I have always wanted to try designing up a 3D printable screw on a shaft. Last weekend I thought I finally had a user case for such a design. A quick Google search coughed up this DS Mechanical tutorial: http://www.rs-online.com/designspark/electronics/eng/knowledge-item/designspark-mechanical-thread-creation. After 30 minutes of playing around, I found I could easily design up a thread on a shaft. I started with a thread outline:
I tried using the designed up inner shaft to cut out a corresponding void in an outer cylinder:
In Simplyfy3D I then tried enlarging the outer nut by 1%, 2 %, 5% in the X & Y axis, but each printed nut still didn't quite fit properly. Each one was a bit better than the last, but in the back of my mind I knew that perhaps the correct method, might be to redesign the inner shaft to be a bit smaller (especially the thread), and print that out, ie: use the initial build to cut away the outer nut, then another build to print out as a working inner shaft. But that seemed like a lot of extra work.
In the end, it turned out to be quite easy. I just took off the bottom 0.5mm of thread height and re-pulled the thread around the inner shaft and printed it out. One trick I used was bury each end of the thread into the inner shaft. That way it makes it easy to engage the thread onto the nut. You can see the transition as a black line near each end of the thread:
Here it is the prototype all printed out:
The nut screws onto the inner shaft very easily, and moves smoothly up and down, even with the polygon based cylinder/shafts. Note: the above thread is 10mm long, with a pitch of 5mm, so there are two complete revolutions in 10mm of inner shaft length. The ends where tapered off at 50 deg. angle.
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