What is a draft angle?

Beside this, the McNeel help is not really helpful. There is no draft vector shown in the image, which does not indicate the usecase of such analysis. The geometry shown is to be drafted (?) in Z direction, otherwise the pictures shown doesn’t make much sense.

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Assuming anyone has waded through the faff this far to try and learn something…

Also, beyond the “What…” is the “How…” It’s one thing to ‘analyze’ draft angle, and another to ‘create’ proper draft in the first place.

Generally speaking (unless there are newer tools in 6 / 7WIP I’ve missed) in Rhino, as a surface modeler at least, draft has to be planned and explicitly drawn, e.g. extrude tapered, whereas draft can often simply (relatively) be ‘added’ (depending…), where draft does not exist, in most higher end MCAD, such as Creo.

As always, depends on form, etc. I’ve literally surface modeled parts manually with a draft angle (crazy as that may sound to some) AND paid no attention to creating draft in Rhino - but cognizant of its need. Draft is then downstream added (sometimes to rather sophisticated Rhino Polysurfaces) as a imported solid part in MCAD, with a magic - create draft tool. (OKAY, takes some knowledge/skill, but still seems kinda magical!)

Basic Concepts - Creating Draft Features
https://learningconnector.ptc.com/content/tut-405/creating-draft-features

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I think the problem with drawing draft surfaces is that is not so easy to do. At least on car models all flanges/draft surfaces are slightly curved. Blending inbetween slightly-curved draft surfaces may cause a blend be locally under the required tolerance. This is critical if you create draft surfaces with low draft angles, which is not uncommon.

One example are part seperations. If you want small and precise gaps, you need exact draft angles.
Another example is the radiator grill, take a grill made of hexagons, which is a typical example for radiator grills. You want extremly small draft angles, because the higher the draft angle, the higher the material usage and less air can go throught. So if your minimum draft angle is 3 degrees your maximum is maybe 3.2. So there is not much space and lot can go wrong at the corners.

Also, if you curve a surface normal to the draft direction, your inital angle is even higher. You also need to find the place where it goes below the tolerance to cut the part there… Analysis is really a must have in my oppinion.

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Agreed! Was trying to imply same with - “depending…” and yes, robust analysis is always key. Really does depend on the part. In some cases draft can be drawn manually, and in other it either shouldn’t, or is really hard.

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DraftAngleAnalysis in V7 WIP has much more capability than in V6. New capabilities include several methods to specify the reference direction and drawing curves of constant draft angle. Previous versions are much more limited in capability. Help in V7 WIP has not been updated with the new capabilities.

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That “magic draft tool” works only either on simple surfaces or if surface quality is not important

Just to add:
A draft angle of 3 degrees is much for small, shiny objects, but not for big, rough ones. I have made parts with 0.5 degrees draft, but that is not so common.

So things that affect the draft angle requirements are:
-Time to spend in the mold for cooling (the warmer the plastic the uglier the surface becomes if draft angle is low)

  • Size of object
  • Quality of surface
  • Structure of surface (many plastic surfaces are bumpy and these bumps affects the draftangle)

Time in mold affects price, so it’s allways good to go for a decent draft angle if it doesn’t affect the aestetics in a bad way.

Automotive design is Class A and Class B surfaces where Class A is visible AND important to the aestetic experience, so keep in mind that rules for Class A may not apply to not visible surfaces. So if you are modelling an internal part then visual quality will be less important, but production speed might be even more important, resulting in the same demand form a 3% draft angle :slight_smile:

Hope that helps, and please correct me if you disagree and think I’m wrong.

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Just to note for the record that is is possible to mold parts with 0 draft angle or even undercuts, but for rigid part materials that requires an injection mold with slides or movable parts that open to release the object, making the mold much more expensive to manufacture and maintain.

If the part material is flexible enough, it may be possible to pull parts out of the mold with 0 or slight negative draft angles as well.

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Absolutely, good point as always Mitch!
And if the part is soft like silicone it can be pulled out of pretty bad cavities, just like silikone molds can be folded back to reveal it’s content.

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This is really informative. Actually I was doing pure class A modeling for a limited amount of time, just about 2 years. It was actually more some sort of an extra educational step I had to take in order to do my initial job better which was about automating things. Because of this, I never really got deeper insights about the why. The requirement was made by the people which massproduce. And they took things very accurate. Very often it seemed more about satisfying the system rather than having real arguments. However, I believe that having everything angled does also improve the look. But this could be subjective…

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Well, ya know, magic is an illusion after all… :wink:

That said, it DEPENDS…and I’m not sure anyone is really interested in the anecdotes, as well as my energy to tell them.

But anyway, it’s all good. Happy weekend, ‘drafters’, wherever you are.

Google is your friend.

You could answer like this to 90% of all threads in this forum. To sum it up:
As said the german/bulgarian translation of this term could also indicate other meanings, since a direct translation may refer to something mechanical as well. It also not fully explains the need for an analysis.

And obviously a lot of those tutorials are simply not accurate and shouldn’t be blindly trusted. As it has been said, it is possible to produce plastic injection molds with undercuts, something I personally did not know and probably the guy implying 2 degrees is a constant, doesn‘t know as well.

You can debate on how to ask questions, but there are no dumb questions!

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Tool ‘actions’ as a means to deal with undercuts are often inevitable, however, they can be frowned upon too, as such will increase tool cost and complexity. Age old designer - engineer - bean counter discord ensues in cases where undercuts may be designed-out or eliminated with draft, i.e., it depends…

More from the source above for the inquisitive:

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The DraftAngleAnalysis topic has been updated now.

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Very well done!

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I love those animations in the help file, especially that one for DraftAngleAnalysis.

Thank you. :grinning:

(Ent-)Formschräge

By the way wikipedia is a great way to translate technical terms.
search the englisch word, than use the Language selector to translate.
Screenshot 2024-02-24 at 17.16.10