How to make a knob 3d-print friendly?

[rickyvanswaal]

Hello, I've designed the following part: 

It is for inside a housing, attached to a rotary encoder. Kinda like a computer mouse wheel, but not quite!  It is dimensionally correct and I love the way it grips on your finger. The problem with it, however, is that since it has two curved surfaces on both sides (and a larger cylinder on one of them) I can't 3d print them in resin, since the overhangs would be too large/sharp. So that got me thinking, and I came up with this:

Both parts have those teeth that interlock, making it 3d print friendly. I did this with (stiched) surfaces, but because it's such a complet geometry it failed a lot of times to calculate basic things like splitting the model alongside the surface, to downright taking 30 minutes to calculate splitting off those peaks to make a platform to rest on. When I finally got to printing this, it didn't fit at all, because of my limited knowledge I'm not able to make a tolerance in this design. My point being- I think I'm on the right track, but I can't go any further. I keep failing, making too much of a complicated mess. 

So I would like to ask you kind people if you could help me get back on track! I've attached the model file of the knob down below. For information: The maximum freestanding overhang without deformation is 55 degrees, and the first printed millimeter is bigger than the others because of "elephant foot"/overcuring/limits of my printer (roughly +2%)

I am desperate, and I can't wait to hear some opinions from some of the smarter people around here 🙂

Thank you!

EDIT: I forgot to mention I've been scratching my head, prototyping and designing only for this part for over 16 hours. Of course I can print it and sand it in the correct shape, but now I'm determined to learn how it should be done the correct way!

[raihanrobin1602063]

You can scale the model to make the tolerance according to your printer.

[TheCADWhisperer]

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@raihanrobin1602063 wrote:

You can scale the model to make the tolerance according to your printer.

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I don’t think this is a viable solution.

Can you demonstrate your “solution”?

[raihanrobin1602063]

i did this in the cura slicer.In 3D printing process the material expands inward and outward of circle.So calculate the tolerance and scale it.Like my printer tolerance is .05mm.

[TheCADWhisperer]

Can you Attach the *.stl file here?

[rickyvanswaal]

Here is the .stl file

Thank you all for thinking with me! I still haven't figured it out on my own haha

[dankG2KYJ]

Maybe I'm failing to understand the issues here, but can you try to list out your main concerns? Are you trying to avoid supports because of the threaded/toothed geometry? What is the desired orientation to print this in for the best resolution of certain features, I am guessing "Z-Up" instead of the way your model is currently oriented? Or is it simply the base of the print is over cured etc. and need to scale it in that specific region?

If the interlocking teeth are a workaround for one of those answers it seems to me that it is a pretty "complex" solution that adds more headaches. 

[etfrench]

The knurling is always going to exceed the maximum overhang angle no matter which direction the item is printed. 

If the problem is the rounded bottom, then redesign it.

You can also try rotating the model 45 degrees, then print it:

[dankG2KYJ]

@etfrench "The knurling is always going to exceed the maximum overhang angle no matter which direction the item is printed."

This is part of what was confusing because it sounds like it was printed and the knurl came out the way it was intended to. I know SLS (PA12) could handle this, but not sure if SLA could just squeak by.

@rickyvanswaal 

As suggested the model could be rotated and the fillets/rounded geometry could be redesigned into 45-degree chamfers or multiple chamfers around that angle which will optimize printing and come close to being aesthetically pleasing if that's part of the goal. 

[GRSnyder]

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@etfrench wrote: The knurling is always going to exceed the maximum overhang angle no matter which direction the item is printed. 

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That may be the case with the part as shown, but I don't think there's any reason you can't create knurling that accommodates any particular maximum overhang angle.

This very similar knurled differential screw shown below prints just fine for me on FDM. I made it with the standard Coil command using the Triangular (internal) profile. I haven't thought this through enough to prove it, but my geometric intuition says that since the grooves are cut by an equilateral triangle that is always normal to the tangent, the maximum overhang will be 60 degrees no matter how you duplicate or reflect the grooves. 

But there's no reason you have to use an equilateral triangle as your cutting shape; just use a right triangle and you'll reduce your maximum overhang to 45 degrees, which any printer ought to be able to handle.

This isn't a directly-supported option in the Coil command, but you should be able to generate the helix using Section Position = Outside and the triangular profile, then sweep the cutting profile of your choice along the edge.