Solved Model Tolerances

@Ryan TeGantvoort Here is the problem for us as makers. If you design a model in CAD you generally model it in the exact dimensions it should be. As a part designer the actual measurements are sacred and we don't mess with those. In my experience if a designer messes with the dimensions to 'make something fit' then I move on to something else, because I know that part is poorly modeled.

Printing is different, we are not LEGO and do not have all the material scientists they have so we cannot account for all the aspects of material properties. We only make rash generalizations of how parts need to fit together as well as the print settings we need to get there. Depending on the output material we make minor adjustments to account for our specific printer precision. I am most familiar with PETG and my Robo so I can only speak to it specifically. I know that if I have a part that must be a very tight tolerance and dimensionally accurate I must take my model and in the slicer of choice enlarge it by 0.2% in the X/Y Axes (my printer is very accurate in the Z axis). In S3D you double click the model and uncheck Uniform Scaling then I put 100.2 in both the X and Y Scale (%) boxes. For ABS which I almost never use I recall the number was 100.8 for my printer. For PLA I leave it at 100. You will need to experiment until you find what scaling factor works best for your particular setup.

If you plan to share your model with others please note that adjusting for your printer in the model will almost certainly make it the wrong size for the next person who tires to use it. If it is dimensionally accurate they can apply their own scaling factor and make it work for them.

If you don't want to read the whole message the gist is, good modelers do not adjust the CAD model to make it fit.

 

@WheresWaldo is correct, that is the best approach.
Since all of our models are for printing we have taken to allowing a certain leeway in sections where overlap or fitment is critical.. How much depends on the filament. For most of what we do (PLA) about a 1mm tolerance on the fitment areas is fine.

This is going to be material, modeler, environment specific I would expect

Materials other than PLA (Nylon,PET, PC, etc.) have their own adjustment factors since every one shrinks/swells when put down a little different. Find a number that works for you and test/Adjust.

 

This. If you are not failing you are not trying much ...

 

You will likely find that if you care to fine tune it the numbers you get will vary a bit from filament types.
1mm works well for our stuff with PLA (most of the time). I would experiment and then dial it in using a small model.

Create a smaller version of your model that contains just the elements you want to test (a hinge for example, or just the overlapping planes). Print just that portion. By small I don't mean scaled down, I mean just a portion of the entire model. You want something small so it doesn't take hours to test or a lot of filament either

 

You could for example create a small dowel and a circular plate for it to fit through and keep tweaking the sizes and printing until you are satisfied with it.

 

The plate and dowel will get you close. If you are a perfectionist (I know we are) then you may still want to use SW to extract the problematic section and do further testing/fine tuning. Eventually you will come up with some general 'fudge factors'that work for each filament type

 

@Ryan TeGantvoort Sometimes if I am just testing fit, I might print the object with 2 perimeters and no top or bottom layers and a low percentage of infill. It will look funny but can print very fast. Sometimes I even cut the part and only print a section.

 

Here is why I like scaling in the slicer and not the actual model. We work with plastic and different plastics have different shrink rates, so you cannot use a universal model adjustment and make it work for every situation. Also what if you have someone that wants the model and will be using a CNC machine or some other disruptive manufacturing process (SLA, SLS, SHS). No CNC operator wants to mess with the model just because it was ADJUSTED in the CAD program. If you are building it only for yourself, then by all means do it however works. If you ever plan to share it, then do it better!

 

If you put up a model with the correct dimensions and people complain just remind them to calibrate their printers better. I have had only one complaint regarding size on all my models. Told the guy to calibrate his printer precisely and never heard from him again. It is not your problem to fix other people's calibration issues.

In the same token it is your responsibility not to add to calibration issues either. That is what you do when you share models that are not dimensionally accurate. That is also why I have started to share a STEP file with my models. If you don't like my measurements you can modify it yourself.

Just my opinion.

 

Actually yes they do, in PLA.
in PETG I need to scale a bit as I mentioned.