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Drive Stepper (4th axis) off of PWM Spindle Control Ouput

Sorry 'bout this one, I am just sorta "thinking aloud" here, feel free to brainstorm with me, there is no real specific question here.

I want to use the 4th axis (horizontal rotary axis) as a lathe on my CNC. The X,Y,Z would operate normally, the only difference would be that they would be grinding away at a chunk of something spinning, as aposed to sheet-goods clamped down to the table.

This requires me to spin the 4th axis as a "motor" --with no regard to an "indexed" zero-point nor the total number of steps completed. I need set it and forget it, and just keep it spinning. Mach3 has no provision for this.

I occured to me today that the PWM frequency used for the spindle speed control might be what I need to keep my stepper stepping. --I mean, a pulse is a pulse, right? It would just be a matter of putting a scope on the stepper, jogging it as fast as it will go, and taking a look at the pulse rate coming out of Mach. From there, I could do the math required to adjust the PWM frequency and duty cycle to match. -Both of these are very adjustable via Mach3.

Now, for the life of me, I can not figure out why this won't work, but before I start un-wiring things and scoping etc., does anyone have any thoughts?

 

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AC servo motor/driver I believe. Little pricey but I think mach3 handles them.  Need a mechanical brake for indexing/milling I think because they dont hold like a stepper. You can get them in NEMA frames and theyll spin fast plus index.

With 4 positional axis you can make a lot more than what's possible on a lathe. Do you really need to spin the stock so fast when you are spinning the tool at 20000rpm?

I'm a bit confused by your description. On one hand you seem to be talking about using a stepper motor as a standard motor. On the other hand you seem to be talking about just using PWM to drive it.

As far as I know your 4th axis is a stepper motor so you must drive it with a 2phase pulse pattern that cannot be replaced with PWM.

For the purpose of your lathe where the 4th axis is just rotating continuously why don't you use a stand alone driver circuit with a pot to control the speed of rotation?

Just unplug your stepper motor from one controller and plug it into the other.

The only issue I can think of would be the physical limitations of your lathe motor. I know from some motors do not like PWM, causing them to excessively heat up. If the PMW idea doesn't work then your might use a servo or stepper motor to automate a router speed control. Basicaly attach the stepper to your the dial and  let it turn it for you.

 

Just an idea though.

I am having search term overflow. I can't seem to find the right combination of search terms to weed out all crap and links to ebay listings. I figured I would have a better shot here in that folks here speak pulse-width fluently. Really, I am just trying to avoid a bunch of un-wiring / wiring (even just to test) because I am missing something obvious.

see if this helps your search http://tinyurl.com/kdgqb64

After doing a bit of searching myself, I ran across http://groups.yahoo.com/group/CAD_CAM_EDM_DRO/message/85262 . The suggestion there is to tell the axis to move a given number of revolutions at a given speed in G code.

Did no one at CNCZone ask a similar question, or, offer any ideas as to a work around?