Tweaking the geometry
I recently posted an idea for a 3D printed depth setting jig for use on my Myford Big Bore lathe. A couple of readers had run the STL files and struggled with the fit of the mounting boss thread (M35 x 1.5) that mates with the thread on the end of the lathe spindle. This is a known problem with 3D printed threads where the accuracy of the 3D printer and the size of the thread being printed can interact and have an impact.
Fusion 360 does not have a tolerance tweak in the thread creation tool. This is not a problem in that you can use the Face Offset tool to adjust the thread geometry. This does not take long to do. The process also allows you to add fillets to the thread peaks so they are less aggressively ‘sharp’ and therefore more likely to survive longer.
Select the Inspect/Section Analysis to view the cross section of the thread to be adjusted. Choose any axis for this. Manipulate the view so you can see the cross section face and the around to the side of the 3D model. Do the tweaks shown below by selecting the appropriate faces of the thread and making an Offset Face adjustment and then adding a chamfer. The difference is very minor but it makes the thread less ‘sharp’ and aggressive to its mating half which is likely to be a metal component. If you are working with a modelled threaded hole rather than a rod then the changes are the same. The values shown are nominal and will change with the modelled thread size. If you overdue the offset the thread will become very sloppy.
The only tricky part is Manipulating the view in Fusion to allow the appropriate face selection otherwise the Offset command is straightforward.
To a degree some of this could be achieved in your 3D slicer but adjustments would become global rather than specific to just the thread geometry.
If you want a more detailed explanation then I suggest you watch Kevin’s post on Product Design Online.
I have modified the geometry of the Depth Setting boss threads to give more tolerance and reposted the STL to match on the link below.
If you have a Myford Small Bore lathe and would like to send me the bore size and end thread I can create a new version of the depth setter to match.
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