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Silly drive screws..worth upgrading to a ballscrew setup?

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gamble
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Silly drive screws..worth upgrading to a ballscrew setup?

Postby gamble » Mon Sep 19, 2016 9:43 pm

These stupid drive screws and anti back lash nuts are bothering me. They have maybe 5 hours of use and today I got 2 jacked up cuts then the gantry wouldn't move. The anti back lash nuts unscrewed themselve. Any idea on why that happens?

Worth upgrading to a ballscrew setup?
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sphurley
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Re: Silly drive screws..worth upgrading to a ballscrew setup?

Postby sphurley » Mon Sep 19, 2016 10:10 pm

The ones I have the plastic backlash screws into a aluminum housing. Installed it with blue locktight and then it has two set screws that also hold it. (lock tight them also).
Not had one problem with it.
Steve
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gamble
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Re: Silly drive screws..worth upgrading to a ballscrew setup?

Postby gamble » Mon Sep 19, 2016 10:24 pm

Your drive screw? How fast have you cut? Wondering if me cutting over 200ipm wears them out faster or makes them loose.
I will try some blue lock tight


where did you buy yours from? wondering if they are the same as mine
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Dingo745
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Re: Silly drive screws..worth upgrading to a ballscrew setup?

Postby Dingo745 » Tue Sep 20, 2016 9:17 am

The cost of real ballscrews off Ebay is not that much, but the improvement is massive, so just do it.

gamble
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Re: Silly drive screws..worth upgrading to a ballscrew setup?

Postby gamble » Tue Sep 20, 2016 9:30 am

Will I be able to go faster with them?
Any advantage to going to a larger diameter than what is on my current setup?
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Larry83301
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Re: Silly drive screws..worth upgrading to a ballscrew setup?

Postby Larry83301 » Tue Sep 20, 2016 11:53 am

gamble wrote:Will I be able to go faster with them?
Any advantage to going to a larger diameter than what is on my current setup?


Going faster depends on how much weight you are moving around and how strong the motors that drive the screws are. The faster you go the harder it is to change directions, your motors need to stop all that weight and then start moving it again in a different direction which takes a lot of power. I upgraded to ball screws several years ago and I love them, I also went to larger screws which makes for a smother movement I think. You didn't say how big your screws are now, so it's hard to say if bigger ones would be an advantage, but I'm sure you will like how smooth they run and ball screws repeat much closer then regular screw do. IMHO :D :D


Larry

gamble
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Re: Silly drive screws..worth upgrading to a ballscrew setup?

Postby gamble » Tue Sep 20, 2016 12:06 pm

Larry83301 wrote:
gamble wrote:Will I be able to go faster with them?
Any advantage to going to a larger diameter than what is on my current setup?


Going faster depends on how much weight you are moving around and how strong the motors that drive the screws are. The faster you go the harder it is to change directions, your motors need to stop all that weight and then start moving it again in a different direction which takes a lot of power. I upgraded to ball screws several years ago and I love them, I also went to larger screws which makes for a smother movement I think. You didn't say how big your screws are now, so it's hard to say if bigger ones would be an advantage, but I'm sure you will like how smooth they run and ball screws repeat much closer then regular screw do. IMHO :D :D


Larry

The torchmate drive screws are small, I think the thread is 3/8 and the overall size can't be much bigger than 1/2" od. I'll measure when I get home
The motors (forget the size) but they can handle 500ipm and I also upgraded my cards to 5amp cards
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Re: Silly drive screws..worth upgrading to a ballscrew setup?

Postby tcaudle » Tue Sep 20, 2016 2:56 pm

Speed is determined by the Motor RPM divided by the TPI of the leadscrew. The best CUTTING RPM of a stepper is in the 500 RPM range or less so you still have reserve torque left for acceleration. SImple fact: The faster your feedrate the more acceleration you need to hold a tight toolpath track at 225 IPM (for say 14ga at 45A) you need greater than 25 IPS/sec acceleration to get good tracking.

So....lets say you have 5 TPI then your top cutting speed where you can still get acceptable performance is about 100 IPM (500/5) So you would think that the solution is to externally step UP the RPM of the motor using belts and pulleys; In essence spin the leadscrew faster, BUT that brings other consequences. First there is lead whip that limits the max RPM of the lead-screw based on the length, the rigidity (diameter) and the type of mountings on each end. You have to stay under that RPM or the leadscrew deforms and causes the friction to increase dramatically (even on a ballscrew). Second , the type leadscrew you describe uses opposing nuts and usually nylon cores to cut down on backlash but at the expense of greater friction. There is a definite RPM limit before the lead nuts overheat and start to bind. The further you have to move in a continuous line the quicker the nuts overheat.

So you need to look at your calibration of the number of steps per unit to travel one inch and that divided by the microstepping number X 200 will give you the true TPI. True ballscrews would work better (lots less friction) but I doubt you would get to 500 IPM unless the TPI is 2 or less.

You have to fight the Physics of the stepper motor that predicts that torque drops off with RPM to a point that it does not have enough torque to move the load. Max motor RPM is a factor of the DC voltage , the drivers and the actual motor inertia . Most smaller steppers top out at about 1000 RPM running at 48Volts with a moderate load but after about 500 or 600 RPM , torque drops of in a non-linear fashion. (Translated: you don't have enough torque to rapidly change direction)

There is a reason most plasma tables use rack & pinion or toothed belt final drive rather than ballscrews.

jimcolt
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Re: Silly drive screws..worth upgrading to a ballscrew setup?

Postby jimcolt » Thu Sep 22, 2016 10:54 am

Excellent and correct analysis from Tom (tcaudle). Ballscrews (and I have been involved with design and build of a number of ballscrew cnc plasma tables over the years) are great on motion systems up to about 4' in length....as the ballscrew diameters can be kept within a reasonable size.....as the length increases the ballscrews will twist from the torque during high acceleration and become somewhat irratic as well as less accurate......so increasing the size of the ballscrew seems like the solution. With larger ballscrews comes the requirement for a larger drive motor as well as dramatically higher cost.

On high accuracy industrial plasma machines with more than 4' of travel....expect to see helical rack and pinions with mechanical anti-backlash mechanisms and zero backlash planetary gear boxes. These are the most accurate, cost effective and reliable methods of moving large machines with high travel speeds and high acceleration in a dirty industrial environment for many years!

Again, on small machines (I think the OP Gamble, has a 2' x 2' machine) ballscrew drives are achievable assuming the motor rpm is properly calculated per Tom's (more experience than most of us) suggestions! Jim Colt Hypertherm


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