The band saw is one of the most versatile tools in the wood shop. It can handle straight cuts, curved cuts, large planks and tiny pieces. This versatility makes the band saw a wonderful tool for hobbyists and professionals alike. Everyone who owns one of these gems will tell you that the key to making great cuts on the band saw is in how you tune the saw.
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Proper blade tension keeps the blade from flexing while cutting. This can ruin a piece, especially if you're trying to cut a straight line.
Before ever turning the saw on, check to make sure the blade is tensioned correctly. You can adjust the tension on most band saws via a knob on the back of the saw. There will also be a gauge showing the ideal tension for each blade width. Ideally, you want to turn the knob until the gauge indicates the correct tension for the width of the blade you have installed. Then, turn the knob a quarter to half of a turn further. This allows for any variance in your saw's blade tension scale.
You can also check for proper tension by pushing on the blade with your finger. Right above the band saw table, press on the blade with your finger. The blade should deflect a bit, but no more than a 1/4 of an inch. If it deflects more than that, apply more blade tension. If the blade doesn't deflect at all, then loosen the tension on the blade just a bit.
The blade in a band saw has a tendency to drift to one side a bit as it cuts. This makes it impossible to cut a perfectly straight line when using the fence. The way to compensate for this is to adjust the fence laterally (side to side) until it matches the angle of the blade drift.
Most fences that come with the band saw have screws on the front and back that can adjust the lateral angle. Simply tighten or loosen the screw on either the front or back depending on the angle or drift of the blade cut when cutting a straight line.
If you're using a shop made fence, then you can easily clamp one end down and adjust the angle of the fence before clamping the other end down. It may take a few tries before matching the angle perfectly.
A quick tip is to draw a straight line on a scrap board, and cut that straight line on the band saw. You'll notice that you have to angle the board slightly to get the blade to follow the straight line. Halfway through the cut, hold the piece still and turn off the saw. Keep holding the board in place, and trace the edge of the board on to the table with a pencil. Make sure it's the edge closest to the fence. Remove the board and adjust your fence to that angle.
Position Guide Blocks
The guide blocks are the two small squares that are on either side of the blade. They keep it from twisting under the tension of cutting. For maximum results these need to be adjusted properly.
Each block has a small hex screw that holds it in place. Loosen the screw and move the guide block until it's almost touching the blade. The best way to do this is to slip a piece of paper between the blade and the guide block. Once the block barely touches the paper, tighten the hex screw. Do this for the other guide block as well.
Once you have the blocks in place on the sides of the blade, you will need to adjust their back-and-forth position. There is usually a screw on the back of the guide assembly that moves both guide blocks back and forth. The front of the blocks should ideally sit just behind the curve of the blade tooth, or "gullet."
There are a set of guide blocks in the bottom wheel of the band saw too. Adjust them just like you did the top set. Now, your blade will run smoother and cut cleaner.