2016 3D surface snapping to a circle

Hi All,

I have an issue where I want to draw a circle on the edge of cuboid which is exactly 5mm diameter. I set some guides 2.5mm in so my finished circle will be 5mm diameter. However, when i draw the circle the side of the cuboid gets pulled in changing its size.

You can see in the above pic how the sides of the cuboid has wondered into the circle out of square to the guidelines.

The circle is 90 sectors as I want to 3D print it round.

How can it stop this from happening? I am working with a scale of 0.0mm as I can print to 0.1mm


for best results…

scale up X 100 while modelling, scale down for export… [i.e. use meter for mm]

use multiples of 4/8/12 for your circles so that you can easily find the cardinal points…

align circles to major axis…


So this is an issue because SU can’t handle small measurements? It seems a slight flaw in a CAD program…

Buildings are usually ± 5mm, so it pretty accurate in those terms…

the fact we can work around it is a bonus, SU can handle much smaller edges once they are created…


It’s a huge issue that shows up in all kinds of situations. One form or another of scaling up and down is the only real workaround for producing tiny objects at full size.

SU, however, does not position itself as a CAD program, and cites this as a reason for what would be several obvious deficiencies if it did. Also, SU was born as a tool for designing architectural-sized objects, and this trait seems to be woven into its DNA.


Thanks for taking time to respond,

Can anyone recommend a free piece of software that can handle this sort of accuracy? I used to CAD precision work in AutoCAD but I am just playing around with 3D printing at home so I can;t afford to buy a license…however, i wonder if i am looking for the impossible in free software…

mind you scaling is always an answer if that works and is reliable?

The point is it can handle the accuracy, simply by working at a larger scale as necessary.
You can even use components to make this work for you, make you original the correct size and edit a larger copy, then delete the large one when finished and the small one will retain the accuracy.

There are a great many posts about SketchUp’s tendency to lose very small edges as it “cleans up” after a drawing operation. The situation you show is a less well-known instance of the same thing: the edges of your circle pass within SketchUp’s tolerance of the edge of the cuboid, so it decides you meant them to be parts of the same line. To accomplish this, it bent the edge of the cuboid over to join the vertices of the circle - but the next ones over from where the circle actually touches!

As has been pointed out, the workaround is to scale the model larger while drawing this detail and scale it back to correct size when you are done. The small geometry can exist, but can’t be drawn at size due to the cleanup action.

I have also played around with the circle sectors and this too seems to affect the results…

I typically keep my units set in decimal feet and pretend I’m working in decimal inches. This can be scaled to actual size very quickly and seems to survive the transition intact. That handles most cases of too small geometry. The small geometry can exist; it just can’t be created.

In your case, since you’re smart enough to use metric, you could set the scale to meters and pretend it’s millimeters (which @john_drivenupthewall has already mentioned).


Yes, that will affect how closely the next vertices on the circle come to the edge.

Thanks for all your help people :wink:

I will soldier on and scale down.

I believe the cuboid is moving because you are using guides. I have noticed that any geometry close to guides (or hidden lines in files imported from CAD) automatically snap to the guide or hidden line.

I would suggest drawing an actual line that is 2.5 mm; use one end as the center and the other as the radius to draw your circle, then delete the line.

I already explained why this happens. It has nothing to do with guides, only with SketchUp’s cleanup action when you draw small entities.

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You are right. My post should be re-stated to say that the opposite of the stated problem could occur in the presence of hidden lines.

For example, if the cuboid was an imported model with hidden lines, his circle could snap to the hidden line.