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Clearance for mating threads

Esrever_reenigneEsrever_reenigne Member Posts: 6 EDU
edited August 2018 in Drawings
Hi everyone, I'm new to Onshape & this is my first post.

I feel a bit silly asking such a simple question but I've had several failed attempts so I thought I'd ask the pros.

I'm trying to model some mating threads (metric) for 3D printing and they all seem to look good in the drawing but the clearances are too tight and the nut never fits the male thread when I print them.

I've tried modeling the male thread and then creating the female part by doing a boolean subtract on the female mating part. Setting a bigger offset on the boolean subtract seems to destroy the thread form and I can't seem to set an offset bigger than 0.4mm with the thread I'm trying to offset.

I've also completely modeled the female part from scratch (allowing clearance on the root diameter of the male thread) and swept a thread with the same thread form and it still didn't fit after it was printed. I'm sure I could tinker with my clearances and get it to work without too much trouble this way but I didn't pursue that as it seemed like a lot of extra work.

Here's one I tried yesterday by doing a boolean subtract from the male part. I used a 0.25mm offset when I did the boolean subtract and the nut won't even go close to screwing on. My thread forms aren't on spec, I'm just using rudimentary thread forms until I can get my clearances/fit worked out.

I'm not up to assemblies in the learning path yet, no doubt I'll be more enlightened by the time I get through that but I need some threaded parts now.

I'm obviously doing something wrong, if anyone can point me in the right direction I'd really appreciate it.

Edit: I just found the thread creator. I'll run some tests with that, it looks like it's just what I needed.

Edit 2 : I just drew another model using the thread creator tool (that was much easier than modeling a thread)  and after printing it wouldn't screw together either. I made both the parts a nominal 30mm diameter, applied the threads to both parts and then did a boolean subtract with 0.2mm offset between the parts. So I will need either a larger offset value or draw one part with the appropriate clearance added or subtracted from the nominal diameter before applying the thread.

I know this is all really simple stuff but maybe my mistakes will help someone else some time in the future if this comes up in a web search.


Best Answers

  • owen_sparksowen_sparks Posts: 2,259 PRO
    edited August 2018 Accepted Answer
    Hi and welcome to the forum.

    Firstly nice model :+1: 

    3D printing threads is not easy!

    A couple of thoughts in no particular order.

    (a) If you're using a regular filament printer then metric threads are going to be tough.  They're designed to be really precise. 

    (b) When we print them (on average at a 0.2mm layer height) then we're already making an approximation of the thread.  ie for this 0.2mm space insert filament here yes or no? So a 0.2 mm offset is doomed

    (c) It'll then be a bit out due to printer calibration.

    (d) It'll then be a bit further out as the part warps due to cooling.

    (e) Usual practice is to print slightly oversize and then run a tap or die over them to clean up the threads, but who happens to have an M30 tap and die set lying about?

    So unless you need your parts to mate to existing real M30 threads I'd say ditch that thread profile and use something that has a wider pitched sloppier fitting profile.  The very sharp tips of threads offer very little in the way of extra strength but require precision to make, so I'd suggest something more rounded.  Take a look at a plastic coke bottle lid thread for inspiration, and bear in mind that that will be made with more precision than we can manage.

    Hope that helps,

    Owen S.
    Production Engineer
    HWM-Water Ltd
  • mahirmahir Posts: 852 ✭✭✭✭
    edited August 2018 Accepted Answer
    Definitely start with known thread forms. They've been around forever, and there's no reason to reinvent the wheel. Here's a quick guide a la Google for the minor (innermost) dia of a metric female thread and the major (outermost) dia of the matching male thread.


    Without getting too into details, the first thing I noticed when looking at your last link (under Edit2) was that on your female thread has very sharp crests on the minor dia. To avoid interference, make sure the peaks are cropped and the valleys have a minimal radius. This applies to both male and female thread. See my redline below.


    Also, don't forget that your 3D printer has some tolerance. To insure clearance, add that tolerance to either your male or female part, or split the difference and add half to each. The .010in (.25mm) you were using seems about right if you have budget 3D printer.

Answers

  • owen_sparksowen_sparks Member, Developers Posts: 2,259 PRO
    edited August 2018 Accepted Answer
    Hi and welcome to the forum.

    Firstly nice model :+1: 

    3D printing threads is not easy!

    A couple of thoughts in no particular order.

    (a) If you're using a regular filament printer then metric threads are going to be tough.  They're designed to be really precise. 

    (b) When we print them (on average at a 0.2mm layer height) then we're already making an approximation of the thread.  ie for this 0.2mm space insert filament here yes or no? So a 0.2 mm offset is doomed

    (c) It'll then be a bit out due to printer calibration.

    (d) It'll then be a bit further out as the part warps due to cooling.

    (e) Usual practice is to print slightly oversize and then run a tap or die over them to clean up the threads, but who happens to have an M30 tap and die set lying about?

    So unless you need your parts to mate to existing real M30 threads I'd say ditch that thread profile and use something that has a wider pitched sloppier fitting profile.  The very sharp tips of threads offer very little in the way of extra strength but require precision to make, so I'd suggest something more rounded.  Take a look at a plastic coke bottle lid thread for inspiration, and bear in mind that that will be made with more precision than we can manage.

    Hope that helps,

    Owen S.
    Production Engineer
    HWM-Water Ltd
  • mahirmahir Member, Developers Posts: 852 ✭✭✭✭
    edited August 2018 Accepted Answer
    Definitely start with known thread forms. They've been around forever, and there's no reason to reinvent the wheel. Here's a quick guide a la Google for the minor (innermost) dia of a metric female thread and the major (outermost) dia of the matching male thread.


    Without getting too into details, the first thing I noticed when looking at your last link (under Edit2) was that on your female thread has very sharp crests on the minor dia. To avoid interference, make sure the peaks are cropped and the valleys have a minimal radius. This applies to both male and female thread. See my redline below.


    Also, don't forget that your 3D printer has some tolerance. To insure clearance, add that tolerance to either your male or female part, or split the difference and add half to each. The .010in (.25mm) you were using seems about right if you have budget 3D printer.
  • Esrever_reenigneEsrever_reenigne Member Posts: 6 EDU
    edited August 2018
    Thanks for the in depth replies guys.

    I think you are both right about tolerances and thread forms etc. 

    I ended up printing a batch of test nuts for the 30mm male part to see how much clearance the standard metric fine thread generated by the thread generator would need for a reasonable fit. I thought  .25mm was a good starting point but it was just way too tight, so I printed them in increments of .2mm (starting at .3mm oversize) until I found one that would fit. The first one to fit was 1.1 mm larger. After a bit of a rub with a thread file (both parts) the .9mm oversized nut fits nicely and feels quite good but I'm just not happy with such a small amount of thread engagement.

    So I've learned that my printer prints slightly out of round (about.2mm) across a 30mm diameter even though a 40mm test cube is within .07mm of being true in all directions ( maybe it's partly due to warping as @owen_sparks mentioned) as well.  So I have a double tolerance issue you'd never see on machined parts. Allowing tolerances for oval parts is just something I've never had to contend with.

    So I'll take the excellent advice offered above on board and switch to a different pitch thread and try to eliminate those sharp points on the peaks and radii that may bind on the valleys.

    I wasted a bit of time and filament today but I've learned a lot so it was well worth it.

    Thanks guys!

    Thanks to the Onshape people too, this is awesome software.
  • Esrever_reenigneEsrever_reenigne Member Posts: 6 EDU
    Sorry to drag this up again

    I thought I'd found a suitable thread in thread creator tool (the ISO DIN thread) so I printed a mating pair and found the chamfer tool created extra material where I expected it to remove material. The extra material is enough to prevent the parts from screwing together. Maybe this was part of my problem before?

    If I turned the part on a lathe the thread cutting process would remove the extra material, when I use the thread creator it seems to want to preserve the chamfer profile even though it shouldn't be there and created an extra raised sector that spans about 230 deg around the start of the threads.

    I modeled an example in a drawing I made public earlier today in a separate parts studio.

    Is there an easy way to trim this extraneous raised area the chamfer creates?

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