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View Full Version : Need help with Ball Screw Choice



samualt
09-05-2004, 06:48 AM
I'm trying to pick a ball screw for my Y-axis. I thought I might ask for some advice. Here are my specs:

Motor: Servo with 1500 ozin Peak, 2000 rpm
Y-rails: THK rails 53.5" length
Target IPM (Inches per minute): 200 IPM
Target movement: About 48" movement for Y-axis

My Many Questions:
1. Is it really a big deal to have two ball nuts connected by a preload adjustable coupler? They say it helps with backlash. Worth the extra money?
2. On a ball screw, is 5/8" O.D. (outside diameter) too small for my specs? Is 3/4" O.D. too small? Should I go all the way to 1" O.D. ? I don't want it to whip around and such. Any advice?
3. Wouldn't a fatter screw mean more weight and thus mean it would be hard to start/stop? Should I not worry about this?
4. About how much does it cost to have the ends turned-down on a screw. Some I'm looking at haven't been turned down. It just shows thread all the way to both ends!
5. What Lead or Pitch should I be looking for? Is a Lead of 0.5" good (2 turns per inch of travel)? What about a pitch of .2 (5 threads per inch)? Ehhh, I'm confused, especially because I'm not sure what is good for accuracy/speed. Any hints?
6. On a related note, is 200 IPM too slow for a light-weight aluminum router? Should I aim for 300 IPM? I don't want it to dance around like a ballerina in the nutcracker!

Thanks for any replies............Sam
:cheers:

dmgdesigns
09-05-2004, 06:46 PM
Sam
I have the same motors and useing 66, 42, 18, inch screws. I am deciding on scrrew size and pitch myself.

I would suggest this great read . http://www.cnczone.com/forums/showthread.php?t=5866 It is a well written and very informative article. This will give you a good idea of the screw you need. After this check out the specs on the different screws. They all have suggested max speeds.

I will be watching this thread for other suggestions and answers to the other questions I was about to ask about ballscrews.

Graham S
09-06-2004, 05:09 AM
1. Depends how much accuracy you really want, compare backlash specs for standard and anti-backlash pairs. You decide.

2. Pass

3. Yes is will have higher inertia. Unless you want super performance then no.

4. Pass

5. Pitch effects the resolution and the force applied to the workpiece at given speeds. Read the document linked to above to understand that relationship.

6. Should I aim for 300 IPM? I don't want it to dance around like a ballerina in the nutcracker!

200IPM I think is very fast indeed

Graham

arvidb
09-06-2004, 06:00 AM
"1. Is it really a big deal to have two ball nuts connected by a preload adjustable coupler? They say it helps with backlash. Worth the extra money?"

Backlash is a problem not only because it worsens your accuracy. If you cut metal with backlash you will get more noise, worse surface quality, and you might get considerably shortened tool life. This is because with backlash, there's nothing to dampen vibrations.

Preloaded nuts actually totally eliminates backlash in the screw/nut interface. They might not be of much use unless the rest of the transmission is also backlash free: preloaded angular contact ball bearings for the screw, backlash-free gearing and couplings etc...

Arvid

ESjaavik
09-06-2004, 06:24 AM
Just to clear up a misconception: Two nuts is not necessary to eliminate backlash.
Another way to go is to use special nuts where the offset is built in. Often oversize balls are also used to achieve preload. That makes them much more compact than the 2-nut variety. Go to the web sites of THK, NSK, Bosch-Rexroth (Star), and others. There you can find specifications and scetches of how they are built.
There is also a cheaper solution from Isel where the nut can be adjusted by squeezing it just like we know from Acme screws. Probably good enough for hobby use.

IJ.
09-06-2004, 08:02 AM
HiWin do an affordable 0 backlash Ball nut as well.