Aligning joints to other parts workflow

[mah6786]

Hello,

   I'm trying to build a toy walking robot with the legs driven by linkages, and I'm using the "design in place" method--I first design the hip joint, then align the thigh segment to the hip, then align the shin to thigh, and after I design the limbs, I project the parts into another sketch to create the linkages.  The issue I'm having is that I don't know how to set it up so that if I edit the pose of an upstream limb segment, the positions of the downstream limbs will update as well.

For example, when I create the shin, I move the origin to the knee joint, and then use the "Align" tool to align the part with thet high by aligning the Z-axis to thigh's origin point, and then hitting "capture position".  This works great until go back in the timeline and change the position of the thigh, since the "capture position" doesn't update.  I thought of creating a sketch immediately creating the part so I could create a revolute joint and attach the part, but then I'd still have to use the align/capture procedure to line the the parts precisely since it's not yet fully constrained, so it seems like I'm back to the same problem.

 I would love to hear any ideas.  Thanks!

Example of misaligned joint after editing an upstream limb segment.

Joints misaligned after editing upstream segment.

[laughingcreek]

If you using align, position capture, etc... then your not using a "design in place" work flow.

[mah6786]

Ok, so what should I be doing?

[etfrench]

Use joints to align components. When you drive a joint, all previous related joints will be updated. 

[mah6786]

I'm trying to align my part coordinates before I've actually design the part, because I need to match up features between the parts (pin/slots, screw holes, etc).  I line up the sketches, and then I project from the old part onto the new.  So I can't use joints, because the part doesn't actually exist yet.  It seems like there is an inherent joint center around the Z-axis when I create a new part, but even if I use a joint, how do I get the alignment?

[Beyondforce]

Hey @mah6786 ,

As my fellow EE said, you should build the parts in-place "Top Down" design.

This way you will only need to use As-Built Joint and not Joint. Please check out this tutorial, it will help you understand the concept of the top-down design method and the differences between As-built Joints and Joints.

https://help.autodesk.com/view/fusion360/ENU/?guid=GUID-929B8937-A1AA-4B97-AEB9-3E1E9FBCFDAE

Cheers / Ben
---------------------------------------------------------------------------------------------------------------------------
Did you find this reply helpful? If so please use the Accept as Solution or Kudos button below.

Need a new computer? Fusion 360 Hardware Benchmark: TESREG.com
Check out my YouTube channel: Fusion 360: NewbiesPlus

[mah6786]

Hey Ben,

   Thanks for the tip.  Everything in my project is in the same design, and I am using mostly as-built joints.  Maybe another screenshot with the origins and axes shown will explain the issue better.  The main leg segment of this robot was made first, and is constrained by the linkage above, which tilts the entire part with its coordinate system at an angle.  The lower leg segment was made after the first leg segment was already in position, and right after I created the part, I moved the coordinate system to the joint where it attaches the larger leg segment, and I aligned with the larger leg segment so that everything would be straight.  This works great, up until the point where I make a change to the linkage at the very top, which alters the constraints on the large leg and its position and angle changes.  However, the coordinate system of the lower leg segment doesn't change.  It seems like when you align two objects (in this case, coordinate axes), Fusion 360 still treats that align as an absolute position, not a relative one.  

What I'm trying to figure out is if it's possible to reference the alignment of one coordinate system with another, without making it its child.  Thanks!

[Beyondforce]

Is it possible to reference the alignment of one coordinate system with another? Yes, it is. But why would you want to do that!? Why can't you just reposition or Align the sketch instead? This way, if you go back in the Timeline, the position won't change! I hope it makes sense?

[mah6786]

Doesn't the sketch take on the coordinate system of its parent?  So if the parent isn't aligned, wouldn't the sketch be misaligned too?

[Beyondforce]

Just give me a few minutes. and I'll make a video.

[etfrench]

The forum ate my previous reply.  Here is an example of creating a joint between a sketch and another component.  This allows you to project existing geometry to a new sketch in the sketch component without changing the timeline. 

If you need to move sketch items to an earlier object in the timeline, create a temporary sketch at the top level.  Project the geometry (with no links).  Move the sketch to the beginning of the timeline.  Create a new sketch (or edit existing) in the target component.  Project the geometry to the new sketch.  You can delete the top level sketch when done.  It's a bit tedious, but can save a model suffering from bad planning.

a6bc4835-325b-4e66-9c05-a9ecf415e19c,640,620

[etfrench · Accepted solution]

The forum must be hungry tonight.  It's eaten the screencast twice now 😣

https://knowledge.autodesk.com/community/screencast/a6bc4835-325b-4e66-9c05-a9ecf415e19c

[Beyondforce · Accepted solution]

I hope this video will help you understand Top-down design concept:

[mah6786]

Thanks Ed! I never thought of mating a sketch to a physical part using connectors.   I'll give that a try and see if I can get it to work for my design.

[mah6786]

Hi Ben,

   Thanks for the very clear explanation of the design concept.  I think I ran into some trouble because I cheated a bit: it was difficult to calculate the orientation of all the pieces of the linkages, so I design the parts off in a corner and the n assembled them, so I didn't actually know the final position until I assembled everything.

But I understand what you're trying to teach me here, I'll think a little ahead next time on how I can get everything lined up automatically.

[Beyondforce]

Don't worry about it, I made that mistake too when I started 😉

There is a technique that I use if I don't know the final position of the components and is also helps to control many elements in one place.

I'll make a video and upload it to my YouTube channel. If you are not a subscriber yet, then go ahead and don't forget the Bell button to get a notifications.

Let us know if you need more help.

Cheers / Ben
---------------------------------------------------------------------------------------------------------------------------
Did you find this reply helpful? If so please use the Accept as Solution or Kudos button below.

Need a new computer? Fusion 360 Hardware Benchmark: TESREG.com
Check out my YouTube channel: Fusion 360: NewbiesPlus

[mah6786]

In the end my model was too far gone to set it up the way BeyondForce is suggesting, but I was able to use etfrench's method to string all by linkages together.

For simple cases where you're just trying to orient the origin, you can also add and reorient a Joint Origin instead of a sketch.

Thanks guys!