I know X-acto handles are very common, but I gave up on them for the same reason the collet would loosen and the blades fell out.
I purchased knife handles from Excel. They are far superior. First they hav two slots at right angles in the collet unlike the X-acto handles that have a single slot. The double slot allows for a tighter grip on the blade. And to tighten the blade in the collet the handle has a knurled knob on the end of the handle, that makes it easier to tighten the blade The Excel knife handles are a pleasure to use and change blades. Check out the Excel website.
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