Printing screws and lids

[Brulti 177]

Hello,

 

I'm looking for some advice on how to properly print screws and lids that can be screwed on a box or whatever. So far, when I print files found online from Thingiverse and such, the lid or screw printed at 100% will never fit correctly. I have to increase or decrease the size by 1% or 2% to make them fit, but it's always a guesswork so they often end up with too much slack. Plus there are sometimes alignment problems, for example the lid will not align properly with the geometry of the box, same with the top of the screw.

 

I usually print them in PLA, but advice for every kind of material (except TPU and the like, a flexible screw makes little sense 😂) would be welcome.

 

Thank you.

[yellowshark 153]

I will assume there is nothing really wrong with your printer and that is has decent dimensional accuracy, i.e. a maximum tolerance of 100 microns.

So firstly, your screw/bolts and the screw holes in the lid and box. These are circles; circles never print accurately; 3D printers do not prints circles they print straight lines; so a circle is made up of very many small straight lines and it is the method used to print them that causes the dimensional error. Now if you have two holes, a screw and a screw hole you might expect them to have the same error and therefore fit, but as you see this is not always the case; what hidden law of physics is acting there I have no idea. Mine are normally smaller and if I am printing a hole for a steel bolt to go through I will normally start with the diameter of the hole modelled as bolt diameter + 300 microns. But this will not always work first time so yes it is somewhat suck and see but with some experience you can get it spot on pretty quickly.

 

The box/lid alignment really should not be a problem; at the worst a quick file on a print defect in the wrong place should do the job. If the designer has a) designed with wall widths of 400 microns and b) left a100-200 micron (gap between the two mating surfaces) and c) you are doing nothing silly with your nozzle width and wall width, then it should fit. 

 

In my experience which is predominantly PLA based, PLA is fine

[Brulti 177]

You're pointing out something that I knew but forgot and should have checked: whether the designer did leave a bit of a gap between screw and hole, or lid and box. I'll give a look at some of the stl I downloaded online to check this.

 

Also, lid/box alignment is sometimes important and annoying when it doesn't match. I downloaded and printed a point counter for a game, better than writing and erasing on a sheet of paper or a dry erase board, and the misalignment is big enough that the arrows on both sides of the number wheels are not aligned when all the pieces of the counter are properly tightened. It makes reading the score a bit difficult since you don't read the same number depending on which arrow you're looking at... 🤔

[Smithy 1,144]

51 minutes ago, Brulti said:

when I print files found online from Thingiverse and such

My experience is that there are a lot of none working files on Thingiverse and I gave up to print such files if not really needed. Also, files with a lot of makes don't fit every time. I don't know why people are able to print such files successfully, but I tried some on all my 3 printers and got similar none working results.

 

So normally I design my on models and then I do it like yellowshark and give them a tolerance about 300 microns. For screws, I use the thread function in Fusion360 which works great and self-designed screws and holes work perfectly.

 

I often thought that there must be something wrong with my printers, but there is everything ok, it is just the model which has problems and there are a lot of them on such platforms.

[yellowshark 153]

Yup there is a lot of naff stuff out there in the cloud. If you can always better to design it yourself, then you can match your build environment and make sure it all fits properly :)

[JohnInOttawa 104]

Depending on what the thread is for, I often have a mate in metal somewhere. In that case, once I clean up any strings or tags, I'll run the metal complementing thread (bolt, nut, camera filter, etc) and use it to 'chase' the thread more accurately.  I've had pretty good luck doing this.

 

One thing that I have had bad results with is using PVA supports with threads. The PVA just messed up the thread layers so it looked like a comb from the side.  Better to go with no supports in that case.

 

Best of Luck!  Cheers

John

[geert_2 557]

It also took me quite some trial and error to get tight fits working. Layer lines always extend a bit; and sometimes there are little blobs. Also, ringing around corners gives inaccuracies; and elephant feet on the first layers do increase size. This makes fitting difficult.

 

So, usually I provide a gap of 0.1mm to 0.3mm between parts that have to fit, the 0.1mm gap giving a very tight fit (really stuck), the 0.3mm barely contacting. Usually 0.2mm is best for my designs, giving a nice fit without play (after removing any defects like blobs or elephant feet).

 

I tried printing threads, but they are so weak in PLA that they are worthless for practical purposes with some load and movement.

 

Making a thread with thread cutting tools didn't work at all: it just melted everything, even when moving extremely slowly, and with good lubrication. At least in small sizes of M3 to M6; I didn't try large sizes like drink bottle caps. And the resulting threads were too weak anyway.

 

So I mainly use alternative ways to fix things: screws, snap-fits, glue.

Snap-fits lockings in PLA do work well if freshly printed. But after a year the PLA gets harder and more brittle, and then they break when trying to unlock or relock. Snap-fits in PET keep working.

 

For printing *horizontal* holes for screws, I sometimes give them a teardrop shape to avoid sagging (see picture). For printing small *vertical* holes, I design them at the desired size + 0.5mm (e.g. 4.5mm to let an M4 screw through), and then drill them out with a manual 4mm drill to remove any blobs or so.

 

Glueing with cyanoacrylate works very well on PLA, even on flat unprepared surfaces. The glue seems to chemically eat into the PLA: fractures are usually in the PLA, not in the glue. On PET or NGEN, cyanoacrylate glue works less well, and here the bonding-interface fails (=place where glue and NGEN meet).

 

Often I also use nylon screws to fix things. I usually design a case around the nylon nut, so it doesn't fall out, as shown in two of the images below.

 

With a nut-cage, nut and screw don't fall out when released.

 

Teardrop shaped horizontal holes prevent sagging of the overhang.

 

Another sort of nut-cage, now with tiny retention tabs. Even when the screw is totally removed, and the blue spoon is removed, the nut stays in place.

 

Handy: this drill chuck gives a very good grip, and allows careful and manual drilling with good feeling, to avoid melting of the PLA. (Electric drilling nearly always results in melting.)

 

 

[Brulti 177]

Thanks all for your answers.

 

To sum it up:

- Put in a tolerance of 0.2mm to 0.3mm in the design,

- No support is better,

- Clean the hole afterwards with a metal thread or a drill the size of the screw.

 

Looking a bit more closely at the design on Thingiverse, it would seem that there is no tolerance at all between screw and hole, thus the problems I have making them fit.

 

I'm not yet at the stage where I can design stuff, still learning. Although, I'm using Blender and I've found add-ons that adds premade mechanical objects like screws and gear that you can then edit via a menu. I guess I'll have to give it a closer look soon.

 

@geert_2 Teardrop shaped holes is an amazing idea!

[geert_2 557]

1 minute ago, Brulti said:

...

I'm not yet at the stage where I can design stuff, still learning. Although, I'm using Blender and I've found add-ons that adds premade mechanical objects like screws and gear that you can then edit via a menu. I guess I'll have to give it a closer look soon.

...

@geert_2 Teardrop shaped holes is an amazing idea!

 

The teardrop-concept is not mine, I borrowed it from somewhere, but I don't remember where I saw it first. But it works well indeed. The basic idea is not going over 45° overhangs.

 

For designing technical and geometric models (=with flat and cylindrical surfaces only), I would recommend DesignSpark Mechanical. This is free and only requires registration. It is very easy to learn, and there are good tutorials on Youtube. The user-interface is somewhat similar to the push-pull concept of SketchUp, but it produces good and printable models and STL-files (contrary to SketchUp). It is not suitable for organic models.

 

I would say: search for it in Youtube, and see if it appeals to you.

 

[Brulti 177]

7 minutes ago, geert_2 said:

 

The teardrop-concept is not mine, I borrowed it from somewhere, but I don't remember where I saw it first. But it works well indeed. The basic idea is not going over 45° overhangs.

 

For designing technical and geometric models (=with flat and cylindrical surfaces only), I would recommend DesignSpark Mechanical. This is free and only requires registration. It is very easy to learn, and there are good tutorials on Youtube. The user-interface is somewhat similar to the push-pull concept of SketchUp, but it produces good and printable models and STL-files (contrary to SketchUp). It is not suitable for organic models.

 

I would say: search for it in Youtube, and see if it appeals to you.

 

 

Thanks, I'll give it a look.

[Smithy 1,144]

Or Fusion 360 which is also free for private use, I like it a lot.

[SteveCox3D 85]

Agree on the use of Fusion 360 for modelling threads that have to be 3D printed, I use that all the time.

Some of the problem with threads comes from the fact that the outside thread will tend to be 3D printed slightly larger than the nominal size, and the inside thread will be slightly smaller than the nominal. The result is in an interference fit rather than the running clearance required for a thread. 

I usually engage the two threads together and then work them against each other and gradually they bed-in and work as they should. It can be useful to introduce a lubricant to make this easier (as long as you use a lubricant doesn't affect the material).

[Smithy 1,144]

@Brulti

For Fusion360 you find a lot of tutorial videos on Youtube and on the Autodesk site itself, but I found the following courses really great and helpful. The instructor has a very good knowledge and you learn by doing.

 

https://www.udemy.com/user/vladimir-mariano/

 

I have seen that the courses are on sale currently, but if you miss it, there are always some discounts, so don't buy it at the regular price, it's too expensive.

Edited November 22, 2018 by Smithy