by Luke Davenport
I’ve been doing a fair amount of update training for Inventor 2014 recently, and the new ‘Joints’ tools are one of the biggest talking points. I thought I’d have a bit more of an in-depth look at them myself. So what difference do they make over your common or garden Inventor constraints? Well to summarise - they don’t do anything that CAN’T be done with constraints, but they’ll do it in half the time or less AND will be a heck of a lot easier to analyse and make changes to at a later point.
So why are they quicker? Well perhaps you remember that an insert constraint is a combo of a face to face constraint plus an axis to axis mate?
Insert Constraint = Face Mate (with Offset) + Axis Mate
This is great, but if we take the example of a cylindrical joint we see how this concept of combo constraints has been taken further – it’s like doing the following:
Cylindrical Joint = Insert Constraint + Angular Alignment
All in one go. Very nice indeed. How much easier it is to make all your changes to the joint in one place.
What about if I wanted to produce a rigid joint between two moving components? Remember that’s not the same thing as grounding a component! Rigidly joining two components (assuming you’re not using a skeletal modelling technique) would traditionally have taken at least 2, or possibly 3 constraints (as well as all the effort of turning on the visibility of all the work planes you need), but you can now do it with a single joint.
Rigid Joint = 2-3 Mates.
How about a sliding joint? That would have involved 2 mate constraints (and a third one with limits applied if you wanted a range of motion), but is now just one hit.
Sliding Joint = 2x Mates + Mate for Limits
So joints make it much easier to bring the parts together with minimal fuss. When you also consider that you can add all your limits (angular AND linear limits on a cylindrical joint for instance) in the same operation, and have all this joint information stored in one place for future edits it’s a huge benefit, and eases one of the largest CAD headaches in existence – COMPLEX MECHANISMS!
But enough chit chat. Now for a couple of tips if you get stuck with joints (also see David Gates blog on the subject of diagnosing joints.
1) If you aren’t getting the ‘limits’ dialogue box shown above, hit this button:
2) You can hover over the name of the joint and it will give you some information on the degrees of freedom the joint allows (for instance, a slider joint has 1 linear degree of freedom (and zero rotational):
And finally – how about a topical example of a simple mechanism created with some joints? I thought I’d create my own version of the famous table/chair mechanism designed by Welshman Alex on The Apprentice UK a few weeks ago.
I modelled it using only two joints:
1) A slider joint between a sleeve and the frame:
2) Then a rotational joint between the seat back and the sleeve:
Max and Min Limits were set in both the joints. And then I added a little iLogic equation so that when the slider joint parameter is changed, the rotational joint will update to suit. Check out the video of the results
Hope you find this helpful.