Bending machines work by pulling on the tubing. The longer the tubing, the harder it is for the machine to move it. This problem is compounded with smaller-diameter tubes, as the force needed to bend them increases proportionately. As a result, using a metal mandrel that is too small can cause it to heat up and contract, damaging the tubing near the mandrel and within the machine itself. Similarly, using an oversized mandrel can cause excessive wear on the machinery itself and can also cause galling (wooden fibers rubbing against each other).