How to mate a cross beam to slide loose / loosely in a cross slot

james_newton903

I'm trying to figure out how to mate a cross beam (a long beam with a cross section of a cross, plus sign, etc...) so it accurately slides in a slightly larger cross cut in a plate. The goal is something that can move up and down, tilt slightly to the left, right, forward and back, but not rotate (although in reality it would rotate slightly, this does need to be precise). This picture should explain it better than I can with words:

Currently, I have a washer hidden in there, and I've ball mated it to the origin, with a 20 degree limit. The I mated the cross beam to the washer with a slider. So far so good, but... It can rotate. Facepalm. The ball mate allows rotation as well as "tipping" (is that the right word?). The same sort of thing would be true of a box tube in a slightly larger square hole; it would slide, tip to the side a bit, but not rotate. This allows more tipping, less rotation, but it's the same idea. Any thoughts on how to make that move like it really would if those parts were real?

MichaelPascoe

Here is a combination of planar, ball, linear, and tangent mates:

https://cad.onshape.com/documents/56e99002182c670ef3ccd6bd/w/a85ec12e648b07e29cde7e93/e/373f0fd52ad5d0590392d9d6?renderMode=0&rightPanel=namedPositionPanel&uiState=63efaf7316c19e0b2f60239e

Setting up the mates:

• Create a variable studio with a variable for your planar mates in case you need to adjust all at the same time.
• Create planar mates with limits on side of the slot part. This uses the dummy surface technique @eric_pesty mentioned except I used surfaces instead of sketches.
• Use another dummy part that has a straight edge running along its center and a 90 degree face for clocking. Create a ball mate, no limits, between this part and one of the mate connectors belonging to a dummy surfaces (One of the surfaces with the planar mates)
• Create a tangent mate from the center edge of the dummy part to the center vertex of the other dummy surface.
• Create a tangent mate from the 90 degree surface of the center dummy part with on of the orthogonal vertices of the side dummy surface.
• Finally, create a slider mate with limits. 

eric_pesty

I think you are on the right track but you would need to use two "washers" mated to each other to allow your +/- rotation along one axis, and one of them mated to your sliding part also allowing +/-20 rotation but on a perpendicular axis and then the first one like you have with a slider.

The "best practice" for this not to use a physical part like a washer but instead use a sketch or "dummy surface" to avoid having extra part(s) that could be "in the way" in the future.

Now that I think of it you can probably do it with just one part: use a pin slot mate instead of the slider for it (re-orienting the mate connectors and using limits) and then a revolute with limits (axis 90deg from the pin slot rotation axis) instead of your ball mate.

MichaelPascoe

Here is a combination of planar, ball, linear, and tangent mates:

https://cad.onshape.com/documents/56e99002182c670ef3ccd6bd/w/a85ec12e648b07e29cde7e93/e/373f0fd52ad5d0590392d9d6?renderMode=0&rightPanel=namedPositionPanel&uiState=63efaf7316c19e0b2f60239e

Setting up the mates:

• Create a variable studio with a variable for your planar mates in case you need to adjust all at the same time.
• Create planar mates with limits on side of the slot part. This uses the dummy surface technique @eric_pesty mentioned except I used surfaces instead of sketches.
• Use another dummy part that has a straight edge running along its center and a 90 degree face for clocking. Create a ball mate, no limits, between this part and one of the mate connectors belonging to a dummy surfaces (One of the surfaces with the planar mates)
• Create a tangent mate from the center edge of the dummy part to the center vertex of the other dummy surface.
• Create a tangent mate from the 90 degree surface of the center dummy part with on of the orthogonal vertices of the side dummy surface.
• Finally, create a slider mate with limits. 

MichaelPascoe

@eric_pesty using the sketches as guides is great input! On a similar note, here is a Fake mate connector feature for instances like this. It creates those dummy mate surfaces and excludes them from the BOM and sets them to Not revision managed. 

eric_pesty

@MichaelPascoe
You went all out with this! I like the "fake mate connector", arguably you could just have one a document somewhere and keep re-using it but it's convenient to be able to create one on the go like this!

james_newton903

@MichaelPascoe
That is truly amazing! You are setting an excellent example. I'm a tiny big confused as to how you decided on the value for planerLimit? Is that based on the gap between the two parts? Or also based on the thickness of the first part? e.g. if I wanted to make the plate less thick, what adjustments would I need to make to retain accurate limits?

MichaelPascoe

You could draw a sketch representing the beam at its max angle. Measure the Z distance for the tolerance, then divide that by two. 

Or, you could change the size to what you want, then just eye ball it . I think that's what I did for this. That's why I placed the tolerance as a variable so I could easily change it.

Also, depending on how often you need this kind of connection, you can just insert this assembly into whatever other assembly you need it in.