Do I need a new Z Axis Ball Screw?
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.
https://youtu.be/IrV84djodew
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
Re: Do I need a new Z Axis Ball Screw?
just about any free hanging shaft will wobble - the question is - 1) does the screw hang straight and in line with where the nut mounts - 2) if you reinstall the screw - and take off the motor - and turn the screw by hand - does the screw bind up anywhere - especially anywhere around the point where the problem occurred - and it shouldn't wobble so much when installed - if it does wobble or bind when installed then might have to remove the screw to see if its straight and see if the nut alone binds at all - then make sure the fixed screw bearing is straight in the mount and make sure the fixed bearing mount is straight in line with where the ball nut mounts on the slide - if it doesn't bind or wobble excessively when turned by hand then the ball screw might be ok
'
when it "cut a few thousandths deeper" - did it lose position (lost steps) or did it just cut deeper than expected in that one spot
Re: Do I need a new Z Axis Ball Screw?
Thanks for the reply SignTorch. I'll pull the screw and roll it on a flat surface to see if it is true. If it is, I'll see if I can reinstall it to get less wobble.
I can probably get some pictures up later, but imagine milling a large, very shallow bowl with linear passes along one of the horizontal axis. I used z step roughing, which looked to be just fine. The problem was with the planar finishing runs. The mill started at run end running along the x axis with the z moving up and down to follow the contour of the bowl, removing very little material. About a third of the way into the process, the mill all of a sudden starts to cut just a few mils deeper. The next section of the bowl it mills stays consistent with the new Z, but sits a few mils lower then it should. Then the CNC will skip a step again and the bit drops again, milling the next section a bit lower. Then at one point, the mill would trigger the reset and stop while my program kept running. I also did another run with the linear direction running along the Y axis. I got similar results, with a couple of missed steps, but a bit before the mill triggered the reset, it looked like it lost some down steps rather than up as it milled that last section quite a bit higher, maybe as much as 100 thou, than it should have. So it is clearly bombing out somewhere. Just wish I knew more what I was doing or I could hire the Maytag repairman to come and fix it!
Re: Do I need a new Z Axis Ball Screw?
A ball nut full of gunk could be the cause of the wobble. Could it be the coupling thats causing the z height issue? They do need replacing from time to time just cant see how the ballnut & screw could play much part in loosing position unless say the nuts lost most its ball bearings or the screws visibly damaged. Ive had one of the cheap sfu type ball nuts wearout the plastic lugs that return the balls and consequently balls were getting in the no ball zone which caused all sorts of issues. Swapped it out for a german screw and nut with tube returns.
Re: Do I need a new Z Axis Ball Screw?
since it is loosing steps - do clean and check the ball screw to turn as freely as possible - but then all you can do is lower the acceleration - short of increasing motor size or gearing down the z axis more - if it's direct drive then adding a timing belt reduction of 3 or 4 to 1 would probably work better than increasing the motor size - note: as you lower acceleration you may also have to reduce feed rate - btw what is your acceleration and feed rate ?
Re: Do I need a new Z Axis Ball Screw?
Thanks Gents. I had posted my original thread over in the K2 sub forum in hopes of finding someone somewhere that could help. I have had a few replies over there. But still not clear on next steps. This evening I pulled the ball screw. 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 clearly 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 or as you noted Jon, get a new screw and better bearings. What would you recommend?
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 another 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. I believe this is whay SignTorch referred to, 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 so passes, It then decided to cut deeper and eventually stalled again. I was thinking the skipped steps were due to rapid movements. But I wonder if the sudden rapid movement actually caused less missed steps and the quick movements were better able to move the rough bearing around the screw. I got all sorts of missed steps when I really slowed things down and reduced the acceleration. I really wish I new what I were doing ;)
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