Introduction: Sketchup -- How to Draw Lines Tangent to a Circle

Sketchup doesn't have a function that allows you to draw tangent lines to a circle so you have to do it the old school geometry way. Here is how.


Brogmoid (author)2015-12-07

Good method but what if it was 1 3/16" X 2 31/64"?

TrevorW4 (author)2015-04-26

I appreciate the video. Sketchup gets a large user-base
because its free, but it sucks it requires so many workarounds to achieve basic
results, as you demonstrate. I use it and it has its purpose. Its purpose is to
have reasonable 3D presentations for basic uses but it is not and cannot be
accurate. It is completely unusable for any purpose where 10ths, hundredths, or
thousands of an inch matter.

Sketchup was designed badly from its inception. At its core,
is flawed math. Whenever you draw a circle in Sketchup, mathematically, any
circle/arc is immediately translated into straight line segments. I design
parts and assemblies and it was very disappointing to realize it’s impossible
to draw anything with accuracy using Sketchup.

It got worse when I realized it’s hard-coded into 24
segments per circle. While Sketchup does allow the user to manually change the
number of segments, that feature is worthless as it breaks all the internal
math calculations for creating faces, trimming at intersections, or, frankly,
anything normal in making a drawing. A user is stuck with 24 segments per 360
degrees or, if you change the number of segments, you will soon learn how Sketchup
breaks down.

It is also broken with its hiding feature. Incessantly, I
hide groups or components only to discover the auto-trimming and/or auto
creation of faces and/or multiple other features are affected by objects that
are hidden or already moved. For example, I havea group in the background with arc segments
and not only should Sketchup not trim my newly designed lines/arcs using
elements from a group (its supposedly
designed in behavior), but I also find my elements are continuously
trimmed/modified by elements that are hidden.

I hope my comments don't get reflected badly on you (they
shouldn't). I appreciate all your effort to help with creating lines from
tangent to tangent. You’re doing the only method that will work in Sketchup and
it is very helpful. I wish you would show users what happens when you create a
solid by clicking "closely" as you described at the beginning. Yet
another bad feature of Sketchup is that it does NOT ensure closed endpoints.
When you click closely, there is really a gap. That gap doesn't stop Sketchup
from allowing planes to be created (the drawing appears okay graphically), but
the gap does cause numerous features to break; especially those related to

For example, if you try to use the join or subtract features
they, very often, fail. I have experienced crazy bending/warping, inner
contours that can't have their planes deleted, or I couldn’t join two solids simply
because they are broken in some undiscoverable way without a plugin helper like
“Solid Helper2”.

To be honest, Sketchup is simply not a professional software
product. It is impossible to design anything with accuracy and it’s loaded with
buggy/limited features.

NOTE: Having said that, the free version can be very useful
and its very helpful to a great number of people who don't have money. It’s
really not bad for something that is free. It is not worthy of the Google name.

Seriously, thanks for assisting, it does take crazy stuff
like your example to draw basic shapes, but knowing these type of tips/tricks
allows the product to be more useful. I do hate that it’s impossible for
Sketchup to ever be a truly professional product (IMO) as the core engine is
mathematically flawed, but I do still use it knowing all its problems.

Thanks again and sorry if I want other users to be
wary/careful of using it. It’s truly not a professional grade product and
cannot possibly be professional grade the way it mathematically works
under-the-hood. That is why it doesn't support simple features like snapping to
a tangent. The math formula that calculates a tangent requires a true arc or
circle's dimensions. A series of straight line segments simply don't have the
information required to allow that kind of feature to work. It’s also why it
doesn't have a fillet option or chamfer options or any of many options that robust
3D/CAD systems offer. In summary, it’s a useful toy under the right
circumstances and the word pro belongs nowhere near it.

Solidworks is the real deal for actual real-life production
purposes. For gaming or other purposes, there is a long list that are the real
deal, but that's not my territory. Again, for real-life professional purposes,
get Solidworks, or AutoCAD Inventor. Inventor LT is far superior to Sketchup
and costs less than Sketchup Pro.

Mark Chambers (author)2015-02-18

Thanks. Very useful.