Tolerance Considerations and Moving from 3D-Printing Prototypes to CNC Machining

Anonymous · ‎12-20-2016

Greetings, Community.

This is my first post and certainly a worthy one as I'm fairly new to CAD, 3D-Printing and CNC. The question is fairly straightforward:

I have designed the product model in Fusion 360 and iterated changes with 3D-printed prototypes. As you may know, there are certain dimensional tolerances when FDM printing, for me it's 0.2mm. And this adjustment allows for more exact dimensions and fitment on the final print. BUT, now that the prototype is finished and we're ready to move to CNC and have this milled professionally, what tolerance is needed? Surely I can't just remove the 0.2mm FDM tolerance and call it a day, can I?

Any help, guidance or tips are greatly appreciated.

TrippyLighting · ‎12-20-2016

No, because CNC milling machies also have natural limits in reproducability and repeatability.

What tolerances are needed depends on the requirements of your products. More info is needed 😉

Anonymous · ‎12-20-2016

Fair enough. We basically have a lower "cup" and a lid that fits down onto it, securing and pressing an o-ring (water tight). The lid is then screwed down into the cup. So, obviously size matters quite a bit for the screws, o-ring and fit everywhere. Is it based on the machine? Tool being used? Material?

TheCADWhisperer · ‎12-20-2016

@Anonymous wrote:

... Is it based on the machine? Tool being used? Material?

Yes. Yes. Yes.

It is not possible to manufacture perfect parts.

As a general rule you should specify clearances (and tolerances) as generous as possible that still results in design intent function.

I recommend that you sit down with an experienced machinist (and/or manufacturing engineer) and go over the various dimensions.