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View Full Version : Question for the CNC lathe spindle speed control experts.



DICKEYBIRD
02-24-2011, 05:44 PM
I'm thinking about (down the road a bit) adding a Homann DigiSpeed-XL to the treadmill motor/KBIC speed control on my little cobbled up lathe project.

Initially I'd be happy enough just coding the motor speed and on/off functions via TurboCNC. Do you guys think that it would control the speed accurately enough to do threading via Mach 3? Just curious.

edit: Don'cha just hate it when you mis-spell the thread title and can't fix it?:(

edit 2: Don'cha just love it when a bloody Engishman comes along and edumacates yer...even after a couple o'pints down t'pub!:D

John Stevenson
02-24-2011, 05:57 PM
If you go to edit post and choose advanced you can edit the title

Black_Moons
02-24-2011, 06:50 PM
Oddly enough, While you can edit the 'post' title, You can't edit the thread subject.. thats initialy set to the post title.

tyrone shewlaces
02-24-2011, 07:04 PM
It will control the basic RPM of a motor by replacing the potentiometer with a PWM signal from Mach3 or the like. That alone is pretty cool to have and would be worth doing probably. But it won't be accurate enough to thread on it's own. To thread you'll need some kind of encoder feedback on the spindle.

I'm not sure how that works with Mach 3. Surely it would somehow, but I don't know enough about Mach to advise about how they deal with feedback loops and the like. It might also require a whole different drive with a servo motor or something. Of course a servo motor is essentially a DC motor with an encoder, so the drive is the trick. Wish I had something to point you toward because I will someday soon be looking into it as well. So far everything I've found has been pretty pricey.

macona
02-24-2011, 07:12 PM
Yeah, the control is accurate enough for mach to do threading. But to do threading you need to add a spindle position sensor that tells mach how fast the spindle is turning and gives it the timing info needed to start threads at the same spot every time.

Also using the feedback allows Mach to close the loop on the spindle. So when the spindle slows mach commands a higher setting to compensate. On my mill and lathe it keeps the speed pretty steady. Also it allows the use of the speed setting in g-code with a whole lot more accuracy.

Mach does a spindle profile where it monitors the spindle speed as Mach slowly increases the spindle speed command and creates an internal profile so that when you select a speed it knows what signal to give it and it reaches that speed much faster.

The index position can be very simple. Just an opto-interrupter and a disc with a slot in it. Or a hall effect sensor and a magnet.

One issue I have had on the few machines I have added spindle feedback to is the length of the spindle pulse. I ended up adding a small one-shot timer based on a 555 to stretch the pulse to a length where mach will pick it up.

DICKEYBIRD
02-24-2011, 07:37 PM
Thanks Jerry! That gives me the incentive to start putting some hobby money aside to pick up a used 'puter and Mach 3 down the road to go along with the DigiSpeed.

I keep reading good things about EMC/Linux which would fit my budget better but I have enough trouble already with Windows.:rolleyes: