Yes, 6 mm collet. No, it will not fit 1/4" bits. No, a 1/4" collet will not chuck a 6mm bit. Does the collet taper fit? I don't know. Ignore the 1 star reviews from people who don't read the collet size descriptions. Also, any light tool that turns over 20krpm can break the wire or change it very quickly, when the stone or spike holding the disc is slightly out of balance and the correct harmonic vibrations are produced. Any die grinder may sometimes require some adjustment and some repair/maintenance is also to be expected. However, this particular grinder has not caused me any problems. I bought from another seller. If it's the same product and batch, then. It's not the prettiest dancing girl. But it's a really reliable, tight-tolerance tool that gives it very little runout, adequate power, and a tough, high-temperature-resistant, fiberglass-reinforced plastic body. It will be a time of mourning. In the meantime, pass the ketchup. It's good stuff. The vibrations are good and the tool is light and easy to use. Bonus: Carbide and stone cutters are typically cheaper than 1/4" compared to 1/4". There are much better, more powerful grinders out there if that's what you're looking for. It's a cheap, lightweight tool that ranges anywhere from a $100 to Dremel and a grinder lies.
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