Precision thread cutting

[llrjt]

I'm interested in running some experiments to see how precisely I can cut threads for a worm gear.

As I understand it, cutting a perfect thread helix would require perfect synchronisation between z-axis movement and spindle speed. Leadscrew, belt transmission and non-constant spindle speed all contribute errors to the cut helix, as does temperature, vibration, machine alignment etc.

It appears that EMC2 can sync threading with BOTH spindle encoder pulses and an index mark (there are several YouTube videos showing the lathe chuck being manually rotated and the tool moves in sync), resulting in more updates to the z-axis speed, and consequently finer sync during threading.

Mach3 accepts an index pulse, but can it use spindle encoder pulses like EMC2 to further improve sync with the z-axis for threading?

Richard

[samco]

The only way I know of mach being able to use more than just an index pulse is to use one of the external motion cards.. (and you would have to do your research) Dynomotion | Motion Control Boards comes to mind - but like I say - you would have to research it as I am a EMC person.

Emc would allow you to do it with just printer port hardware. (to a point - you are limited to the speed that the printer port can count - probably maxes out around 40khz or more depending on the computer). Emc has pretty interface hardware also that moves the encoder counting external fromt the computer. mesa makes a 48i/o board for around $90. (would also do closed loop servo with an analog daughter board)

You can do stuff like this also...

Hobbing (Gear cutting) on a Mini-Mill with EMC2 - YouTube

Hobbing (Gear cutting) on a Mini-Mill with EMC2 - YouTube

sam

[llrjt]

Thanks for the link Samco - I think EMC might be the ultimate answer, but involves me setting up a Linux box, which I've not done before. I like the idea of handling the encoder inputs outside the PC on the motion control board.

The Dynomotion board has an interesting section on rigid tapping - the sync requirement is similar: http://www.dynomotion.com/Help/Mach3...gidTapping.htm

My experimental rig is a Wabeco CC-D6000E lathe with steppers, Wabeco or Xylotex control and no spindle speed control. I'd like to upgrade this bit by bit, and see what difference it makes to the thread I can cut (I've got some accurate Renishaw measuring kit) - I guess the upgrades are (in no particular order):

1. Ground ballscrews
2. Spindle encoder with EMC
3. Closed loop spindle speed control with spindle servo motor

Your video application is very interesting - the boundaries blur between spindle and indexable 4th axis. The key question is, which aspects of control are most important for most accurate helix when single point threading? In theory, sensing small errors more often and correcting for them quickly is what I need, so high resolution encoders, and high frequency servo loops are the way to go - am I missing anything though?