pushing the work piece hard enough into the blade. If the blade is not yet touching the rear guide then I can probably be more aggressive. If I am rubbing against the rear guide hard or the rear guide wheel is spinning very fast then I am probably pushing as hard as I can get away with.
When pushing this hard the blade will move or bow straight back, and probably is still not touching the properly adjusted side guides. If I continue to push harder then the blade may start to twist or buckle, then it will hit the side guides. The side guides will assist in keeping the blade cutting straight, but they are not an iron lock on the blade forcing it to cut straight.
When diagnosing a machine that is not cutting straight look at the guides. If the rear guide is touching the blade even when not cutting, then the rear guide is probably set too far forwards. This is forcing the blade forward of its natural position on the blade wheels and can cause poor quality cuts. If the rear guide is seriously out of position it may even push the blade off the wheel crown, hence the earlier suggestion to check the blade guides before making any adjustments to the wheels. After correctly adjusting the guides you may find the blade is tracking properly on the wheels.
If the blade is pressing hard on the side guides it may lead to poor quality cuts, but it will certainly lead to shortened blade life and shortened guide life. Having the guides press hard against the blade will cause the guide and blade to get very hot. Hot enough to distort the blade and wear out the side guide wheel or block. Also, having the blade rub hard on the guide is basically the same thing as asking the blade to wrap around a very small radius wheel and will cause the blade to break. The ideal blade setup is to wrap a dollar bill around the blade then adjust the guides in until they just touch the dollar bill, then take it out. This leaves a close but free running fit. In truth you can be more sloppy than this with acceptable gap ranging from just kissing to 1/32 inch. It is very important that the side guides never be both touching the blade at the same time, i.e. zero gap.
Once the blade guides are properly set up go back and do a quick recheck of the blade tracking on the wheels.