"Safety" X-acto Knife

Please be careful when handling blades. There is a considerable risk of injury.
I only provide the design. The handling of the item and any blades used is your own responsibility. I assume no liability for property damage or personal injury.

Vimeo Link - Video Demonstration

Background

As someone who works in a packaging prototyping lab, I find myself using an X-acto knife pretty much every day.  I've always used the stock, pencil-shaped X-acto knife handle assemblies in the past and those generally work fine while you're using them.  One issue I have with these stock handle assemblies is that the safety caps often break or get lost and you end up with a knife with an exposed razor blades.  Another issue I have with the stock handles is that, due to their cylindrical pencil shaped design, they often roll off on you.  I've tried out some of the models people have designed as an alternative to the stock handles but never found something that ergonomically satisfying to use or that adequately secured the blade enough that I was confident it was safe to use. 

Use case

This X-acto knife build delivers a no. 11 blade on demand from a package that is comfortable to handle, simple to operate, has minimal blade play, and should be safe to carry in a pocket or bag when in the locked position*.

• *Printed components MUST print correctly for the build to function and be handled safely.
• *Final assembly MUST include non-printed components listed below for the build to function and be handled safely.   

Non-printed components

This build includes non-printed components that are necessary for the assembly to function as intended.

1. Blade Lock Mechanism
   • I tried to design a printed alternative for this portion of the build but I couldn't create something that adequately secured the blade without growing the overall build to the point it was uncomfortable to handle.  I may revisit this in the future to see if I can solve for it but for now this design uses blade lock mechanism from a stock X-acto knife handle assembly.
   • If you already have an X-acto knife handle assembly you can just use the lock mechanism for this design.  If you don't already have one, you can get what you need HERE.    
   • 
2. 6 x 3 mm Magnets 
   • The magnets are necessary for this build.  The magnets are used to provide an additional layer of safety so that even if the blade carrier will remain secured inside the handle even if it is in the unlocked position. This is very important if you wish to use this build as something you could carry in your bag or pocket without slicing yourself to pieces.  
   • If you don't already have some of these magnets on hand, you can get them HERE.
      
3. Blades
   • I've been using X-acto brand no. 11 blades and they've been working well so far.  I suppose this should work for generic brand blades of the same type or blades of different type but relatively same shape.  You can get the blades I've been using HERE.

Printing

I used PLA for all the printed components and that seems to work fine.  I tried to use ABS for this but did not have sufficient layer adhesion and the threaded portion of the handle body snapped off under pressure.  Support varies by part but the constants between the three components was 25% infill with a resolution of 0.2mm.  If you're like me and don't know how to customize supports for multiple parts on a build try, I would recommend printing the components individually to avoid trouble. 

1. X-acto Handle Body
   • Print with the threads up and supports that look something like the picture below.  Supports shouldn't be needed on the threaded portion of the part but they don't hurt and I don't know how to get rid of them.  
   • 
2. Blade Carrier
   • Print with the threads up and supports for the knob.
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3. End Cap
   • Print with the threads up and turn off supports. 

Finishing / Sanding

This build was designed to ensure the security of the blade and to minimize blade play.  Therefore, things are going to be a little tight right off the printer and you'll need to touch up some areas by hand.  I recommend putting the printed components together as is, WITHOUT THE BLADE, testing it out to find points of resistance, doing a little sanding / finishing, then doing that until it feels right.

Because the layer lines conflict with the intended dispensing function of the design, you'll want to sand down the layer lines on the blade carrier component until it's relatively smooth.

You'll want to touch up the track and lock mechanisms of the handle body with a knife or sandpaper to remove trapped supports and stray overhangs.  

You'll want to touch up the knob rod on the blade carrier to remove stray overhangs. 

Assembly

1. Insert Magnets
   • The holes for the magnets are pretty tight.  You may be able to force them in by hand but I've been able to insert them both using a vice / clamps and also heating the magnets up with a lighter and then inserting them into place.  Take care to be sure you're inserting the magnets in the correct orientation so that they are attracted to each other.  
2. Blade, Blade Carrier, Blade Lock Mechanism 
   • Very carefully insert the blade into blade lock mechanism and screw it into the blade carrier until it's as tight as you can get it by hand.
3. End Cap 
   • Move the blade carrier knob to the locked position and begin screwing on the end cap.  If you're using the end cap with clip you'll instead need to move the blade carrier up the track so that it clears the clip when screwing the cap on.  Please be careful.  There are dimples in the end cap and knobs in the handle body that will interact to prevent the cap from unscrewing.  Continue screwing on the cap until the dimple / knob interaction prevents the cap from moving.  You will hear it click and about 30 clicks from the dimple / knobs should be enough.
4. Dispensing the Blade 
   • The hole in the handle body that the blade is dispensed from is slightly smaller than the overall size of the blade to ensure minimal blade play.  Therefore, the blade will not dispense correctly when you first try.  What you'll want to do is move the blade up towards the opening, note where the edge of the blade is relative to the opening, and use another knife or file to create a very small channel for the blades edge to pass through.  

Extended Use

I've been testing and using this design daily for about a month now both as a tool and a fidget device.  Everything still functions well and nothing has broke but the locking interaction between the handle body and blade carrier have lightened up over use time.  After extended use it could be expected that the lock mechanisms may wear down to the point where they no longer function as intended.  At that point I would recommend printing replacement components and building a new one.  

I hope you find this design helpful and usable in your crafting endeavors.  Once again, please take great care when working with knives.  I've tried to make this as safe as possible but, in reality, nothing with a razor sharp edge is safe.