My most recent knot idea is the 3D print a fid handle in steel which can then be attached to a traditionally lathe turned fid spike. I spoke with my knot buddy, Shane Marks, who does metal and lathe work to create awesome marlin spikes (check out his site, Rhino Ropework). He said if we were going to join a 3D printed knot with a spike that he turned on his lathe, he’d have to tap threads into the knot. What a great idea! I had no way to test it, though. I knew I wouldn’t want to 3D print the final threads, but I wanted a plastic prototype printed with my Makerbot Replicator 2. I set aside the threads idea for the time being, while I first figured out the knot piece.

I had done 3D printed globe knots in steel before like the one below, so I knew it was doable.

3D printed steel globe knot.

3D printed steel globe knot.

 

To make it a handle, though, I wanted it to have a tapered end. In Freakin’ Sweet Knots, the globe knots are simulated so the spacing of the knot resolves well around the entire globe. I needed something similar for arbitrary shapes. I adapted my globe knot code to project the knot onto an arbitrary Unity mesh collider (though, being roughly convex is really necessary) and simulate over it instead of a sphere. The result was very promising!

To get the handle shape I wanted, I started in blender. Starting with a cube, a couple extrusions and a subdivision surface modifier I quickly had a reasonable handle shape.

blender_mockup

 

After a bit of refining, I had my finished handle shape.

handle_shape

 

After bringing it into Unity, assigning it a mesh collider and feeding that to my newly minted relaxed collider code. I had a knot in the rough shape that I wanted it!

 


This is the same knot as the 3D printed stainless steel globe at the top, just mapped to a fid handle shape. This isn’t quite what I wanted as I still needed something to tap the threads into. I relied on my knot tying experience to choose the ideal knot for this situation. The type of knot that knot tyers commonly tie around a fid handle is an asymmetrical pineapple Turk’s head knot (or something like that). You might see this kind of knot tied around a gear shift as well. I pulled out my handy Advanced Grid Maker to design the knot I needed.

handle_knot

 

This knot will mostly close up at one end with a pentagon shape and have a large circular hole (or at least a 15 sided shape) on the other. That was what I was hoping for at least. I ran it through Freakin’ Sweet Knots and I was right!

 

 

This knot started to taper down and allowed for an extra piece to be added which could have threads in it. I cut up my original fid handle shape in blender and added a section on to the end of the knot. Here are a couple test renders.

fid_test fid_test2

 

The next step was getting threads into it. I knew this piece would be handled by the machinist anyway, but I still needed to make my own prototype! I did a search online for 3D models of screw threads and found a few OpenSCAD solutions that were very slow and I didn’t really know what size threads I wanted so I wanted something responsive and fast that I could experiment with. Those OpenSCAD solutions linked to the ISO metric screw thread standard on Wikipedia, which was exactly what I needed to get started on stl_threads. After a couple days of hacking, I had it, my latest addition to stl_cmd.

Several months ago I started putting together a git repository called stl_cmd. stl_cmd is a suite of command line tools for generating and manipulating binary STL files, a very common file format for 3D models when 3D printing. The format is so simple because it’s essentially a list of triangles. To avoid any complicated conversions or high memory overhead, the stl_cmds deal only with binary triangle  data (the stl_cmds never need to read in the entire file into memory as it can assume its dealing with a stream of triangles).  I started the repository as a way to teach basic command line usage in the 3D printing space. It’s still in its infancy, but I think it could be a handy tool in many 3D printers’ belts. I bring up stl_cmd because stl_threads is its latest addition.

stl_threads generates 3D models of both male and female screw threads of any size (standard or otherwise). This was exactly what I needed to prototype my fid handle. After my couple days of hacking, I needed to test it. Of course my first test had the same dimensions for male and female, which of course didn’t work. I needed some tolerances in there. After a couple tests I decided 12.5% larger female threads gave decent results with the scale I was working with on my Makerbot Replicator 2. Also, the ISO standard defines the thread angle as 60 degrees, which could cause overhang issues on the Makerbot, so stl_threads can change that angle. I used 90 degrees, which results in a 45 degree angle with the ground plane, rather than a 30 degree angle by default. Here’s my first successful threads print, which you can download from thingiverse here.

DSC_0315 DSC_0320

 

They were pretty snug after I screwed them together that far in the picture, but now that I have it in my pocket just about everywhere I go to fiddle with it screws nicely together all the way.

I downsized the threads a bit and incorporated them into my fid handle design and gave it a shot on my Makerbot Replicator 2.

DSC_0003 DSC_0011 DSC_0014

prototype

Worked great! Now my order is in for the steel version and will be shipped to Shane Marks over in Australia to see what he can do with the spike. If anyone has information about a machinist local to Bozeman willing to try out cool projects like this, let me know! I’ll keep you updated. Can’t wait!

 

 

2 comments

  1. John Fullman says:

    Hi, I’m so glad to have found an opportunity to talk with you and say thanks for your generosity. I’m just 69 years young and have been involved with knots since I had my first pair of lace up shoes. In between times I got involved in scouting, so was pleased to find Stan Pope’s pages and gr8knots too. Later I needed a few knots during and after seamanship training with HM Royal Marines. Now I need knots more than ever to slow down my arthritis. I admire the e-tech you have applied to this hobby and appreciate what is involved in that too. I have even played with the math of dividing the circle into awkward numbers of bight pin divisions. Seven and 11 were interesting. Even used Excel to help produce a table for any size diameter mandrel for any size turk’s head knot. I have made my first TH jig, just a flat board with pins (metric I’m afraid although I did get the idea from one I saw using American thread pins size 6-32).
    Now I’m moving on to cylindrical ones. I find the making of the mandrels as challenging as the knots themselves and the calculations in Excel nearly beat me until I converted my angles from degrees to radians.
    Your other contributors have helped me a lot too and the page you contributed, which offers to list the available colours, had me beat for a while but on inspection there is a typo in the url which needs to have the letter ‘s’ inserted just before ‘colors’. This isn’t a criticism, I offer it in the spirit of helpfulness that you have started by giving the software away for free. It does look a lot like recipe books already, or no longer available, have used your software to produce ‘Recipe’ books and ‘Workbooks’ and indeed they sometimes use Ashleys’ book of knots for illustrations which look to have been converted to line drawings and photoshopped. Maybe I’m cynical but I think of myself as a helpful cynic.
    Once I have mastered the Turk’s Head knots I hope to progress to the Globe Knots that they can become. I have tied Monkey’s Fist knots for years as toys for the various dogs in my life. Oh, if only my kids were so easily satisfied!
    Take care and kind regards and many thanks,
    John

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