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Amethyst

## Looking for an advanced Draft command technique

What I from time to time need is to be able to draft 2 sides dependently but carry at least the minimum draft value to both sides.  The problem is that many times the thickness from the depth from the parting line along a part might favor the top side as well as favoring the bottom side.

There is a hinge in both the top and bottom, and the draft has to be additive.

If you draft each side independently there will be an overbite noticeable.

If you look near the red lines you can see that by drafting independently the overbite is on the bottom impression in the above picture.

Looking to the next picture however you can see in the red circled region that the overbite is on the top side.

What I need to do is match draft to where a minimum value of 5 degrees is held on both sides.

If I draft these sides dependently I could either favor the middle picture side or the bottom picture side.  The problem is that one side will be exactly 5 degrees while the other will have a region or regions less than 5 degrees.

In our line of work we have to have a minimum of 5 degree draft in order to achieve separation from the dies.

If there is no solution for this I imagine that I could draft both sides 5 degrees, build a sketch with the outermost draft extent from each side taken to the parting lock split.  Then I would need to generate an intersection to the parting face with that sketch.  After this I could build a swept blend or independent ruled surfaces that join at the parting lock.

This certainly is possible but a lot of extra work.

21 REPLIES 21

## Re: Looking for an advanced Draft command technique

The way you are doing it the top and bottom of the part remain the same size and the center is growing with the draft.  What about creating a curve at the parting line intersection and using that curve as the draft hinge so that the top and bottom get smaller with the draft?  Then you could use a single draft feature, split at draft hinge, dependently at 5 degrees.

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Doug Schaefer | Experienced Mechanical Design Engineer

## Re: Looking for an advanced Draft command technique

Doug,

I have tried this type of technique but I don't believe this would work this specific application.  The customer won't want the extra stock that would be incurred by drafting back the largest combined profile.

## Re: Looking for an advanced Draft command technique

Perhaps I'm not understanding your concern, but I would think the geometry is the geometry, regardless of technique.  If you require a certain minimum or maximum size and a certain minimum draft, you're going to get what you get.  How you get there is immaterial, I would think.

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Doug Schaefer | Experienced Mechanical Design Engineer

## Re: Looking for an advanced Draft command technique

Sometimes, geometry drives what's needed regardless of customer "wants".  No getting around that and they'll have to understand that.  What it seems you want is somewhat, but not too unusual, it's just that you either get the step, or you sweep a cut, or sweep a protrusion.  There is no other way to get the draft you want.

## Re: Looking for an advanced Draft command technique

Draft one side, the one that makes sense, then sweep a cut around the other curve with the section normal to a plane parallel to the surface.  This will give you a parting line wherever it ends up.  If you absolutely need the parting line in a particular location, you could sweep a section adding material using the surface you have shown and the upper and lower edges as trajectories (turn on variable secion option).

Also, realize that the geometry simply may be impossible to create like you wish, you might have to adjust your design.

## Re: Looking for an advanced Draft command technique

Frank, I like your idea.  In some cases this would work, perhaps even with the one I've shown with the snapshots.  There are some reasons why we do have to cling to a specific designed parting line.  I would like to try this sometime to see whether it would work as I think I understand you are describing this.

I have come up with the drafted part design with 2 boundary blends as I specified above.  I had 3 sets of curves so it was fairly easy to go top to parting and from parting to bottom.

What I really would hope for would be a true match draft capability with the Creo software.

## Re: Looking for an advanced Draft command technique

Just FYI I will show you what I end up with by combining 2 boundary blends (which were solidified out of the original shape).

In our line of work this would be a typical application of match draft.  Where you hold the top and bottom lines in the die and you have to adjust draft to meet at the parting line.

## Re: Looking for an advanced Draft command technique

Can you share the file?  I might be able to play with it a bit, assuming it isn't Creo 5.

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Doug Schaefer | Experienced Mechanical Design Engineer

## Re: Looking for an advanced Draft command technique

Doug: Normally I can't share customer specific models, but this example is very generic and only is the very start of the model.

Hopefully the file will come across.

What I am trying to do is hold the hinge curves (see 1st picture in my post) and have at least 5 degrees draft where each side meets the parting lock.

## Re: Looking for an advanced Draft command technique

Cool. I'm pretty busy at the moment, but I love geometry challenges like this.  I'll try to take a look soon.

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Doug Schaefer | Experienced Mechanical Design Engineer

## Re: Looking for an advanced Draft command technique

Here are 3 options that each produce exactly 5 degrees on each side of the P/L.  None maintain the exact top & bottom profile because I don't think it is possible given the geometry.  The closest, I believe, is matchdraft_5-exact_2.prt.

Hope they help.

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Doug Schaefer | Experienced Mechanical Design Engineer

## Re: Looking for an advanced Draft command technique

Doug,

Sorry but for some reason I can't retrieve your files.  I get the below message.

I appreciate you checking into this.

Without seeing your models, the truth does appear that you can't hold top and bottom while matching at the parting line and guaranteeing 5 degree minimum draft.  There may be times when going to the largest extent at the parting split and just doing a simple 5 degree draft back in both directions would be allowable.  I chose the route that takes longer, but does meet the requirements.that I'm looking for.

I would love to see the software have a true match draft command.

## Re: Looking for an advanced Draft command technique

What version of Creo are you using?  I assumed Creo 4, so that's what I used.  I knew I couldn't open it in Creo 2, I guess I should have tried Creo 3.  Sorry about that.

1. Draft the full side surfaces at 5 degrees based on the lower edges as the draft hinge.
2. Create a curve at the intersection of the drafted surfaces and the split quilt
3. Extrude a cut using that curve to trim the drafted surfaces back to vertical on the upper side of the P/L quilt.
4. Draft the new upper surfaces at 5 degrees using the intersection curve as the draft hinge.

The bottom is exactly the same size, the top is slightly different. I hope that helps.

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Doug Schaefer | Experienced Mechanical Design Engineer

## Re: Looking for an advanced Draft command technique

Doug,

The only thing that doesn't work with your method is that the top draft has a small region on the right side that would undercut.  I do like your method.  It would be simpler than my work around.  It is good to have someone look at alternative solutions.

If only true match draft were an option with Creo.  It almost pulls it off with their dependent draft option.  If you choose the bottom hinge as dominant you can match the hinge from the top side.  Only thing is that in the same area as your method undercuts the dependent draft ends up too small.

## Re: Looking for an advanced Draft command technique

Doug:  I forgot to add that I am also using Creo 4.  I am unsure why I can't open.

## Re: Looking for an advanced Draft command technique

@pimm wrote:

The only thing that doesn't work with your method is that the top draft has a small region on the right side that would undercut. ...

Where are you thinking?  I don't think there are any undercuts.  A draft analysis shows a nice clean split:

(I used 4.9 degrees for the analysis because, for some reason, Creo will show that the draft isn't quite adequate at 5.0 degrees, even though the draft features are explicitly defined as 5.0 degrees to the TOP plane.)

Now, the top is smaller than the bottom and that upper rear edge is no longer parallel to the RIGHT plane, but it's 5 degrees everywhere, despite what Creo believes. 😄

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Doug Schaefer | Experienced Mechanical Design Engineer

## Re: Looking for an advanced Draft command technique

Doug:

I am speaking of the upper rear ridge.  Your results would sit slightly inside mine in that area.

If the width was not critical to the customer in that area your idea would work.

I am starting out with a rectangular box prior to the draft that gets applied.  My intention is to have a perfect match to the edge I've shown in the 1st picture as the draft hinge.  I don't want to use up any of my tolerance inside or outside of the draft hinge.  That beginning rectangle is the size we are being held to.

Regular Member
(in response to pimm)

## Re: Looking for an advanced Draft command technique

When using the "split by split object" inside the draft feature, you can select more than one draft hinge.  Then set the side options to "draft sides dependently".  While this can work well on simple geometry, it's not very robust when the geometry gets complicated.

In your case, most of the part looks good, but the two sharp corners get a little funky.

## Re: Looking for an advanced Draft command technique

Can't you get the right result with the "Split" --> "Split by draft hinge" option?

## Re: Looking for an advanced Draft command technique

I would do similar, except that since if I recall the degree(s) of draft is more important on the shorter side of the parting line, I'd draft the top part 5deg, then do an "intersect" curve where the parting surface is to get the parting line.  Then I'd simply sweep a cut using the intersect and bottom edges at trajectories, making sure the section is normal to a plane parallel to the bottom.

## Re: Looking for an advanced Draft command technique

Yup!  Exactly what I've had to do on occasion.  Good job!

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