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A fair number of reviews here discuss issues that buyers had with the collet on this Dremel. I just bought this model, and think I may understand the problem that at least some users ran into. First, a word about terms. The solid shaft that the cutting or grinding tool is mounted on is the mandrel (or Dremel also calls it an accessory shank). When you install the cutter on the Dremel, this mandrel slides into a silver-colored hollow tube called the collet. The dark-metal nut that encases the collet and screws onto the unit is the collet nut. All of the accessories that come with this Dremel model use 1/8"-diameter mandrels/shanks. Most other accessories that Dremel sells also use this size, although some use 3/32", 1/16" or 1/32" shanks. When my Dremel 4000 arrived, I noticed that none of the included 1/8" shanks fit into the collet. I think some of the other buyers who ran into this problem concluded at this point that their Dremels came with the wrong-sized collet. However, when I took it apart a little I found that the collet had just been jammed tightly into the collet nut, closing down its diameter so that the shanks wouldn't fit. I unscrewed the collet nut from the Dremel and used a small screwdriver to push the collet out of the back end of the collet nut. This allowed the collet to open up to its full diameter; all of the included shanks now fit. So, if you buy one of these and it seems as though the shanks don't fit into the collet, give this a try before you conclude it's the wrong size. Dremel could probably stand to do a little quality control on this to make sure everything works smoothly. If you do actually need a different collet size for other mandrels/shanks not sold with this kit, Dremel sells a 4485 Collet Nut Kit (about $6-$7 in 2015 at hardware stores) that has the four collet sizes listed above.