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t-squareddesign

Tolerancing for fitting parts?

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So I'm still new to my 3D printer. I printed out two different parts that fit together. One part was a square peg that fit in a square hole. Using Solidworks, I set the dimensions of the square peg to be exactly 6mm, and the hole to be the same dimensions. When the print was done, the peg was way too thick to fit in the hole, and the hole had a thickness of 5.6mm. This means 0.4mm too small.

 

What tolerance do I need to set my printer to, in order to get perfect fittings from Solidworks?

 

Is my printer not callibrated correctly?

 

Should I set the print thickness to the thinnest setting at 0.6mm?

 

Please let me know.

 

 

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Welcome :)

 

Yes, you will need to design some tolerance into your objects. With 3D printing it is very unlikely that if you make a design with no margin, it will not fit. The layers are never perfectly aligned so you will never get a perfect hole and a perfect peg.

 

You can do a few things:

  • Square corners can be very difficult to print exactly right, especially inner corners. Make sure to add some fillet to the corners.
  • Check Thingiverse for calibration objects. There are complicated models, but I like this simple one: https://www.thingiverse.com/thing:342198 
  • If you have an Ultimaker, you really shouldn't have to worry about E-steps and all the explanation that's on the Thingiverse page. It has all been done for you and you can just ignore all of it.
  • The only thing you will want to fine-tune in Cura for each type of filament is the flow rate (in other slicers it's also called extrusion multiplier).

This is my typical way of tuning for new filament:

  1. Set flow at 100%
  2. Print the s-plugs
  3. If I can push them together and remove them again without damaging anything, I'm done (a little bit of force is needed, but nothing extreme)
  4. If they are too loose, increase the flow by 1 or 2% and go back to 2
  5. If they are too tight, decrease the flow by 1 or 2% and back to 2
  6. At the end, always save a new profile with the name of the filament. You don't want to have to do this again later on.

Good luck!

 

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Couple of things here. I am surprised that the square hole has an error of 0.4mm. If it were a circle then yes, circles always print smaller and you need to probably try modding the dimensions several times to get a circle right - experience make it quicker!. Notwithstanding your 0.4mm error for the moment I suggest you dimension your peg to be slimmer than the hole by 0.2mm. You may get away with 0.1mm. Anything up to 0.5mm will normally work depending on the level of tightness you require.

 

Coming back to your 0.4mm error. You should be able to achieve an accuracy of better than 100 microns on the X/Y axes. You do not say what your printer is (importantly is it old or new) and what your print settings are. As a general rule to get good accuracy you want to print slow not fast; personally I would not exceed 30mm/s on my engineering prints requiring dimensional accuracy. I use Solidworks too so I do not believe it is a Solidworks issue;

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Tolerances can be a funny thing as it can also be material dependent and even brand dependent. But, I do agree that it would not be a Solidworks issue considering it is an engineering level design and manufacturing product and proven in many industries.

 

FFM/FDM is not the same as injection molding for repeated accuracy.

 

I am just now finishing a project that:

  • Tolerances actually changed when I changed materials.
  • designed the tolerances wonderfully when raw plastic but too tight when paint applied thus causing the paint to scratch when parts slid past each other.
  • Am finishing the project using one type of material and brand also to keep differences from creeping in.

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Most of the time I make a gap between parts of 0.1mm for a tight fit (requiring force to insert), and 0.2mm for a normal or loose fit. This is usually sufficient for the normal irregularities due to layers, rounded edges, ringing effects, occasional small blobs, slowing down on corners, etc... For small gaps, I take larger tolerances, based on experience, or after trial and error. For example, a 4 mm hole usually ends up as a 3.5mm hole. Then the best solution is to go through it with a 4.0 mm drill (manually!!! Never electrically, this melts PLA). Rounding bottom edges with a 0.5mm round, reduces elephant feet.

 

Printing slow and cool improves tolerances. Printing hot and fast makes it worse, since the pressure can not change immediately in the nozzle upon stops, retractions, etc. So that causes more irregularities in the print.

 

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Printing faster/slower can also affect the amount of plastic being extruded, thus changing tolerances slightly. I usually recommend that people print a series of test holes at varying amounts of tolerance added to see which works best with that particular cura profile.

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Depending on the type of fit you want. Designing something the exact same size shaft and hole will always be a transition fit which would require some heating, cooling or tooling to make them fit together. So it is normal to add some clearance to your designs but you will need to increase the amount of clearance to suit 3D printing. As suggested make a profile that you use for all prints that require a certain fit. the print some samples and measure them.

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On 12/21/2017 at 2:50 PM, IRobertI said:

Printing faster/slower can also affect the amount of plastic being extruded, thus changing tolerances slightly. I usually recommend that people print a series of test holes at varying amounts of tolerance added to see which works best with that particular cura profile.

Unfortunately, someone will still find fault in that it will not be the same as a tooled part no matter what.

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