topic Assign a sketch dimension a variable but maximum value in Fusion Design, Validate & Document Forum

Assign a sketch dimension a variable but maximum value

Anonymous — Sun, 10 May 2020 14:02:49 GMT

I wonder if anyone can assist with this little problem

This is a birdsmouth cut on a rafter, I want value A to be as large as possible upto the following constraints: It must be no larger than StudDepth (95mm) and Value B must be no larger than RafterDepth/3 (50mm).

The reason I don't want to just have a specific value for A or B is that if A is set to StudDepth, then with a roof pitch over a certain angle B becomes too large. If I specify that B is 50mm then with shallow roof pitches A becomes too large and won't sit correctly on the top plate.

I'm probably asking too much but I thought I'd ask anyway!

Re: Assign a sketch dimension a variable but maximum value

g-andresen — Sun, 10 May 2020 14:31:43 GMT

Hi,

If conditions like if,then,else or <> are required, this is currently not possible with the equations and expressions in Fusion.

günther

Re: Assign a sketch dimension a variable but maximum value

laughingcreek — Sun, 10 May 2020 15:12:50 GMT

you can find the functions available to you here-

https://help.autodesk.com/view/fusion360/ENU/?guid=GUID-76272551-3275-46C4-AE4D-10D58B408C20

try something like min(A1,maxstuddepth)=A2

you can also test if a value is greater or less than another by subtracting them and using in the sign(expr) function, which returns a value of 0 if negative.

Re: Assign a sketch dimension a variable but maximum value

Anonymous — Mon, 11 May 2020 15:23:05 GMT

Thanks for the pointer on using MinMax but I've not managed to work out how to do it yet.

From what I can see MinMax can be used to pick one of two values either the min or the max, but I can't constrain both A and B in the sketch as it would then be over constrained.

günther is probably right that I really need a > or < or If to do the job I want and sadly can't. Its not the end of the world as its an easy thing to check and change manually if needed.

edit:

In one of those glorious epiphany moments I realised that if I calculated distance B based on StudDepth and RoofPitch, I could use the min() function to choose whatever value was lower, either the calculation or the 1/3rd rafter depth measurement. Works perfectly.

So, from the sketch above B is now defined as

min(BirdsMouth; rafterdepth / 3)

This picks whichever value is lower, the BirdsMouth parameter or the 1/3 rafter depth.

Birdsmouth parameter is now set to be

StudDepth * sin(RoofPitch) / sin(90 deg)

So now if the roof pitch is high like 45 degrees, the value B is limited to 50mm (1/3 RafterDepth). If the pitch is low like 14 degrees, the value B is chosen as 22.98mm which in turn will limit value A to a max of StudDepth.

Thanks so much for pointing me in the right direction!