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Backlash on X axis changes to A axis
« on: October 23, 2017, 08:22:38 AM »
Hi there! This is my firs post, excuse me if i'm postint in the wrong section... :)

I have some backlash in my CNC, and I don't know exactly what happens to the machine and what I have to do. The machine is 2 months old, and has been working perfectly since a couple of weeks ago.

The X axis of my CNC has two stepper motors (X and A) and has ballscrews in all the axis. When I move the machine along the X direction, it turns back aprox 6mm on the A stepper motor and then it goes to the correct direction. This happens the 95% of times I try to move it with the software, and it happens at every point of the table (600 x 600mm). If I change the Stepper motor speed the backlash is alwais the same. Both stepper motors of the X axis have the same speed and parameters, so I don't know what can be wrong. I was carving a little piece of soft wood and when I was moving the machine to take of the piece of wood I discover that backlash.

What makes me crazy is that I've tried to change the plug connection between X and A Stepper motors in the electrical box, and after do that, the backlash changes from A to X axis. This make me think that maybe there is some software issue that makes that problem on my CNC instead of beeing some mechanical problem, but I don't know what to do. I've reinstalled Mach3, and reloaded my original parameters in the machine but the problem goes on. I've also sent the electrical box to the seller and they told me that they checked it and everything it's ok. Now, before sending the whole machine with the electrical box and the laptop to the seller I want to try if there is something that I can do to solve this. Can someone tell me something to do?

Thank you so much and excuse my poor english  :)

Marçal
Re: Backlash on X axis changes to A axis
« Reply #1 on: October 23, 2017, 12:13:47 PM »
Hi.
Quote
The X axis of my CNC has two stepper motors (X and A) and has ballscrews in all the axis. When I move the machine along the X direction, it turns back aprox 6mm on the A stepper motor and then it goes to the correct direction.

That does not sound like backlash. Backlash is where the nut slops backward and forwards because of the gap in the thread. The whole point of ballscrews is to
eliminate backlash. Can you post an up-close picture of one of the ballscrews?

In the lower right hand side of Mach3 is the name of the profile. In the Mach3 folder find the file <profile name>.xml. Create a copy <gibventure>.xml and post it
so we can have a look at how your machine is set up.

Craig
My wife left with my best friend...
     and I miss him!
Re: Backlash on X axis changes to A axis
« Reply #2 on: October 23, 2017, 12:26:26 PM »
Hi,
do you have backlash enabled?

Config/Backlash   all distance settings should be zero and <Backlash Enabled> should be unchecked <OK> and restart Mach3

Craig
My wife left with my best friend...
     and I miss him!
Re: Backlash on X axis changes to A axis
« Reply #3 on: October 24, 2017, 02:27:09 AM »
Hello Craig,

First of all, thank you for your response. I've checked the backlash option on Mach3 and all distance settings are on 0 and the <Backlash Enabled> option is unchecked.

I attach a couple pictures of the A axis ballscrews and also the <Ventura.xml> with the CNC configuration. Hope it can be useful.

Thank you so much for your time,

Marçal
« Last Edit: October 24, 2017, 02:29:41 AM by GibVentura »
Re: Backlash on X axis changes to A axis
« Reply #4 on: October 24, 2017, 02:55:48 AM »
Hi,
the first pic is the ballscrew and I would expect if it has any backlash it would be very small, measured with a micrometer if you can measure it at all.
We can safely say the fault is not backlash.

Please double check that the X and A axes are slaved together. Config/Slave Axis....A axis slaved to X axis  <restart>

Do both steppers turn the same way, at least when they are working properly? The reason I ask is because I have a test that might point you in the right
direction. If both motors turn the same way then take both Direction wires from the two drivers and hook them both to X Dir pin of the BoB. Take the two
Step wires from the two drives and hook them both to X Step pin of the BoB.
My wife left with my best friend...
     and I miss him!
Re: Backlash on X axis changes to A axis
« Reply #5 on: October 24, 2017, 03:01:15 AM »
Hi,
oops.. will finish the post.

Both steppers should now behave identically.  If they don't, then one of the drivers is faulty. If they do, then Machs slaved signals are screwy.

Can you post some pics of the BoB and the stepper drivers? What sort of motion controller are you using; parallel port, USB, Ethernet?
Can you post your xml file?

Craig
My wife left with my best friend...
     and I miss him!
Re: Backlash on X axis changes to A axis
« Reply #6 on: October 24, 2017, 06:04:39 AM »
Hi Craig,

I have checked and A axis is slaved to X. I also made the wire changes and the stepper motors behave identically. Could this mean that maybe the problem is in somewhere of the software?

I send the xml file (Ventura.xml) and some pictures of the electrical components. The motion controler is a USB box with all the electrical components inside.

Thank you so much,

Marçal
Re: Backlash on X axis changes to A axis
« Reply #7 on: October 24, 2017, 12:50:43 PM »
Hi,
possible but unlikely, thousands use Mach3 without problem.

I see one fault with your setup.
You have assigned pins 12,11,10 and 13 as homes for X,Y,Z and A respectively. You've then assigned Estop to pin 13 ie you have repeated assignment
of pin 13. In the first instance assign A home to pin 12. Then the X home switch will trigger both X and A home.

Shouldn't hurt but disable your probe input.
Again shouldn't hurt but disable output#2 and output#3

Also intrigued that you have for X,Y and A your steps per set at 40. Seems very low...how did you arrive at that number? How many microsteps have you set for your
drivers?

Looks like your drivers and BoB are all on the one board, how did you shift the input signals to the X and A drivers?

Craig
My wife left with my best friend...
     and I miss him!
Re: Backlash on X axis changes to A axis
« Reply #8 on: October 25, 2017, 04:57:36 AM »
Hi Craig,

I've assigned the A home to pin 12 and also I've disabled the probe input and outputs #2 & #3. At the moment the problem persists.

The number of steps per mm was set on 40 when I purchased the machine. The number of microsteps are 2500.2 (named "units per minute" on the program) if I'm not mistaken.

The first test I did was switch X and A plugs on the electrical box, and the issue changed from A axis to X axis. Following your instructions I put together both pin wires of X and A axis inside the X plug and the machine moved without that "backlash/ back movement" on A axis.

Thank you so much,

Marçal
Re: Backlash on X axis changes to A axis
« Reply #9 on: October 25, 2017, 06:04:49 AM »
Hi,
I think we may have to revisit some of those settings. A two phase stepper has 200 1.8 degree steps per revolution. A conventional driver set to 'full step'
when pulsed 200 times the motor will turn one revolution. Most drivers offer microstepping which subdivides each full step into commonly 4,8,16 and sometimes
more microsteps. For CNC routers microstepping allows a very much smoother motion than full stepping which can be quite harsh. Most use 8 and sometimes
16 microsteps per fullstep. Do you have any documentation for your controller? You will need to set the dip switches for 8 microsteps.

With 8 microsteps now the driver will be pulsed 1600 times for the motor to complete 1 revolution, I'm going to guess that your ballscrews have a pitch of 5mm
per rev and that the steppers are directly connected, ie no belt reduction, 1600/5=320. That is to say that 320 pulses would cause the motor to turn 1/5 of a
revolution and the axis to travel 1mm. You might consider that the resolution of your machine is 1/320=3.125um. That is an optimistic assumption, for a number
of reasons the practical resolution will be about 12.5um, just a bit over 0.01 mm, pretty damn fair!

Stepper motors lose torque in a big way the faster they go. Probably fair to consider their max speed to be about 500 rpm. You can of course make them go faster
but unable to drive an axis with authority, ie it will lose steps or stall and be generally hopeless for CNC. 500rpm x 5mm pitch=2500mm/min, so your velocity setting
sounds about right. A general rule of thumb for acceleration is about 10% of velocity so 250mm/s/s.

Unfortunately the rewiring did not result in testing the drivers as I'd hoped. You changed the wires from the output of the drivers to the motors. I was hoping you
could change the inputs to the drivers and so establish whether the A and X axis drivers are good and the same. Because the inputs to each driver are determined
by the copper traces of the circuit board you can't realistically change them.

Can you take some close up pics of the board, I want to see if there are any markings which would indicate the microstepping available with different DIP switch
settings? Can you also check or measure the pitch of the ballscrews. Turn one by hand 1 turn and measure how far the axis moves. Be accurate. The difference between
0.2 inch, a common imperial pitch and 5mm, a common metric pitch is only 0.08mm, ie not much between them. Can you post any manual you got with the
machine?

Craig
My wife left with my best friend...
     and I miss him!