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Betreff: Variable pitch helical curve inacurate?

Gears Esteemed Contributor Gears Esteemed Contributor
Gears Esteemed Contributor

@The_fat_Hobbit  If I remember correctly the helical curve is an approximation based on caclulated points and the accuracy of that can be adjusted in Variable Table after the curve has been created.  You might try increasing the accuracy to see if it helps.  I believe it is set to a lower accuracy by default as the helical features are compute intensive.



Ken
Production: SE 2019 MP8, Testing: SE 2020
http://Grundey.blogspot.com

Betreff: Variable pitch helical curve inacurate?

Gears Esteemed Contributor Gears Esteemed Contributor
Gears Esteemed Contributor

@JasonTitcomb  Don't fix it for my sake but that part won't open as a step using a part template. It will open in an assembly template as a part in an assembly but the part contains nothing.

Bruce Shand
SE2019 MP8 - Insight - Win10 - K4200

Re: Variable pitch helical curve inacurate?

Solution Partner Phenom Solution Partner Phenom
Solution Partner Phenom

I thought it would be fun to try Goal Seek on this.

If you use a start of 252.0 or an end of 427.5 or an axis length of 105.62 you will get your 100deg sweep.

Maybe Goal Seek can help you find the error.

LMGi
www.TLMGi.com

Betreff: Variable pitch helical curve inacurate?

Solution Partner Phenom Solution Partner Phenom
Solution Partner Phenom

@bshandI bet you have Import curves and points turned off in your step import settings.

LMGi
www.TLMGi.com

Re: Variable pitch helical curve inacurate?

Thanks for all the help guys, this one is really starting to cook my noodle.

With a given start and end pitch, and a constant rate of change of pitch in between, to calculate the helix length the rate of change of pitch must be integrated. I was not sure how these programs evaluate such an integral. I was suspecting it was done numerically and hence giving a small error.

I have replicated the 94.67° result with NX10 and Solidworks 2015 which is really strange.

Plotting the helix in polar coordinates with Mathcad and Excel gives me the correct 100°.

I think a bit of head scratching is in order. I will post back here if I get anywhere.

Betreff: Variable pitch helical curve inacurate?

@KennyG I did try adjusting this, I suspected this geometry may be evaluated numerically as an integral of a rate of change of pitch must be evaluated. I tried changing this accuracy but the result was the same.

Thanks for the pointer, I was unaware of helix accuracy!

Betreff: Variable pitch helical curve inacurate?

Gears Esteemed Contributor Gears Esteemed Contributor
Gears Esteemed Contributor

@JasonTitcomb wrote:

@bshandI bet you have Import curves and points turned off in your step import settings.


You would win that bet. thanks

Bruce Shand
SE2019 MP8 - Insight - Win10 - K4200

Variable pitch helical curve inacurate?

Hi all,

 

So I have plotted a law curve in NX10 with a start and end lead angle equal to the start and end pitch of 292.5 and 468 respectively.

 

Then I performed a wrap curve to turn this into a helix. Sure enough the number of turns equates to 100°.

 

Then in the same part file I make a variable pitch helix with the start and end pitches equal to 292.5 and 468 respectively.

 

I then observe the number of turns and this equates to 94.675°.

 

To my mind this means one can make a helix with a start and end pitch of 292.5 and 468 whilst simultaneously having an axis length of 100 and a number of turns equal to 100°.

 

I then return to my original query, why does the variable pitch helix not produce the same number of turns?

 

I have attached the part file, if anyone has any idea of what is going on...

Re: Variable pitch helical curve inacurate?

Also, attaching solid edge part if people don't have NX10