There are at least 3 things that are critical to the durability and therefore functionality of the knife. They are blade geometry, material selection (for the blade) and speed of use of the knife.

The blade geometry is quite interesting. A sharper blade edge is obviously nicer to use but at the same time is inherently less durable. So I have included 3 versions (geometries) you can select from depending on how you want to use the knife. I have been using the double-sided 45-degree edged blade the most and very successfully. I have also included a single-sided 45-degree edge and a single-sided 33-degree edge. The 33-degree edge is so fine that it can only be used for cutting thin films when a strong filament is used. But I wonder how long it will be before we can 3d print in metal or metal-like materials. Which material you plan to use as well as how you plan to use the knife also plays into the blade selection.

Material selection is pretty straightforward… you want to use the strongest and highest temperature filament you have. In my case, I used an ABS (actually eSun ABS+) and it was quite successful so I have not tried anything stronger. I do have a few different carbon fibre-enforced materials that I have never used and thought I would try them for this but so far I have not. But that might be another way to improve the durability and strength of the blade. As I noted above, if you can print with metal then that would obviously be the best solution.

Lastly, how you plan to use the knife is more important than you might initially think. Of course, a plastic knife is not a great solution for most knife applications. But cutting relatively thin materials such as box tape to open packages received from online shopping or opening envelopes is quite suitable. In fact, cutting rolls of thin films like plastics and paper is also another good application. However, the speed at which you use the blade is also a major deciding factor in the knife’s appropriateness. Cutting through a box at full speed will melt PLA for sure and the knife will only survive a few uses regardless of the blade geometry.

The entire knife prints as 3 pieces. The blade and 2 halves make up the handle. One of the halves has an integrated compliant spring which provides the safety detent for the blade in the open and closed position.

Printing

Functional designs (such as this one) generally require more strength than non-functional designs and more strength than the typical default slicer settings provide, so please review your slicer settings carefully.

This product was designed for a 0.4mm nozzle, a 0.2mm layer height and the following settings:

• Wall Line Count = 4 ( 4 x 0.4mm = 1.6mm walls )
• Top and Bottom Layer Count = 5 ( 5 x 0.2mm = 1.0mm top and bottom )
• Infill = 40%, Cubic
• Ironing = ON, Everywhere (to assist in separating the layer components)
• Ironing = OFF (not needed)
• Support = All required support is designed into this product
• Brim = NO (a brim can complicate details of print-in-place parts and parts printed close together – If you have to use a brim, set it to “outside only”)
• MATERIAL: PETG recommended (any material should work fine)
• PRINT ORIENTATION:

Assembly Notes

No post-print processing is required. To assemble it, simply align the parts as shown and press them together.

The post Folding Box Opener Knife first appeared on logicalplanet.

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.

The post Print-in-Place Folding Scalpel first appeared on logicalplanet.