09 Dec When the Band Saw Won’t Cut in a Straight Line…
The number one reason is that the blade is dull. It doesn’t take much to blunt the tips on an inexpensive blade. Plus, most of us actually use the band saw more than we realize, so the blade may have some serious mileage on it. And we often ask it to plow through hard materials such as countertops or plastic so we can rough size parts, or maybe make waste small enough to chuck in the garbage. Those plastics and resins are hard on tips. And as blades get dull, we tend to push harder because we’re used to feeding at a given rate. When that happens, the dull teeth can’t handle the pressure and they look for a way out. They find a path with less resistance, which is usually off the line.
The next most common reason is incorrect blade tension. If it’s too loose, the blade will wobble and that delivers an erratic track. If it’s too tight, it can slide across the tire, heat up wheel bearings and even break under a load.
Make sure the blade guides or bearings are set correctly and are still spinning (if they’re bearings, not carbide guides). They can get gummed up, especially if they’re not sealed. If blades have worn a groove in the face of the bearing behind the blade (and check both the top and bottom bearings for this), the guide is no longer spinning and the blade is both locked into a track and generating heat. The rear guide shouldn’t actually touch the blade when the saw isn’t cutting. It should be maybe 1/8” back, so that it only engages when the blade is under load. If it’s too far forward, it’s pushing against the back of the blade and the front of the blade will wobble because it isn’t being constrained in the same way as the back.
If the side bearings are set wrong, they may be forcing the blade to the left or right. They should be set the thickness of a sheet of printer paper from the blade, so they rarely make contact during a manual test rotation. If one bearing is too close to the blade and the opposite one is too far away, the blade won’t travel in a true orbit and the saw won’t cut in a straight line. It’ll wander toward the far guide. And if they’re too far forward, they’ll be running against the teeth, which can quickly deform the blade tips and destroy the guides, too.
Next, check that the wheels are aligned properly. If, for example, the top wheel leans in at the top and the bottom wheel leans out at the bottom, the blade will still travel and won’t derail, but it will have a somewhat erratic path. You can tell by unplugging the saw and manually rotating the wheels. The blade should track just about dead center on each tire (check both). If the rim of the wheel has a crown, the blade should be staying on top of it.
Are you using the right blade? Check with your supplier about the width/depth of the blade (1/8”, 1/4”, 1/2” and so on), and the number of teeth per inch (TPI). 4 TPI is coarse, 6 to 8 meets most needs, and anything above 10 is fine. The more teeth per inch, the smoother but also the slower a blade will cut, so don’t push too hard. Those small teeth wear quickly when asked to do coarse work.
Ask your supplier about set, too. The set is the angle of the teeth, and this can be wide (aggressive) or narrow. It can also affect how well waste (chips and dust) are removed from the kerf, so you’re not cutting the same material more than once.
On long rip cuts if the kerf begins to bind (the two halves start pressing together), pop a small wedge/shim in the cut to keep it open. Don’t force it or the board may split along the grain. If the wood pinches the blade, it will cause a heat build-up and that can dull the tips and cause an erratic path.
Finally, it’s surprising how many people get hurt on a band saw. They almost universally say afterward that they didn’t really take it seriously because it wasn’t as intimidating as a table saw. But keep in mind that butchers use band saws because they’re so efficient at cutting through flesh and bone…