A serviceable printable hobby knife.
I've uploaded the individual parts for those who need to print a part separately. They are the exact same models though, so you still only need the "Blade_Holder_V20-_No11_Hobby_Knife.stl and "Cap_and_Handle_V20-_No11_Hobby_Knife.stl" files for a full knife.
I'm replacing the original files with this V2 version. The design has been modified to strengthen the fingers that hold the blade. Much more robust now. Also, I scrapped the cap design I had originally. The original cap design forced you to orient the blade a certain way to get the blade in. This new design can be put on in any orientation. The only drawback is that it doesn't socket onto the tip of the handle when not in use, but honestly I found that I never used that feature anyway (I doubt anyone else did either - it was just cumbersome). Note: the cap will be pretty tight when you first put it on, so BEFORE you put a blade in (for safety), screw on the collet as normal and then snap the cap on and off a few times. It will clearance itself and be a lot easier to use.
I modified the cap design and how it snaps onto the handle (due to some feedback that the cap was cracking on some prints). This new version has no slots on the side, which is where the crack was forming. I also moved the retention dots down on the base so now instead of the cap snapping onto those, it uses those dots as a "stop". It is now totally an interference fit and uses the layer ridges as the friction surface for retention. I did a test of taking the new cap on and off about 200 times and after the first few, it didn't seem to develop any additional slop. Feedback appreciated!
I wanted a bunch of spare hobby knife handles to distribute through the various places in my shop, but none of the printable versions I found were very robust and tended to let the blade slip pretty badly under normal use (for me that is cutting foam board, balsa, and other lightweight material).
This design grips the blade from both the sides and the edge and has a detent inside the collet to help keep the blade from slipping out. Also, the collet shaft is thick enough so that there is a decreased danger of the knife breaking when you are applying pressure.
I also wanted it to be quick to make in its most basic form, so it is designed so that you can use a 3/8" dowel as a handle.
There is also printable handle which is designed to decrease the propensity for the knife to roll around, and also an optional cap. The cap sockets onto the end of the handle when not in use.
I've had a couple of people mention the "gap" between the top portion that compresses the collet and the collet base: that is actually kind of needed. As the knife ages and things loosen up over time, that space will take up the slack. Over time you'll be able to screw the top part on tighter and tighter and without that space, you'd bottom out the top portion pretty quick.
Obviously this is a printed hobby knife, so don't go bananas with it. It's strong, but not as strong metal one. Be careful.
I'd recommend 65% infill at least and 3 shells with 4 top and bottom layers.
This was designed to be printed in PLA (I used Hatchbox, which I love!). I've also printed it in ABS with good results. I'm going to try it out PETG too, but the PLA version works really well.
It was designed to be printed with cooling and no supports. If you add supports, you're probably going to have to do some cleanup to the threads.
The tolerances of this model are designed around the normal .1 - .15 mm oversizing you see on most FDM setups when you are going for solid layer adhesion. If things are too loose, you may want to bump up your extrusion just a smidge.
A note on infill: usually, you can get away with low infill on most models without affecting the strength noticeably. This is not one of those times. Because the fingers of the collet are actually acting as a spring, I wouldn't recommend going below 65% infill. I tried a few of the prototypes below 50%, and the shaft also becomes too weak for me to trust it (especially since it is holding a razor blade. Next to my fingers.)
You can print the Blade Holder Base and use it as-is (stubby style!) or add a 3/8 wooden dowel for a handle. There is also and optional handle and cap.
Put it together
Screw the ferrule partially onto the collet base. Slip the end of the blade into the slot until the base of the blade is completely in the slot and tighten the ferrule.
If you're adding the handle (printed or wooden), I'd recommend gluing it in.
There are a set of raised dots that act as a stop to indicate how far the cap can go down safely. You might have to snap the cap on and off a few times for it to clearance itself. (safer to do this step with no blade installed.)
Be sure when you do have the blade installed to line it up correctly with the cap.
The Blade Holder Base
The Blade Holder Base - Assembled
Made in Autodesk 123D Design