Arc segments for small models?!?!


This is driving me crazy. I keep getting this error:

as soon as I chose the 3rd point in an arc.

I know that I can change the number of segments using ctrl +/- but when? It seems like anytime I try it it tries to draw an arc where the cursor is and gives me the error. I know that you can use the {#}S to tell it to use a specific number of segments but that only works after you have an arc but it won’t let me draw my arc, and it seems like if I draw a larger arc and do it it doesn’t stick. The sketchup help center also says that you can use a {#}C to make it segments of a total circle (this seems to really make sense to me, but I cant seem to figure out where to set that, no matter what I try it takes the C as a “you want to use the circle tool” and changes to it.

I know I have a few arcs drawn but I haven’t figured out what I did before it to get it to let me draw them, I have no idea how many segments they have and I can’t seem to make it do it again.


You’ll get that message when the segments in the arcs are too small to be created. The fix is to work at a larger scale to create those sorts of details. The scale down. Or better, use the “Dave Method” which Box has nicely illustrated in a couple of animated GIFs.


We’ve gotten into the larger scale discussion before (at one point I was designing a ring that I was machining out of a nut), I didn’t get comfortable with it then, but for a bit I was just doing everything at 100x, but I haven’t tried it yet with models for 3D printing, I feel like that can only induce errors in something that I’ve already been trying to find where errors are coming from.

Any idea how I’m got some of the arcs drawn but not all of them? Or how to deal with it as is so I can just get this done? (I though I’d be printing a new part after 15 min… I’ve fought this for close to an hour or more)



I use this method all the time for small stuff and have never found any errors.

It depends upon the radius and length of the arc. The radius and length of the arc (or included angle) work together to determine how long the segments must be for the given number of segments.

I guess I would expect that too but I’d have worked with what SketchUp can handle instead of trying to fight it.


Try this: press spacebar (selection tool activated), then press ‘a’ ( for Arc-tool) and type the number of segments you would wan’t it to be ( just the number(s), not ‘s’ ) then hit 'Enter’
The number of segments is now set for the Arc-tool and you can start with the first point.
All the Arc tools and Shape tools work like this and keep their individual number of segments in memory.


Some things to be aware of with small arcs and tiny models.
Think about the level of detail you need for your model usage. Are you going to do close up renders or is it just a rounded edge in a distant shot? or is it for 3d printing? is the detail below the level that your printer is capable of? or do you want lovely rounded arcs even when tiny?
Work out what you need from the arc and adjust the segments accordingly, fewer segments can make a significant difference to how a model behaves. But on the other hand using the Dave method allows you to use a much greater number of segments which will be retained in the tiny arc, therefore making it a smoother arc.


That did it. I’m not sure why I couldn’t figure that out. Even the {#}C (for the equivalent number of segments in a full circle worked. Thanks!

I realize that I probably need to start scaling things, but that’s just an extra step for a quick “I’m going to draw something up and print it,” thing. Maybe I’ll get comfortable with it some day. In the mean time I do find this a bit frustrating since Sketchup even offers a 3D printing template, which of course will be doing things to this kind of scale.


I’ve noticed a smaller number of segments often results in more errors in the solid shape (things like open segments and stuff), and the fact is that I’ve been able to show differences down to .02mm with 3D printing and you can easily hold tolerances to .1mm, so some detail at that level is appropriate for this kind of a model


Ok, I did take a good look at that link and tried it and there is one really big problem with it in this case: I was starting with an existing shape (in this case the pic that I linked is essentially a silhouette of the original shape) and scaling it to get different final proportions. When you’re finished you end up with a correctly proportioned but incorrectly sized part and you have to go and fix that again, not really saving you any time over either just dealing within the arc/circle segment constraints or resizing the overall part and then resizing it back the same amount when you’re done.

Interestingly the 2 examples used in the gifs are good examples of things I’ve had problems with. Especially the round corners tool which can quickly turn your small model into a mess to clean up into something printable…


Why do you get an incorrectly sized part? That doesn’t happen to me. I think you’re missing something.

How about sharing what you started with and its real dimensions?


You can calculate the appropriate maximum segment count for a given radius using the 'Versine’
See here…
There are two code snippets you can copy/paste into the Ruby Console [inches and mm].
Then fill in the values for radius, tolerance [default 1/1000"] and swept arc [default 360°] in the dialog, and it reports in the Console the best segmentation for the given radius, with an alternative for a %4 count too.


You might be missing the main point of this method. You scale up a second instance of the component, edit that and then throw it away.
The large edits will be there in the original small component.

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