Y axis stalling

[richie00boy]

I'll have to strip the machine more to get at the leadnut - there is grease on it so I can't see what it's made of. I woudn't have thought it was excessively worn as it's only run about 10 hours.

It does vibrate and stall.

Is the current leadscrew threaded rod that I can buy easily and get the machinist at work to do the ends for me? I don't want to spend more than 50 quid really, but the primary use of the machine is PCBs so I need to get low backlash.

[JAZZCNC]

I'll have to strip the machine more to get at the leadnut - there is grease on it so I can't see what it's made of. I woudn't have thought it was excessively worn as it's only run about 10 hours.

It does vibrate and stall.

Suggest you take it off completely and check screw against flat surface to see how much it's bent. It may be possible to straighten if not bad.

You also need to know why it's bent.? Has it bent because the alignment is off and one end is higher or offset to the other. If so what happens is the leadnut which is fastened to the gantry bends the screw when it approaches the end which is higher or lower than other. The screw fixed ends and leadNut need to be perfectly aligned on the same plane. This is a common cause of bent screws.!!

Is the current leadscrew threaded rod that I can buy easily and get the machinist at work to do the ends for me? I don't want to spend more than 50 quid really, but the primary use of the machine is PCBs so I need to get low backlash.

Like I say hard to see from picture.!! . . . BUT it won't be thread rod like you buy at local hardware store.
It could be what's called Acme Lead screw which is often used on cheaper machines, if this one of the early Chinese machines then chances are it is.? If so they used plastic lead nuts which wore out very quickly and could have done so even with 10hrs use that's how crap they where.!!!!

The later slightly better Chinese machine are coming with 5mm pitch ballscrews, Looking closer at the pics I'm tipping it's lead screw not ballscrew.

You will buy a ballscrew with nut from china complete with end machining for around £50 delivered.
TR14 lead screw will cost about £15-20, lead nut will be £10-15 then you'll need to machine the ends. You could be up and running for £20-£30 but you'll still have crappy screw with some backlash in the nut unless you buy better anti-backlash nut which will cost much more.!!. . . So you can probably tell were I'm going.? Not worth IMO so just Bite the bullet and buy a ballnut you'll get much smoother machine with better accuracy and probably more speed.

[richie00boy]

Thanks for the help. How can I tell if the alignment is off to bend the leadscrew? And is that fixable?

I do remember noticing that the leadscrew was not totally concentric when I first got it. I assumed it was just due to the leadscrew being made from really long thread which got slightly bent at some point in the Chinese factory.

[JAZZCNC]

Thanks for the help. How can I tell if the alignment is off to bend the leadscrew? And is that fixable?

With careful measurements and visual checks.!! Aligning ballscrew and nut is tedious job but very important to both prevent bending and getting the best performance.
If alignment is off enough to bend the screw it should easily be visible by manually running the gantry along it's length carefully watching the screw to see if it's bending. Also if you turn the screw by hand with motor disconnected and you feel it stiffen has it nears ends this is good indication some things off.!!

Can't really comment on what would be needed to fix without seeing how ends are fixed and how much off it is.?

[FatFreddie]

Thanks for the help. How can I tell if the alignment is off to bend the leadscrew? And is that fixable?

If you take one of the bearings off and turn the screw by hand, as the nut approaches the other bearing it should be fairly obvious if it's misaligned, particularly if you can get the nut quite close to the bearing.

[richie00boy]

OK, before I take things apart a bit more and get lost, here is what I have found.

1. The motor knob becomes stiff when the leadnut under the gantry can be at almost any point along the length of the leadscrew, i.e. not just at the ends.
2. I have unscrewed the leadnut from the gantry and can see the leadnut housing bobbing about relative to the gantry again at almost any point along the length of the leadscrew.

I'm thinking that there could be two ways for bending the leadscrew. One if the leadnut was not mounted centrally, and the other if the chassis is out of square or the leadscrew not exactly in the middle of each end piece. How can I check for all of these? Because there is also the possibility that a bent leadscrew has been fitted at the factory - which I'm hoping is the case and my machine is fundamentally sound. Also I would not be that bothered if the leadnut was not mounted centrally because if I convert to ballscrews I would be fitting a new mount.

[JAZZCNC]

OK, before I take things apart a bit more and get lost, here is what I have found.

1. The motor knob becomes stiff when the leadnut under the gantry can be at almost any point along the length of the leadscrew, i.e. not just at the ends.

Ok you can't feel for stiffening properly with the motor connected because of the Detent of the motors to do it properly you MUST remove motor.

I'm thinking that there could be two ways for bending the leadscrew. One if the leadnut was not mounted centrally, and the other if the chassis is out of square or the leadscrew not exactly in the middle of each end piece. How can I check for all of these? Because there is also the possibility that a bent leadscrew has been fitted at the factory - which I'm hoping is the case and my machine is fundamentally sound. Also I would not be that bothered if the leadnut was not mounted centrally because if I convert to ballscrews I would be fitting a new mount.

Without seeing the exactly how the ends are fixed and the leadnut mounts to gantry then it's hard to say. But if it's how I think then to get a rough idea you'll need to remove the bearing at one end and the run the gantry to that end with leadnut attached and see if the screw is central in the hole.? This should give you some indication if it's misaligned.
The most common reason for bending is because the leadnut is either lower or higher than the ends in middle of it's travel where it's the most bendy it then lifts or pulls down on screw when approaching ends bending screw in process.

To check if screw ends are equal is a simple case of measuring from one reference edge to screw centre(or edge) for both vertical and horizontal. IE: Machine side to screw for horizontal and End plate edge to screw for vertical.

Then if ok loosen the leadnut mount from the gantry so there's no pressure on it and run the gantry to one end. Now check for any gap between Mount and gantry.? Now Run the gantry to other end and see if the gap is the same. Do the same at several points along the screw.
If all is ok the gap should be same or very very close to none, ideally you want light interference fit.! Now it's just a case of shimming the gap so when tightened there's no pressure or pulling on the screw.
If there's no gap and there's tight fit between leadnut mount and gantry then you'll need to remove some material from leadnut mount.
If your leadnut mount doesn't bolt onto flat surface and instead has drop bracket that which fastens to gantry cross brace then you'll need to loosen the bracket to slip fit and put witness marks on bracket and brace. Then run gantry up and down it's length and watch if marks move.? If they do then try loosening lead nut from the bracket and find balance using both bracket and leadnut mounts, failing that you'll have adjust the end fixings.!!

If the machine is out of square then you'll feel it going tight when pushing gantry up and down it's length with leadnut disconnected. Being honest thou with the sloppy linear bearings on these machines I doubt you'll feel it. That said for it to be bad enough to bend leadscrew you'd most certainly be able to visually see it.!!

Obviously I'm giving adjustment suggestions blind has I can't see your machine and how things are fixed but hopefully should give you an idea of what i mean.!!! . . .. . . OR I could have just blown your mind. .

[richie00boy]

The stiffening is a lot worse than other axis though and I can see the leadscrew is bent so I think I'm satisfied with that as the fault diagnosis. The gantry runs nice and free up and down the rails with the leadnut disconnected, so I'm happy that things are square. I can't feel any play.

I don't think I can do these other tests being mentioned because the leadscrew is bent, therefore it's going to make the leadnut not align with the bearing hole when looking through, etc. If it was the leadnut alignment that bent it, well I'm ditching that anyhow.

So what I need now is to spec and purchase a suitable replacement ballscrew system. Then once I have that I can do these trueness checks. I'm going to have to do them anyway to work out the mounting for the ballnut aren't I?

I'll start a new thread for the ballscrew.