Is the Z Axis Ball Screw the cause of my skipping?


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    Default Is the Z Axis Ball Screw the cause of my skipping?

    I purchased my K2 machine used a few years ago, and it has preformed well since then. Yesterday I was milling the recessed seat area for three rocking chairs, and I every so often with the first couple of seats, it would periodically cut a few thousandths deeper. Not so much but enough I will have to sand a bit to smooth the area. When I got to the third seat, the machine did the same thing but then the spindle stopped moving even though Mach3 just kept sending instructions. Something had caused the machine to bind and knocked the reset.

    At this point, I wasn't sure which axis was causing the grief, so I checked that all the mounting screws were tight, and that all the belt pulleys were tight. No issues. So I changed my code to run parallel to the Y axis rather than the X axis. Part way into the program, the machine skipped some steps then later stalled again. At this, I was feeling the issue had to be with my Z direction, so I pulled the spindle and unhooked the controller motor. The spindle bracket slid easily up and down the assembly. I couldn't feel any spots of extra resistance or binding. I removed a few more brackets to expose the ball screw. When I ran the motor to raise and lower the spindle mount, I noticed a pretty good wobble at the bottom end of the Z ball screw, maybe about 1/16" or move of movement. The Z ball screw is only anchored at the top of the Z assembly, and the table end just floats.



    I was surprised to see this much movement in the ball screw, and believe it might be enough to cause the axis to hiccup or stall during a micro move command. I am not well versed with these machines as I never built this one and have only done routine maintenance on it. I am hoping someone here can offer some insight as to whether this is likely the source of my issues. If so, where is the best place to get a new ball screw assembly? K2 is no longer in business.

    The ball screw may have been like this since I got the machine. I mainly cut inlays and do Z level milling, which minimized the Z movements or moved the Z in larger increments. With the chair, I had a lot of very fine Z movement. Or maybe I damaged the ball screw when I moved the machine. I'm not sure. I'd just like to get the problem fixed.

    Thanks for your help.

    Ken

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    Default Re: Is the Z Axis Ball Screw the cause of my skipping?

    It doesn't sound like the problem is really in your ball screw. (And that doesn't actually look like a ball screw in your picture; it looks more like a gas spring.) It sounds like you're losing steps, which is a common problem in stepper-based systems. That might be because your gas spring, which is supposed to counter-balance the weight of the spindle assembly, has failed. They don't last forever, but fortunately they are a lot cheaper than ball screws to replace.

    [FONT=Verdana]Andrew Werby[/FONT]
    [URL="http://www.computersculpture.com/"]Website[/URL]


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    Default Re: Is the Z Axis Ball Screw the cause of my skipping?

    Hi Andrew,

    Yes, I have a gas spring (black rod on the left part of the tower that functions fine), but look at the ball screw and bearing in the center of the tower as the bearing goes up and down. Notice the high degree of wobble, especially near the bottom of the ball screw. The ball screw wobbles by at least 1/16" off center. I am losing a few steps, but I can't get the program to fully mill my project. I even cut my milling speeds in half to run at 75 ipm horizontally and 35 vertically to minimize the missing steps. Made no difference, the mill still missed steps and stopped moving about half way through the project. I use a ball mill to rough out the seat base use Z steps down. No issues here. I then run the ball end mill back and forth across the seat base in a planar finishing pass using a slight step over. The planer passes follow a straight X or Y pass depending on which way I run the program. The mill rides up and down over the wood along the straight horizontal axis. At some point usually about half way through, it binds and triggers my reset, leaving the program running but the end mill stopped. I can't find any place where the X, Y, or Z is binding. With the motors disengaged, I can run the tram along the rails in any direction, and it is nice and smooth. So it has be somewhere on the ball screws, and the Z axis seems like the likely culprit. But I don't want to replace the ball screw if this type of wobble is normal. If it is, I'll keep digging for something else.

    BTW, thanks for replying!

    Thanks,

    Ken



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    Default Re: Is the Z Axis Ball Screw the cause of my skipping?

    if it gets deeper and deeper then it loosing steps , very possible due to motor can not deal with weight..
    what you saying """binding""" that is the sign, motors are on the limit..

    they might designed for smaller spindle, and after installed larger spindle they put on a gasspring.....

    however motor should handle without gasspring too...

    check on Z axis driver, and look for if you can put on larger motor..

    I had same issue.. it always lost a very minimal.. I just noticed when many pocket came up within one job...
    when I cut individual pieces, and changed tool frequently it didn't came up so bad..

    so I changed my Z axis motor.. and forget all..
    ================================================== =============

    that wobbly screw is not normal... however it can run long before it gets really worst..

    binding always was present a little.. and simply moving machine wont bend that screw...

    so first just check on how much would cost replacing motor to a larger..
    don't worry on folks telling larger is slower.. it wont be slower so bad.. if anyway slower..



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    Default Re: Is the Z Axis Ball Screw the cause of my skipping?

    Don't worry about the wobble; the unconstrained end of the ball screw is likely to do that. I would check that gas spring, but if that's okay the next thing to check would be the Z axis driver. Lost steps are rarely due to a motor going bad.

    [FONT=Verdana]Andrew Werby[/FONT]
    [URL="http://www.computersculpture.com/"]Website[/URL]


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    Default Re: Is the Z Axis Ball Screw the cause of my skipping?

    motor is not bad,, I didn't say motor bad,...

    motor is small, compared to the weight it has to lift.. neither motor nor its driver not bad... it simply lost steps due to the weight of spindle...



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    Default Re: Is the Z Axis Ball Screw the cause of my skipping?

    If it had always acted this way I'd agree that the motor's just too small. But it used to work fine and now it doesn't. To me, that means that either there's more resistance to motion in the mechanical assembly (like a bad gas spring would cause) or something happened to the driver.

    [FONT=Verdana]Andrew Werby[/FONT]
    [URL="http://www.computersculpture.com/"]Website[/URL]


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    Default Re: Is the Z Axis Ball Screw the cause of my skipping?

    Andrew, if you looking for the video, then you can see theres a large spindle.. that is heavy...
    I know of my experience, it was visible working right..
    but... as I thinking I have almost everytime jobs, that I just made a couple of pockets on signs.... there wasn't visible lost steps, till I got a job where was so many tiny pockets, that the many upn down immediately showed..

    but again I was in the faith it working right.. you don't notice immediately a big failure.. but tiny bitty lost steps..
    that's why I suggested the motor changing.. this machine visible wasn't set on the side.. it looks it was just about used...

    and again, it should lift the spindle itself without any stout.. who knows they might set on a larger dust shoe.. or used a spoilboard mill... then the extra weight pulled up the lost steps more..

    we just cant say it was 100 percent sure.. a couple of lost steps not immediately visible.. we will see..



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    Default Re: Is the Z Axis Ball Screw the cause of my skipping?

    question is if the driver can handle larger motor..



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    Default Re: Is the Z Axis Ball Screw the cause of my skipping?

    Thanks Gents. Okay I pulled the ball screw this evening. As I rotated the bearing around the screw, it was really rough. Some places on the screw seemed to be okay but others felt like there was sand mixed with the bearings. I don't know how much resistance there can be before the motor will miss steps, but a few points required a really good grip and some force to rotate the bearing around the screw. The bearing at the head of the shaft also does not work too smoothly but it isn't as rough. I rotated the bearing off the screw and checked the screw shaft on a flat surface. It rolled true. I also gave the screw a good wipe down, and I couldn't see any damage to the screw. Do the ball bearings wear out after awhile? My machine doesn't get used a lot, and I can't imagine I have worn out the bearing as I do keep them lubed, but they clearing aren't working as smoothly as I would have thought they should.

    I don't know how big a deal it would be to clean and repack the bearings or whether I should simply get a couple of new bearings.

    I am attaching links to a few pictures that show my issue. The first picture is of an oak chair bottom. The ball mill dropped deeper a couple of times. One is clearly visible in the image. I ran one other chair of maple, and got three or four areas where the ball mill dropped a bit deeper. I played around with my code prior to running the last chair bottom of walnut. I cut the X,Y, and Z feed speeds by half. I also went into Mach3 and changed the acceleration and deacceleration to spin up and down more slowly and also increase reduced the steps/IPM by just a touch. When I ran the program on the walnut chair bottom the mill stopped in the position shown in the picture. I changed my program to run the planar passes parallel to the Y rather than the X, and slowed down the acceleration even more. This time I got an area where the cutter actually didn't drop low enough for about a dozen or passes, It then decided to cut deeper and eventually stalled again.

    Is the Z Axis Ball Screw the cause of my skipping?-oak-jpg
    Is the Z Axis Ball Screw the cause of my skipping?-walnut_1-jpg
    Is the Z Axis Ball Screw the cause of my skipping?-walnut_2-jpg



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    Default Re: Is the Z Axis Ball Screw the cause of my skipping?

    best I can tell - "really rough" on the screw and nut means it needs to be cleaned or replaced - on the bearing it means that probably needs to be replaced - and that's probably what's causing the problem - it won't hurt to try repacking the nut - at any rate it should turn smooth and easily from one end to the other

    www.signtorch.com


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    Default Re: Is the Z Axis Ball Screw the cause of my skipping?

    Think I am going to order a new ball screw assembly. I don't like how rough this one felt. If the machine works, then I have fixed it. If it doesn't, I'll have a new ball screw and will look elsewhere. But I'm optimistic a new ball screw will be the cure.

    Ken



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    Default Re: Is the Z Axis Ball Screw the cause of my skipping?

    Back again. My ball screw was really rough, and I couldn't get it running smoothly so I ordered a new one. It finally came, and I was able to install it a week ago. Despite the screw running more smoothly, the Z is still losing steps. I swapped the Z motor and controller with my X motor and controller and still have the same issues. I have also double checked all my connections and put dielectric grease on the control pins. The gas shock still seems strong. I have a program to mill an acoustic guitar neck, which takes about an hour to run. The Z will lose about 0.04" throughout the routine.

    I think the really serious issues I had with the Z stalling may have been due to the ball screw. Now I seem to just be losing steps. I have been wondering perhaps if this has always been an issue, and I just didn't catch it. I have run small jobs in the past when the Z cut a bit deep, but I used to set my Z height based on the top of my stock and just presumed I wasn't taking into account the thickness of the stock correctly. Now I set the Z at the spoil board for many of those jobs. My neck routine used to be multiple programs, and I would set the Z with each tool change and program. Now I am doing more with a single ball mill, so the program runs much longer. The problem may have always been there, but for these two reasons, I never saw it.

    So I am back to trouble shooting. Are there some settings in Mach 3 I should check? Is a stronger motor the next option. By the way, I did remove the gas shock and ran the machine without it. Within about 30 seconds, the Z stalled while the program kept running. If a motor is the next step, where is a good source, and can it be swapped out and still work with my main controller and Mach 3?

    Thanks for your insights.

    Ken



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    Default Re: Is the Z Axis Ball Screw the cause of my skipping?

    What are your Z axis velocity and acceleration settings?
    What are the specs of the motor, and what drive is it using?
    What voltage are you running at?

    Gerry

    UCCNC 2017 Screenset
    [URL]http://www.thecncwoodworker.com/2017.html[/URL]

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    [URL]http://www.thecncwoodworker.com/2010.html[/URL]

    JointCAM - CNC Dovetails & Box Joints
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    Default

    Quote Originally Posted by KennethC View Post
    Back again. My ball screw was really rough, and I couldn't get it running smoothly so I ordered a new one. It finally came, and I was able to install it a week ago. Despite the screw running more smoothly, the Z is still losing steps. I swapped the Z motor and controller with my X motor and controller and still have the same issues. I have also double checked all my connections and put dielectric grease on the control pins. The gas shock still seems strong. I have a program to mill an acoustic guitar neck, which takes about an hour to run. The Z will lose about 0.04" throughout the routine.

    I think the really serious issues I had with the Z stalling may have been due to the ball screw. Now I seem to just be losing steps. I have been wondering perhaps if this has always been an issue, and I just didn't catch it. I have run small jobs in the past when the Z cut a bit deep, but I used to set my Z height based on the top of my stock and just presumed I wasn't taking into account the thickness of the stock correctly. Now I set the Z at the spoil board for many of those jobs. My neck routine used to be multiple programs, and I would set the Z with each tool change and program. Now I am doing more with a single ball mill, so the program runs much longer. The problem may have always been there, but for these two reasons, I never saw it.

    So I am back to trouble shooting. Are there some settings in Mach 3 I should check? Is a stronger motor the next option. By the way, I did remove the gas shock and ran the machine without it. Within about 30 seconds, the Z stalled while the program kept running. If a motor is the next step, where is a good source, and can it be swapped out and still work with my main controller and Mach 3?

    Thanks for your insights.

    Ken
    The Z stalled? Do you think that is purely because of too much weight. Or being that it ran for 30 seconds did it initially lift fine? If so coukd it be a ballscrew alignment issue.

    Attaching a dti to the nut block and touching screw as you run up and down will give you the run out. Doesn't take a lot to jam these up.

    Just read you have one end unsupported and one guy says don't worry about wobble. I find this quite disagreeable, wobble will have the same effect as screw whip. Too much and it will jam.



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    Default Re: Is the Z Axis Ball Screw the cause of my skipping?

    Had a similar issue with Z axis of my K2 a few years back that drove me crazy. Ended up being the control board inside the stepper motor was loose causing me to loose steps and/or stop. Found the problem by just shaking the leads coming off the motor when not running to simulate the vibrations of running the machine and it did the same thing. Ran some glue into the encoder and has been working fine ever since.



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    Default Re: Is the Z Axis Ball Screw the cause of my skipping?

    i had an issue with a machine axis losing steps, was solved by setting mach3 to half pulse sherline mode which overrides the uS pulse length to half of the kernel driver frequency... for 25 or 37.5 Khz, this effectively turns 5uS into 15us. the same machine had problems with the X axis losing steps only in one direction, as this is how i became suspicious of the problem. y axis didn't have that problem for reasons i don't know.

    another problem with mach3. if you have backlash enabled. the velocity at which backlash is added to the motion.. there is no acceleration compensated for when adding that backlash...the backlash is added after the axis has already changed direction, instantaneously at the velocity set in the program. if the change in velocity is too high, you will lose steps.



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    Default Re: Is the Z Axis Ball Screw the cause of my skipping?

    Thanks folks for the reply. I have been trying to catch up on my projects and haven't had time to tinker with my machine. Rather I have been doing my best to keep making progress and put off sorting out what is wrong with the Z Axis when I have time in a couple of weeks. I have needed to use the CNC, so I have broken my programs into a number of smaller routines and reset my Z axis after each smaller program runs. This is a pain as I have tend the machine rather than let it run and go work on something else. But I am catching up. I have one more fingerboard to mill, and I will be good for now. Even cutting fret slots, the end mill will drop enough after cutting a half dozen frets, that the Z needs to be reset. I'll look into your suggestions and report back.

    Gerry, Where do I chekc the voltage?
    Jon.N, The machine hasn't stalled when doing all my fingerboard and inlay work. It will lose steps simply raising the spindle with the end mill cutting nothing but air
    Cerk, I don't think it is the stepper as I swapped the X and Z motors and controllers and the problem persisted
    Eldon_Joh, Your suggestion sounds simple enough. I'll look for those settings in Mach 3.

    Ken



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    Default Re: Is the Z Axis Ball Screw the cause of my skipping?

    I had to run a couple of acoustic guitar bridges today, so I thought I would tweak a few settings prior to running my program. I found where to selection the Sherline mode, so I made that change. I also slowed down the acceleration for the Z axis. I ran my program on a piece of maple, and it milled the bridge without apparently losing steps. I ran it on a couple of pieces of ebony, and it milled the bridges and saddle slot just fine, but it stalled three times when drilling the bridge pin holes. These have a lot of jerkiness in the motion, and I suspect that may be the cause. Maybe if I slow the acceleration for the X and Y a bit, I can eliminate those. But just milling the bridge without the ball mill dropping during the finishing passes was huge.

    Ken



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Is the Z Axis Ball Screw the cause of my skipping?

Is the Z Axis Ball Screw the cause of my skipping?