Knife - Folding Utility. From a Secateur Blade and 3D Printed Parts

Knife - Utility - Secateur Blade

Background:

We had a secateur which failed due to a broken spring, yet the blade still had some decent life to it. I don’t like waste, so I made a simple ‘knife’ by mounting the blade into a small wooden stick. It makes for a nice handy package opener and such at my office desk.

I was musing, as I do, that although I liked my one-off craft made knife, I felt it would be possible to make a repeatable pattern knife, as I have all the tools at my disposal. Furthermore, as much as I like my original, having a fixed blade means it is very much a desk-bound piece, with no possibility of pocket carry. 

My aim, therefore, was to design a small folding blade knife made up of a 3D printed handle which would hold a secateur blade securely and safely.

So I set out to design and make a knife utilising Tinkercad online 3D facility and my 3D printer.

Weasel Words:

This is no EDC, prepping, survival or moose-skinning tool! It’s just a lightweight little folding blade useful for opening boxes from your favourite online store, sharpening pencils (anyone remember a wooden pencil?) and other small tasks where a sharp blade can be carried and utilised in safety. It’s not meant to do heavy duty tasks, but for light uses I imagined, it works fine.

It’s only a 3D print, after all. This is in PLA, but with thick walls and a dense infill. It would be stronger in PETG, or ABS, but this is sufficient for my needs. I honestly believe the obvious weak points in the design should not fail if the thing isn’t abused.

Also, when it comes to the actual steps I have taken in Tinkercad - I don’t profess to be a maestro at either 3D design or Tinkercad - I like Tinkercad very much because it’s dead simple to use, amazingly flexible and a great price :)  Please, don’t tell me I’ve gone about the process in a naive or dimwitted way, I wouldn’t be the least bit surprised. But I got through the design in a way which I found acceptable, and for me, it doesn’t matter if there are much better ways I could have done it. I have 56 current designs on Tinkercad, some of which have a number of sub-designs. Over the years I have had many more than my current 56. As I say, I don’t profess to be an expert, but I can get done what I want, and I am happy with my methods.

Design Process:

What I present here may look like a well thought out design process from concept to final product. It is not that. This version is the result of a number of iterations. There were attempted designs which didn’t work, design ideas which were rejected, and layouts which I found impractical. In other words, this is the final version in quite a long line of attempted designs. I go through the process of building the shapes I needed, but there were rabbit holes along the way which would be pointless to document.

Tinkercad:

Picture 0. The product I wanted to make is shown in this picture. It’s in its folded arrangement, and the secateur blade is held tight by the large nut. The nut is loosened off, and the blade pivoted into its operational position. The nut is then re-tightened. The nut should not be loosened to the point of it coming off the thread.

Picture 1. Shows the simple oval primitive shape which was my starting point.

I needed to remove material to get to a handle shape, so I used simple boolean operators to cut away sections - shown in pictures 2, 3, 4 and 5.

Picture 6. This shows the beginning of fashioning the actual part which holds the blade. In picture 6, I first cut out a cylindrical shape, into which I will construct the blade holder. Picture 7 shows the result.

The secateur blade is to be held onto the handle by means of a large threaded locking nut which is tightened onto a robust thread.

The thread, shown in Picture 8 is borrowed from a Thingiverse file  -  ‘G-Clamp, fully printable,’ by Johann517  -  thank you Johann.

Picture 8 shows the beginning of the blade holder which is to be refined in later processes. Picture 9 shows the method of ‘cropping’ the thread to the length I wanted, with the result in Picture 10.

Pictures 11 and 12 show the first step to making the recess for the pivoting hole in the secateur blade. 

Picture 13 shows how the space for the blade is carved out of the rough holder shape.

Picture 14 shows the finished holder, in position where it will eventually be joined to the handle section.

I drew the profile of the secateur blade I wanted to use, in Affinity Designer, by simply tracing a scanned blade. I brought the profile into Tinkercad and simply extruded it 3mm to make the ‘blade volume’ I needed for both checking my design, but also to make the void I needed in the handle for the folded blade. In Picture 15 I arrange the blade volume, made into a ‘hole’ in Tinkercad, into the handle section in its correct location. A simple boolean operation removed the material for the blade recess.

Picture 16 shows the holder attached to the handle, which has the recess showing.

In Picture 17 I cut off the top of the handle, to make the blade recess ‘open-topped.’ I decided on an open design to reduce material and to simplify 3D printing. I was certain the amount of material remaining would be of adequate strength for my design needs.

Picture 18 is the completed 3D printed part, which constitutes the handle and blade holder of the knife.

Picture 19 shows the constituent parts with the dummy blade. The screw knob is simply made from a Tinkercad ‘cog’ primitive, with the screw thread cut from it.

Picture 20 is the same as Picture 0, showing the end result of the project.

Picture 21 shows the knife in the open blade arrangement, with Picture 22 showing the knife with the screw knob in transparent to show how it all fits together.

Pictures 23, 24, 25, 26 are photos of the actual finished product.

Pictures 27 and 28 - I intend to add nice wooden scales to the knife, but haven’t done so yet, as I’ve only just recently acquired my simple laser etcher/cutter. A wooden scale would look nice, but then, so would thin acrylic. The top scale is that shape because the scale cannot extend below the nut, as it would prevent the nut from tightening down to properly hold the blade closed, or open.

Conclusion:

As I said earlier, it was a journey of many wrong turns and mistakes - without which I would have not achieved the end result with which I am actually very pleased.