Laser cutter tool paths - interiors first?

[matt]

I've been using Fusion 360's preview laser cutting feature:

http://www.acemonstertoys.org/fusion-360-for-laser-cutting/

When cutting parts that have interior voids, it's a good idea to cut the holes first; then the perimeters.  That way, if the part shifts as it falls free of the material sheet, it won't louse-up the part.

In my first project, Fusion seems to be doing this for me automatically.  Is that by design, or just a case of beginner's luck?

If the latter, how do I control the order of the cut paths?  ...Is there a way to do it, short of creating two contour operations?

-Matt

[Steinwerks]

In the Passes tab select the "Preserve Order" chcekbox and select the geometry in the order you wish it to cut.

[matt]

Okay...

So that would be the thing to do if I wanted to control it all explicitly.

But, if I leave that unchecked, what happens?  ..the tooltip merely states that "...the program optimizes the cut."

Does this mean all interior contours will be cut first?

[Steinwerks]

---

@Anonymous wrote:

Okay...

So that would be the thing to do if I wanted to control it all explicitly.

But, if I leave that unchecked, what happens?  ..the tooltip merely states that "...the program optimizes the cut."

Does this mean all interior contours will be cut first?

---

No.

Optimizing the cut in this sense simply means the CAM will do its best to calculate the shortest path in linking moves between the selected contours. If the optimal order happens to cut everything on the inside first, then yes. But that can't be assumed, which is why there is a Preserve Order option. Sometimes the shortest path is not the one that is needed.

[matt]

Hm... are you sure about that?

I've just done a few experiments (with 'preserve order' UNchecked).  I've tested nested parts and weird shapes.  In every case, the generated tool path seems to start with inside contours and progress toward the outside.  

That's great, if I can always count on it.

Can you precisely describe (or better yet provide an example of) a case where the generated toolpath might _not_ always progress inside-to-outside?

[Steinwerks]

Am I sure? No.

I don't use the sheet metal cutting feature so sure, I suppose it could be ordering based on your part layout. I wouldn't bet money on it though.

Generally speaking, it's usually best practice not to rely on "maybes" and just program what you want. You're selecting all the contours anyway, why not force it?

[xander.luciano]

Generally speaking, it can figure out at least what the outer profile is.

I wouldn't say to trust it for nested contours, as it will try to find the shortest distance for internal cuts. This shouldn't be an issue in practice as the rest of the part should still be fixed, but if you want a fully in-to-out toolpath, you may as well just preserve the order and do it yourself so there's no question about it. Here's an example where it will correctly identify the outer contour, and do the outer contour last, but you may want areas 3 and 5 cut before the surrounding regions. The numbers indicate the order it cuts:

In this example, it's just finding the fastest route to cut all the interior contours, which again, should be fine for most applications. However, if you wanted to specify a specific order, that's what the preserve order option is there for though 🙂

Hope that helps clarify things
Best,
Xander Luciano

[matt]

Great example. Thanks!  

But, I guess what I'm really wondering is whether the tool path generator attempts to do the interior contours first.  My experiments (and your example), seem to suggest that it does.

Correct?

[xander.luciano · Accepted solution]

Correct,

It attempts, and usually (in all my tests with it) succeeds in at least separating interior and exterior contours. No matter the selection order, no matter if it's a combination of top and bottom contours on a flat plate, it tries. 

Best,
Xander Luciano

[Anonymous]

I am new to this so I ma not sure if there is difference between laser cutting and contour routing?  However the first sample test I created had two parts nested in the "scrap area" of a larger part.  It did not cut from inside out at all.  (That was why I came to this forum.)  I guess based on other comments it is simply best to specify the cutting order and you know what will happen.