Variable pitch screw

[ynnek]

Looking for some info on how to program such a monster. It has become a tough task trying to create a 3d model in TurboCAD v15 Professional Mechanical to generate the code from. But I think I may be over complicating it. I am begining to think it would be easier to code by hand, I just need a starting point.
I know this was a very vague post, I am merely trying to understand what needs to be done. The screw I will cut will made from UHMW or Delrin with a stainless steel thru shaft, the machine I will cut this on will be a CNC router (to be built or bought in the near future) with a 4th axis. I may be building one if I do not get one purchased soon, as of now I have no machine specs, just looking for theory.

here is a link to the turbocad user forum thread on this subject
Can anyone help?

[HuFlungDung]

Someone else was asking about this not too long ago. 4th axis wrap will do the trick and you are correct, it is quite simple to hand code it once you understand what is going on. A full scale model does very little good when it comes time to machine it other than unwinding the helix on each side of the groove.

Have a read here to get started:

http://www.cnczone.com/forums/showthread.php?t=60323

[Geof]

Are determined to do it with a router?

G32 on a lathe allows variable pitch threading.

[ynnek]

Originally Posted by Geof Are determined to do it with a router? G32 on a lathe allows variable pitch threading.

Yeah, cuz I will be getting a 4x8 router to build the setup parts for our filling lines. I just figured with a machine that large I could install a 4th axis & leave it in place just for making the variable pitch infeed screws. With that being said, can I not control the 4th axis like a lathe? Or is it not that simple?

Newbie to CNC, but been doing machine work for 26 years.

[jamesweed]

are you talking about a feedscrew, like a augar? or a timing screw?

[ynnek]

Originally Posted by jamesweed are you talking about a feedscrew, like a augar? or a timing screw?

Yes, it is an infeed screw to a rotary filler. It has to be timed with a starwheel. The profile of the thread varies with the shape of the bottles being fed into the machine. We run 3 basic shapes of bottles, they are round, oval, & rectangular.

[Geof]

Originally Posted by ynnek ....install a 4th axis & leave it in place just for making the variable pitch infeed screws. With that being said, can I not control the 4th axis like a lathe? Or is it not that simple? Newbie to CNC, but been doing machine work for 26 years.

Controlling a 4th axis like a lathe is not possible.

Hu's approach will work, some machines actually have this ability in the control to map the Y axis onto the A axis.

You could even program it by hand the hard way, which could be worthwhile if you are doing a lot and quick machining is important. But it would be very tedious to fake a continuously variable lead by using a lot of 90 degree rotations each at a constant lead; I think it could be done using several incremental subroutines.

[jamesweed]

I work on a rotary mill producing feedscrews for the plastics and food industeries. My B axis can rotate 99000 degrees before it has to be unwound. ive made a couple of "fake" variable lead screws but what I did was change pitch every .500 inch. I dont understand the y axis fakeout...what we do is this...360/2.0=180multiplier. so if Xaxis moves 5.0 then A axis will rotate 900 degrees to maintain 2.0 pitch(5*180)and so on. im using mostly carbide endmills to mill with. i have 4 axis but my Y axis mostly stays on centerline of part. Am I on the same page as you with this line of thought? If I can be of more help, let me know.

[jamesweed]

This is the macro I used on these "variable" pitch screws. This macro did all the math to find all the A axis moves. Lots of math to find degree moves on a screw. I use programmed Casio caculators to help with the tons of math to find all the degree moves on feed screws. Not hard math, just lots of it. Not that I expect that you can use this macro on a cnc router, but just to show it can be done. Our 3 custom made 3-4 axis rotary mills are 100% programmed and operated with macro type programs.

%
(PROGRAM SETS FLIGHT WIDTH)
(ON BACKSIDE IF #33=1)
(**MILL BACKSIDE?** 1=YES 0=NO )
#33=
(**ENDMILL DIA.?**)
#32=
(ACTUAL O.D.?)
#11=
(FLIGHT WIDTH?)
#12=
(SMALL LEAD MULTI ?)
#13=
(LARGE LEAD MULTI ?)
#14=
(LEAD CHANGE START?)
#16=
(LEAD CHANGE ENDPOINT?)
#17=
(HOW MANY LEAD CHANGES?)
#15=
(FLIGHT LENGTH?)
#2=
(PROGRAM DEPTH?)
#20=
(FEED RATE?)
#10=
(SPINDLE SPEED?)
#18=
(**********EXIT EDITOR**********)
M98P4
G90
#9=1
#500=#13
#510=[#17-#16]
#510=[#510/#15](X(?)PER LEAD CHANGE)
#511=[#13-#14]
#15=#15+1
#511=[#511/#15](MULTI CHANGE)
#15=#15-1
#500=[#500+#511]
#1=#16 (CURRENT X POSITION)
N1 (MATH TO SET FLIGHT WIDTH)
#500=#500-#511(SMALL LEAD)
#505=#500-#511(LARGER LEAD)
IF[#33EQ0]GOTO2(FRONT OR BACK?)
#29=[#12+#32]
#26=[#11*3.141593]
#27=[360/#500]
#27=[#27/#26]
#28=ATAN[#27]/[#26]
#27=#29/COS[#28]
#512=#27
#27=[#27*#500](A ANSWER)
#8=[-#27]
#30=[360/#505]
#30=[#30/#26]
#31=ATAN[#30]/[#26]
#30=#29/COS[#31]
#513=#30(LARGE X ANSWER)
#514=#513-#512(DIFFERENCE)
#512=0
IF[#514LE0]GOTO2
#27=[#514*#505](BARRIER MATH)
#28=[#514-#28]
#28=[#27/#500]
#28=[#514/#28]
#27=[#514*#28]
#3=#27 (FINAL ANSWER TO - )
N2
IF[#9EQ1]GOTO4
#1=[#1+#510-#3](ACTUAL X POSITION)
#25=[#510-#3]
GOTO5
N4 (SETS METER ZONE)
#1=[#1+#513-#3]
#25=#1
#513=0
N5
#25=[#25*#500]
#6=[#6+#25] (ACTUAL A POSITION)
#3=0
#514=0
IF[#9EQ0]GOTO9
M3S#18
M00(**READY TO START**)
G0A[#8](*SETS ACTUAL FLIGHT WIDTH*)
G70A0
GX-#1A[#6](**RAPID TO START**)
G1F1Z-#20
#9=0
#8=0
N9
G1F#10X-#1A[#6](*PROGRAMMED MOVE*)
IF[#1GE#17]GOTO3
#504=[#504+1](RESTART COUNTER(?))
GOTO1
N3
#7=[#2-#1]
#25=[#7*#14]
#6=[#6+#25]
G1F#10X-#2A[#6](*PROGRAMMED MOVE*)
G0Z0
X0A0

#500=0
#510=0
#511=0
#505=0
#512=0
#513=0
#514=0
#504=0
M2
%

[ynnek]

Originally Posted by jamesweed I work on a rotary mill producing feedscrews for the plastics and food industeries. My B axis can rotate 99000 degrees before it has to be unwound. ive made a couple of "fake" variable lead screws but what I did was change pitch every .500 inch. I dont understand the y axis fakeout...what we do is this...360/2.0=180multiplier. so if Xaxis moves 5.0 then A axis will rotate 900 degrees to maintain 2.0 pitch(5*180)and so on. im using mostly carbide endmills to mill with. i have 4 axis but my Y axis mostly stays on centerline of part. Am I on the same page as you with this line of thought? If I can be of more help, let me know.

This all looks very promising. I will have to learn more on CNC programming before I completely understand this. My brother has some experience on HAAS & Citizen 7 axis machining centers, I will be working with him on this. I will see what he thinks Monday. I am sure this will be a very big help.

Thanks a bunch. This is starting to remind me of when I was learning Visual Basic. I went from forum to forum asking questions.

[Kiwi]

ynnek
I'm working on a VB program to generate variable helix code. If you post the specs of variable pitch screw, I'll try and produce some code for you.
eg. Start Pitch, Mid Pitch, End Pitch and Spacings.

[ynnek]

Originally Posted by Kiwi ynnek I'm working on a VB program to generate variable helix code. If you post the specs of variable pitch screw, I'll try and produce some code for you. eg. Start Pitch, Mid Pitch, End Pitch and Spacings.

I will get you some specs. As I said earlier in the post the profile of the thread will be either round, rectangular, or oval. Is your VB program going to be capable of generating the code for 4th axis milling?

BTW, thanks for the reply. I have downloaded a trial of MeshCAM & it looks promising as it has 4th axis capability. I will just have to be able to build a 3D solid model to work from.