Need advice on design tolerances  

RE: Need advice on design tolerances

I have found that adding a 0.2mm gap is sufficient for large threads using PETG with 0.24mm or greater layer heights. I select the thread surfaces in Fusion 360 and do a push/pull to separate them. 0.2mm seems to work for snap-fit parts in PLA and PETG.

RE: Need advice on design tolerances

I believe there are some tolerance tests on Thingiverse you can print. They will tell you what amount of separation is required between fitting parts for varying degrees of fit; I'm betting this is not a one-size-fits-all, since the accuracy of the gap is also dependent on the assembly of the printer and slop in any of its components.

RE: Need advice on design tolerances

I've used a few of those, and you're correct that you need to get your extrusion multiplier right. I use far smaller prints for that tuning. I've been able to get 0.15mm clearances to work, but 0.20mm seems to be an easier target. Linear Advance is also important.

Here are some of the big & chunky threads that worked with PETG with 0.2mm additional clearance added to the threads.

RE: Need advice on design tolerances

Thanks, @bobstro, @vintagepc, @bobstro for your input and suggestions.

Does the live "z" height have anything to do with the final print sizes?

Is there any shrinkage with PLA after or during printing?

@bobstro, What does "extrusion multiplier right" mean?

Thanks all,

RE: Need advice on design tolerances

Posted by: canquest

[...] @bobstro, What does "extrusion multiplier right" mean?

Tune Filament Settings->Filament->Extrusion Multiplier to avoid over-extrusion which will interfere with parts.

RE: Need advice on design tolerances

Thanks @bobstro.  Could you please explain more about "Extrusion Multiplier", or point me to an article that helps me understand when & if I should adjust?

RE: Need advice on design tolerances

Posted by: canquest

Thanks @bobstro.  Could you please explain more about "Extrusion Multiplier", or point me to an article that helps me understand when & if I should adjust?

This is from 3dhub.com:

• Print out a cube
  • (20mmx20mm would be nice)
  • in vase mode (single outline, no infill, no top or bottom layers)
  • set your extrusion multiplier to 1
  • set your extrusion width to equal the width of your nozzle.
• Measure the walls with a caliper in several places (at least 8) and get an average.
• Change your extrusion multiplier: new multiplier = old multiplier x (extrusion width / average measurement)
• Repeat until you are happy.

The idea is that when the printer is commanded to extrude filament, it is assumed to be exactly 1.75mm in diameter.  This is used to calculate how much molten filament will exit the nozzle for a given length of filament input.  The nozzle is also assumed to be exactly with specified size.  More math, including the nozzle size, layer thickness, and print head velocity, translates this to an extrusion width.  So, if you tell the printer to print 0.4mm widths and the printer prints 0.41mm widths, it is extruding too much material, in which case you need to reduce your extrusion multiplier.  If, for the same 0.4mm requested width, you end up with a 0.39mm wide extrusion, then the printer isn't extruding enough material and you need to increase your extrusion multiplier.

The extrusion multiplier lets you adjust for variation in nozzle aperture and overall average filament diameter.  It doesn't help you with variation in filament diameter within a length of filament. 

RE: Need advice on design tolerances

Even when you get cubes to print "perfectly", you will find that interior cylindrical holes oriented along the Z axis will come out under-sized.  The effect is worse for small holes (less than 10mm) and practically disappears for very large holes (greater than 30mm).  This is an inevitable consequence of the FDM method of printing:  as the liquified plastic trace around the perimeter of the hole cools, it tends to shrink (making the radius of curvature smaller) before it completely solidifies.

This is even true of commercial high-end printers the size of refrigerators that cost near $100k.  Their recommendation is to print a bit small and then bore the hole out to the desired diameter.

You can, of course, adjust the size of the hole in your model before you slice it.  But doing so is empirical; you have to print several trials to zero in on the exact correct size.  And then the perfect size in your model actually depends on your using the exact same filament at the exact same temperature and speed, corrected exactly for the true filament diameter on your spool, with the environmental temperature being consistent, the moisture content of your filament being consistent, the phase of the moon, etc. etc.  In other words, your "adjusted" hole diameter is useful for most purposes, but don't ask for metal tooling precision out of an FDM plastic model.

RE: Need advice on design tolerances

By the way, canquest, you may have noticed that people are answering you in millimeters, not inches.  The 3D printing world is metric.  I live in the US and grew up with inches, but I have adapted and now think in millimeters for objects that I model and print.  (I still think in inches and feet if I'm doing a sketch of a room or a building... I know, it's hard to be consistent.)

RE: Need advice on design tolerances

Great explanation @william-m51, did not know there was this level of fine-tuning.

But does the "Live Z-height" make any difference?

RE: Need advice on design tolerances

Thanks @retireejay, living in Canada I am very familiar with both Metric & Imperial.  I will be designing using both conventions but mostly still prefer Imperial.

RE: Need advice on design tolerances

Thanks @retireejay, great comment "don't ask for metal tooling precision out of an FDM plastic model".

Glad you reminded me of that.  I have been involved with high-precision projects most of my working life.  Time to just have some fun & design & build accordingly.

Here's my second project.

RE: Need advice on design tolerances

Posted by: canquest

Great explanation @william-m51, did not know there was this level of fine-tuning.

But does the "Live Z-height" make any difference?

Only on the first layer.  The amount of extruded filament is the same on all layers, assuming that you have set all layers to have the same layer height.  The first layer is relative to the bed.  The live z height controls what the actual distance between the tip of the nozzle and the bed surface is, when you tell the printer to go to 0.15mm or 0.2mm, or whatever your first layer height is going to be.  If the actual height is less that the expected layer height, the extrusion is going to be thicker that specified.  If the actual height is higher, then the extrusion will be thinner than expected.  Because there will be variations in the bed surface, in some places it will be closer and in some farther, meaning that you have a second variable, on top of variations in filament diameter, impacting the extrusion width for the first layer.

The default print settings usually have a slightly wider extrusion width for the first layer, 0.42mm versus 0.4mm, for example.  Because bonding of the first layer is so important to the success of the print, an slight bit of over extrusion is generally ok.  Lots of prints will have a very small lip on the first layer due to the wider extrusion.  For parts that fit together, a quick trim with a sharp knife will help with the fit.

RE: Need advice on design tolerances

Thanks @william-m51, good explanation, now I am starting to understand the Live Z much better.