Jerky CNC Motion

[Cartierusm]

OK, I've used my cnc for a while now, about 1 year, but now I'm finally cutting my guitar stuff and I want to make sure everything is working the way it's supposed to.

When moving during a run the sound of the motors are jerky sounding and when looking at the ballscrews during the run they move in a somewhat jerky motion. Now the cuts seem pretty smooth and since I've never owned or used a CNC machine before I'm not sure if this is correct movement. So if you all can look at the video and tell me that's normal or not I would appreciate it. The sound is going to be the key as it's hard to see the motion of the machine, it's just cutting air I've turned off the router. The video is 15mb. I'll include my G-code for this particular run that you are watching to see if it's that. Its not jerky during straight runs.

Video: http://www.smokeproductions.com/cnc.avi

A little background on the machine, home built. Keling 425oz steppers, Keling 8078 Drivers, 48V 6.5A PS, Roton Ball Screws, Mach 3 V2.63, 10,000 steps per inch, 72IPM maximum and 10 acceleration for motor tuning, CV is turned on. I just thought during a run it would run smooth during curves in a fluid motion?

[martyb3000]

In the Mach3 "General Config" screen. Make sure your Motion Mode is set for "Constant Velocity" and not "Exact Stop".

This one seems common for what you're seeing. If its not, then hopefully someone else can help a little more.

[springlakecnc]

Hi, i am not a stepper motor guru, but i have retrofit about 75 milling machines with steppers in the past. BTW, nice vidio. It sounds to me like the steppers are doing exactly what the control software is telling them to do. I would believe the problem is in the software settings. Do you ever lose posistion? it sounds like there is NO ramping time set for the motors to ramp up and down to the next speed they need to go. If you ever lose posistion, i would say thats exactly what the problem is. Or the "constant velocity" like what Marty said. Anyway, im sure its in the software settings. Typically steppers need way more ramping time than servo motors with encoders do, and your system is just instantly to the new motor rpm with no ramping. Good Luck

[Khalid]

If you are using CV mode on and still you are getting jerks then
1- check for any misalignment in lead screw...
2- Check the ball nut for any debris/dust..your balls may be sticking inside
3-Check for the vector from which you made G-code, it should be not littered with point data
4- Check for any loose/misalign coupling

[warrenb]

G64 will set to constant velocity.

[Cartierusm]

Thanks for the responses. I know it's not with anything sticking, everything works smoothly. I'm sure it's some kind of setting. As I said in my first post CV is turned ON, but there are a few settings for CV.

springlakecnc, I'm not sure I know what you mean by ramping, as far as I know ramping is a protocal used to help a bit enter a piece of work at an angle to aviod plunging the bit straight in, so please elaborate, thanks.

Khalid, I'm by no means even a novice at G-Code, if you know what Point data is I've posted my G-code in the first post.

[ger21]

Turn off all the CV options on the settings screen and in the general config window. Some are turned on by default and can cause problems.

[Khalid]

Originally Posted by Cartierusm Khalid, I'm by no means even a novice at G-Code, if you know what Point data is I've posted my G-code in the first post.

Sorry Cartierusm
I didnt looked the G-code...I can't help anymore except what solutions came in my mind, i mentioned above..
Again sorry if something hurt u

[Cartierusm]

Khalid, I'm not sure what you mean in your last sentence, did something I say come across rude?

[Khalid]

I felt i hurt you so i apologize
thanks

[Cartierusm]

No, I wan't hurt....I'm a bit confused...anyway no hurt feelings here, hell your the one who's helping me and I appreciate it.

P.S. Gerry, once I turn of the CV stuff do I enable G64 (where ever that goes) in my G-Code?

[wyldesyde007]

Originally Posted by Cartierusm Thanks for the responses. I know it's not with anything sticking, everything works smoothly. I'm sure it's some kind of setting. As I said in my first post CV is turned ON, but there are a few settings for CV. springlakecnc, I'm not sure I know what you mean by ramping, as far as I know ramping is a protocal used to help a bit enter a piece of work at an angle to aviod plunging the bit straight in, so please elaborate, thanks. Khalid, I'm by no means even a novice at G-Code, if you know what Point data is I've posted my G-code in the first post.

What springlakecnc is talking about is on the motor tuning page in mach3 it's the acceleration box, changing this value will cause the servos to either gradually build up speed or accelerate instantly depending on which way you set it. By gradually building speed it helps prevent losing steps by giving the servo a chance to receive the siginals rather than saturate the phases with full power possibly causing it to stutter which now mentioning that I do recall mentioned in the setup manual that if your servos seemed to stutter or miss steps that you might have to play around with the pulse widths (found on the motor tuning page also) hope any of this info helps.