Best settings for sturdy parts..?

[shoe 1]

I want to print my own CNC. Parts should be very sturdy. Is there a recommendations table how to increase wall and bottom/top layer count and infill?

 

For example I doubt it makes much sense to increase walls to 10, have on bottom/top layer and leave infill at 20%. Is there a recommendations table how to gradually increase those 3 parameters to get sturdier and sturdier printed parts?

 

I Cura's default infill is Cubic. Never used anything else. I noticed if you go with infill above 50%, Cura automatically switches infill from Cubic to Lines. What is the reasoning behind this? Is Lines better above 50% or does Cubic not increase sturdiness above 50%? I am confused. What would you recommend for my CNC parts? Cubic 50%? Or is 75% much sturdier?

[GregValiant 1,342]

"I am confused."

Welcome to the club.

 

I like to use Grid as it gives strength in all three axis.  With the Infill Density at 15% set "Infill Line Multiplier" to 3.  You will want to play with the infill type and the line multiplier to see what works best for a particular model.  I also increase my "Retraction Minimum Travel" to a number (15?) that keeps retractions down.  That keeps the number of z-hops down and can decrease printing time.  Lots of infill means a slow print.

Walls 2.0 thick and Top/Bottom 2.0 thick.  With the Grid infill at 15% and multiplier at 3 you can drive a car over a printed block.  In tension?  Now that is a whole nother story.

Edited December 31, 2020 by GregValiant

[shoe 1]

When is the retraction an issue? During printing Infill?

 

Why is increasing the multiplier better then increasing the infill density?

 

"In tension?" What do you mean? Isn't driving a car over a block something like "tension"?

 

Did you ever experiment with "overlap"? For example "infill overlap"?

 

Very detrimental to a prints "sturdiness" is how much print elements "overlap". Not only within a layer, but also between layers. Things like a slightly blocked nozzle (you wouldn't probably notice if it is not extremely blocked) can lead to much worse overlap. Especially between layers. In an extreme case you notice, because a print only have 50% of its calculated weight. But something like 10% you wouldn't notice.

Edited January 3, 2021 by shoe

[GregValiant 1,342]

2 hours ago, shoe said:

"When is the retraction an issue? During printing Infill?"

Not retraction itself - the number of retractions in a small area can cause a lot of unnecessary movement, especially if z-hops are enabled.  The printer would be constantly retracting and priming and jumping up and down, as opposed to just moving to the start of the next extrusion.

 

"Why is increasing the multiplier better then increasing the infill density?"

They work together.  The multiplier gives thicker internal walls.  The density increases the number of walls, but they are thin and weak.  As the density approaches 100% the fact that the walls are thin becomes less important since they get very close together.  At lower densities and a higher multiplier, your internal grid is spread out but each internal wall is a rib (or gusset if you prefer) in it's own right.  It's a question of balance.

 

"In tension?" What do you mean? Isn't driving a car over a block something like "tension"?

Driving a car over a print is compression.  Consider that concrete is excellent in compression, but if you grab both ends of a concrete tube and pull on it, it will come apart pretty easily because it is poor in tension.  They might make roads out of concrete, but you would never want to make the cables of a suspension bridge out of concrete.  You can do things (like you are) and adjust the internal structure to increase compression strength.  When it comes to tensile strength, you are pretty much stuck with layer adhesion as a limit.

 

"Did you ever experiment with "overlap"? For example "infill overlap"?"

If you have a print of 3 walls or less, infill with a high overlap can leave artifacts (marks) on the outside of the print.  This is more noticeable at high speed as there is going to be some wobble in the print head as it stops to make a turn.

 

"...Things like a slightly blocked nozzle..."

That's a printer / maintenance problem.  Increasing the nozzle size to a .5 or .6 can alleviate the problem.  They just don't clog as easy.  The price is that the prints aren't as "crisp" and details can disappear.  Moving down to a .2 will increase the likelihood of a clog, but the prints are "sharp" and details better defined.

 

[shoe 1]

Thanks for your help. For the CNC the most important aspect is that the parts don't twist. They need to be inflexible. Flexibility in the parts leads to 'play', which results in a not very accurate CNC. That's what I meant with sturdy.

[GregValiant 1,342]

PLA is less flexible than PETG.  I'm not sure about ABS.  Regarding torque loading, wall thickness is your friend.  Consider the driveshaft on a rear wheel drive vehicle.  It's hollow.  

[shoe 1]

Yes. That's what I wanted to find out. How to gain the maximum "stiffness" for a part, given a certain amount of Filament (weight).

 

Fully solid should result in maximum stiffness. Let's say, this needs 200g of Filament. What settings should I use to get maximum stiffness with 80%, 70%, 60%, 50%, 40%, 30% of 200g Filament.

 

My hope was that there is a table that shows me somehow the importance for each parameter. For example it could be, add infill until you reach 50%, after that adding walls makes much more sense. Or a general rule, like for every 10% extra infill, you add an extra wall.

[GregValiant 1,342]

Now you are going off into the realm of Finite Element Analysis and Strength of Materials.  There might be tables that would describe what you want.  There have been some posts here from people who were researching FEA and trying to export the models from Cura with different infills.  That isn't possible, but maybe they continued by altering the CAD models they were using.

The strength also depends on the type of loading the model sees.  Compression, Tension, Torque moments, or a combination of the three.  The part geometry comes into play.  Are there holes?  How to the holes affect the physical properties?

At any rate, you are well beyond what can be expected of slicing software.

[geert_2 558]

All plastics tend to creep under load, and deform permanently, especially PLA. Try printing a hook as cloth hanger, and after even a few days, you will see the deformation. Can be quite severe.

 

So I am not sure if 3D-printing is the best for CNC-machine parts? Or maybe if you use glass-fiber or carbon-filled filaments? But I have no experience with them, so I can't give recommendations. I read that they tend to clog nozzles easily, and you need a hardened nozzle and feeder, as they are abrasive.

 

Another option could be to print moulds in 3D, and then cast the parts with fiber-filled or stone-filled epoxies? These can be made very rigid. I know of industrial laser photofilm printers for the printing-industry that used a base of marble-filled epoxy, to give them the required stability and stiffness. That base was as hard and rigid as a block of marble itself.

 

If you use sand as filler, it has to be pre-processed (washed with some chemicals) to make the epoxy stick better to it, making the cast model much stronger under tensile loads too. But I don't know the details of it.