Using coincident constraint

jose_popoca

Depress the coincident constraint, select one circle then the other. The second circle is 3 times larger then the first. Question; Why does the second circle selected disappear? 
Additionally, placed various entities, various size lines, circle, arcs... on first sketch, used the dimension tool on a circle, when hitting enter all the entities shifted outward, like an explosion. Why? Guess, really need to learn everything about dimensions vs constraints. At this point of question in learning due to stuck at a boolean usage. You know, back tracking to find what went wrong. Thanks in advance for help. 

michael_mcclain

This is because the circles are most likely concentric and when you use the coincident, it cause the circle's edges to touch each other. Concentric centers+edges that touch causes the circles to overlap each other and it seems that one has disappeared.

Right click on the circle and use "select other" and you should have two items in the list representing one of each of your circles. I would suggest to delete one and try again

Play around with some circles and the "coincident", "concentric", "equal", and "tangent" constraints so you can understand their behavior and you can understand whats happening and why. Be aware that circles can be constrained by their center and their edge.

michael_mcclain

Check out this video: https://www.onshape.com/videos/dimensions-and-constraints

jose_popoca

Thank you Michael but no, the second circle is approximately 3 times larger then the first

michael_mcclain

If you share a link to your document, then you'll have an army of experienced users troubleshooting the issue and offering advice.

The first dimension you place in a sketch scales the whole sketch. I.e. the line is ~3cm and you place the dimension and choose 6cm. The whole sketch will scale itself by 2 based on that first dimension placed.

jose_popoca

I really can need help. Want to do as asked, Michael, but don't know how to shear a link?

jose_popoca

Just figured it out. https://cad.onshape.com/documents/e994593886de5d442dbfa9fb/w/68f75f55d1e99e9e2811199b/e/0c3030599876a138fcaeab0a

michael_mcclain

When you are inside your document look in the top right. There is a blue button marked share. Inside that dialog you will see a tab marked link. Copy that and paste it into a comment in this thread

michael_mcclain

I'm on my phone and I'm having trouble with bad connection where I'm at. If someone else doesn't come by and help you out, then I will check out your document in about an hour when i get home and let you know whats going on with it. In the meantime, I recommend the Onshape tutorial videos and the Onshape learning center.

jose_popoca

Appreciate it Michael.

Have completed many Onshape tutorial videos in sequence. Working on the Cantilever arm currently. Made it up to the usage of the boolean feature. Unable to complete this feature successfully lead to evaluating all steps to the beginning of the first part of the cantilever arm exercise. So created a new document, placed various size lines (solid and construction ), circles, arcs. Leaning to manipulate each move, size. The arc is the only one that can't be moved...

Maybe using/access a different sequence of videos? 

Thanks for the video link!

Well, thanks for your time.

michael_mcclain

Ah I forgot to tell you that when you share a link like that to the public forum, you have to also set it as public for me to go into it and be able to edit and see the features in depth. To make it public is in the same 'share' dialog box

jose_popoca

Just made it public.

michael_mcclain

jose_popoca said:

Just made it public.

Thank you, I am able to check it out now.. Sorry for the late response, but I've been busy these days.

Have you figured out your issue or learned more about how constraints behave? Anything you need help with?

paul_chastell

Note that coincident means "put the two things on top of each other" and for circles that means Onshape sets the center and radius to be the same for both.

I expect the second circle looks like it has disappeared because it is on top of the other one. If you had first dimensioned both circles to different sizes before adding the coincident constraint the sketch will complain because it can't simultaneously make them different sizes and coincident. Or if you make two arcs coincident you will more clearly see that the radius and center of both arcs are the same but you still have two arcs.

What were you trying to do when making the circles coincident? Did you want them coincident at a point? There's a tangent constraint which will make two circles touch.