ballscrew question

[mobus1]

hi folks,
and now, my next question of perhapes many more!
I was fortunate to aquire an ADEPT xyz setup.
nice quality servo/ballscrew drive setup.
the y axis motor mount casting is damaged beyond repair, and replacement parts are too costly.

my plan is to remove the y axis (and perhaps even the x axis) ballscrew and use it (them) in my homebuilt setup i'm starting to work on.

The ballscrew threads are a slow spiral, about 2 "threads" per 1" of shaft length. one rotation of the shaft yeilds about .875" of travel.
I can guess that with a fast moving DC servo drive, this setup must work just fine.

here is the question;
Please help me understand the relationship of the rotation speed vs. the travel distance /speed.
Will the slow spiral work ok to be driven by a stepper motor?
Will the rotation vs. motion travel distance be compatible with a stepper motor?

the ballscrew/shaft setup is so sweet, I'd sure like to use it and save some cash.

Thanks

[Al_The_Man]

Originally Posted by mobus1 The ballscrew threads are a slow spiral, about 2 "threads" per 1" of shaft length. one rotation of the shaft yeilds about .875" of travel. I can guess that with a fast moving DC servo drive, this setup must work just fine. here is the question; Please help me understand the relationship of the rotation speed vs. the travel distance /speed. Will the slow spiral work ok to be driven by a stepper motor? Will the rotation vs. motion travel distance be compatible with a stepper motor?

That pitch is usually considered rather coarse by most CNC standards, Typically pitch of 4 or 5 is normal (.25 ~ .2"/rev).
A pitch of this nature is usually reserved for low load/low inertia applications, such as in a CNC punch where the only relatively light sheet material is the load to be moved, not the table.
With a ballscrew, the mechanical advantage increases with a decrease in pitch or an increase in diameter.
Al.

[ger21]

Originally Posted by Al_The_Man That pitch is usually considered rather coarse by most CNC standards, Typically pitch of 4 or 5 is normal (.25 ~ .2"/rev).

Not necessarily with a router though. Our heavy machine at work uses ballscrews with about a 1" lead. And a lot of guys building routers here are starting to use 2 turn per inch acme, finding that they get much better performance. This is especially true with steppers, because they have much more power when spinning at slow rpm's. Although as Al said, the mechanical advantage is less, so you may need more powerful motors. One other issue is that you lose resolution.

[Al_The_Man]

Originally Posted by ger21 Not necessarily with a router though. Our heavy machine at work uses ballscrews with about a 1" lead. And a lot of guys building routers here are starting to use 2 turn per inch acme, finding that they get much better performance. This is especially true with steppers, because they have much more power when spinning at slow rpm's. One other issue is that you lose resolution.

I guess I am spoilt working with servo's, also the resolution issue usually applies to steppers, as this is overcome by servo encoder resolution.
Al.

[ger21]

Originally Posted by Al_The_Man also the resolution issue usually applies to steppers, as this is overcome by servo encoder resolution. Al.

I was going to point that out, but he said steppers so I didn't bother.

[mobus1]

Thanks folks,
This info makes sense, the ADEPT setup was basically a pick and place application...all it was moving was the weight/mass of itself, plus a pneumatic gripper/ No a large load.

Too bad. The screws are in great shape. I was hoping to use them.

[ger21]

Originally Posted by mobus1 Thanks folks, This info makes sense, the ADEPT setup was basically a pick and place application...all it was moving was the weight/mass of itself, plus a pneumatic gripper/ No a large load. Too bad. The screws are in great shape. I was hoping to use them.

I'd use them if it were me. Maybe I wasn't clear, but I don't see a problem with them, provided you have enough torque.

[LazyMan]

Another advantage of using a higher lead is you may get away with a smaller diameter ballscrew because it wont have to spin as fast. Otherwise you can easiley run into whiping issues with a long screw.

[mobus1]

yes, i guess i did not understand. Seriously, I am i bit dyslexic and I have some trouble grasping written concepts. You can imagine how difficult a time i have with equiptment manuals and docoumentation.

I have learned most things by actually setting up a demo or prototype setup and then playing with it. Thats the best way for me to understand functional relationships, especially with mechanics.

So, bottom line, because of the lesser amount of grooves(threads per inch), it results in a different torque curve? Meaning the motor will tend to "lug" without a higher torque rating?
Sort of like a car trying to start from a stop in 4th gear?

Don't laugh, I sometimes need to use analogies in order to understand.
In any event, it sounds like it would be worth it for me to remove the ballscrew from the broken mount, set up a test bench with my stepper motor driving it and observe/measure results.....Yes?
thanks

by the way, since i have the servos with encoders, maybe i should look into that direction instead of steppers?
I had come across stepper drives large and small, so i just figured i would go that route...any pros and cons offered?

Are the servo drive any more expensive than stepper drives?

Since cost is an issue, I'm thinking ballscrews will cost me more than the servo drives will.

thanks

[Oldmanandhistoy]

Hi,

Just wondering why no one has mentioned gearing?

John

[ger21]

Originally Posted by Oldmanandhistoy Hi, Just wondering why no one has mentioned gearing? John

If you size the motors correctly, you shouldn't need to gear it with steppers. I'd only consider gearing it if you already had the steppers, and needed a little more force.

[ger21]

Originally Posted by mobus1 So, bottom line, because of the lesser amount of grooves(threads per inch), it results in a different torque curve? Meaning the motor will tend to "lug" without a higher torque rating? Sort of like a car trying to start from a stop in 4th gear?

Not a different torque curve, just a different location on the curve (for the better), but less force delivered by the screw. If you don't have enough torque, it won't "lug", but it might just stall, or sort of "sputter" (lose steps).