constrain and edge in a plane in assembly

robert_hirsch

I have a planter and a parapet. I have suyccessfully mated the bottom of the planter to the top of the parapet with an offset. yay. Now I want to align an edge in the planter to a vertical plane of the parapet. But I don't see how to do this. The top part is the planter, you can see that the front plane is not parallel to the parapet shown at the bottom. I want to constrain the edge of the painter between the front pane and the bottom fillet to the highlighted plane shown on the parapet. Any idea how I do this?

andrew_troup

If you add a mate connector to each edge you are trying to align, and choose a "fastened" mate, that will do both the things you are trying to do in one hit, provided you reorient one of the mate connectors (if necessary) to line up with the other, and apply any offset (if needed) to reflect your positioning intent.

andrew_troup

The forum software is playing up so that the comment I just added has vanished. (sigh)

It will probably reappear: I was going to add this explanation: The mates in Onshape are more powerful than in other MCAD modelers: generally it takes only one mate to position a new part relative to an existing part.

robert_hirsch

I think I see your post. I will try

andrew_troup

In fairness to the (nevertheless badly behaved) forum software:

 on this occasion it was my wifi which had switched to a weak repeater, not the fault of the fs

Here's a quick rundown on the fastened mate: good luck !

https://cad.onshape.com/help/Content/mate-fastened.htm

robert_hirsch

so now I am retying this, but I dislike it. The reason is that the result is not parametric. I have to fix the offsets to some numbers. The way I would like to do it is the plane mate, which I did. This will make it such that I change the position of either plane and the mate is still good. 

Them I would like to constrain the edge I was talking about to a plane. This way if I change either geometry, the constraint will remain good. With the way you proposed, if I change the width of the parapet I will have to change the mate offsets as well for each planter.

robert_hirsch

ok, using your suggestion of creating mates and orienting as I like then using a plane mate seemed to work as I expect. Thanks!

andrew_troup

A planar mate used alone will not position your planter : you will be able to push it around

You can capture design intent with a fasten mate to give a desirable response to (foreseeable) parametric changes if you think about where on each edge to locate the mate connectors.  For example, don't use offset to centre the planter on the edge: instead locate your connectors at the midpoint of each edge.

robert_hirsch

Ah, I meant I used two plane mates. That worked as I expected. Now I have more questions if you dont mind:

The firs planter is in position. I want to copy and paste more planters. When I copy and paste the mates copy also. Great, because the free direction with no mate lets me move the copies where I like. Until I get to a place where they need to move in another direction. Do I find the correct mate on the correct copy, delete it and then make anew?

andrew_troup

I would recommend you proceed with caution when using multiple mates to position one item.
You will get away with it for very simple models but it will lead you into a quagmire further down the track.

Two planar mates is an inherently bad idea, because each mate inevitably trespasses on a degree of rotational freedom already locked down by the other mate#, and you are in any event still left with a translation degree of freedom.

For the situation you describe, I would either create a mate connector on the parapet for each instance of the planter, and use a single "Fastened" mate per planter...

Or (perhaps not quite yet, but soon) forget about using mates altogether and model in a Part Studio (one containing a master of the parapet, but not the master Planter) and derive multiple instances of the planter into that part studio. For the moment you can use "Transform" to move the planters, but this is not parametric: we've been told that this workflow will soon be possible, however. 

Mates in Onshape are really optimised for controlling relative motion. The process is unnecessarily laborious if you are trying to capture static relations: these are generally better dealt with in a part studio wherever possible.

# this is because each planar mate locks down two rotational and one translational degree of freedom. So two planar mates lock down four rotational DoF, out of a maximum possible of three. And they lock down only two of the three translational DoF.

máté_rangics

andrew_troup said:

The forum software is playing up so that the comment I just added has vanished. (sigh)

It will probably reappear: I was going to add this explanation: The mates in Onshape are more powerful than in other MCAD modelers: generally it takes only one mate to position a new part relative to an existing part.

No. OnShape mates are simply horrible. You can call it "powerful" as marketing bullshit, but it is simply horrible. It works well up to everything is aligned perpendicularly, otherwise it's crap.