Unexpected Parting Surface Behavior (at least to me)

[ryan-feeley]

Hello everyone,

I came across something I don't understand about how the Solidworks mold parting surface feature works. I've reproduced the effect with a simple model.

In short, if I create a part with a non-planar parting line, omit the drafts features, and then create the parting surface feature, it works as it should. [Top solid in the image below where I have curvature displayed].

However, if apply parting line drafts to the solid, and then create a parting surface feature, the parting surface has very irregular curvature and roughly 3x smaller minimum radius. [Bottom solid in the image below where I'm showing curvature]. Introducing the 2nd parting line draft appears to trigger the issue. If I draft only one side, it is ok.

I've attached the model (Swx2020) if the feature tree isn't obvious from the screen capture. Does anyone see anything weird about my workflow? I don't do much of this sort of work. Surely the parting surface feature in solidworks can't be this bad?

Perhaps the parting line draft feature is just not up to the task and is scrambling that edge at a microscopic level? If I convert that edge to a 3D sketch and view the curvature, it doesn't look any different before and after the parting line drafts are applied. Are there any higher-resolution ways to examine edge quality to see if it degrades?

Thanks!

(p.s., this is using the non-manual mode of the parting surface feature, but the behavior is the same with manual activated)

[zxys001]

Hello Ryan, I think you've answered your questions in that the drafted surface becomes dirty and the parting surface amplfies it. As you raise the image quality it's not terrible but yeah, if you want to use your manual surface to shutoff on that would be better.

[ryan-feeley]

Thanks for looking at this @zxys001!

I explored a little more, and yes, you've nailed it with the "amplification". I'd previously converted the PL edges in a 3D sketch and extruded them at a 45-deg to inspect the result. I didn't see any visual differences in curvature, even with max image quality.

However, if I inspect the curvature of those edges with the curvature comb scale at the maximum, there are very small interruptions, that do get amplified by the parting surface feature.

It is interesting that the amplification is self-correcting -- it is worst in the middle of the parting surface, but then disappears at the edges. That makes a workaround pretty easy.

1. Create the yucky parting surface.
2. Use face curves to extract the inner and outer edges, where there is only the initial small interruption in curvature from the parting line, but no amplification.
3. Use face curves to extract a few horizontal segments normal to the line of draw
4. Create a boundary surface from the above and replace the yucky aspects of the parting surface.

[ryan-feeley]

I'll look into this more, but this may be a situation where I should be activating the "allow reduced angle" option with the parting line draft. That may give the numerics enough flexibility to prevent the edge from getting the small interruptions in curvature in the first place.

I believe technically "allow reduced angle" shouldn't be required, but I could see this being a situation where a general purpose drafting algorithm can't quite achieve best mathematical solution -- one which would take me a bunch of tedious surfacing to achieve.

[zxys001]

yeah, if your need is greater precision or if the assessment is that the surface/edge are that dirty (it kind of is but?) for your application,.. I'd reconstruct as well..otherwise if it falls within the tolerance noise...ship it!

[ryan-feeley]

yeah, if your need is greater precision or if the assessment is that the surface/edge are that dirty (it kind of is but?) for your application,.. I'd reconstruct as well..otherwise if it falls within the tolerance noise...ship it!

Yep! My concern in this case was the tight curvature of the parting surface in those little regions. I don't know enough about CAM software to know if it would try to force a tool with a 0.75mm radius, or if it would be smart enough to know that the intended minimum radius of the parting surface was more like 2.5mm. [This is just a toy part that illustrates something I found with one of my actual models]

I explored this a little more, and here is what I learned:

• Activating allow reduced-angle on the second PL draft does eliminate the waviness and small minimum radius of the parting surface feature. It results in a draft angle reduction of say 10% in places
• Ruled surf, with the "tapered to vector" option, has an implicit "approximate-angle" behavior and will both reduce and increase the draft angle relative to the taper angle entered by the user. It gives the smoothest drafted sidewalls of the options I tried.
• Extruding a closed-loop 3d sketch with draft as a surface is able to produce nice edges at the PL that translate into a nice parting surface. It also gives constant draft angle. (Use a closed loop for robustness as it makes the software more consistent in assigning face sense -- if you extrude open loops there are situations where the face normal can flips and that takes the draft angle with it)
• Converting a split-line on the sidewall of a part in a 3D sketch is a little lower fidelity and gives lower curvature smoothness than creating an intersection curve within the 3d sketch using the original inputs to the split-line feature

[ryan-feeley]

In case it wasn't clear, this is the indicator from Solidworks that caused me to explore this in the first place. I got tripped up thinking "why is the min radius of my seal-off < 1mm?? I designed the stepped PL so it could easily be above 2.5mm."