Print-in-Place Folding Scalpel

I know, there are lots of scalpel designs out there and plenty of folding scalpels too. But I could not find many (if any) folding, print in place and no hardware required quite like this…

This folding scalpel is small, super light (about 8g) and handy. I have added a pressure point on the top of the blade which conventional scalpels do not typically have. This helps to give you a safe place to put your index finger and also allows you to comfortably apply pressure when cutting. It is important to note this knife, like all scalpels, is designed to be held like a pencil.

I really like the #18 blades and they are super cheap in quantity. This knife is specifically designed for the #18 blade. I have now made a modified version for the #24 blade also and will confirm if that works as soon as I get some #24 blades.

Since scalpel blades are inexpensive this folding scalpel makes a great, convenient and safe knife that you can keep handy at all times. And, it is designed to be re-used with new blades (as opposed to being disposable). But cheap enough to make that you can print a bunch.

At the heart of this product design is tolerances. This knife would be of little use if the pivot of the knife was loose. So I have designed it in such a way that the pivot should be very tight. Because of that and my experience with people occasionally telling me a design is too loose (or too tight), I have included 3 versions of the design with varying dimensions of the rotating point. There is one with 0.35mm gap in the radius, another with 0.40mm and a third with 0.45mm. The models are differentiated by dots on the handles indicating which one is which as follows:

• 1 dot – 0.35mm
• 2 dot – 0.40mm
• 3 dot – 0.45mm

I have tested the design on 6 printers, using 5 different brands/types of PETG. For me the 0.35mm (1 dot) was only useable with a mat finish material. The glossiest material I loaded in was eSun PETG and that one worked well with the 0.45mm gap. Using a medium gloss PETG it worked well with the 0.40mm gap. I recommend printing all three knives at once and see which one works the best for you. If you find they are all too loose you are likely under-extruding. And, if all of them fuse and are unusable you are likely over-extruding.

Printing

Because the rotating part of the knife is printed in place and in the closed position it is important to ensure you have ironing ON for all layers for this design. This will help to free the parts after printing.

I have tried to make my main print settings visible on the Cura screenshot but in case they are difficult to read I have summarized them again here…

Material: I choose to use PETG for its durability and flexibility.

My Setup:
Nozzle = 0.4mm

My Settings:
Layer height = 0.2mm
Line width = 0.4mm
Wall count = 4 (1.6mm)
Infill = 40%
Ironing = ON

Support:
Designed to print without support.

Print Orientation: As shown in the screenshot(s).

Assembly Notes

No assembly is required (other than inserting a blade) but the design will be partially fused due to the close tolerances. So freeing the part to rotate can take a little bit of work initially. Please see my video for notes on how to get the part free.