"Center" Mate Connectors are Dangerous/Useless

S1mon

I'm really baffled by mate connectors. In some ways they solve a lot of issues that other CAD systems have but in other ways they are practically useless.

Case in point:
I've been trying to take imported ECAD parts and add connectors to help re-position parts on PCBs. I was trying to put a mate connector on this ZIF connector:


It looks like it's centered from left to right (X-axis of MC), but it's actually 0.003mm off. In the Y-axis of the MC, I couldn't figure out what it's doing. So I made a simple part:


The mate connector here is not centered, except if we assume it's at the centroid of the area of the face. To me this is beyond useless, because it deceptively seems like it's doing one thing but it's actually doing something else. It's taken me hours to figure out what Onshape is doing. To be fair, the word "centroid" is in the help, but somehow I assumed that it would be the centroid of a bounding rectangle that encloses the face. The centroid is only useful if the shape is already symmetric, otherwise it's pointless.

I have figured out how to use the "between entities" option to get what I want, but it's far from obvious that would be the right way to do this. It might be helpful to add more examples of "between entities" to show the power of that option. I have other mate connectors on other parts where I added a sketch to get the MC where I wanted. Now I might be able to skip the sketch step. I wish this was more clear.

S1mon

Here's another example things I need to do which I can't do with a mate connector. Below is a FAKRA connector. I want a mate connector at the intersection of the axis of the center pin, and the plane defined by the four highlighted squares. 

In another CAD system (Solidworks or Creo and I assume countless others), I could easily create a coordinate system with a few clicks which would be where I need it. In Onshape, unless I'm missing something, I have to add a sketch or manually add an offset dimension. I tried a bunch of "on entities" and "between entities" picks, and I can't get what I need. Why is this so hard?

S1mon

Another example:

Here's a simple block, created with a centered rectangle sketch located on the origin. I've added a mate connector at the center of the face. It lines up with the origin - as one would expect.



Now I add a hole before the mate connector and it moves:


In this case the change is large enough that I would likely catch the issue. But what if the hole is smaller in proportion? Then there's a high likelihood that mistakes will be made. With a smaller hole which only moves the centroid slightly, I might never catch the shift:


Now that I'm aware of this insane approach, I can work around it, but I wonder how many models out there have strange little errors due to issues like this?

MichaelPascoe

Looks like it is calculating the center of mass and placing it there:

There are a few things you could do to work around this. 

1. If using sketches to create your part, have a reference point. (Not ideal, but will work.)
2. When placing a mate, use "Between entities" instead of the center of mass. See the gif below.
3. Create a custom feature that places the mate in the center of the bounding box instead of the center of mass.

Mate connector & center off mass:

Placing a mate connector between entities. (Optional solution):

S1mon

@MichaelPascoe Thanks for those GIFs (BTW what tool do you use to record them? Are you on Windows or Mac?)

It's definitely the 2D centroid of the face, not the Center of Mass of the part projected on the face. See this example:



I had already come to the same conclusions as your #1 and #2 solutions. #1 is not ideal with imported geometry. I would typically need to add sketches to do what I want. Certainly I've been thinking about #3.

My main concerns with highlighting this issue are:

• What was the logic for this behavior in the first place? I see very little use for the centroid of a face. I see a ton of use for the center of the bounding box of the face (or even faces).
• Why isn't this behavior explained clearly in the training materials? At least for me, this cost hours of frustration.

MichaelPascoe

I'm not sure, perhaps Onshape will share their reasoning with you.

If you decide to go with option #3, let me know. I have some lines of code that will be a quick copy and paste to make that feature.

nick_papageorge073

I agree completely. The centroid is absolutely useless as a mate connector, and dangerous, because you can't tell you are not 100% on center many times. I did a feature request on this topic a few months ago with the built-in helpdesk system. Please do one also. I think they give more weight to the helpdesk requests than these forums.