This saves a chuck changing the Arbors at Dremel. This is usually required if the diameter of the tool to be clamped deviates from the Dremel standard. If you like working with drills, despite the price should not shy away from the purchase. It is worth it! The only downer: If you want to use on the Dremel the chuck always, the Dremel does not fit in the storage box. So tight once. But the imprecise (sprayed?) Plastic storage box is already the largest and ME only shortcoming on Dremelset.