Unexplained lines!

New to sketch up so apologies if this super obvious / has been explained multiple other places.

I have created a very simple model (attached) of a bent cylinder. In order to do this I:

  • Drew a rectangle
  • Used the Arc tool to created a curved joint rather than a 90 degree angle
  • Double clicked in the opposite corner to create an equal curved joint there
  • Erased the bottom line of my rectangle so that my shaped resembles an upside now ‘U’
  • Drew a circle at the base of one of the lines
  • Used the Follow Me tool to extrude the circle along my curved line

This is all fine except for one thing - on one of the curved corners there are two circular lines. I’m worried that these are appearing on one curve but not the other.They’re supposed to be identical, and both arcs have the same dimensions. an anyone please explain these mystery lines? The Hidden Geometry doesn’t show anything untoward.

Thanks in advance!

test.skp (137.8 KB)

I’d say the corner with the lines is the one you drew the arc on first. You probably didn’t make the arc fully tangential, look for the Magenta inference which means the arc is equidistant along the two edges. The other corner was a double click so it came out slightly cleaner so it doesn’t leave the lines.
Try making it again and look for the Magenta/Pink inference on the arc.

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Ah yes - you’re right. The arc I drew wasn’t fully tangential, but I’ve had real issues with the Magenta inference - sometimes it appears, sometimes it doesn’t so I just get impatient and draw an arc that lokos about right.

Any idea why the inference is appearing so inconsistently?

The arc isn’t tangential at all.

I haven’t noticed any inconsistencies with inferencing. Perhaps the next time you run into an issue, you could upload the SKP file so we can see exactly what you are trying to get.

Instead of doing that, you might consider using guidelines to lay out and help you draw the arc correctly.


I don’t have any issues, you just need to hit the right spot.


I attached the skp file to my first post, but can see that it’s not even worth you looking as you’ve both diagnosed the issue straight away without having to do so!

I did use guides to draw the arc (30mm from the corner on each line), but I couldn’t get the Arc tool to show magenta - it got to a certain point then snapped to position where some of the arc was overlapping the lines.

I will start from scratch and see if I can get the arc tangential…

Try placing only one guideline at 30 mm from the adjacent edge. If you place two, the inferencing is to the intersections of the guidelines and the edges instead of just the edges. Tangent to a point doesn’t make sense geometrically so no magenta.

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When I draw a two point arc, I don’t get the option to drag it like you do in your example. I can only seem to click two mark the two end points of the arc, then create the bulge from there. Is there a modifier key or setting I can change to make it work like yours?

Are you clicking and releasing the mouse button after the first end of the arc is placed? If not, do that.

Sounds like you are using a pre 2015 version?

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Yep, just tried and it seems that making the two guidelines/guide points was really messing things up. Great to know for future :slight_smile:

I’m using version 17.2.2555 - it is only Make though, not Pro.

Yes I am, but instead of stretching out an arc it just shows a dotted line.

The right way for what you’re trying to do is the one they’ve told you. However, if you only want to hide those lines is also possible, can use eraser tool (e key) and hold shift to hide them or make a selection, right click to show the context menu and choose hide.

Yeah, I’m trying my best to understand the correct way of doing things while I learn the software rather than rushing ahead, taking shortcuts, and forming lots of bad habits like I do with most things in life!


That happens if the first end of the arc isn’t anchored to some other geometry. If it is anchored to a guideline or point, you’ll get that result.

I have just worked out (based on @DaveR’s previous advice) that for some reason, I you draw an arc from a guide point you get the dotted line rather than seeing an arc that you can drag to various size / position you want.

I guess there must be a sensible reason for this, but it was my expectation that the way to make an equal precise arc would be to draw guide points. This has been my downfall! I can now draw arcs as demoed by @Box.

Thanks all :slight_smile:

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You’re welcome.

Now you’ll be arcing like Jacob’s ladder. :smiley: