Tip #47
Resawing Techniques (continued)
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Notes on Cupping and Blade Tension

Figure 12. One of the most common resawing problems is “cupping” — the blade bows in the stock. Usually, this problem can be cured by increasing the blade tension. The piece of wormy chestnut on the left was cut with the tension too low. The piece on the reight was cut after the tension had been readjusted.

One particularly common problem — cupping —needs a bit more of an explanation. Cupping is when blade bows in the wood, cutting a curve through the thickness of the stock. (See Figure 12.) This can be caused by many different things - too much feed pressure, a dull blade, a blade that's too flimsy for the job. But the most common cause is not enough blade tension. This may surprise and worry you, particularly if you've already adjusted the tension to the recommended setting on the scale. Can you safely adjust the tension higher? Yes, as long as the blade and the weld are in good condition.

Blade tension is not an absolute. You don't necessarily want to run a particular blade at a particular tension all the time. Some cutting jobs may progress better with the blade tensioned higher than normal, and resawing is one of those jobs. Most blades will withstand higher tensions than recommended with no damage. But how high can you go - safely?

I talked to an engineer who designs woodworking tools, and he recommended that I sue this method: Mount the blade and crank up the tension to the recommended setting on the scale. Cut a test piece and, if you're not satisfied with the result, give the tension knob an extra turn or two. Stop when you get a good, smooth cut with no cupping.

By the way, whenever you adjust the tension you should have the wheel covers closed or secured. If the blade should break - whether because of a bad weld or too much tension - you want something sold between you and the teeth. Also, don't adjust the tension so high that it collapses the tension spring (that is, the coils must not close completely). When the coils are open, this spring provides a “shock absorber” to dampen the vibrations of the machine. Once the coils of the spring have collapsed, minor problems with your bandsaw quickly become major ones. Both the blade and the machine may be over-stressed.

Finally, don't use flat washers or a piece of pipe as spacers to shore up the spring. Contrary to a popular bit of woodworking mythology, this does not raise the tension. All it does is make it easier to collapse the tension spring.

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