How to calculate part dimensions VS nozzle diameter to predict fit tolerance on printed parts?
Guys, what I'm trying to get at is... Say I have an object that I want to make a part that would snap into, or over it. I'm having difficulty determining my part dimensions VS what my slicer is trying to do about adding material that is smaller than the diameter of my nozzle. In this case .04mm. I'm using Simplify 3d as my slicer, and it acts like unless I add a multiple of my nozzle size it will not add that dimension to my model when slicing. say, I have a wall .04mm thick and I increase it to .06mm it does not show that it adds another tool path to cover that .02 difference in the preview. But if I increase it to .08mm it will add another pass. So, I guess I'm asking that if I'm making adjustments in my part so that it friction fits just right, Do I have to add in multiples of my nozzle diameter? Or is this just not showing up in my preview?
Obviously, I do have some over and undersize printing of objects and holes, and those don't seem to be linear either, but I'm working on getting that part figured out.
Does anyone else have this issue with tight fitting parts?
5 Answers
Not sure about Simplify, but Cura attempts to offset the nozzle center exactly half the extruded width. Not necessarily half the nozzle diameter because the extruded material that lays down is usually wider than the nozzle diameter. There are various settings in Cura to adjust this offset compensation. It’s never exact as there are many factors that can affect the extruded width. I could list pages of factors that affect it and no doubt still miss some. Unfortunately some factors don’t even have a setting in the slicer that can be adjusted for compensation. Trial and error can get you close to cancelling out all the factors but no matter how well you hone the settings, the next part will always be slightly different. It’s just the nature of FDM.
I’ve designed some FDM parts with close tolerances for press fits and snap fits. It’s tricky. What would work for an injection molded part will fail miserably with FDM. It’s a two fold process: 1. You have to learn how to control the FDM process to minimize the errors as much as possible. 2. But even so, you have to design your parts with very loose tolerances in mind to begin with.
I’ve found to get the desired fit, I have to use a combination of clearances between parts in the design coupled with the right settings in the slicer. I set the slicer to minimize the variances and get as close to the nominal geometric part dimension as possible. Once that’s done and I have a good idea of the tolerance I can achieve, I then go back to the design and adjust the part clearances accordingly.
While I think it can be done, I personally would not try to adjust my clearances using slicer settings by themselves. Tolerances (variables) are minimized in the slicer. Clearances are set in the design.
Would it be better (easier and quicker) to just print the part oversize, and then carefully drill or file the parts to fit?
Well, the idea was to come up with a set of rules or a process where by one can calculate this upfront and then not have to post process, or print samples for fitment.
I try to post process as little as possible. What I've done in the past is print a test piece with holes (and pegs if you will) of a known measurement (model) and then next to that and then print a set that is .001 through .003 over or undersize, so you can determine just how much over or undersize your printer is printing. this does vary with material though and so you end up with lots of test blocks. However, this is not linear. the larger the hole or peg the less it is over or undersize, and the smaller the hole or peg the larger the difference. I'm sure somewhere, someone has an algorithm that can describe this effect and predict it.
Another approach to deal with printer tolerances is to change the design. Depending on the diameter, tapper diameters with half rounds male and female ring fit. In this case is all about the pressure applied when snapping both parts, and off course the larger the diameter the more flex you have.
Good points made by all !
I appreciate your input and never thought about dragging or stretching distance of different filaments. I also do agree designing the part with shapes that more easily lend themselves to needing lesser tolerances is a valuable tool as well!.
I wonder if anyone has done a "best practices" for snap fit type attachments for 3d printed parts?
