microstepping and skipping steps

Projects General Guidance
1

Good Y'all

I am new to this forum, please excuse if in the wrong area or topic.....

I have uno R3 and cnc shield on my chinese 2417 cnc engraver...i blew up the woodpecker board...another story.

Anyway, my stepper motors (nema 17 motors),are skipping steps, x axis and z axis.

I have setup quarter step microsteps, i get a "grinding" type noise on the steppers, assume skipping.

Current grbl settings are:

2018/12/29 08:32:27 - $2=0 (step port invert mask:00000000)
2018/12/29 08:32:27 - $3=5 (dir port invert mask:00000101)
2018/12/29 08:32:27 - $4=0 (step enable invert, bool)
2018/12/29 08:32:27 - $5=0 (limit pins invert, bool)
2018/12/29 08:32:27 - $6=0 (probe pin invert, bool)
2018/12/29 08:32:27 - $10=3 (status report mask:00000011)
2018/12/29 08:32:27 - $11=0.010 (junction deviation, mm)
2018/12/29 08:32:27 - $12=0.002 (arc tolerance, mm)
2018/12/29 08:32:27 - $13=0 (report inches, bool)
2018/12/29 08:32:27 - $20=0 (soft limits, bool)
2018/12/29 08:32:27 - $21=0 (hard limits, bool)
2018/12/29 08:32:27 - $22=0 (homing cycle, bool)
2018/12/29 08:32:27 - $23=0 (homing dir invert mask:00000000)
2018/12/29 08:32:27 - $24=25.000 (homing feed, mm/min)
2018/12/29 08:32:27 - $25=500.000 (homing seek, mm/min)
2018/12/29 08:32:27 - $26=250 (homing debounce, msec)
2018/12/29 08:32:27 - $27=1.000 (homing pull-off, mm)
2018/12/29 08:32:27 - $100=200.000 (x, step/mm)
2018/12/29 08:32:27 - $101=200.000 (y, step/mm)
2018/12/29 08:32:27 - $102=188.976 (z, step/mm)
2018/12/29 08:32:27 - $110=5000.000 (x max rate, mm/min)
2018/12/29 08:32:27 - $111=5000.000 (y max rate, mm/min)
2018/12/29 08:32:27 - $112=800.000 (z max rate, mm/min)
2018/12/29 08:32:27 - $120=10.000 (x accel, mm/sec^2)
2018/12/29 08:32:27 - $121=10.000 (y accel, mm/sec^2)
2018/12/29 08:32:27 - $122=10.000 (z accel, mm/sec^2)
2018/12/29 08:32:27 - $130=290.000 (x max travel, mm)
2018/12/29 08:32:27 - $131=290.000 (y max travel, mm)
2018/12/29 08:32:27 - $132=200.000 (z max travel, mm

I have a vref voltage on a4988 drivers at 0.77V, about 1.5A.

Can someone tell me what is wrong with my setup please

Cncnut

2

A motor skips steps when it is going too fast for the torque the motor can develop. Solutions are:-

  1. Don't go so fast.

  2. Supply more current to the coils.

  3. Use a higher voltage so the current gets into the coils more quickly. This will need a chopping regulator driver.

Note you have supplied very little real information here. No link to what you have, no description of any voltages, no description of what motors you have.

my stepper motors (nema 17 motors)

All that says is what size flange the motors have, it says nothing about the electrical characteristic of the motor.

3

Sorry for leaving out, i am using nema 17 motors, with TR8 lead screws, am using a 24 volt 6A power supply. which i assume is ample for running 3 axis' simultaneously.....

4

cncnut:
Sorry for leaving out, i am using nema 17 motors,

Maybe you missed what Grumpy Mike just wrote

5

am using a 24 volt 6A power supply. which i assume is ample for running 3 axis' simultaneously

Well not if each motor takes more than 1A per winding.

6

Hey Guys'

From what i could find, the steppers are the JK42HS34-1334 ones, "although no markings on motors".

I have the A4988 stepper drivers, set on 1/4 step resolution on a cnc shield on the uno R3 board.

I run a 24v 6A power supply, my stepper Vref shows 0.77V, would you advise to increase current still?

By going too fast, meaning cutting speed, or travel speed too fast? or even acceleration?

Sorry for ignorance, i try understand......

cncnut

7

By going too fast, meaning cutting speed, or travel speed too fast? or even acceleration?

Yes all of those.

The JK42HS34-1334 motor should be run at 1.33A per phase. That is two phases per motor and three motors so a total of 8A and you only have a 6A capable power supply. That would be where I would look for the problem. Do you get skipping when only one motor is moving and the other two are disabled? By disabled I do not mean not moving, a stepping motor takes maximum current when it is not moving. By disabled I mean disconnected.

You said:-

I have a vref voltage on a4988 drivers at 0.77V, about 1.5A.

By the same token 1.5A per winding would give you a total demand of 1.5 * 6 = 9A.

So by your own figures the power supply is not up to the job.

8

6A at 24V will give more than enough power for the stepper drivers to push the appropriate current through each motor. Remember they act like a switchmode power supply so their output amps will be higher than the input.

9

Thanks for the replies, G_M, the skips happen with one motor at a time, my x axis, then the z axis does as well, just after each other.

My machine is the 2.5D, so i assume that there are only two axis' active at any one time, so the power supply is good enough?

The machine had worked perfectly when i got it with the woodpecker control board, then i tried a laser on it and it went to happy hunting grounds, then onto arduino due to relatively inexpensive cost.

I had engraved on glass successfully, with the arduino setup,but with very low feeds and speeds, i try to cut mdf board, all goes pear shaped, skips....

I use a diamond bit on the glass, i use a v-bit or a fishtail bit on the wood (some useless info), may that be where the problem lies?

10

Remember they act like a switchmode power supply so their output amps will be higher than the input.

Well no, I don’t think so. True the peak current will be bigger than the avrage current but you need large capacitors on the supply rails to make up for such a large peak over continuous current. And the capacitor’s current rating has to be up to it. What sort of large capacitors do you have on the supply?

the skips happen with one motor at a time, my x axis, then the z axis does as well, just after each other.

Not sure what that last bit means. Are you sure the other motors were disabled and not just not moveing during this test?

My machine is the 2.5D, so i assume that there are only two axis' active at any one time, so the power supply is good enough?

I don’t know what 2.5D means, but remember that a motor not moveing draws the maximum amount of current.

11

I have no large capacitors on the supply line, i have just plugged in the arduino uno, cnc shield, a4988 stepper drivers, put grbl 0.9j on the uno, plugged in 24v supply and thats it.

up to now i have upped the current to the motors, tried both full step and half step and now quarter step, the x motor still skips, the motors (all of them) now make a "grunting" sound when stopping, not a smooth stop.

This is the motor spec: 34MM,Current 1.33A, 12v.Torque 0.25N/M (found on Aliexpress) (where i bought the machine from).

When "skipping" the motor makes a "grinding, rough sound" and can see it skipping.??

Still the current?

What gives??

12

Still the current?

Yes. And it woudn't hurt to up the voltage as well, looks like it can go up to 36V max.

Are you using the same power supply to run the spindle?

Has anything changed to increase the friction since you use the initial controller board?

This link suggests their are two power supplies http://forum.linksprite.com/topic/4706-schematic/

13

Hey G_M

yes i am running the spindle on the same power supply and no nothing has changed since the initial controller board....

attached is my setup....

my power supply connects to the cnc shield on blue connector, i also connect it to the relay for the spindle, which is controlled by the shield. The uno gets power from the usb..

This setup i got from the interweb and some of my local guys here in sunny South Africa....would this work or still add another supply??

IMG_20190101_161657.jpg
IMG_20190101_161657.jpg600×800 163 KB

14

Can you try higher microstepping rates? Resonance issues are typically to blame for missing steps in
a lead-screw system (belt drive is much better damped mechanically). Adding mechanical torsional
damping (ie friction) is another way to reduce resonance). If its a resonance issue it should get worse
with coarser steps, better with smaller steps, and happen mainly at particular stepping rates.

15

my power supply connects to the cnc shield on blue connector, i also connect it to the relay for the spindle, which is controlled by the shield.

Well if we were questioning the capacity of your power supply before, running an other motor off the same supply is only going to make matters worse.

16

Thanks MarkT and G_M

I will try on a higher step rate, fyi i did clean the lead screws (all of them) and put a thin layer of oil to make it move better/easier....maybe not such a bright idea??, i shall do as mentioned and get feedback asap..

Thanks again for the ideas and input...

Cncnut