Motor tuning in mach 3 mill for mini CNC?

[PaulMakesThings]

I'm well into building my first CNC, it's a mini 3 axis mill. I've got the drive board set up and I have mach 3 mill set so the motors seem to be working, though I haven't quite figured out how to adjust the speed yet and I don't know if these settings are anywhere near optimal, so I'm looking for tips from those more experienced than I. Here's a bunch of info on my setup.

The motors:
Datasheet. Pulled from printers, 7.5 deg/step 7.5 ohm, 2-phase bipolar, 1A peak current, 24V.

The driver:
One of these ones that are all over ebay, based on the TB6560AHQ driver chip with a peak output current of 3.5A per motor. I have it connected to a 24V 10A power supply. This board has dip switch settings and currently I have it set to:
current: full (options are full, 3/4,1/2,1/4)
decay mode: fast (options are fast, 25%,50%, slow)
microstep: full (options are full, 1/2,1/8,1/16)

The software:
I'm using mach3 software connected through a PCI parallel card. The settings are what I'm posting to ask about. I have so far configured the output pins for my board, which got the motors moving a little but sometimes they would just shake. Then I went into motor tuning and set the steps per inch to 960 because I'm using 20 tpi leadscrews, currently the velocity is 120 in/min the acceleration is 2 in/sec/sec and the step pulse and dir pulse are both 1 us. Those are some of the settings I'm not sure of, I don't know what to base them on. I have the native units set to inches because my lead screws are in inches. Under ports and pins>port setup and axis selection, my kernel speed is set to 25000 Hz, because that is what the manual for my drive board said to use. Other than the above I haven't really touched the settings. When I run a test set of code the motors whirr quickly to the start point but then move very slowly, like a step per second, along the path, which if hooked to a lead screw at 20 tpi would mean it was moving pretty dang slow. I realize I might not have provided some relevant info or checked things I should have, but that's part of why I'm here asking questions. The motors seem to get pretty hot after 2 minutes or so also. Thanks for reading this, I will update as I figure out more.

[PaulMakesThings]

I've noticed that the motor sometimes spins in the wrong direction, also sometimes the torque is very low for a step, if I hold the motor it will push so lightly that any resistance at all during these steps will keep it from moving. I'm also concerned about how hot the motor is getting. The board has setting to use less current, but I'm not sure if that is the right option to help with the heat or not. I have also been trying different decay mode settings, I'm not sure what is best for those, they all seem to work.

[PentueR]

Hello Paul,
I use same driver board (but different motors) and first of all, I have recommend to you set the current to 25%! You have 1A motors and current settings should be similar to motors maximum. For second, your velocity is too high I think, I would recommend to start with velocity about 20-30 in/min and then try higher values if everything going well. Also more fine steps are recommended than full microsteps, you will get a minimum vibrations and better accuracy.

Good luck.

Martin

[PaulMakesThings]

Hello Paul,
I use same driver board (but different motors) and first of all, I have recommend to you set the current to 25%! You have 1A motors and current settings should be similar to motors maximum. For second, your velocity is too high I think, I would recommend to start with velocity about 20-30 in/min and then try higher values if everything going well. Also more fine steps are recommended than full microsteps, you will get a minimum vibrations and better accuracy.

Good luck.

Martin

Thank you, the lower current definitely seems to be helping the heat, I have it set to 25% now and the motor is only a little warm where before it would be burning hot. The lower speed seems to help some, I set it to 20, and 1/2 step. It still has the problem where sometimes it starts with almost no force and doesn't always turn the right way though. Maybe a finer step would help pull it in the right direction? Thanks again for responding.

Update:
I've tried 1/16 and the direction responds better, but still not always the right way, even at speeds as low as 4 in/min, it also bumbles around for a few steps before taking off into a spin. Could it be something with pulse length or acceleration?

[PaulMakesThings]

I've switched to this motor, which is unipolar, but I've wired it to the controller in a high torque mode, with the center tap left not connected. And it seems to be working a lot smoother. Maybe this controller isn't capable of 2-2 phase drive? This one still gets hot but I'm getting much more steady and accurate rotation.

[doorknob]

I know that you said that your machine was small, but the motors that you're using are really small with little available torque. Your motors are probably rated at around 20 to 40 oz-in. holding torque, which is not very much at all.

If you decide to get some more powerful motors, you might want to look at some 200 step/rev motors instead of the 48 step/rev ones that you are currently using.

You might try increasing the step pulse width to 5us or more to see whether that makes a difference.

Also, if you're handy with a soldering iron, you might consider bypassing the optoisolators on the step and direction inputs (because the optoisolators on that board have a reputation for being "slow", and it is possible that your wrong direction problems are occurring because the direction signal is being delayed compared to the activation of the step signal - see the thread at http://www.cnczone.com/forums/genera...y_chinese.html for discussions of that and other issues).

[PaulMakesThings]

I really appreciate the advice. The board that he modified in that thread is very similar to mine. My background is electronics and fabrication, so I should be able to modify the board, and possibly make my own improved one. He seems to have had the exact same issues with it that I have.

I know the motors I'm using are very weak. I agree an upgrade is probably worth while, and it would cost a hundred or so to get motors for a machine this small (it's going to have a 9"x9" work area). Right now this cheap driver and motors are a learning experience, and the scope of my plans will grow with my knowledge. I'll look at the kind of motors you recommended, there don't seem to be many small ones with 1.8 degree step, but the ones I see aren't too expensive.

[doorknob]

One other thing to note - sometimes when you operate some stepper motors without any load (beyond the built-in shaft), the motors can tend to vibrate in place and not turn the way that they would work if a load were connected.