Skeletons for Mechanism

[james.lynch]

Hi,


I was wondering if anybody had any pointers for developing skeleton models to define components and motion of components (if possible) for use in Mechanism.


I recently watched one of PTC's webcasts on an intro to skeletons ( I havent used them before) and the presenter said that the use of skeletons can be very benificial when delaing with mechanism. He mentioned that I should "include in your skeleton design any datum curves / surfaces / planes / axis etc.. to actually be able to simulate the complete motion of your design" but how would you go about this?


for example, usignTop Down design with skeletons, look ata double door, where one door is hinged off the other door which in turn is hinged off the wall. (I can remember the name for that type of door) how would you create the skeleton for that, keeping in mind that the hinge inbetween both doors moves withe the door attached to the wall? (hope I havent confused anybody

)


I'm only getting in to this side of ProE at the moment and I would greatly appriciate any help or pointers ye have.


James

 

[billwillett]

Hi James,


I'm no expert, but here's how I do it. I create an assembly for my mechanism and add a skeleton model to it. Then I add datum curves for the parts in my mechanism. For the double doors, I would create a sketched datum curve shaped like a V with one end starting at the origin.


Next, I would create door 1 part and merge the skeleton model into it. I would use the first part of the V in the merged skeleton as a reference to set the hinge location and width of door 1. I'd merge the skeleton into a another model called door 2 and use the second part of the V to create it.


In the skeleton model I would create an axis at the origin and one at the joint between the doors. You could just as well create these axis in the door parts, but by doing it in the skeleton, I only have to make 1 axis. Note, if you depend on datum items from the skeleton, be sure to 'copy datums' when merging.


WhenI assembly my mechanism, I make my pin joints selecting the axis in door 1's merged skeleton model and the axis in door two's merged skeleton.


Bill

 

[james.lynch]

Bill, thanks for the reply, I just have a few quick question..


When you speak of merging a skeleton in to a model, I 'm afraid I dont quite undersand what you mean. Do you use the Edit/Merge command same as you would for merging 2 surfaces?


Also, the skeleton that you would create, how would that be constrained? as in would the sleleton move with the door part?


Also is this the technique you'd use for all designs, as in, create an assembly, create skeleton, add datum, create parts, then merge to skeleton..


Thanks for your help Bill and sorry for the newbie quiestions!


if anybody else has any hints, pleaseadd them in here.


Cheers,


James
Edited by: james.lynch

 

[Brian_Adkins]

James,


TDD Skeletons have been the 'traditional' method for creating kinematic assemblies. In this scenario,a single skeleton part is composed of datum features that represent kinmatic links. Actual parts are then assembed to this skeleton and fully constrained. The skeleton part changes shape when regenerated (i.e. the door-alignment datum changes) which causes the attached parts to also move.


Mechanism Design is another method for achieving kinematic assemblies. In this situation, the components are not fully constrained, but left 'unhinged' in desired degrees of freedom so that they can move.


I'll abbreviate these methods as TDDSkel and MDX.


At first glance, the two methods actually seem to be opposites:


A. In TDDSkel, parts are fully constrained and don't move until regeneration. In MDX,the parts are 'packaged' and are free to move at any time by dragging the mouse.


B. In TDDSkel, the entire mechanism can be simulated without any actual parts present... I.e. you can test the motion before you design any real parts. In MDX, you can only actually test the motion once the parts have been designed and assembled. This, in my view sort of defeats the purpose.


PTC has a document that details how both of these methods might be used at the same time, but I'm not 100% convinced (not even 50% actually...). It calls for creating a separate skeleton part to represent every component in the kinematic assembly and then assembling these skeletons together using MDX. This sort of mitigates the problem of needing to design all the parts first in MDX, but causes some other issues...


Here is the document: http://www.ptc.com/cs/cs_23/howto/apm5903/apm5903.htm


-Brian






Edited by: Brian_Adkins

 

[james.lynch]

Thanks for the info Brian, I see your point!


I had seen that suggested technique before but was a little confused as to what they were doing but your post clears it up a bit.. I'll go throughit later today..


So, in your opinion, is it worth pursuing a TTDSkel approach? or just use MDX? I have to create a 25 DOF mechanism and it's quite complex so I was hoping to simplify it with the use of Skeletons..


thanks


James

 

[JBAUER]

Hi James, I'm glad you posted the ?... I have just started a new job and have been debating the best method to use on a new project... I have not had recent experience with mechanism, but during my last encounter with it we found out that it did not behave well with some of our existing assemblies... In other words std assys would not convert over well to mechanism type joints,and vice versa... Has anyone had recent experience going back and fort from mechanism to a std Pro assy. At any rate here's my description of a skeleton... It's intended to be a wireframe layout that mimics your criticals... It reminds me of techniques I have used back on the old 3dwireframe systems... You go to work defining boundaries, critical mounting points, axis, etc. Often the linkages are represented with curvs...This allows you to lay out the criticals before worrying about allot of parent/childs, failing features, and getting involved with the details of part modeling... An added advantage is that often times the skeleton can be used as a base component in other assemblies as well... Also the inherent "Top Down" advantages to an actual skeleton allow you to control the criticals from an assembly viewpoint. Allows perhaps easier use of family tables, layouts, and even pro-program to drive your assembly... Has anyone used any of the above successfully with mechanisms... The only other option I know of is to create some quick "dummy" parts and to start assembling them... That is if the mechanism is not to complex. That being said, oftentimes even than you will need some features in your assembly that mimic a skeleton to allow proper motion...

 

[Mloew]

I

 

[Brian_Adkins]

I submitted an enhancement request about a year ago requesting this:


Let us asssemble our components to our assemblies using conventional constraints AND MDX joints. It would be awesome if you could redefine a component's placement and there could be two check-boxes:


[] Use constraints
[] Use joints


By selecting one option, the other option's settingswould not disappear forever, just get ignored until it was toggled again. I think this would make it easier to use MDX in our existing assemblies without destoying the existing constraint schemes.


-Brian
Edited by: Brian_Adkins

 

[james.lynch]

Thaks for all the help guys!! I think I'm maybe starting to get theidea.. But please if you have any more tips for a newbie, please post them here!

Mathew, I was as USER myself last year, first time, absolutley loved it, the University sent me over after winning one of the awards.. probably have to self fund it this year though!!

.. anyway, I actually attanded that presentation and it was excelent! A little over my head at the time but I'm referaceing it at the moment as part of my research. Now you obviously dont have to but would it be possible to get a copy of those part files you used in the Demo? And also, what refinements have you made to the technique?


Thanks for the help,


James

 

[raj_1]

Bill, thanks for the reply, I just have a few quick question..


When you speak of merging a skeleton in to a model, I 'm afraid I dont quite undersand what you mean. Do you use the Edit/Merge command same as you would for merging 2 surfaces?


Also, the skeleton that you would create, how would that be constrained? as in would the sleleton move with the door part?


Also is this the technique you'd use for all designs, as in, create an assembly, create skeleton, add datum, create parts, then merge to skeleton..


Thanks for your help Bill and sorry for the newbie quiestions!


if anybody else has any hints, pleaseadd them in here.


Cheers,


James

 

[djyou3000]

 

[deepakbhat_nie]

Hi Guys,


U can simulate the mechanism by writing a relation and every time you regenerate it simulates the motion of the mechanism and if the mechanism fails then you will get the famous regeneration fail;ure error of proe.please let me know wether u want to try this example if so then i can prepare and send an example of it.


Regards,


Deepak Bhat