CNC: lead screw and load question 3

[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

 

[SERVOCAM]

I took a look at the sizing, guess on some of the missing information. Sizing for a 67" long ball screw with a 0.2 in/rev lead with a load of 230 lbs and making a 24 inch move with a velociy of 4 inches/second (260 ipm) using a 0.25g accel rate (profile is pretty square) and I came up with it only requires 82 oz-in (using 23 frame 1 stack step motor). Now I don't know what your opposing force is, so do you know that?

As far as screw whip, I calculated the critical speed of your screw but I'm not sure on the root diameter of your screw or your bearing supports from the pictues. I'm going to use a 0.480 inch rood diameter and a simple-simple bearing for the fixity factor. Based on that I came up with 508 rpm. 508 rev/min * 0.2 in/rev = 101.6 in/min. So your 240 ipm will be whipping the screw pretty good.

Going up to a 1 inch diamter and a 0.5 in/rev lead, its critical speed would be (using 1.0 Fixity and 0.830 inch root diameter) 880 rpm. 880 rev/min * 0.5 in/rev = 440 in/min. Deffinately be able to pick up speed. Now on torque, resizing using the same move profile, same load (no opposing force) I calculate we only need 220 oz-in of torque @ 480 rpm (using 34 frame 2-stack step motor) and only a ~5:1 inertia mismatch.

Tell me what mfg and part number of your motor. Also, do you know the bearing coefficient of friction, screw preload, opposing force from machine tool? Also, what are you using for an accel/decel rate? Are you using time-based or rate-based? Do you know the bearing fixity factor?

-cam

 

[SERVOCAM]

Another thing to consider when selecting ball screws is compression loading or also known as column loading.

I checked the column loading for your exsting screw using a 0.480 root diameter and a simple-simple fixity factor. It can handle about 166 lbf before it will buckle. So depending what your using for an accel rate, you could be buckling the screw as well. If you accel rate is higher than 0.72g's, you will buckle your screw. Of course, I didn't figure in the 20% margin either, so could be lower.

Going to a 1" diameter screw with a root diameter of 0.830 inches, your column loading will go up to 1485 lbf. No problem there.

 

[cnc123]

Hi Servocam
Thanks alot for the detailed help. I can`t supply any motor data because it seems the motor company does not
provide that type of data. That is a troublesome issue because I can`t readily tell what to expect from the motor. Here is all the info I have:
Holding Torque (2 phases on)......8.7 (1208) Nm (oz/in) +/- 10%
Rated Current per Phase.6.0 (Amps DC)
Phase Resistance...0.45 (Ohms +/- 10%)
Induction (mH) +/- 20% typical..5.1
Motor Length (mm)118
Rotor Inertia (g-cm^2)2700
Weight

Also, do you know the bearing coefficient of friction, screw preload,(NA) opposing force from machine tool?,(NA) Also, what are you using for an accel/decel rate? I am
using Mach2 software for the accel/decel. But I`m not certain how they designed the accel curve. Are you using time-based or rate-based?NA) Do you know the bearing fixity factor? One end has two bearning and a radial bearing and at the far end one bearing and a radial bearing. Note:
I just tried to tension the ball screw.It seems the screw
is running at 200 plus ipms with very little whip. I am still thinking about up-grading to a larger diameter. I am a bit concerned about losing resloution if I go to a .5 lead. I believe if I`m correct that a half-step setup would
provide a single move of .00125. That`s probably close enough for woodworking and some metal machining.

Thanks again for the great help!!!!!!!!!!!!!!!!!!!!!!

 

[SERVOCAM]

Yes, with a 0.5 in/rev screw and 400 step/rev your resolution will be 0.00125 inches. Keep into consideraion the mecahincs of the system as well (coupler windup, backlash in the ball nut, bearing stiffness, lead error) that will effect your accuracy & repeatability.

For example, on lead error, if your ball screw gains 0.005 inch/foot, over your 67 inches, you will have an error of 0.027916 inches. Maybe your software has lead screw compensation and backlash compensation.

I would think your step motor would hold its torque to 500 - 600 rpm....but you don't need that much torque anyways. What feed rate do you expect to hit? I would expect you could hit 500-600 ipm (not counting on opposing force from cutting and your voltage/speed limitation on drive/motor).

It sounds like you were able to improve the loading on the screw with the bearings, so using a Rigid-Simple fixity factor, a 0.830 rood diameter, I calculate 1293 rpm critical speed.

Sizing using the 230 lbs and 0.5 in/rev screw at a feed rate of 600 ipm w/ 0.25g accel rate, you will need 250 oz-in @ 1200 rpm. (this is also based on the step motor I'm sizing off of that is capable of higher speeds and has an 160VDC bus. I see you have a transformer in there, 75-80VDC??)




-cam
"Green Eggs & Ham" by Dr. Suess
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