Unable to Select Centre of Circle to Draw 3-pt arc

[Anonymous]

I tried doing a search for my problem and had no luck (probably the wording).  I just started working through the documentation and have already ran into an issue.  I am following the first tutorial, Creating Sketches and Converting them into 3D features:

http://help.autodesk.com/view/fusion360/ENU/?guid=GUID-E24C20F4-E5E0-4585-A1DF-0BE8CDDC787D. 

Activity 3 is where I am stuck.  I need to draw a 3-pt arc using the centre of two small circles and then use the tangent constraints on the arc and the inner edge of cylinder.  I cannot seem to snap onto the centre of the small circles; so, my arc is all wrong (fails!).  The arc ends do not stay clamped to the centre of the small circles.  I have attached a jpg showing my result as well as the 'f3d' file.

 I have tried changing different selection filters, snap to grid off, snap on/off over in the Sketch Palette options.  I even tried the slice option, just to try.  I am at a loss as to what I have done wrong.  Any help would be greatly appreciated.

[davebYYPCU · Accepted solution]

It’s probably a Preferences setting as a root cause, but 

in the new sketch, use Create > Project > Project, Click the top faces of the 2 Cylinders, (Maybe All 6 of them) press Ok, and then,

Create > Project > Intersect, Click on the inside wall, press Ok.

Hide the body, with its eyeball, to confirm that worked, Eyeballs are necessary.

You will get purple copies of those articles in your current sketch, now you have the tools and data to continue.

Preference setting is to Project on Reference, on or off, either does what I mentioned automatically or not.  Half the world will have it turned off, then you can do it manually for just what you need.  So nothing wrong, except,

you can only snap to current sketch items, with that setting turned off.

I can get to your file later, see if it’s something else, thanks for that, does help, but for now you can keep going.

Might help....

[jhackney1972 · Accepted solution]

I think you have a tangent between one end of your arc that is giving you a fit.  Take a look at the Screencast and you will see me get rid of it, then apply the required sketch constraints.

[Anonymous]

Thanks Dave, my preferences had "General > Design > Auto project edges on reference" unchecked.  I checked it and now everything works properly.  I deleted my sketch 4 and redid the arc etc.  Thanks a lot.  I was going crazy with this.

[Anonymous]

Thanks jhackney1972.  Your solution worked.  I have not used the coincident constrain before (I am very new to Fusion 360).  That is a workaround for my mistake.  I think the tangent constraint that I originally had was because I could not snap to the centre of the small circle.  Now that I changed my preference settings for "auto project edges on reference" to on, I am able to snap to the centre of the small circles.  I tried it out in another saved file and it worked.

[chrisplyler]

Right. You were unable to snap to the center points of the two small circles because those small circles were in a different sketch than the one you are drawing the arc in. When working in a sketch, you can only snap to stuff that exists IN that sketch.

Projecting makes linked copies of things in the current sketch. You can manually project just the two circles you want to snap to, or, as you have discovered, you can have Fusion automatically project an entire face when you create a sketch on that face. I personally don't like having anything automatically projected, because it projects a bunch of stuff I might not need and clutters my sketch unnecessarily by doing so. I like to keep that setting turned off, and just manually project the occasional item(s) that I need.

[Anonymous]

Thanks, chrisplyler.  I kind of wished all the tutorials were in video format so that we can see their browser on the left side; this one is text only (with images).  In the tutorial, they created a new sketch for the arc and there was nothing mentioned about having the "auto project edges on reference" checked or not, or manually projecting.  I have watched some of the videos where they mention about projecting; but, have not used it yet.  I will have to look up how to manually project something and see if that is how I want to tackle these things. 

[chrisplyler]

When you're in sketching mode, down towards the bottom of the Create menu, one of the tools is Project. And it has a sub-menu with five different types of Project operations to choose from.

I think sometimes the folks who make tutorials have turned on (or off) a preference setting, and used it like that for a while, and they're just used to it, and so they don't have the thought that maybe they ought to mention it to their viewers.

I found it very helpful when I was a beginner...to look through every menu and every preference option, and read all the pop-up tool tips that display when you hover your cursor over a tool, etc. So, for example, maybe I didn't know what the various Projection tools were for specifically, but I knew they were there, and so if I saw someone talking about them, I would think, "Ahhhh....THAT'S what they're talking about."