[largas]

I’m a first time CNC milling router builder. My CNC is a small 2’ x 2’ tabletop machine. I’m using Mach3, Lazy CAM (free versions) with a 3 axis, TB6560 controller and all stepper motors are the same.
My problem is the z-axis.
I’ve successfully motor tuned the z-axis to move positive one inch but when I instruct it to return, (negative one inch) it moves one and a quarter inch.
The z-axis has however, a different screw and gear ratio than the other two axis but I thought the motor tuning had the final say.

x, y axis = 10 rotations per inch
Z axis = 16 rotations per inch
TB6560 switch settings = all the same
Stepper motors = all the same

Thx, Larry

[awerby]

This sounds like mechanical slippage. Are your screws attached to the motors with setscrews? Do the setscrews have flats underneath? If not, they can twist, and lose position, incidentally making them very hard to remove. Check all your mechanical connections in the Z axis. If it's doing this repeatedly, try making marks across the shafts, screws and couplers so you can tell if they move relative to each other.

Andrew Werby
ComputerSculpture.com — Home Page for Discount Hardware & Software

[mactec54]

argas

It sounds like you have not done the steps per or Axes calibration
Go to the setting tab, then just above the Reset button you will see Axes calibration

In the Z axes Negative is down Positive is up

[largas]

thx guys,
I checked all mechanical and found no slippage.
By motor tune , I mean the steps per unit calibration. weird thing is I can do the calibration in either direction for exactly an inch and the opposite is long by a quarter inch,,,or I can calibrate it so both side are evenly short of an inch, offset, but I can't increase the gain so both are an inch.

[mactec54]

largas

Are you doing the calibration similar to what this Hoss video is doing, this is really the only way to get it spot on, There are other videos on doing this as well

Improved Mach 3 Axis Calibration - YouTube

[largas]

yes. That's the method I used to calibrate all the axis but z-axis is the only one out of the three axis that is having a problem.
That method will only successful calibrate in one direction, leaving the other direction long by a quarter inch.

[adprinter]

yes. That's the method I used to calibrate all the axis but z-axis is the only one out of the three axis that is having a problem.
That method will only successful calibrate in one direction, leaving the other direction long by a quarter inch.

Are you using a ballscrew on your Z axis? If so, it sounds as though you may have a few balls missing from the ballnut. If it is just a lead screw, then it may simply have that much back lash happening. Which direction of travel is giving you the extra quarter inch, Up or Down? If it is happening in the down direction of travel, then gravity (and back lash of your lead screw/nut assembly) is your problem. Try setting the feed rate to a very slow setting, then navigate to the MDI screen in Mach3 and enter Z.25. Does it move UP a quarter inch? Or does it move UP at all? Gravity should already have the back lash taken up before the move.

Now enter Z-.25 does it move DOWN a quarter inch? Or half an inch? Mounting a dial indicator to measure the distance actually moved, to compare to the commanded move is essential in diagnosing any mechanical problems. Also, the 16 versus 10 turns per inch will be different on the Z, changing the ratio used as compared to the X and Y axis. (I.E. Steps per calibration will be different on the Z, than your settings on the X and Y axis).

If you are making these calibration adjustments from the Motor Tuning screen in Mach3, don't forget to click the Save Axis Settings before navigating to a different axis. Otherwise, any changes made on the axis will be lost! (I.E. it will remain unchanged from it's previous setting). Hope this helps you to sort it all out.

[hemsworthlad]

Don't think it will be missing balls because to give 1/4" of error then you would easily feel that in the nut when building or even just turning the screw by hand as it would be very sloppy and grumbly.! . . . Same goes for lead screw.! . . . 1/4" movement in any nut does not equal backlash . . . It = FUBAR.!

If the error is repeatable then I can see why it could thought to be backlash but 1/4" is a massive amount (4 full turns on a 16tpi screw) and should easily be felt by turning the screw by hand.!

To me this sounds very much like mechanical stickage or under powered steppers, possibly both.?

The fact your using the TB based controller concerns me has I've seen nothing but trouble with them thru my experiences with the cheap chinese 3020 type machines.!!
One of the issue's I've had is the wire connections causing strange happenings even thou every thing appeared to be correct and in order.? Things like Z axis changing direction mid move or jog in one direction then try jogging in other and it still goes in same direction.
All these issue's where semi lose connection related either at the controller or in the stepper wiring and seems TB controller is very intolerent of even mildly lose or poor connections.!!
Not saying this is your problem and pretty sure it's not but certainly wouldn't hurt to carefully check the connections or wires.!

In which direction does the error occur IE: upwards or downwards direction.? Like Mactec says usually positive = upwards. It's most common for Z axis to only or mainly work in the negative with Zero being at the top and all downwards movement negative.

Is the error accurately repeatable every time or does the amount change being some times more or less 1/4".?

Can you give more details on the machine, both mechanical and electrical.
IE: Motor size and how wired Bi-parallel or serial etc
Voltage supplied to motors.
Stepping amount.
screw type
Linear bearing type
Construction material (IE: wood.metal etc) Basicly want to get an idea of how heavy or sticky this machine could be.?

[largas]

To test for backlash, I turned the machine off, manually turned the screw and noticed no backlash. I held the router and moved it up and down and it was tight with no signs of any slop.
I don’t have a dial indicator but I do have a needle riding on an engineering ruler. I actually took that Z-.25 you suggested down to .05. I ran that in both directions and the results were a mini move of the problem. (.05 in one direction and .06 in the other) Also, If I calibrate the +Z axis first, the -Z axis is off or if I calibrate -Z axis first, the +z axis is off.
I know I can fix the problem with changing out the screw and nut to match the other two axis but I hate cutting a 24” screw down for a 8” job and I won’t sleep at night until I find the problem.

[OCNC]

I ran that in both directions and the results were a mini move of the problem. (.05 in one direction and .06 in the other) Also, If I calibrate the +Z axis first, the -Z axis is off or if I calibrate -Z axis first, the +z axis is off.

You keep using the concept of "one direction and then the other". It would be better if you used the concept of the 'loaded direction' when you are moving the router against gravity and the 'unloaded direction' when you're moving the router with gravity. Alternatively explain how you're defining plus (+) and minus (-) Z. At this point I still can't decide if the longer dimension is in the loaded or unloaded direction. Please clarify. I agree with hemsworthlad that this can't be backlash.

[louieatienza]

I’m a first time CNC milling router builder. My CNC is a small 2’ x 2’ tabletop machine. I’m using Mach3, Lazy CAM (free versions) with a 3 axis, TB6560 controller and all stepper motors are the same.
My problem is the z-axis.
I’ve successfully motor tuned the z-axis to move positive one inch but when I instruct it to return, (negative one inch) it moves one and a quarter inch.
The z-axis has however, a different screw and gear ratio than the other two axis but I thought the motor tuning had the final say.

x, y axis = 10 rotations per inch
Z axis = 16 rotations per inch
TB6560 switch settings = all the same
Stepper motors = all the same

Thx, Larry

Larry, I'm assumin that when you say move the Z axis 1 inch in the positive you mean UP and negative you mean DOWN? As in G0Z1.0 to go up, and G0Z0.0 to go back down?

If you're going UP then the weight of the carriage will take up any slack, going DOWN can release that. I would tend to think it's your motor coupling slipping; or some connection is loose. If it was missing steps you'd be returning a shorter distance than you went up (provided you're moving UP exactly 1") since the mechanical advantage of the 16tpi screw won't cause the carriage to backdrive.

I would start with the carriage in the UP position, then do G0Z-1.0, measure that, then do G0Z0 and see if it comes back up 1/4" short. You can try lowering your rapids on the Z since the high speed needed to evevn movve the carriage slowly would result in a big drop in torque.

[largas]

Sorry, +Z is up, -Z is down.
Also I'm using, Set Steps per Unit under the Settings tab to calibrate.

[OCNC]

Sorry, +Z is up, -Z is down.
Also I'm using, Set Steps per Unit under the Settings tab to calibrate.

I would want to be certain that it isn't a bad drive so I would be inclined to switch the Z motor to the X or Y drive and see if the problem persists.

[OCNC]

Sorry, +Z is up, -Z is down.
Also I'm using, Set Steps per Unit under the Settings tab to calibrate.

As far as the tuning in Mach goes I'm not aware of any way that you could get a particular axis to have by design a different pulse train count in the + versus the - direction. Steps per inch setting would only determine the absolute accuracy of the movement but whatever the movement was it should be the same in both directions.

[bbasiaga]

Was there a resolution to this? I am having the exact same problem with my LMS mill, have done much of the same troubleshooting, and still haven't got this figured out yet.

-Brian