Constraining rectangle size with rounded corners

[mahboudz]

I am designing a shell for a hardware product. I'm at an early stage, and am also new to Fusion 360.

I have a rectangle inside a larger, rounded-cornered rectangle. The inner rectangle will represent the size of the PCB and battery that will eventually be contained. Because of the rounded corners of the final shell, I wanted to be able to have a way to ensure that the inner dimensions were at least what I need, regardless of what the shell's size and shape end up being.

What I am trying to figure out is how to make the outer rectangle be formulaically constrained to the inner rectangle. The variables I want considered are:
- the length and width of the inner rectangle

- the radius of the outer rectangle's rounded corners

Doing so now means that when I have final numbers for the PCB and battery dimensions, I can just alter the length and width, and the outer rectangle would resize accordingly. And if we also decided that we wanted rounder or less round corners, then adjusting that dimension, would readjust the outer rectangle accordingly.

Once I get this done, then the next step would be to create an extrusion of the outer rectangle into a 3D shape. (there I'd want similar constraints on the height, plus there is a chance we might want rounded corners on the top of the shell too, not just the sides.

Here is the shape I am currently working with. 

I've used a multiplier of 1.15 on both the length and width for now, but that isn't perfect. I end up with cases like this:

There's too much wasted space in this one, as you can see from the spacing between the corners and the round corners.

[hamid.sh.]

If you want that corners of the inner rectangle always touch the fillets you can do like this screencast.

File is attached.

[mahboudz]

Mersi Hamid.

I noticed that you drew a Center Rectangle instead of a 2-point. Is that important in this instance?

Also, should I have the two rectangles on separate sketches so that later the inside rectangle doesn't impact the end-result?

Eventually, I want to create a hollowed out shell that looks like the below, and the dimensions of that are based on that inside rectangle, but when I go to 3D print this, I don't want the inside rectangle to impact the thickness of the shell or anything like that.

 

[HughesTooling]

If I was creating that I'd have a sketch with one rectangle with sharp corners, no fillets. Extrude, add fillets as a feature to the box then shell. Rule is keep sketches simple and fillet the body where practical.

Mark

[hamid.sh.]

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@mahboudz wrote:

... I noticed that you drew a Center Rectangle instead of a 2-point. Is that important in this instance? ...

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I used center rectangle to easily center it on origin. Also in what I did in my screencast you notice I needed the center of rectangle. Even in your own picture, as far as I can see, you have added construction lines to have the center.

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@mahboudz wrote:

... Also, should I have the two rectangles on separate sketches so that later the inside rectangle doesn't impact the end-result? ...

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I'm not sure why are you worried about having two rectangles in a sketch. Whatever operation you want to do you can select the profile that you want.

Now with these all being said; in my previous reply I just tried to answer your specific question about how to make the constraint you wanted. If I were you I would listen to @HughesTooling 's advice. Keep the sketch as simple as possible.

[HughesTooling]

@mahboudz See attached file for some ideas on this. Step along the timeline features to see how it's made.

[TheCADWhisperer]

I recommend that you Attach your completed *.f3d file here and one of the experts here will then demonstrate how they would have completed the geometry.

[mahboudz]

@HughesTooling That's a pretty good example for me. What I'm still missing is how to constrain the inside dimensions to a hight, width and length. As my PCB size becomes more finalized, I want to modify those three numbers and have the shell around resize to fit it. The hard part for me was the rounded corners since they encroached on the internal size. When I started out with sketches of the rounded corder rectangle, that was constrained to an internal rect, it was clearer. With your design where you start with square corners and then do fillets, I'm not sure how I can constrain to the internal size that I need.

Where I get lost is whether I need to have the measurements and constraints all done in the sketches, in 2D, before the extruded shapes are created.

[davebYYPCU]

If the internal size is the most important, use them, instead of guessing.

Change parameters and see if it is what you were thinking of.

Might help....

[mahboudz]

Thank you. How do you get the Parameter listing? The table with all the mm values.

[HughesTooling]

@mahboudz The parameter manager is on the modify menu. Attached is another version where you can set the inside sizes and the outside is calculated with clearance.

Mark

[mahboudz]

Thank you all so much, this has been very eye-opening. What part? The help you've all provided (and yes the Fusion training too).

Is this thread a good place to ask the following questions or shall I open a new thread some place else?

- shall I add a bead around the body/lid so that it snaps shut, or should I use hooks? (I know it would be cheapest if I used glue, but I'd rather not - I want the enclosure to be somewhat permanently closed once snapped shut, but glues seem like a mess). A bead could make ejection of both parts more difficult unless I use more flexible material. I'm looking for material that looks and feels like Lego blocks.

- instead of making the USB port (the little opening) a slot to make ejection easier, should I use a nicer clean USB-sized hole and have side-action on the injection-molding? The slot is actually much higher up on the lid side of the shell, so the slot going up to it is a little unattractive. Alternatively, I can have that opening milled out later.
- Same thing for a resent button hole and a small LED light exit.

- Any recommendations on injection molding services in the US? I've already reached out to ProtoLabs and Xometry.

[HughesTooling]

Does the PCB have holes in it to fix it to the enclosure? If it does the cheapest option for fixing the 2 halves together is just simple pillars and bushes. 

Something like this with a 2mm pillar on one side and a 2mm bush the other is surprisingly difficult to get apart. If you make the base fit inside the cover it will almost impossible to get apart. The bushes need to be at least 5mm long to work well. File's attached.

Mark

[mahboudz]

Thank you again. I've been working hard to get the PCB as small as I can, and I had to leave the mounting holes out. And that also means that the pillars are gong to make the enclosure too big, since they'd take space outside the PCB dimensions.

Looking at the options I have left, that leaves me hooks (barbed features) and slots (I apologize if I don't know the terminology), and also an indent on the outer shell, and a bead on the lower feature to make a snap fit type of closure. The latter is the less permanent closure, but it might not be a bad option if rechargeable battery replacement is a desire.
Is there a site where all practical mechanisms are pictured? I'd like to see what other options I have.

Also, do I need to worry about draft angles, or will the injection molding companies help out with that?

[mahboudz]

Thank you again. I've been working hard to get the PCB as small as I can, and I had to leave the mounting holes out. And that also means that the pillars are gong to make the enclosure too big, since they'd take space outside the PCB dimensions.

Looking at the options I have left, that leaves me hooks (barbed features) and slots (I apologize if I don't know the terminology), and also an indent on the outer shell, and a bead on the lower feature to make a snap fit type of closure. The latter is the less permanent closure, but it might not be a bad option if rechargeable battery replacement is a desire.
Is there a site where all practical mechanisms are pictured? I'd like to see what other options I have.

Also, do I need to worry about draft angles, or will the injection molding companies help out with that?