Simulation : Linear Global Load

[laurent.vidonne]

Hello,

I've been learning and working with Fusion 360 for a couple of months on and off, and doing good progress on the design of a sort of CNC mill that I want to build. Now that I have the basic concepts and design decisions done, I would like to refine the definition of some critical parts, with the help of Simulation.

The machine is similar to a router, except that it has an extended Z arm, giving me about 1000 mm high work volume. The spindle and, if things go well, B/C head to which it is attached, will weigh about 15 kg. So, when the mill is lowered to its maximum depth, I have this mass dangling at the end of the 1000 mm free Z arm. For now, the Z arm is made of a square steel tube, 150x150x5 mm section.

I'm now trying to dimension the Z beam so that it doesn't flex too much when the mill is moved in the X/Y direction but still remains not too heavy. I've tried several ways to represent this case with Simulation, with results varying by an order of magnitude.

This is what I've done :

- Constraints : the z beam is attached to the rest of the machine through 4 linear motion blocks. I have set fixed constraints to the back and inner faces of these blocks

- Loads : I have activated the Gravity, and, in order to represent the effects of inertia, I have also :

  - added a Manual point mass of 15 kg set to the lower end of the z beam, that represent the spindle and B/C head

  - applied a Linear Global Load to a corner edge of the beam, with a magnitude of 7 m.s-2 to represent a combined maximum acceleration of 5 m.s-2 in the X and Y directions

- Contacts : I've run the Automatic Contacts command and left the resulting contacts as is (an attempt to define the contacts between blocks and rails as Sliding resulted in a model that could not be solved)

This gives me a max. displacement of +/- 0.1 mm. (zBeam Simul 1.jpg)

I have also tried to represent the inertia as forces applied to the beam and beam end, equivalent to mass x acceleration, in the X and Y directions. This gives me about 0.01 mm (zBeam Simul 2.jpg)

Please note that I have not represented the effects of the cutting forces that apply to the end of the mill tool : as the machine is meant to work on soft material, I assume that I can safely ignore these at this stage.

Questions :

- is any if these models relevant to what I'm trying to measure ?

- which one would be closer to what happens in reality ?

- what would be a better way to create a more accurate model ?

- what exactly is a "Linear Global Load" and how different is it from a Structural Load ?

Being very new to Fusion 360 Simulation and FEA in general, I appreciate any guidance, and thank you for your help

[AndrewSears]

Hi Laurent,

If you are able to add your file to this thread, I will take a look and offer any suggestions I can.  If this is something you do not want to share publicly, can you add me to your project?  The best way is to create a new project and copy your design to it so I am not messing up your production model. We can continue to use this post for communication so others can benefit from the discussion.  

Send the invite to andrew.sears@autodesk.com.

Andy 

[AndrewSears]

Hi Laurent,

Here is the link to the Simulation help/Linear and Angular Global loads.  http://help.autodesk.com/view/fusion360/ENU/?guid=GUID-DEFFD272-36B6-482B-A917-7C8ADECD86CE

Linear Global load is constant acceleration in a straight line, acting on the entire model. 

We can cover the rest of your questions better if you can share the model with me.

Andy 

[laurent.vidonne]

Hello Andy,

Thank you for your reply. I'm trying to export the file now so as to post to this thread, but File > Export doesn't seem to do anything. ?

[laurent.vidonne]

Andy,

The link posted in your last message seems to be broken, I get a 'Learn from the experts / 404 " page

Is this just not my day 😉 ?

[laurent.vidonne]

Hello again

Sorry that it took me so much time to get the design to export, here it is finally.

Thanks again for your help

(I have clicked the 'Accept as a solution' button on a previous message by mistake, any way to un-accept it ?)

[laurent.vidonne]

Correction to a previous msg above : the link does work, it simply had an unwanted trailing space in it.

OK, so I've read the article, and gained a better understanding of the Linear global load, which comforted me in the idea that the two methods (applying linear global load or equivalent structural loads) should give similar results. I revisited my Load cases, and here it was : a stupid error in the value of the forces applied. Fixed it and guess what : the results of both methods are now consistent, with only a 10% discrepancy in the max. displacement value, which is fine with me.

As I move onto simulating the other parts of the machine, I think I will stick to using equivalent structural loads, so that I can stay closer to the actual X and Y movements, rather than uniform acceleration.

Thank you for your help, and my apologies for the time wasted