68 - Our All-New Router Jig, страница 10
BLADE TOOTH DESIGN IN THE SHOP Jig Saw Blades ▲ Blade Clearance. To prevent a blade from binding in its kerf, the teeth are set or the sides of the blade are tapered. When I bought my first jig saw years ago, it came with two blades. This made it pretty easy to pick which one to use. I just used the first blade until it finally broke. Then I used the second one. But times have changed. If you go to a hardware or tool store today, you're likely to find a wide assortment of jig saw blades available. In fact, there are so many blades to choose from that it can be a little overwhelming. Fortunately, selecting the right jig saw blade isn't too difficult. To start with, you need to know what type of material you will be cutting (wood, metal, plastic, etc.). Beyond that, you need to know what kind of cuts you will be making — rough cuts, tight, scrolling cuts, or both. Then it's just a matter of knowing a few of the basics of blade design to be able to select a blade that is right for the task at hand. Number of Teeth - The most important thing to consider when selecting a blade is the number of teeth per inch (tpi). Wood-cutting blades typically have anywhere from 6 to 12 tpi, and metal-cutting blades can have as many as 36 tpi. As a general rule, the fewer the teeth, the faster the blade will cut. But this speed comes at a price. Blades with fewer teeth also tend to make rougher cuts than blades with more teeth, see the drawing at right. One other point to consider is the thickness of the material you will be cutting. In order for the blade to work properly, you should always have at least two teeth in the workpiece at all times. So thinner materials should be cut with a blade that has finer teeth. Blade Clearance - But regardless of the number of teeth, in order for the blade to cut without binding, it has to cut a kerf that is wider than the thickness of the blade. And different types of blades use different strategies to accomplish this, see drawing at left. Most wood-cutting jig saw blades have teeth that are "set" —just like you might find on a hand saw. The tips of the teeth are bent to the left or the right in an alternating fashion. This way the blade creates a wide "path" as it cuts. On other blades, the teeth aren't set at all. Instead, clearance is created by tapering the sides of the blade so that the back edge of the blade is thinner than the cutting edge. These "tapered" blades cost a little more than "set" blades and will cut slower. But generally they leave a smoother cut. A third type of jig saw blade has "wavy-set" teeth (like a hack saw blade). The teeth on these blades are ground straight but are then bent in groups to create an undulating or wavy edge. These blades cut the slowest of all three, but they are the best choice for cutting metal, plastics, or many other non-wood materials. Bi-Metal Blades - Most of the jig saw blades you will run across A Teeth Per Inch. Blades with more teeth per inch (tpi) will give you a smoother (but slower) cut. 10 ShopNotes No. 68 |
