Jump to content
Ultimaker Community of 3D Printing Experts
Sign in to follow this  
Titus

Dimensions not matching files

Recommended Posts

So I've read about holes being smaller in real life than they are on the drawings, due to shrinking material. But I've had several parts now where dimensions were totally off.

Example one

 

The ulticontrollerwheel just didnt fit. It was a 5mm hole with a 4mm flat end, rather than 6mm and 5mm

http://www.thingiverse.com/thing:31989

Example two

 

 

Here you see the UltiFee

Nor the toe would fit in the foot, nor the ultimaker would fit in the foot(due to the tiny part with the air gap I think)

It is strange that the toe and foot don't fit, since they "should" have had the same error...

http://www.thingiverse.com/thing:25950

 

Example three

 

Here the holes that were supposed to fit M3(they should have been 3.2mm), measured roughly 2mm.

http://www.thingiverse.com/thing:388752

Example four

This nut calumet didn;t fit an M3 nut like it should:

http://www.thingiverse.com/thing:44118

I can hardly believe this all is due to shrink, thus I suspect an error in the machine. Any ideas?

I can try to find at which settings I printed the parts if it helps.

Thanks! Titus

 

Share this post


Link to post
Share on other sites

You have backlash issues. Notice how your circles aren't even close to being perfectly circular? And also how your infill isn't touching the perimeters properly? You probably need to tighten your short belts.

http://support.3dverkstan.se/article/23-a-visual-ultimaker-troubleshooting-guide#circles

http://support.3dverkstan.se/article/23-a-visual-ultimaker-troubleshooting-guide#wallspace

 

Share this post


Link to post
Share on other sites

I'm not convinced it's backlash but it might be. Backlash on UM2 is much more rare.

Anyway, the feet are smaller for only one reason - shrinkage (possible 2: backlash) but circles are small for 3 reasons and so one expects them to be much smaller than desired.

1) Shrinkage

2) STL doesn't support circles so instead you give it a bunch of lines but for some reason CAD software always puts the lines *inside* the circle and never outside. Don't know why. So the fewer the lines the larger the error. Yet if all your circles are 50 sided polygons that's just silly.

3) Rubber band effect. For PLA it has a long way to cool before it gets close to glass temp (unlike ABS). This starts happening the moment it comes out the nozzle so it is already cooling a bit and shrinking so the PLA comes out kind of like liquid rubber bands - pulling towards the nozzle which pulls in on the part and especially for inside circles, there's nothing to keep the PLA from pulling in but mostly air. On outer perimeters it's not a problem as the part keeps the "rubber band" from slipping in as much.

 

Share this post


Link to post
Share on other sites

Hi Robert, Gr5,

I'm on a UM1. I tried to tighten the short belts to counter any backlash, but I also installed the dampers and they introduced a little play(is play the same as backlash?), as you can see here:

http://umforum.ultimaker.com/index.php?/topic/4479-sound-dampening-ultimaker-original/page-5&do=findComment&comment=62163

Though the short belts feel not that tensioned, I can't really get more tension on it.

 

  • I've checked that 12 pulleys are tight
  • The belts are above each other, so the pulleys should be properly aligned

Things that I haven't done yet:

 

  • check the alignment of the axes, as described here: http://wiki.ultimaker.com/Calibrate
  • investigate the ticking noise I'm hearing, which sounds like one of the axes is sliding to the back and front. I tried to fasten the endcaps, resulting in nothing, unfasten them, resulting in more movement.
    Ticking noise at the end of the clip
     

Right now I'm printing another controller wheel. Will post updates on how that goes once it's done.

 

Share this post


Link to post
Share on other sites

backlash and play are the same thing. There is an article defining these words on wikipedia.

That damper definitely looks to be a problem. I recommend you remove it. That looks like around .5mm play right there. So I would expect .5mm error on your Y axis on your circles meaning they would be ellipses that are shorter in the Y direction.

 

Share this post


Link to post
Share on other sites

 

The circles seem alright, dampers installed on both X and Y, therefore the circles should look circular again?

The dimensions are 4mm and 5,5mm instead of 4,4mm and 6mm.

So I guess this is a shrinkage issue. I'll print the ultifeet again, to see if they fit now.

Or it could be a cooling issue, since the top layers are printed slow as well?

 

Share this post


Link to post
Share on other sites

Those circles are quite square. And your infill is attaching to the wall at some points, while being completely lose at others.

So, still backlash/play. Sorry. Dampers won't do you good. You need stiffness, not springiness.

 

Share this post


Link to post
Share on other sites

Ok, this one is interesting:

 

I removed the dampers on X/Y(still there on Z, planning to remove that one too.)

Result: Round circles, but still to small ( 5,6mm vs 6mm and 4,4mm vs 4,5mm)

Getting there! So, what's next?

 

Share this post


Link to post
Share on other sites

Next step is to modify the cad drawing to be the size opening you want. In other words if hole is 1.1mm too small increase in cad by 1.1mm.

Your backlash may be gone but you will always have smaller holes than desired for 3 reasons but the main one here is that when the PLA extrudes it starts cooling instantly and is already shrinking and acting like a liquid rubber band and it pulls inward as it is laid down and ends up laid down inside of the desired (commanded) position.

This is not a problem for the exterior because the layer below holds it better in that configuration.

 

Share this post


Link to post
Share on other sites

EDIT: For the record,

I print most of my prints at 210 degrees C now

Layer : 0.1

Shell: 0.8

Bot/top: 0.7

fill: 20%

speed 75 mm/s

So now it's just a case trial and error? I can hardly believe that you would need to print every file you take from youmagine/thingiverse and print it, measure and rework it, before it is usable. That just doesn't sound right :(

For example. I wanted to print this:

http://www.thingiverse.com/thing:126716/#files

And the big hole on the Chariot that should fit a M4 bolt, just fits an M3, and the M3's don't.

 

 

Could there be something else wrong?

 

  • Flow?
  • Friction in the printhead(hard to move on it's own)?
  • Print settings?
  • Cooling fan shroud shape?

From another topic:

 

 

Both images seem to show infill not meeting the shell which implies play/backlash. More play than my UM Original.

 

 

I can't put more tension on my short belts I believe. Could it also have something to do with the friction in the long belts(Mine's pretty high)

It could be fixed by a higher overlap I guess, but that would also introduce new problems right?

 

Share this post


Link to post
Share on other sites

I can't put more tension on my short belts I believe

 

Don't. This has nothing to do with "play" at this point. If it was play your outer dimensions would be just as bad as your inner dimensions. It has to do with what I call the "rubber band effect". This happens to everyone printing in PLA. I suspect it's not as bad with ABS because ABS hits glass temperature much sooner.

With practice you don't have to design everything twice - you get really good at predicting the adjustment the first time.

You are not the first person to report this. Most demand that Cura compensates. And maybe it should but the affect is complicated and changes if you change: fan speed, print temperature, bed temperature, color of PLA or brand of PLA.

The affect is also a problem for example on gears - the inner part of the tooth - the gap between the teeth. For the same reason. But most people notice it on vertical holes.

 

Share this post


Link to post
Share on other sites

With my UM2 printer, I oversize holes by 0.2 mm in diameter and they generally come out very close to correct. Print head 225 C, bed 60 C and then 45C after the first few layers, speed 20 mm/sec, "Ultimaker Blue" PLA filament.

The very first layer is set to 0.3mm thick and gets squished out more, so I do have to ream out holes but it's mostly just that first layer edge.

 

Share this post


Link to post
Share on other sites

Just scale the holes with a few percent. This is why modeling with oSCAD is so awesome; its very easy to change the parameters.

 

Thanks! I just discovered how that works, after installing OSCAD, which is some kind of PCB design tool :p

I'd have to use that more often! But do you guys open the SCAD file before even printing it and sizing everything up?

It doesn't sound really user friendly, especially for UM2 users that want a desktop printer. It's still hard to believe

 

Share this post


Link to post
Share on other sites

It's just the nature of the beast. People who work with injection moulding have to deal with shrinkage as well, and it's not even uniform throughout the mould.

That said, if you have to adjust the model or not depends on, well.. if you have to :p The guy who made the model might have already adjusted it so that it works well in his printer. Or it could be someone who hasn't compensated at all.

 

Share this post


Link to post
Share on other sites

There are many "parametric" cad programs. Like solidworks. Or openScad. It simply means you can assign a parameter to a particular dimension so that other's can later adjust that parameter.

openSCAD is what power most of thingiverse's "customizable" objects. In customizable objects you can modify lots of parameters such as hole diameter, part height, diameter, length, width, height - whatever the designer included as a parameter for the object.

For example my favorite designer, emmett, does everything in openscad - he did the "heart gears" and his gear bearings which you can set the number of teeth and gears and some diameters:

http://www.thingiverse.com/thing:53451

But 90% of the stuff out there is has no parameters. Anything done in openscad is infinitely parameterizable as it is more of a programming language than typical CAD software.

 

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  

×

Important Information

Terms of Use Privacy Policy