worm gear cad design

[sparkyengle]

    Does anyone know of a cad program that can be used to design a wormgear that has a gradual increase in it's pitch the longer it is. The gear I am looking to design will have a 2" diameter, and be approximately 2' long. It will have a starting pitch of about 1/4" on one end and gradually increase to about 3/4" on the other end. Any design help would be greatly appreciated. Thanks...Andy.

[Dan13]

Should not be a problem if you can write a mathematical expression for the helix you're describing

Daniel 

[Tweakie.CNC]

Can't help being curious but what would you use a variable pitch screw for please ?.

Tweakie.

[RICH]

You could create one using 3d of a cad program ( haven't tried ) ie. you do a helix with a decreasing end diameter.
Now to get the gcode to cut it is a different story.....Mach threading is based on a constant pitch,  set by  feedrate over a distance...
Noot sure a if a math expression applied to something would work...
I don't know ....just some thoughts!

RICH

[Ya-Nvr-No]

how about a logarithm  
http://www.sosmath.com/algebra/logs/log4/log4.html

[stirling]

Here's a quick play around. X is axial and A is rotational Z gives the radius. The idea is that X moves in steps of a thou along the 2 in and the loop calculates the new rotation of A to vary the pitch. A starts at the .25 in pitch and finishes at the .75 in pitch.

pseudo code:

a = 1.44 (degrees)
x= 0.001 (inches)
G91
for int i = 0 to 2000
  G1 Xx Aa
  a = a - 0.00048
next

g-code:

#1000=1.44
G0 X0 Z1 A0
G91
M98 P1 L500
G90
M30
o1
M98 P2 L4
M99
o2
G1 X0.001 A[#1000]
#1000=[#1000-0.00048]
M99

Just kind of a starter describing the general idea. (I use two loops to give 2000 iterations because the limit in Mach of a loop seems to be around 998 or so).

Ian

EDIT: typo: I'd set Z to 10. Of course it should be 1 (as it now is)

[sparkyengle]

Thanks Sterling. I'm soon finding out this is going to be way over my head.(logarithms and such). Maybe after I study more on writing code I will try what you are talking about in your example. Thank you anyway. Do you have any good suggestions on where to start for a noob wanting to learn g-code basics?

[Graham Waterworth]

this might be a good place to start :-

http://www.amazon.co.uk/Programming-CNC-Machines-Student-Workbook/dp/0831133171/ref=pd_cp_b_2

Or here

http://www.amazon.co.uk/CNC-Programming-Handbook-Peter-Smid/dp/0831133473/ref=pd_sim_b_2

Graham