Finding centre of gravity for assembly?

[luresKQFHC]

Hi all,

I've been recreating our fishing lures in Fusion 360, how do I find the centre of gravity for an assembly please? I need to know this so I can place the internal weighting in the correct place. 

[jeff_strater]

the Center of Mass analysis type will put a marker at the center of mass of your assembly:

[jhackney1972]

You use the same method you use with a component, except you select the top level assembly name in the browser.

[luresKQFHC]

Amazing thank you! Just for context, my lures are assembled with 3 different materials, wood, 316 steel wire and lead weights, each obviously have different densities. The theory with this sort of lure is you assemble the wood and the steel wire, then find the centre of gravity and install your lead weight at the lowest point from that location.

Just to confirm what are the steps to achieve this?

Thanks, sorry bit of a noob!

[luresKQFHC]

Amazing thank you! Just for context, my lures are assembled with 3 different materials, wood, 316 steel wire and lead weights, each obviously have different densities. The theory with this sort of lure is you assemble the wood and the steel wire, then find the centre of gravity and install your lead weight at the lowest point from that location. Just to confirm what are the steps to achieve this? Thanks, sorry bit of a noob!

[jeff_strater]

The process is pretty manual.  The analysis just adds a visual marker at the 3D center of mass.  You will have to then position the lead weight visually, based on that COM marker.  If you change the design, then you might have to move the location of the weight.  If you want to share an example here, and show a photo of how the weight would be attached, we can illustrate on that example model.

[luresKQFHC]

Ok thank you! What is the best way to share an offline WIP example?

[luresKQFHC]

Example attached! Weight placement only approximate. Thanks

[jeff_strater]

the easiest way is to export as a "Fusion archive":

and then attach the resulting F3D to your next post.

[luresKQFHC]

done, thanks see attached.

[jeff_strater]

@luresKQFHC - here is the basic idea.  I was not all that diligent with your sketch,  Ideally, you would fully constrain that sketch.  I noticed that you had "fixed" part of the sketch, so I had to start by "unfixing" it, then adding some constraints and dimensions so that the loop (where I assume the weight hangs) can be driven by a single dimension.  Then, I added a vertical construction line, to make that visual adjustment easier.

Then, it's just a question of tweaking that dimension until the alignment is "good enough".  Since this is not an application, presumably, where extreme precision is needed, that seems like it would work.  See the screencast below

Just out of personal curiosity, as a very occasional fisherman, I'm interested in the history/usage of these lures.  The idea of making a lure out of wood is interesting, since it is a material that floats.  That is what the weight is for, I assume.  Anyway, it seems like a good project, and would be fun to hear how you get on with it.

[luresKQFHC]

Thanks!

Yes sorry bit messy, usually try to fully constrain my sketches.

Your instruction is good for positioning the bottom loop at the centre of gravity but sometimes that is not the best place to put it as hook that attach to middle and end loop can get tangled. This is why it positioned a little further forward.

The lead weight I am taking about positioning is the component called 'weight', it is different to the wire and the wood.

These type of lures get their action (swim) due to the relationship between the lures drag (comes from lures shape at front of lure as it moves through the water), the centre of buoyancy (from wood) and centre of gravity (from weight). When positioned correctly you have a lure that swims through the water beautifully imitating a fish.

There is a great Youtube that explains this better than I can:

https://www.youtube.com/watch?v=acqIYMolH6s&t=339s&ab_channel=EngineeredAngler

Here is the page for are our lures, which to date are entirely by hand:

https://www.instagram.com/malosi_lures/?hl=en

Sooo... moving forward we are going to start to CNC machine and resin cast lures so I'm trying to develop some formulas to correctly position the weighting entirely in CAD. Maybe even explore some form of simple fluid sim stuff if suitable?

Any ideas on how I should approach this?

Thanks, very helpful.

[jeff_strater]

Sorry, totally missed the obvious here... I thought that loop was for an external weight.  Of course - that is for the bottom hook...

The process is somewhat the same.  You need to turn on the total COM for the assembly, then another one for the weight alone.  Then, the process is to just move the weight using Component Move, until it lines up how you want it.  Understanding, of course, that moving the weight will make the total COM move as well, so there is a bit of "chasing the COM" here, but you get the idea.

Thanks for the links, I'll check those out.  Machining those will be a good way to manufacture them.

Yes, you should be able to do some level of CFD (fluid dynamics) on the result.  Not my area, but if you get to that point, I'm sure there are experts that can help.

[luresKQFHC]

Excellent, thanks very much for your help.

[luresKQFHC]

Just a couple more noob questions:

1. The centre of mass same as centre of gravity!? Adding hooks to our lures obviously effects centre of gravity and buoyancy, how do I simulate this? It will also effect where to position the weight.

2. How do I find centre of buoyancy?

Thanks,

Jon

[jeff_strater]

This is beyond my knowledge, but I did find this:  the-difference-between-centre-of-mass-and-centre-of-gravity .  This article says:  "The centre of mass and the centre of gravity of an object are in the same position if the gravitational field in which the object exists is uniform", so, if this is correct, then they are the same.

This article explains the buoyancy vs gravity:  center-of-buoyancy-and-center-of-gravity  This seems to be the definition of Center of Buoyancy:  "

• Center of Gravity is the point in a body where the gravitational force may be taken to act.
• Center of Buoyancy is the center of the gravity of the volume of water which a body displaces.

"

I'm not sure how to model that in Fusion.  Is it as simple as creating a volume of the lure with a physical material of water?  I'm not sure, but that's what that definition implies...