Why circular holes are not circular? Which plane produces the nicest looking surface?  

RE: Why circular holes are not circular? Which plane produces the nicest looking surface?

Keep in mind that the nozzle is going to follow the dimension specified in your CAD/CAM file and that the bead of plastic has a finite width, typically 0.4mm. So if you specified a 2mm hole, theoretically it will print as a hole with a 1.8mm diameter. This is always an issue with small dimensions. A "ring" with a specified outside diameter of 4mm, inside diameter of 2mm, and wall "thickness" of 1mm, will print as 4.2mm outside, 1.8 inside and wall thickness of 1.4mm. For this reason, if accuracy is important, I always print small holes "undersized" and then ream them to the desired size as a post-print process. Vertically printed holes will typically be a little "rough" at the top. How rough may depend on various slicer settings such as layer thickness and whether supports are turned on, but again, I would print undersized and ream to finished size.

RE: Why circular holes are not circular? Which plane produces the nicest looking surface?

@reddawg

Thanks. Did you take the treads on the screws into consideration? In my case, for M2.5 screw, I drilled a 2mm diameter hole and ream it. If I want the threads of the screw to grip the 3D printed hole tightly without using nut on the other end (i.e., screw->frame->motor), what hole size do you recommend for M2.5 and M2.0 screws? Is it better to have infill of 90% or even 100%? I always turn on support everywhere.

RE: Why circular holes are not circular? Which plane produces the nicest looking surface?

More perimeters gives way more strength than more infill. The extra perimeters also are better for the screws if you are threading into plastic.  If you don’t want more perimeters on all the model you can just drop cylinder modifiers into your holes and set the perimeters higher there more to localise the effect to the holes you want. 

However threads into plastic are not strong and don’t really bear repeated in/out for the screw. It’s far far better to use a threaded brass heat inset if you can. They are surprisingly easy to do with a soldering iron and a bit of practice. Cnc Kitchen Has a good video on YouTube regarding them. 

RE: Why circular holes are not circular? Which plane produces the nicest looking surface?

Posted by: @neophyl

More perimeters gives way more strength than more infill. The extra perimeters also are better for the screws if you are threading into plastic.  If you don’t want more perimeters on all the model you can just drop cylinder modifiers into your holes and set the perimeters higher there more to localise the effect to the holes you want. 

However threads into plastic are not strong and don’t really bear repeated in/out for the screw. It’s far far better to use a threaded brass heat inset if you can. They are surprisingly easy to do with a soldering iron and a bit of practice. Cnc Kitchen Has a good video on YouTube regarding them. 

Thanks. In the past I just made a hole of 2mm in diameter and then used a drill to create the thread. However, it did not seem to work well. Perhaps because the heat modified the threads on the 3D printed part.

How much perimeters do you recommend? For the threaded brass heat insert, how do I insert them into the 3D printed frame exactly vertically? It seems that the author of that video used the heat from the soldering iron to heat the threaded brass heat insert which indirectly melt the 3D printed part.

RE: Why circular holes are not circular? Which plane produces the nicest looking surface?

I use a lot of M3 screws in my builds, and where they are expected to be removed a few times like to access electronics or for battery compartments I use the brass inserts. Yes the soldering iron heats them up and they melt into the hole and when it cools the plastic locks in the exterior serrations.
You do have to get your hole size correct for your insert size and yes it does take a bit of practice to insert them straight but it’s not as hard as you think. I tend to use a minimum of 3 perimeters on most things as nearly all my parts are functional and that’s enough for the inserts. 
I modelled up a test block with several holes each slightly bigger than the previous one and then actually tested which worked best. That also gives you some practice in inserting them. One thing I did learn is that if you bevel the edge of the hole with a 1mm bevel it helps you centre the insert and makes it much easier. 

For M3 into plastic I do my holes at 2.7mm. As they print slightly smaller that usually gives me a roughly 2.5mm hole. I then if needed ream them to 2.5mm with a drill bit and then use a proper M3 tap to cut threads into the plastic. You can just use the thread on the screw but the proper tap works better and I have a full set anyway that I use for aluminium parts. I usually use 4 perimeters around holes for those and 10-20% infill. Any areas that need higher get a modifier slapped on with higher %. 

RE: Why circular holes are not circular? Which plane produces the nicest looking surface?

Posted by: @neophyl

 

For M3 into plastic I do my holes at 2.7mm. As they print slightly smaller that usually gives me a roughly 2.5mm hole. I then if needed ream them to 2.5mm with a drill bit and then use a proper M3 tap to cut threads into the plastic. You can just use the thread on the screw but the proper tap works better and I have a full set anyway that I use for aluminium parts. I usually use 4 perimeters around holes for those and 10-20% infill. Any areas that need higher get a modifier slapped on with higher %. 

Thanks. Is this paragraph about the simpler method of drilling holes and then inserting the screws? 

Just ordered some threaded brass heat inserts. How come unlike the screws, they are of very limited lengths? I only found 3.4mm and 5.6mm?

RE: Why circular holes are not circular? Which plane produces the nicest looking surface?

If you have a flat top surface, and you have holes in it. The top surface will be in several parts printed.

Do in the holes a 1, 2 or more layers, at the top, so you have a single layer at the top, then then the prints gets more beautiful at the top of the model.

RE: Why circular holes are not circular? Which plane produces the nicest looking surface?

Hi, here are photos of two ought-to-be-circles but ended up like ovals. In person, they look more like (__). How come? Also, as  you can seen, there are notches in one circle.

RE: Why circular holes are not circular? Which plane produces the nicest looking surface?

What caused the (__) shape and the notches? Is there any good way to avoid them?

RE: Why circular holes are not circular? Which plane produces the nicest looking surface?

That's an artefact of layer height.  As holes on that orientation are limited by the layer thickness.  That's what leads to a flat on the bottom and top.  It also looks like you have cooling issues as it looks like its drooping.  You could use support inside the hole but generally its not worth it if its for something like a bolt/screw.  If its decorative as part of the design then it can be worth it sometimes.

Normal practice is to drill them out as mentioned before or if you know what the print orientation is going to be when you design the part to design the hole such that it has a sloped top like a roof ^  That way you get a printable slope at the top and any slight sagging has the room to not interfere with a bolt/screw.  You could also make the bottom slightly pointed too if needed.

RE: Why circular holes are not circular? Which plane produces the nicest looking surface?

You might find this infographic helpful.

https://imgur.com/gallery/SqIdFwB

RE: Why circular holes are not circular? Which plane produces the nicest looking surface?

@neophyl

Thanks for the excellent link. What does red and green in the figures represent? Somewhere it states that "Vertical holes are fine, but horizontal ones should be tear-drop shaped to mitigate steep overhangs." Is the red circle a vertical hole or horizontal hole? 

Usually I just set support to everywhere and 100% infill for PETG. Next time I will set the perimeters to 4. For functional prototyping that needs to support a weight of about 10kg, is 10-20% infill sufficient? For print orientation, if the holes on a surface are important, is it better to print the surface parallel to the print bed?

Due to the smell complained by family members, I put the printer inside the toilet and have the ventilation on and the lid of the toilet seat closed. 

RE: Why circular holes are not circular? Which plane produces the nicest looking surface?

I interpreted the 'vertical holes are fine' to mean those on the XY plane in standard orientation, and 'horizontal' meaning holes on one of the Z planes, or what I would call horizontal pockets.  Kinda backwards, I admit, but I took it in context with the drawings.

RE: Why circular holes are not circular? Which plane produces the nicest looking surface?

@peter-c20

While a picture is worth a thousand words, this one begs several questions, answers to which may help us be more helpful to you: 1) To confirm, is this filament PET/PETG? 2) What layer height was used? 3) What infill % was used? 4) what slicing program was used? 5) Are you comfortable posting the .STL and or .gcode files used? If so, I  think we may be able to suggest some changes to either or both which would be helpful. (The second photo is too out-of-focus to analyze objectively.)

RE: Why circular holes are not circular? Which plane produces the nicest looking surface?

The gray one is PLA and the black one is PETG, both from Prusament. I just used the default settings (layer height 2mm, first layer height 0.2mm, perimeters 2) for the corresponding filaments and set the infill to 100%, 0.15mm Quality, Support Everywhere, Contact Z distance 0.1mm for PLA and 0.2mm for PETG. I use PrusaSlicer all the time.

I took a new photo and attach here.

RE: Why circular holes are not circular? Which plane produces the nicest looking surface?

Both were printed on the vertical plane.