Thank you for this great idea and model. Additional information for others......I built this at 100% and the bit fits but will slide out. as shown in the photo, the bit has a small lip enabling you to push down on the knurl and twist. Works great as designed. if you are using a Phillips, slotted, torx, etc bit without a lip, they will slide right through. i built another at 95% and it was still too large for those small bits. 90% was too tight, it actual split the part trying to hammer it in for a good tight fit. it still works well and has a good tension even though it is split.92% was just right to hold the bit in without it falling out.