544,016 active members*
2,308 visitors online*
Register for free
Login
Page 1 of 2 12
Results 1 to 20 of 21
  1. #1
    Registered
    Join Date
    Jan 2010
    Posts
    21

    Z Axis slipping

    Hi all,

    I have a recurring problem with my Z axis that I am beginning to think may be as a result of insufficient holding torque from the motor.

    The problem is that during the cut the Z axis drops and continues to drop throughout the cut to the extent of approx 0.75mm after 15 minutes and as a result it cuts into the table top.

    The Z axis is the only axis (on a 3 axis machine) that suffers the effects of gravity and to be honest the whole Z axis set-up including the router itself is quite heavy. I've checked the shaft/motor coupling for any slipping and it's solid.

    I can only put it down to the weight transferring too much torque to the motor via the ballscrew.

    I was wondering if anyone has experienced a similar problem or could help me calculate how much torque my ball screw is applying to the motor.

    The holding torque for the motor is 1Nm

  2. #2
    Registered
    Join Date
    Apr 2009
    Posts
    290
    I've had similar results from losing steps due to too fast acceleration settings on my Z axis. Every time the z retracted I would lose a few 0.01's of a mm. The motor tuning in mach3 solved it.

    Mikael
    "Hey! It's that guy you are!" - Phillip J. Fry

  3. #3
    Registered
    Join Date
    Oct 2008
    Posts
    242
    if this only happens during a cut I'd suspect the router bit was slipping. I've had it happen to me a few times, especially with smaller upcut bits.
    ___________________________
    http://jack.works

  4. #4
    Registered
    Join Date
    Jan 2010
    Posts
    21
    Thanks Mikael I'll check the acceleration settings in Mach 3 but mine is a lot more severe than yours so I would be surprised if it was that.

    I don't think the bit is slipping but will check that too.

  5. #5
    Community Moderator ger21's Avatar
    Join Date
    Mar 2003
    Posts
    35470
    Reduce the accel by 1/2 and see if the problem goes away.
    Gerry

    UCCNC 2017 Screenset
    http://www.thecncwoodworker.com/2017.html

    Mach3 2010 Screenset
    http://www.thecncwoodworker.com/2010.html

    JointCAM - CNC Dovetails & Box Joints
    http://www.g-forcecnc.com/jointcam.html

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

  6. #6
    Registered
    Join Date
    Jan 2010
    Posts
    21
    OK I halved the acceleration from 50 to 25 and it made no difference. I tried 15 and still no difference so I dropped it as low as it'd go which was 4. Although this did seem to work it was painfully slow.

    Any other suggestions?

  7. #7
    Registered
    Join Date
    Sep 2007
    Posts
    359
    The norm is to have the most powerful stepper motor on the Z axis.

    Has the amount you are out, changed as you have slowed the Axis down ?

    Is the Z axis being pulled down under cutting load ?

    Phil

    Sent from my HTC Desire using Tapatalk

  8. #8
    Registered
    Join Date
    Jan 2010
    Posts
    21
    All my motors are the same.

    The amount it is out has probably changed as I reduced the acceleration but I have no accurate way of determining this.

    I've tried it with no load and get the same result.

    Do I need a stronger motor?

  9. #9
    Registered
    Join Date
    Sep 2007
    Posts
    359
    Quote Originally Posted by neomagik View Post
    I've tried it with no load and get the same result.

    Do I need a stronger motor?
    1Nm seems a bit on the weak side, and you have a load on the motor which is the weight of the motor and axis, but only you know what the weight is.

    Take the motor off and wind the axis up so you can get a feel for the force required to move it.

    Then with the motor still off but powered up turn the axis coupling to see how much effort you need to overcome the stepper motor.

    You can then compare.

    Phil

    Sent from my HTC Desire using Tapatalk

  10. #10
    Registered
    Join Date
    Jan 2010
    Posts
    21
    I will try your suggestion and maybe rig up a torque wrench to get a more accurate result.

    Am I right in saying that the holding torque, in this case 1Nm, is the highest torque produced by the motor and that as the rpm increases the torque decreases?

    If this is the case then would decreasing the total motor speed and increasing the acceleration to a more acceptable rate acheive anything?

  11. #11
    Registered
    Join Date
    Sep 2007
    Posts
    359
    Yes the motor figure quoted is holding torque.

    Torque comes from amps, but the speed comes from volts.

    Measuring torque is a good idea but the correct gizmo is a spring balance (what a fisherman would use to weigh their catch)

    You need to fix an arm to the shaft you can read all about it here http://www.google.com/m?q=Newton+met...ni&channel=new

    Phil

    Sent from my HTC Desire using Tapatalk

  12. #12
    Registered
    Join Date
    Mar 2009
    Posts
    624
    Hi Neo,

    Has this problem always been there from the day machine was built or just started happening.?
    If it was ok before was it just doing simple 2d profiling etc then started losing steps or position when you've tried 3D work or Z axis intensive code like lithopains or engraving.?

    Can you give some info on your setup IE: motors specs and if wired parallel or serial, drive's used and settings set, PSU size, BOB make and model, pitch of z screw, Velocity & ACC set in mach also Step pulse & Dir pulse settings in motor tuning, what are the active high/low settings in ports&pins/motor outputs.

    Also are you using sheilded cable and what are you using for a spindle.?
    Can you approx guess the weight of the moving portion of the Z axis inc spindle.

    With this info it will be easier to help and trace if it's just under powered motor or whether you have another issue.

    Cheers
    Dean.

  13. #13
    Registered
    Join Date
    Jan 2008
    Posts
    853
    Can you set up a dial indicator on Z and run repeated up/down cycles to track how quickly you are losing position, without making any cuts? Then take the spindle off and repeat with less weight, change acceleration, etc. It is most useful to measure something as you go through this systematically instead of looking at final products.
    Cheers!

  14. #14
    Registered
    Join Date
    Jan 2010
    Posts
    21
    hemsworthlad,

    That’s a lot of information you’ve requested but I’ll attempt to answer as much as I can.

    The problem has persisted since day 1.

    I’ve always used it for pretty intensive 3D work.

    All the motors are Astrosyn MY103H702 wired in Bipolar Parallel (datasheet attached) and I have two of them driving my X axis. (4 in total)

    The driver is a Toshiba TB6560 3 Axis Driver (datasheet attached)

    The Z axis weight is approximately 4kg.

    The power supply is 24V DC 10A

    I am using shielded cables for my limit/home switches but not for the motor wiring however these switches are a recent addition so are unrelated to the problem.

    I'm not exactly sure what the pitch of the screw is but it's reference is RM1605-230mm-C7

    Velocity and acceleration are 1000 and 50 respectively

    Step pulse and Dir pulse are 0 and 0

    Active high/low settings in ports and pins/motor outputs are DirLowActive ticked and StepLowActive unticked.

    The spindle is just a standard router (manually controlled)

    I've just spotted that my drive current output set by the dip switches on the driver board are 50% for all axis. Perhaps increasing them to 75 or 100% would solve the problem??
    Attached Files Attached Files

  15. #15
    Registered
    Join Date
    Mar 2009
    Posts
    624
    Hi Neo,

    First before doing anything else start by checking that you have the motor phases polarity going to the right pins on the outputs, the wiring for the phases should be as below.
    Phase 1 A+ = blk and org/wht . . . A- = blk/wht and org
    phase 2 B+ = red and yel/wht . . . B- = red/wht and yel

    A+ = ZA+ =pin11
    A- =ZA- =pin12
    B+ =ZB+ =pin13
    B- =ZB- =pin14

    If wired differant then just be aware that after changing the motor may move in reverse and you will have to change the Dir active high/low setting in mach.

    Check to see if any change before doing anything else.

    Changing the current will increase the torque but it will also cause the motors to get hotter, possible too hot and going above the rated motor current is not a good idea. The 50% setting is right for the motor rated 1.5A. If you do change then keep a close eye on the motor temp,esp when stood still.

    The drive's max voltage of 24V on the other hand is quite limiting and working on the rule of thumb 20x rated motor voltage then your running the motors at less than 1/3rd their potential speed.
    That said this low voltage wont be causing your problem but I think they are not the best motors for this board given their high voltage requirements and really a bit of a miss match.

    I dont think your motor is under powered concidering your using a ballscrew which is quite efficent and the fact your not lifting a lot of weight at 4-5kg.
    I.ve seen 1.6nm motors running small milling machines with far far heavier loads than your trying to lift without any issue's and at higher speeds.

    Really you should have sheilded cable on the steppers to reduce the risk of noise causing missed steps, but again dont think it's noise causing this but it could easily do so.

    lowering the velocity will allow you to turn up the acc slightly but dont think it will cure the problem just mask it at best.


    Personally I would start by checking the Z axis carefully for sticking linear slides and linear rail miss alignment also sign's of slipping couplings etc to eliminate mechanical issue's also check ballscrew hisn't tight or sticking.

    Replace the cable with sheilded to reduce potential noise issue's.

    If nothing shows any signs of sticking etc then play with the higher current settings but tread carefully and then after re-tuning motors try differant Vel & Acc settings taking a carefull approach and only changing 1 thing at a time checking the affect before moving on to next.
    I would try to accurately measure the lost amount by clocking if possible, this will show clearer if the changes are having an affect.

    What kernal speed are you using in mach.?
    How many steps per in motor tuning.?

  16. #16
    Registered
    Join Date
    Jan 2010
    Posts
    21
    I'm pretty sure I have the motor phases wired correctly as I maticulously followed the Bi-polar Parallel diagram on the motor datasheet but I will double check.

    If it turns out to be a noise issue why would it only effect the Z axis?

    I will check if anything is sticking but I've wound the screw by hand many many times and never found there to be any problem.

    If the drive output is from 1.5 to 3A as stated on the datasheet and you can adjust it in 25%increments, wouldn't that mean 25%=1.5A, 50%=2A, 75%=2.5, 100%=3A meaning I should have the dip switches set at 25% ie 1.5A? I have noticed the motors get quite hot when stationary for prolonged periods?

    If you think the motor is up to the job and assuming there is no phase wiring issue, other than changing the velocity and acceleration, is there anything else in Mach that I could try? I'd rather change settings in Mach than replace the cables at this stage. Were any of my settings suspect? BTW the steps per is 8 but I'll have to check the kernal speed later.

    Thanks

    Rob

  17. #17
    Registered
    Join Date
    Oct 2010
    Posts
    0
    Have you thought about adding a counter balance to your Z axis? Something like a gas spring or a constant force spring.

    I don't know what kind of suppliers you have in the UK, but I'll just link to Mcmaster to show you what I mean.

    Gas Spring (Like those used to hold up the rear door in hatchbacks)
    McMaster-Carr

    Constant force spring (Found in wind up cars)

    McMaster-Carr

    If your problem actually turns out to be a Z axis that is too heavy, I hope that my post may have helped.

    If not, good luck! I'm pretty new to this CNC thing, so as of yet I haven't encountered many of the issues that the guys above me know how to fix.

    Jay Swift
    "The bitterness of poor quality remains long after the sweetness of low price is forgotten"

  18. #18
    Registered
    Join Date
    Mar 2009
    Posts
    624
    Quote Originally Posted by neomagik View Post

    If it turns out to be a noise issue why would it only effect the Z axis?
    Thats wonderful thing about noise it can do this, I know of it happening. The Z axis is working quit hard when doing 3D work making lots of little movements so more open to picking up bits of noise these accumulate due to all the tiny movements causing positional loss and can easily send a unsheild motor off course.

    Quote Originally Posted by neomagik View Post
    If the drive output is from 1.5 to 3A as stated on the datasheet and you can adjust it in 25%increments, wouldn't that mean 25%=1.5A, 50%=2A, 75%=2.5, 100%=3A meaning I should have the dip switches set at 25% ie 1.5A? I have noticed the motors get quite hot when stationary for prolonged periods?
    Erm Yes there's a slight bit of mixed info here. The drive spec is max 3.5A but the docs say 1.5A -3A range.? I suspect the latter should be 3.5A not 3A.
    How hot does the motor get when stood for a length of time.? . . Can you hold your hand on it without getting burnt.?
    It's quit normal for steppers to get hot when stood for lengthy periods but you should be able to hold your hand on without getting burnt but will be quit hot-ish.!

    If the motors dont get so hot they burn then i wouldn't change to 25% it will only make matters worse, after all it's only 1.75A whch hisn't massively over the rated 1.5A.


    Quote Originally Posted by neomagik View Post
    BTW the steps per is 8 but I'll have to check the kernal speed later
    Something is very very wrong here 8 is far to low.?

    I presume your working in metric.? I'm going to presume your using the max micro stepping 1/16.
    With a 5mm pitch screw and 1/16 micro stepping on a 200steps per rev stepper the steps per should be around the 320 mark.

    Check this again and also check what the micro stepping on the board is set to for the Z axis. Also while your at it check what the decay setting is set to on the board.

  19. #19

    Join Date
    Mar 2021
    Posts
    1

    Re: Z Axis slipping

    When I turn on my spindle motor my z axis slowly drops about .250 of an inch. Any idea what could cause this

  20. #20
    Registered
    Join Date
    Dec 2003
    Posts
    934

    Re: Z Axis slipping

    Almost eleven years after the previous post on the thread?Have you checked for a loose coupling or drive wheel?

Page 1 of 2 12

Similar Threads

  1. Slipping Axis
    By Dingo745 in forum Australia, New Zealand Club House
    Replies: 13
    Last Post: 04-04-2013, 03:41 PM
  2. 4th Axis Stepper Slipping Help.
    By Cartierusm in forum Mechanical Calculations/Engineering Design
    Replies: 26
    Last Post: 11-06-2009, 10:11 AM
  3. x axis slipping
    By FUN4ME in forum Fadal
    Replies: 2
    Last Post: 01-08-2008, 09:20 PM
  4. Y Axis Slipping ??
    By twocik in forum Benchtop Machines
    Replies: 18
    Last Post: 12-12-2007, 12:11 AM
  5. Boss z-axis belt slipping
    By jderou in forum Bridgeport / Hardinge Mills
    Replies: 13
    Last Post: 06-01-2005, 07:08 PM

Tags for this Thread

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •