Bug: changing setup stock from center to offset

[jw.hendy]

Well, I just about raged and wept at the same time after finishing up a carving for my wife's birthday. I wanted to take some material off one side of my piece, watching in horror as my bit removed material from the other side instead.

I've reproduced what happened and think this is a bug that should be addressed. Here was my situation:

- stock was already profiled to size using a 2d contour and I wanted to remove material from the back edge only

- since my CNC was already zeroed to the original stock origin, I didn't want to change this. However, the stock was originally centered in my setup, so reducing the overall dimension of my piece would have just re-recentered in the stock, removing half the delta from the front and back

- so, I changed the setup to "offset from -y" which handily puts in the current offset into the field (which was accurate, as I hadn't changed anything yet). I clicked OK, went back to Design and made my through-all cut to the back edge of the piece, went back to Manufacture and exported a path to re-contour (matching the edge on three sides and only material from the back)

- instead it cut a crap ton off the front of my piece! I looked in setup to find that it did not honor the offset value, and upon adding my extruded cut, it auto-updated the value to re-center the part anyway

Here is a reproduction:

- create a 100mm x 250mm sketch, extrude 20mm

- go to manufacture, create a setup with stock 120mm x 300mm x 25mm, centered in x and y, offset 0 from -z, then click OK

- edit the setup and change the y position to "offset from -y" and note that 10mm (our current offset) is automatically filled in. Click OK

- go back to the Design tab and edit the y dimension of your sketch. Mine is anchored to the origin, so this is effectively shrinking the part only on the backside with respect to our setup. I've removed 10mm from the back of my piece (100mm originally, down to 90mm). Finish the sketch to update the extrude.

- go back to Manufacture and look at the setup. Fusion360 auto-updates the offset to 15mm (preserving centered)!?!?!?!?!

- now, change 15mm to 10mm in the "offset from -y" field. Then go back to Design, drop another 10mm off the part y dimension (80mm now), go back to Manufacture, and behold it stays 10mm.

-  think there's another weird variant, where after after some incantation of undos, the setup that says it's centered, but it's clearly not. Toggling from "offset" to "center" doesn't work, nor does toggling to "offset", click OK, re-edit, then change to "center." Editing the sketch (even changing nothing) seems to trigger a refresh of some sort and it's then centered again. This bit me as well, as after my snafu, I lost my original offset (which was some arbitrary value based on the original stock) and I couldn't get back as I couldn't remember exactly what i put in for the stock size.

So, it seems that unless the user manually changes the field after switching from center to offset, Fusion360 won't recognize it. In addition, it has to be different than what is there in some way. Namely, copying and pasting the value will not work, nor will typing it verbatim (i.e. typing "20 mm" does not trigger Fusion to figure itself out, but "20mm" will).

Thankfully my part was salvaged. It was a good 2+hr cut and I'm on a deadline. I'm still bummed as I a good 0.25in more than intended off my original 11in y dimension as a result of this. This is a horrible, potential disaster for someone and it's tough to see if the % change is small!

[jack.holmes]

Sorry to hear about the issues with your carving, good news that you managed to save it! The issue that you've described isn't due to a bug per say, it's because of the expression that has been used behind the scenes to auto fill-in the offset in your setup. You can see this expression by right clicking on the input box for the offset and selecting Edit Expression (this goes for a lot of the properties). You can see here that there is a calculation being used to automatically fill this in, which is why it updates and re-centres after changing your model size, and how the correct value is there in the first instance.

If you type in a specific value it will then overwrite this expression and rather than updating will keep that value when you update your model size. So, to avoid similar issues in the future I would suggest firstly if you need a specific value set then type this in to ensure that it is saved, and always carefully check properties like this after editing a setup or model, as things like this can be changed/updated behind the scenes like you saw.

Hope this helps!

Thanks,

Jack

[jw.hendy]

Thanks for the reply. It's been quite a while since I putzed with this. At face value, your explanation helps understand what was going on. The fact that it doesn't register user entry if you type in the exact converted output of the behind-the-scenes expression still seems like a backfire.

In general, not registering the user's explicit change in reference mechanism (calculated center vs. offset) even if they don't change the actual value still seems like a bug to me, so I'll disagree with your conclusion.

As an analogy, it's like if you put your car in reverse but don't press the gas right away, taking time to adjust the radio before you go. You go to back out of your driveway, finding that the car has shifted itself back into drive! The car manufacturer knows that people spend most of their time in drive, so they automatically put it into drive if you don't actually move the car after shifting the gear. It's a feature, not a bug! They knew what you wanted to do more than you did, right?

Software shouldn't be ambiguous, period. What you're saying is that in Fusion360, there are ways to have settings that appear identically, but function differently. This paradigm is a huge recipe for disaster. If the settings say "offset," they should actually be offset, no matter what's going on behind the scenes. Toggling from center to offset should render that expression completely gone until it's set back to centered.

[jack.holmes]

I may have misunderstood the initial steps you listed, but from what I saw when you explicitly type in the value it does in fact keep this. So if you follow your steps again but rather than accepting the auto-filled in 10mm if you actually type in 10mm, this is where it overwrites that expression. So then after changing the sketch size when you go back into the setup you'll still see 10mm, rather than the 15mm described in your steps. To use your analogy, this is the car being told to stay in reverse!

If this isn't what you're seeing then it could then be a bug after all, but going through these steps myself I did see the 10mm saved. Please double check this and if this isn't what you're getting than let me know and we can look into it further.

Thanks,

[jw.hendy]

There are still two cases to make of why this really is a bug:

• if you re-read the post, you'll see that if you type in exactly what's there, it actually doesn't trigger the expression to go away. Try it out. Type in exactly "10 mm" (not "10mm").
  
  
• more generally, you're saying that the user can explicitly make a change, open up settings to verify that change (close setup, re-open and you'll see that the mode is offset), but that the behavior can be different between these states (it might behave like it's offset if you actually changed the value, but it might also behave like it's centered). There should be only one meaning to settings that set an offset and a value.

You seem to disagree about this. If that's the case, I don't know what to say anymore. I can't see the case for defending this software behavior. Changing the mode from centered to offset should really, truly, no questions asked, switch it into offset mode. It's irrelevant what kind of fancy expressions are at work during centered mode... I changed a setting, showing that I want a different kind of behavior now. Fusion360 should give me that behavior!

[jack.holmes]

If I type in "10mm", "10 mm", or even "10        mm" I find it does save the value, Fusion will ignore any spaces in cases like this so it shouldn't matter how this is entered. Perhaps you cancelled rather than OK'd after typing in the value before which is why you saw it was not saved.

I think you might have misunderstood where the expression is being used, that's my fault for not explaining thoroughly. The image I posted is not the expression for the centred mode calculation, it is what is used when you switch over to offset mode, and is used to calculate the default value so that something is used before you then go and change it. If you do not change the value after switching to offset mode, it will just stick with this default calculation.

I can understand your frustration with it, but it's the same idea as if you created a toolpath and did not change the stepover, just taking the auto-calculated value from the tool size. If you then changed the tool size for this toolpath you'd also expect to see the stepover value change with it, unless you had specified a value before. Same idea here, if you want a specific value you need to enter it, otherwise these defaults will change when you change key settings to do with your setup.

Thanks,

[jw.hendy]

Re. it updating after typing in the exact text, I disagree. Here is proof. I'm on 2.0.7813, Startup plan.

I hear what you're saying, but I still disagree. Centered is a calculated position. Offset is not. I hold to the fact that once the user changes to "offset from foo," they expect a fixed distance from a reference. Again, software should not be ambiguous. If you see offset as the mode and a value in the box... it should only mean one thing, not two.

I think a better analogy would be if there was a "percent of tool diameter" stepover default option. I stand by my opinion: if I switched from "percent of tool diameter" to "fixed stepover" and saw 5mm in the field. I would never expect 5mm to still be acting like an auto-calculated percent stepover if I changed my bit! 

I think we're at a standstill. Can you have a different colleague come offer their opinion? I think the answer is super simple and don't see the justification for defending the default behavior: if you need a calculation behind the scenes when a user switches from centered to offset, use it and then obliterate it. If I changed the mode to fixed, why in the world would I want it to auto-update for me? And why are you putting it on the user to know the difference between "the calculated value that's really an expression but looks just like a value" and "the identically appearing value I typed in"?

[jack.holmes]

OK I can see what is going on now, we were not seeing the same behaviour before which is why we were not on the same page. After following your exact steps in the video I can now see the same behaviour as you, and yes I 100% agree that behaviour is a bug. After a bit of investigation, I've realised why we were both not seeing this before as the steps to create it are fairly specific. This only occurs is you explicitly dimension both sides of the square in the sketch, and then like you say type in "10 mm" rather than "10mm". When I'd tested the 10mm/10 mm thing I'd only dimensioned the Y length of the sketch which is why I'd failed to see the same behaviour.

Thank you for reporting this, I will report it to our development team now to take a look at. Sorry for the confusion along the way, can be difficult to work out exactly what's happening with specific steps like this.

Thanks,

[jack.holmes]

In fact now that I've gone to report it I can no longer recreate the issue following these steps, or following all of the steps in your video again. Every time I think I know which steps create it it then hasn't happened again. There seems to be something quite odd happening here as the issue is not very repeatable. Would you mind sending over the project used in your video? There could be something in there that will point towards what's causing it some of the time.

Thanks again,