Re: [CAD_CAM_EDM_DRO] CNC division

At 01:00 06/04/2001 +0000, you wrote:

>I'm in a similar point in my "4th axis thinking". Yes, even steps per
>degree sounded like a nice idea. As I've just said in a previous post,
>Sherline's 72:1 worm and 1/2 stepped 200 step stepper gives 80 steps to
>the degree. But unless you're doing it "by hand", I don't think it
>matters!

Depends on what you're using to control the 4th axis. For example, in the
MaxNC software, setting up the 4th axis involves configuring the number of
steps per degree. So, if your drive system doesn't result in an integral
number of steps per degree, any rotary moves you make will generate a
positional error. For example, if you had a 40:1 worm drive & a directly
connected stepper, this would give 16000 (half) steps per 360 degrees,
which is 44.4444r steps per degree. The best you could do with MaxNC would
be to set 44 steps per degree, so you would get .444 steps error per degree
of movement - 160 steps in a full rev, approx. 4 degrees out. Absolutely no
good if you were doing anything critical, like milling a gear wheel. So,
the only option with Max is to use a drive system that results in an
integral number of steps/degree. Bad software design in my opinion - if
they had chosen steps per 360 degrees, it would have accommodated any
dividing head/rotary table ratio that you had to hand. As it is, you can
only work accurately with MaxNC if your rotary table's drive ratio is a
multiple of 9, so the "conventional" rotary table ratios of 40:1 and 60:1
don't hack it without using additional gearing between the stepper & the
worm. This makes 72:1 and 90:1 dead convenient for users of the MaxNC
software.

Regards,
Tony