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Can a parabolic surface be created on a rectangular block? (#2445)
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ram1728  
#1 Posted : Monday, July 13, 2009 2:38:01 PM(UTC)
ram1728

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I am new to ViaCAD. I hope someone can help with some simple questions? I am trying to create a parabolic surface cut into a rectangular solid block.

The user's manual says that you can create parabolas (p.359) using the conics tool in ViaCad 2d/3d v6. Question 1: How do you specify that you want a parabola and not some other conic curve?

Then I believe can create a parabolic surface by rotating.

Question 2: But then how do you cut this into the rectangular block?

Any help would be great. Thank you.

Robert
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Steve.M  
#2 Posted : Monday, July 13, 2009 3:04:18 PM(UTC)
Steve.M

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Hi Robert,

1:- A Parabola can be defined with a Conic Rho = 0.5

2: In 2d/3d, you would need to create a solid from the Parabola curve, possibly from a revolve/sweep etc. Then use that for boolean.

If you can give an example of what you are attempting, I could probably help more,


- Steve

EDIT,
Forgot to state,
Welcome to the forum.
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ram1728  
#3 Posted : Monday, July 13, 2009 3:16:00 PM(UTC)
ram1728

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Hi, Thank you. That was fast. The tip for Rho = 0.5 was a huge help.

Here are a few more details that you asked for.

For example if you have a 3" x 3" x 1" solid block and want to cut a parabolic recess into this block say of 2" in diameter and a depth of 0.5". How would you do that.

Robert
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unique  
#4 Posted : Monday, July 13, 2009 3:19:16 PM(UTC)
unique

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Originally Posted by: Steve.M Go to Quoted Post
Hi Robert,

1:- A Parabola can be defined with a Conic Rho = 0.5



Sorry......not sure I understand what you mean here Steve...why is this ?

Just attempting to make a curve as you describe pretty much flattens it !!

Seems almost too easy to make a parabola curve in Rhino....no hassles:p
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Steve.M  
#5 Posted : Monday, July 13, 2009 3:34:43 PM(UTC)
Steve.M

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Originally Posted by: unique Go to Quoted Post
Sorry......not sure I understand what you mean here Steve...why is this ?
Mathematics

Originally Posted by: unique Go to Quoted Post
Just attempting to make a curve as you describe pretty much flattens it !!
Try in software that correctly calculates the mathematics behind Parabola
Originally Posted by: unique Go to Quoted Post

Seems almost too easy to make a parabola curve in Rhino....no hassles:p
Probably better at maths :) I know I am. :D


- Steve
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Steve.M  
#6 Posted : Monday, July 13, 2009 3:38:43 PM(UTC)
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blowlamp  
#7 Posted : Monday, July 13, 2009 3:40:19 PM(UTC)
blowlamp

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Originally Posted by: unique Go to Quoted Post
Sorry......not sure I understand what you mean here Steve...why is this ?

Just attempting to make a curve as you describe pretty much flattens it !!

Seems almost too easy to make a parabola curve in Rhino....no hassles:p


Yes, I get this too. I've found that in ViaCAD you have to enter an rho value of .75 to get a parabolic shape.
It seems to depend on where the devoloper measures the Slope Control Point and Shoulder Point from - see page 186 of the vc3 User Guide for Tim and co's method.

Martin.
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unique  
#8 Posted : Monday, July 13, 2009 3:50:07 PM(UTC)
unique

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Originally Posted by: Steve.M Go to Quoted Post
Mathematics

Try in software that correctly calculates the mathematics behind Parabola
Probably better at maths :) I know I am. :D


- Steve


Yup im familiar with how they are created (even for a CAM engineer):p but what I meant was if I was to generate a parabola on the basis you describe, in ViaCAD, well it would be pretty much useless from what I see ???

However in Rhino it's a piece of cake....so what gives ?
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Steve.M  
#9 Posted : Monday, July 13, 2009 4:01:58 PM(UTC)
Steve.M

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Originally Posted by: unique Go to Quoted Post
Yup im familiar with how they are created (even for a CAM engineer):p but what I meant was if I was to generate a parabola on the basis you describe, in ViaCAD, well it would be pretty much useless from what I see ???


From what I see, Yes.
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Tim Olson  
#10 Posted : Monday, July 13, 2009 4:43:28 PM(UTC)
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Below is a quicktime movie of one way to create this shape.

http://www.csi-concepts.com/Demo/videos/parabCutout.mov


We follow the below convention for conic rho values, which is what we used at GD/Lockheed for the F16, F22, etc.

Conics with rho values greater than 0.75 are hyperbolas, less than 0.75 are ellipses, and equal to 0.75 are parabolas.

A rho value of 0.707 with equal lengths for horizontal and vertical slopes represents a circular arc. The rho value ranges from 0.50 to 1.

Tim
Tim Olson
IMSI Design/Encore
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ram1728  
#11 Posted : Monday, July 13, 2009 7:15:54 PM(UTC)
ram1728

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Thank you for your help. This answered a lot of questions. And even cleared up some on the "workplane"
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Steve.M  
#12 Posted : Tuesday, July 14, 2009 2:38:58 AM(UTC)
Steve.M

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Hi Tim,

Tim Olson wrote:
Conics with rho values greater than 0.75 are hyperbolas, less than 0.75 are ellipses, and equal to 0.75 are parabolas.


I always thought a parabola had an rho value of 0.5, and the searches I have made give the same info. 2 top search results:-

http://www.rhino3d.com/4/help/c...s/conicsectioncurves.htm

http://www.eng-tips.com/viewthread.cfm?qid=160139&page=3


I have not found any info at all that points to a parabola being at a rho value of 0.75.


- Steve
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ianjkirby  
#13 Posted : Tuesday, July 14, 2009 7:03:28 AM(UTC)
ianjkirby

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Hi Steve,
I cannot debate the maths of Rho, but the diagram in your first link shows something I do not agree with. Please note, I am disagreeing with the diagram, not you!
The shown conic section cannot be a parabola because the bottom extremities are converging (because they are forward of the diameter at the base of the cone), and parabolas by definition are everywhere diverging. My understanding of parabolas is that they are formed by the intersection of a plane with a right cone when the plane is parallel to a line on the surface of the cone from the apex (a "generator"). This is not what is stated on the link, so it should be treated with a degree of suspicion.
Regards, Ian.
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Tim Olson  
#14 Posted : Tuesday, July 14, 2009 9:13:00 AM(UTC)
Tim Olson

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Hi Steve,

>>I have not found any info at all that points to a
>>parabola being at a rho value of 0.75.

We use a rho value that is a ratio of distances relative to the shoulder point and slope control point as seen in the image below. This was the convention of draftsmen before CAD at Lockheed and was continued with the introduction of our in house CAD.

To convert a rho value along the line of your references try:

Rho2 = (Rho1 - 0.5) * 2.0;

Tim
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Steve.M  
#15 Posted : Tuesday, July 14, 2009 9:38:45 AM(UTC)
Steve.M

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Originally Posted by: Tim Olson Go to Quoted Post
This was the convention of draftsmen before CAD at Lockheed and was continued with the introduction of our in house CAD.


Hi Tim,

OK, thanks.


Regards,

- Steve
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