Ball Screws for Laser Y axis

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Ball Screws for Laser Y axis

Postby gavztheouch » Sun May 13, 2012 8:38 pm

Has anyone used balls screws to drive their laser cutter Y axis. Im thinking they might cope better with weight of the gantry as I have a larger machine but will they move fast enough for vector cutting.
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Re: Ball Screws for Laser Y axis

Postby macona » Mon May 14, 2012 2:37 am

You can but there are issues. One is the length of the screw. At high speed you get what is called "screw whip" where the screw becomes unbalanced and starts vibrating. On big things this can be quite destructive. A solution to that is mount the screw between two solid points and spin the nut.

Another options is to use a higher lead screw. Something like 1/4" per turn or more. On the laser cutter I have not been building it uses a 1/2" per turn lead screw with a rotating nut.

Belt drives can be scaled up. At work use used it on some very large pieces of equipment. You can also use rack and pinion.
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Re: Ball Screws for Laser Y axis

Postby gavztheouch » Mon May 14, 2012 12:39 pm

Another options is to use a higher lead screw. Something like 1/4" per turn or more. On the laser cutter I have not been building it uses a 1/2" per turn lead screw with a rotating nut.


Yes I have been looking at screw with a large amount of travel per rotation, my only concern was backlash distance might be increased? Not sure about that..

My leadscrew for the Y would be 1M is that long enough to cause whipping issues?

Image

Here is a picture of my machine before I uprated the y axis shafts to 10mm stainless steel.

I think I am getting between 0.1 and 0.2mm backlash on the y axis, looking at my machine do you think it is possible to reduce this backlash.

Many thanks

Gavin
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Re: Ball Screws for Laser Y axis

Postby sports.racer » Mon May 14, 2012 3:24 pm

I have a full-size CNC router with a lead screw that is about 1m long. Whipped and vibrated like crazy until I found this little contraption. http://grumpygeek.com/?p=915

I'm just getting into building a laser and there is no way I would use a lead screw on it.
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Re: Ball Screws for Laser Y axis

Postby Enraged » Mon May 14, 2012 4:13 pm

Why not just run dual motors? You can wire them into the same driver, so you wouldn't get any additional power/speed, but wouldn't that make it easier to balance?
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Re: Ball Screws for Laser Y axis

Postby gavztheouch » Mon May 14, 2012 5:05 pm

Why not just run dual motors? You can wire them into the same driver, so you wouldn't get any additional power/speed, but wouldn't that make it easier to balance?


I like this idea, mainly because it would only mean buying one extra motor. I think I could reuse the couplers I have already and tie the to motors together using a single 10mm shaft, just to make sure they do not move out of sync.
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Re: Ball Screws for Laser Y axis

Postby canadianavenger » Mon May 14, 2012 5:20 pm

Does the backlash reduce/go away if you move slower? I'm wondering if your problem isn't so much backlash as it is forward momentum of the carriage weight.
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Re: Ball Screws for Laser Y axis

Postby TLHarrell » Mon May 14, 2012 6:59 pm

I would first check out your couplers as the cause of backlash. The belt system shouldn't have any backlash if it's tight enough. I can't tell from the picture if you're using solid couplers or not. The best ones are the solid couplers with split collars that tighten around the shaft, not the ones with a single grub screw that presses against the shaft.
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Re: Ball Screws for Laser Y axis

Postby gavztheouch » Mon May 14, 2012 7:45 pm

I would first check out your couplers as the cause of backlash. The belt system shouldn't have any backlash if it's tight enough. I can't tell from the picture if you're using solid couplers or not. The best ones are the solid couplers with split collars that tighten around the shaft, not the ones with a single grub screw that presses against the shaft.


The original couplers were rubbish, Im now onto the solid split collar type that tighten around the shaft, pretty sure they're now doing the job.

I tried a new method today to measure my backlash. I have a light objet dsp which I can use to move my y axis using a command within the software. To measure the actual distance I am using my dial indicator pressed up against the gantry. Then I use the command to move the gantry in the y positive direction say 1mm to take the slack then again 1mm in the +Y direction to move the gantry 1mm. Then I reversed the 1mm movement and bring it back the other way. If the dial indicator tells me it moved 0.9mm I have 0.1mm backlash. This is what I measure about between 0.05mm and 0.1mm.

The next thing I did was move it just 0.1mm back and forth this indicated a movement of 0.05mm. Which does not seem like much.

So now im coming to the conclusion it may be as the canadianavenger suggested forward momentum of the carriage weight. But even on a very slow setting the typical backlash traits still exist?

I decided to run a engraving test using the Y-axis to raster the image. I chose a square box and spread the raster lines apart using a high stepover value of 0.4mm. Amazingly at a very slow speed the box looked more like a diamond, suggesting the carriage was losing steps.
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Re: Ball Screws for Laser Y axis

Postby canadianavenger » Mon May 14, 2012 7:52 pm

If you're losing steps, your belts may be too tight and/or the amount of torque required to move your carriage is too high for your set-up. Are you micro-stepping or full-stepping at each pulse?
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