General guidance on adjusting size of threads for 3D printing

[MRWakefield]

When 3D printing threads there is often a requirement to make threads either smaller (in the case of external threads) or larger (in the case of internal threads). This is usually required due to inaccuracies and/or 'overprinting' that is common with these technologies. The usual way to accomplished this is by offsetting the faces that make up the thread profile. However there is a common misconception that you should select 4 faces (Major, Minor and both flanks) and offset these by the desired amount. This is incorrect and will result in a deformed and weakened profile.

I recently put together a PDF document explaining how to do this correctly, ready for the next time someone required guidance but I've decided to put it into normal forum post format as it should make it easier to find and view.

Fig. 1 Shows a cross section of a screw and nut assembly with no offset applied.

Fig. 2 Shows the same assembly with three faces offset. These are the Major Ø, Minor Ø, and one flank only, which in this example have each been offset by 0.15mm. Note that the widths of the thread crest and root remain consistent with the profiles shown in Fig. 1. This results in a consistent diametral shift of the entire profile (Note: only valid for a 60° thread).

Fig. 3 Shows the same assembly but with all four faces (of each thread) offset by the same amount as in Fig. 2. Note that the threads have now become deformed, with crests becoming narrower and roots becoming wider. You can offset all four faces but you would need to separate it into two ‘offsets’, one for the Major and Minor diameters and another for the two flanks. The flank offset would need to be half that of the Major & Minor offset to maintain the correct form (Note: only valid for a 60° thread - see below for more detail).

As mentioned above, if you need to adjust threads with angles other than 60° you will need to offset the flank by a different amount to the Major and Minor diameters. If you are only offsetting one of the flanks you need to offset it by 2 x Sin(α/2) x Radial offset, or if both flanks it would be Sin(α/2) x Radial offset (where α= included thread angle). As an example: if the thread angle is 55° and you’ve applied a radial offset of 0.1mm to both the Major and Minor diameters you would need to apply an offset of 0.092mm to one flank or 0.046mm to both flanks.

Another way to adjust the diameters equally is to edit the XML thread file, adding or subtracting an equal amount to the Major, Minor and Pitch diameters. If you want to do this with more than a few sizes it would be a bit laborious so I'd recommend using my thread utility which is linked in my signature. Here you can simultaneously add an amount to internal and subtract another amount from the external threads.

Hopefully this makes sense and will help anyone who's looking for information on 3D printing threads.

[g-andresen]

thanks

günther

[MRWakefield]

Thanks @g-andresen!