Designing Threads for 3D Printing

[rsterne]

I am working on a project that requires 3D printed threads of very large diameters and fine pitch.... I would like to be able to go to 70-80mm diameter and a 2mm pitch or finer, about 50mm long.... I found out how to do them in Tinkercad (relatively easy), but the widget for them will only go to 50mm diameter, and up to 20 threads long.... I have tried using FreeCad, with the "Thread Pitch" workshop installed, and long threads simply lock up the program, I can only do about a 10mm length....

I any of you have advice on how to achieve what I need with either of the two programs I have, or know of any widgets on Thingiverse, Thangs, or similar, please let me know....

Bob

[subscriber]

Bob,

I have designed threads in custom sizes and pitches for 3D printing.  Using Solidworks rather than Freecad or Tinkercad.

What I can tell you is that the "recipe" for male and female threads is not the same.  This has to do with the way the filament tracks around the inside of a tube VS the outside.

If you simply want a few parts with threads I can generate that for you gratis.  If you want to learn how to use you software to do it, I can offer you some ways to outwit "the machine":

If your CAD can generate assemblies, these are the steps I follow in Solidworks, that might work the same:
1. stack two or three 50 x 2 mm nuts until you get the overall length you want - or a bit longer. 
2. Mate the parts so the threads are in phase.
3.  Save the assembly as a part file.
4. Open the part file; and combine the solid bodies
Save the part file.
Add you other features by adding or cutting away material as you usually do.

You can imbed the above long nut in your already designed part, by combining them in an assembly and saving as a part.  Then merging the parts, before saving as an STL.

Now, if you can create the thread diameter and pitch you want, you can combine more than one of your own parts as described above, to reach your goal; assuming you can create a building block.  You might be able to make a 25 x 1 mm thread, and then scale it by 2X on all three axiis, to create the building block.

If none of the last paragraph works for you, the first step to threaded nirvana is to visit McMaster-Carr and see if they have threaded parts listed close to what you want:

A broad search does not yield many options, but there is material to work with:
https://www.mcmaster.com/products/nuts/system-of-measurement~metric/thread-size~m70/thread-size~m80/thread-pitch~2-mm/

Here are usable parts links.  Note that just below the green "add to order" button is the facility to download the 3D model for these parts.  I choose STEP format:

M70 x 2: https://www.mcmaster.com/5197N119/

M80 x 2: https://www.mcmaster.com/5197N122/

I have to save the STEO file to a SW part file before I can do anything with it - first image below.  Then I open a blank assembly file; and add stack enough nuts to provide a bit more length than the nut I want.

After combining and refining the Nut, save your part with the thread as an STL and run through your slicer.  Print standing on end - last image

STL created for M70 x 2 nut is attached to next post, below...

[subscriber]

So, that nut STL was too large to attached, even when zipped to 2.2 Mb.  So, I cut it down to about half the length and will try again:

STL attached, below

[TorqueMaster]

I am working on a project that requires 3D printed threads of very large diameters and fine pitch.... I would like to be able to go to 70-80mm diameter and a 2mm pitch or finer, about 50mm long.... I found out how to do them in Tinkercad (relatively easy), but the widget for them will only go to 50mm diameter, and up to 20 threads long.... I have tried using FreeCad, with the "Thread Pitch" workshop installed, and long threads simply lock up the program, I can only do about a 10mm length....

I any of you have advice on how to achieve what I need with either of the two programs I have, or know of any widgets on Thingiverse, Thangs, or similar, please let me know....

Bob

Sometimes I have to get sneaky with Tinkercad.  I am not familiar with the thread widget, but will it let you make the threads at 1/2 the diameter and 1/2 the pitch, then scale up the result 200% in all directions?  Another trick is to take multiples of those 10mm lengths you can get, and merge them.  Duplicate it, move the duplicate up a multiple of pitch size, it should merge perfectly enough for 3D print tolerances.

Another trick is to save objects as a part file, or a shape file, I think that's what it is called.  The part or shape loses it's construction history, so it won't bog down Tinkercad as much as a complexly built object, as threads surely are.

And yes, subscriber is spot on about dimensionally correct male and female threads not printing "the same."  Printed male threads will usually mate decently with storebought female hardware.  But printed female threads often end up too small to fit the storebought bolt that should fit.  Plan on tapping, or some trial and error XY % correction to get a correct size.

[subscriber]

For male threads, McMaster has a number of M70 x 2 and M80 x 2 to choose from.  For instance:
https://www.mcmaster.com/90447A810/
https://www.mcmaster.com/90447A830/

Now, for 3D printing these threads you are probably going to find that the female thread are undersize, and that an actual steel bolt of the nominal size will not screw in.  This, while a male printed thread will be much closer to screwing into a nominal steel nut.  This is an artifact of FDM printing.  It is more pronounced the smaller the nominal diameter and the finer the pitch.

If I am borrowing thread as described above, I will often "shave them" with a thread-form sketch that is a 0.05 to 0.1 mm smaller on both flanks than the CAD model.  I set up SW to follow the thread form to execute the shaving operation.  Female thread require more shaving than male threads because of the way the filament drapes when following a circular path.

Or, I generate female threads in SW from scratch, using a spiral of the right nominal diameter(+0.4 mm), with nominal, having enough revolutions set for the desired length, plus a few extra threads, so that the start and end the thread can be cut to a clean 45 degree chamfer. 

I generate a thread-form sketch that has both flanks reduced by perhaps 0.05 to 0.1 mm.  While I am at it, I "clip" the tips by perhaps 0.2 mm radially to get rid of the sharp points.  A similar treatment is applied to prevent root fouling with the mating thread.

The adjustment from nominal thread form for male threads is much less severe:  Leave the spiral at nominal diameter, clip the tips by 0.2 mm radially, and shave the flanks by 0.025 to 0.05 mm per side.

Now, the above works for the folks I design parts for, using their Ender 3 printers, printing PETG, at their settings.  Depending on your printer, material and settings, you may need different compensation to produce usable thread.  When I say "usable", I mean as printed.  If you plan to chase thread with a tap or die, then you can leave more material to clean up.

A 2 mm pitch thread should print well at 0.2 mm vertical resolution.  You could probably go to 0.3 mm and have a serviceable thread.

If all of the above seems like too much trouble, Bob; and all you want is a given part with an imbedded thread, then email me your part as a STEP file.  I will generate a thread form and imbed it in your part, then save it as a STEP and STL file, emailed back to you.

[rsterne]

Thanks for all your tips.... I found that I can use Tinkercad to make a 50mm x 1.33mm pitch and then scale up in the slicer (Cura) by 150% in all directions.... One nice thing in the tinkercad thread widget is that you can scale the diameter up or down, and also the thickness of the thread profile, to increase the clearance between the male and female threads....

In addition, I redesigned the part so that I only need 20mm of length for the 75mm x 2mm thread.... I will get the extra "travel" by using removable spacers.... If and when I get this project working, I will share it with you all here....

Bob

[subscriber]

Good plan, Bob

[rsterne]

In Cura, I am using "Adaptive Layers" which decreases the layer thickness where there is more detail in the part.... This changes the number of layers in a single thread profile from 7-8 to about 13-14, while only adding about 20-25% to the print time.... Parts without threads are still printed at 0.2mm layer thickness.... You can specify the minimum layer thickness, I am using 0.12mm.... I plan on using PETG, I think it will be stronger than PLA for the threads.... I will do short thread samples to check the fit before printing the parts, the largest one is 12 hours.... 

I found something interesting while checking different wall thicknesses.... On these parts, 4 layer walls print the fastest, with 5 layers a close second, and both are faster than 3 layer walls.... 

Bob

[subscriber]

Bob,

Your namesake above (TorqueMaster) likes to print adaptive resolution for threads too.  He can speak for himself, but what you describe for layers per thread sounds perfectly feasible, based on Bob's prints of my designs that have even 1 mm pitch threads in them.

It would be interesting to compare a larger number of "wall" layers to a thinner "wall" with more crisscrossing "plywood" layers for loads applied in different directions.  I suspect it depends on other settings too.  If you break a test part and the fracture crosses several layers then that is a good sign.  As compared to fracturing between layers.

PLA is actually stronger and stiffer than PETG, but it is also more brittle.  PLA degrades faster in UV light.  It is consider bio-degradable, so will not have the same useable life as PETG.   PLA has much lower creep strength than PETG, and will droop noticeably if parts are left in a car in the sun. 

So, PETG parts are softer with better shock absorption, and better long term stability at constant load.  Especially at somewhat elevated temperatures.  Bob has loads of experience printing threaded parts from PETG.  Based on what he has shared with me, I think that your 2 mm pitch threads should not be difficult.

[TorqueMaster]

I posted some of my thread printing tips here:
https://www.gatewaytoairguns.org/GTA/index.php?topic=196584.msg156465353#msg156465353

Bob, it sounds like you'll get good results.  Using adaptive the way you are -- with 0.20, 0.16, and 0.12mm layers.  As far as i know the Enders are still geared to do best at 0.04mm layer increments.  I get acceptable threads with 0.16mm layers for x1.0mm threads, so yours should look fantastic at 0.12mm.

Most of my prints are thin walled cylinders lately (LDCs) so I use concentric pattern, so it's speedy and sort of "all walls."  If using lines or zigzag, I can see the number of walls having an optimal value, as well as getting more strength from alternating patterns.

[WobblyHand]

I've printed 1/2-20 threads in PETG without much fanfare, that's a 1.27mm pitch.  I have have to confess that I used a tap to clean them up though.  The tap barely shaved anything off, but it did take enough off so the moderator spun on without having to grunt turn it.

For large threads and diameters finding a tap might be tough.  However one could turn a pseudo tap out of metal and cut some grooves in the side.  After all we are tapping plastic.

Or figure out experimentally how to adjust the dimensions of the thread.

For some reason the helix code in FreeCAD has an issue.  I've posted about in on their forum.  Takes far more resources than it should for some helices, and not for others, but I don't know why.  I found this when I was modeling some multi start threads.

[rsterne]

Yeah, for the threads I tried in FreeCAD, the time taken crunching numbers was long at 10 revolutions, and I could not get more than that to work at all....

Bob

[bigHUN]

I am a mechanical designer on a higher CAD platform (Siemens NX) and I have designed threads but for a resin LCD printer which I am considering much more precise in finished parts.
Designing=modelling threads for printing needs some calculations how much to offset the male or female thread. I can do it for you but let me know what file formats your CAD can read (step, igs are the most common for geometry translation in between formats).
I am well inclined designing for resin parts but unfortunately I have no filament printer I don't know the tolerance offset how those materials behave... I know the resins I am using my tolerances are within 50 microns min-max.
These are some of my projects with threads I was working on my one-piece rest (if I can figure out how to upload multiple pics):



 

[subscriber]

Those are some crisp looking parts, Attila.

Resin parts can be very accurate, and while hobby resin printers are becoming more affordable, the resin cost itself seems high compared to filament.

I am sure there are many different resins, with the strength, stiffness and stability of the material being important consideration, after cost per part.  Time to print matters, but no so much, if you can start a print and then come back the next day.

Your question about thread design for FDM depends on the printer, the print mode, the set-up and calibration; and the material.  That said, designs I do for general consumption I generally leave male thread diameters at nominal; possibly clipping the tops by 0.1 to 0.2 mm radially.  This, while I open up female thread nominal diameter by 0.15 mm to 0.2 mm radially.  Then clip the inside diameter thread tips similarly to the OD of male threads.

On both the male and female thread forms, I shave the flanks, for a little more clearance.  Male thread flanks I shave to increase total flank clearance by 0.025 to 0.05 mm.  Female thread flanks I shave to increase total flank clearance by 0.05 to 0.1 mm.

This is for nominal thread diameters between 10 to 25 mm.  The smaller the diameter for a female thread, the more I open the nominal diameter.  Smaller male threads may benefit from adding 0.1 mm to the nominal diameter.

All of the above is for 1 to 2 mm pitch threads.  The coarser the pitch, the easier it is to get a strong thread that does not bind.  The context is to produce threads that do not require being chased with a tap or die; or where only a little fluff is cleaned off by chasing.

Smaller thread diameters and finer pitches should be cut in undersized holes, or chased for best results.  This may be an area where resin beats FDM; although I suspect that a 5 mm thread should be chased, or partially formed by a steel screw. 

Printing male threads much below 1/2" seem to be asking too much, in terms of strength.

[bigHUN]

I was designing tolerance offsets into female minor dia (hole dia in minus makes the root deeper ) and tolerance offset into male major dia (shaft dia in minus makes the crest smaller) in order to get a free sliding on the flanks in between...usually I was targeting at least 50 microns on flank clearance but that is on tight, 0.1 mm to 0.15 shall make the sliding immediate without any post processing.
I don't know the filaments pricing but in Canada the engineering resins are going about $50-80 CAD, that can give you a lot of high precision parts. I am using engineering resins only, and depend of a project I mix resins to get the parts in specs.
I developed a very tight process (temp and humidity controls, washing and curing) and some of my parts annealing depend of requirements.
For my hobbies, including airguns, threads like M3-4-5-6-8 I all print no rethreading with tools.

[subscriber]

Lower roughness and a more true thread form should enable resin prints to use closer to full thread forms.  Thus, more strength for equivalent materials, than FDM.  And lower friction on initial assembly.  "Rocking" FDM thread pairs back and forth, just as they tighten up tends burnish both threads; allowing more rotation and smoother rotation.

What could be useful to the forum is a set of standard thread "blanks", adjusted for FDM printing, in STEP format.   Little studs and tubes of thread, perhaps 15 mm long.  These can then be trimmed in CAD and imbedded in parts that require them.  Longer than that, and the user can place two thread forms in phase, as a stack, and merge the parts.

3D Printer people like STLs, but for me, STLs are a nightmare to edit in CAD.  I would rather use universal format STEP files.  These can be saved as a part file in whatever CAD package anyone is using.

A 15 mm long threaded part will have a surprisingly large file size.  The forum also does not support attachments of all file formats.  Both in terms of size and forum compatibility, zipping the STEP files enables one to attach them to forum posts.  I would suggest you start a new thread, if you want to post "standard thread models" as a service to the forum.  Then ask the Mods to make it a sticky thread, so it is easy to find in the future.

[WhatUPSbox?]

I found this video interesting as it relates to thread printing. I will try that setting in the future.

[rsterne]

The above video was very helpful.... On angled/sloped surfaces, like the profile of threads, the CURA "Slicer Tolerance" setting (found in Experimental settings) makes a big difference to the size, and hence thread fit.... Long story short, using "Middle" produces some interference on shallow angles like threads, using "Inclusive" is worse, but using "Exclusive" gave me workable male threads, but the female threads were still pretty tight.... Here is a drawing of a 2mm pitch Metric thread, using a 0.2mm layer height, that I made up to show what is happening.... The Yellow area shows where the interference occurs....



I printed some M10 x 1.5 nuts and bolts, using the downloaded files from McMaster Carr, using various settings.... The results are below....



On the left is a nut and bolt printed using "Inclusive", and they won't even go together.... Next to that is a nut and bolt printed using the default "Middle" setting, and they will start to scerw together, but jam before the nut is fully engaged.... The picture in the middle is a standard steel nut threaded onto a bolt printed using the "Exclusive" setting, and it is virtually a perfect fit!.... Next is a steel bolt with a nut printed using "Exclusive", and it can be assembled with difficulty, but too tight to use.... On the right is a nut printed using "Exclusive", but also enlarged 0.2mm in diameter.... It is a perfect fit on the steel bolt, and also works perfectly on the printed bolt made using the "Exclusive" setting....

I was using PETG at 240*F, and it is likely that different materials may give different results.... However, this will give me a starting point in understanding how to print threads that will work properly.... In summary....

For a male thread, print using the "Exclusive" setting for Slicing Tolerance in CURA....
For a female thread, print using the "Exclusive" setting, and enlarge the diameter of the threaded hole by 0.2 mm....

Bob

[subscriber]

Bob,

Some of McMaster's threaded part files have theoretical thread forms, without any flank clearance.  In other words, when you mate the male and female thread forms, they meet at the same line, without any gap.  Actual bolts and nuts mate with a tiny clearance at the flanks.

Rather than "shaving" the thread form to make the gaps wider than the "teeth", One can simply increase the diameter of the female thread by 0.5 to 1 %.  Or reduce the male thread a similar amount.   This can be done by scaling the X and Y axis.  Leave the Z axis at 100% to maintain pitch.

When I generate thread forms in Solidworks, I use a spiral at nominal diameter and pitch.  Then use the spiral to sweep a thread form that is shaved on the flanks by about 0.05 mm total; on top of enlarging the female thread nominal diameter by 0.3 mm.

Of course; print setting and material play into this too; so what works for you might be slightly different.

[WhatUPSbox?]

Grainger also has CAD files available.