CNC Project: Ball Screw Load

[cnc123]

Hi group

I`m hoping someone in the group could help me. I am a woodworker building a 3 axis CNC machine. I have my frame constructed with a moving gantry design. I currently have a 5/8 diameter ball screw with a length of 67 inches. Both ends are fixed to double bearing assemblies.
My stepper is capable of running at 1100 rpms. At this speed and even below I am getting screw wipe. I plan to upgrade to a 1 inch diameter ball screw with a lead of .5. I am hoping to increase my traverse speed by the change in lead. I am uncertain if the change in lead may cause me problems moving the gantry. With the .2 lead and a 666 oz*in stepper I was able to move 230 pounds of steel at 240 ipms. I hope to run my .5 ball screw even faster.(ipm)
I have a new motor that I was going to use with 1200 oz*in. Since I am losing mechanical advantage by moving to a .5 inch lead I am concerned the stepper may not be able to move the load.
Another concern.
Someone mentioned that you have to be careful in selecting the size of a ball screw because a large screw will weigh so much that the stepper may have a problem getting the mass of the screw up to maximum speed. I am wondering
if this is something that may adversely affect my design.
Here are a few pics of the machine under construction

http://www.cnczone.com/gallery/showgallery.php/cat/500/ppuser/5962

Thanks for the help

 

[unclesyd]

Here are some formulas to guide you in analyzing your problem.

http://www.roton.com/web/application.6.jsp

 

[SERVOCAM]

see responses on your other thread:

http://www.eng-tips.com/viewthread.cfm?qid=111935

 

[ccw]

Hi cnc123,
You may evaluate velocity based forces by looking at the impulse-momentum equations. If you double your velocity, mass stays same, then the forces can easily double. Your bigger stepper torque goes up to handle it, and the ball screw pitch doubles, oops! more than doubles, so linear speed more than doubles. So, if the drive rotary speed stays same then you may be on the ragged edge.

Look at Parker/Daedel linear ball slide tables published performance data to see if you are in the right ball park. They have a line of preassembled linear positioning slide tables that are used to stack and make pick-place X/Y/Z robots, etc. You may also be entering the unacceptable gantry frame vibration zone of performance if you are attempting to make precision wood cuts with increased positioning speed.

From talking to the Parker engineer(s), if memory serves, it seems that bearing load/life was a major concern in the higher speed, high production applications. If the bearings wear, get loose, one loses position accuracy and invites the chatter bugaboo at the router (I assume) tip.

CCW