Variable pitch helix 1

[SiW979]

Hello People

I have searched through some of the preceeding threads relating to this topic but I've had trouble getting any of the tip to work. Basically I need a helix that satisfies the following criteria

2 turns
Start rad = 30
rad at end of turn 1 = 30
pitch between start point and end of turn 1 = 30
rad at end of turn 2 = 25
pitch between start point and end of turn 2 = 20

I then need to sweep a section along this helix, I also tried doing it in 2 using expressions from a sketch to control the pitch however the 2 were not tangential so it failed.

Any help would be greatly appreciated.

Best regards

Simon (NX4.0.4.2 MP4 - TCEng 9.1.3.6.c)

http://www.jcb.com

If it can't be fixed with a hammer, then it must be an electrical fault...

 

[mmauldin]

Does this help? I created two helixes that fit the description given, fitted point sets to each, then created a spline though the point sets while omitting the points at the joining ends of each helix (this smoothed out the transition somewhat). Its close, maybe close enough?

 

[SiW979]

Hmmmm interesting, thanks for your help, is there any way of making the point set associative to the helices? I followed your route but just used a studio spline with point on curve and this made the sure that if I changed the pitch of a helix, the spline will be associative. I would have thought that siemens would have addressed this issue to make it similar to variable blend where you could specify a different pitch for each turn or specifiec points on the helix.

Best regards

Simon (NX4.0.4.2 MP4 - TCEng 9.1.3.6.c)

http://www.jcb.com

Life shouldn't be measured by the number of breaths you take, but rather how many times it's taken away...

 

[cowski]

Another route to investigate would be law curves.

 

[SiW979]

Yes I've tried playing with law curves a bit, but don't fully grasp them yet, I could do with some decent help material on how to use them.

Best regards

Simon (NX4.0.4.2 MP4 - TCEng 9.1.3.6.c)

http://www.jcb.com

Life shouldn't be measured by the number of breaths you take, but rather how many times it's taken away...

 

[man2007]

Hi JCBCad,

I have created the helix using law curves, hope it helps.

 

[SiW979]

Mann2007

That's pretty cool, but I'm having trouble understanding the math behind it and what to change if you want to alter the pitch or the radius of each turn. Any change you could give a breif explanation.

Many thanks for your help

Best regards

Simon (NX4.0.4.2 MP4 - TCEng 9.1.3.6.c)

http://www.jcb.com

Life shouldn't be measured by the number of breaths you take, but rather how many times it's taken away...

 

[man2007]

As you know the equations for circle are

xt=r*cos(360*t)
yt=r*sin(360*t)
zt=0

similarly for helix
xt=r*cos(360*n*t)
yt=r*sin(360*n*t)
zt=0 or z=f(t)

Where 'n' is the number of turns.

't' is a parameter which varies from 0 to 1.

Here we have to make
(1)'r' as variable (i.e. variable radius) so as to take r1 (=30) or r2 (=25).

(2)'zt' i.e. "pitch" as variable.

Since there are 2 turns in the helix, for 1st turn the value of 't' varies from 0 to .5
similarly for 2nd turn 't' varies from .5 to 1

Let us define
t1=.5-0=.5
t2=1-.5=.5

Now the equations :
xt=(if(t<=.5)(r1)else(r1-((r1-r2)/t2)*(t-.5)))*cos(total_angle*t)
yt=(if(t<=.5)(r1)else(r1-((r1-r2)/t2)*(t-.5)))*sin(total_angle*t)
zt=if(t<=.5)((pitch_first_turn/t1)*t)else(pitch_first_turn+(pitch_second_turn/t2)*(t-.5))

where

pitch_first_turn=30
pitch_second_turn=20
num_turns=2
r1=30
r2=25
total_angle=360*num_turns

if else conditions are used for variable radius and variable pitch expressions.

(1)Variable radius:

For first turn, radius will be r1 from start to end.

For second turn radius should vary from r1 to r2
The rate of decrease in radius is (r1-r2)/t2
For second turn t becomes (t-.5)

the expression for r for second term becomes
r1-((r1-r2)/t2)*(t-.5))

(2)Variable Pitch:

For first turn, pitch varies from 0 (for t=0) to 30 (for t=.5)
So the expression for pitch at any point in the first turn becomes

((pitch_first_turn-0)/(.5-0))*t
i.e.(pitch_first_turn/t1)*t

similarly for second turn, expression for pitch becomes

pitch_first_turn+(pitch_second_turn/t2)*(t-.5)

 

[SiW979]

Man2007

That's fantastic, thank you so much for your time and effort, it's much appreciated.

Best regards

Simon (NX4.0.4.2 MP4 - TCEng 9.1.3.6.c)

http://www.jcb.com

Life shouldn't be measured by the number of breaths you take, but rather how many times it's taken away...

 

[man2007]

Today I saw in Solidworks that this option (variable pitch and radius) is available in the helix command itself, I feel Seimens should incorporate such userfriendly options.