AFFECT OF MISSING BALLS IN BALL SCREW

[Khalid]

I have ball screw installed at all three axis of my milling machine. The ball screw installed at X-axis has some missing balls in it. I set all my 3-axis in half step mode and the steps/unit in Mach3 is 2040.81634 (previously i was assuming my ball screws as 5TPI so i set STEPS/Unit as (200 x 5x 2=2000). I have following questions:

1- Whenever i am machining , my X-axis shifting one side. If it is shifting left side then it contineously shift left side with small increments till the completion of job and i m getting diagonal machine part. Is that the affect of missing balls? The longer the job the greater the accumalitive shift value

2- I tuned all my ball screws with Mach3 AXIS CALIBRATION (STEPS/UNIT) Setup.and found all three with same 2040.81634 steps /unit

3- When i draw circles. The smaller circle has little inaccuracy. As the radius of circle increased i m getting smaller the diameter of circle. i.e If the circle has 20mm dia i am getting 19mm. If circle has 40mm dia i m getting 38mm. as the size increases the greater the difference.The circle i get is complete circle with no distortions.
3- I have no backlash in my ball screws.
4- 2040.81634 is that correct??/ If u have something nearer value then please tell me...
Please tell me what to do. And if this is the problem of missing balls??? what will be the affect if some balls missing in ball screw????

[Khalid]

One thing more.. I have set all 3-axis with 100 IPM(I have experienced no loose steps at 150 even). But whenever i run 3-axis parts (contouring) my machine too slow as i m getting not much more than 12 IPM. Why this happens?? Is that value .81634 after 2040 affect it to move too slow. How can i increase my working IPM in contouring??? I am working on wood currently doing V-carving and embossing work i.e. relief

[Khalid]

In attached figure of THE INITIAL STATE SETTINGS in MACH3 ,all values are default set??? Please till me if i have to change anyvalue to get good running IPM out of my machine??/
Your help in this regard will be highly appreciated.

[LeeWay]

Not real sure I am understanding everything correctly, but I'll give it a whirl.

Missing balls is not good, but not totally bad. If it is only a couple, I wouldn't expect much backlash. It will wear quicker I would think. There is some space between balls in linear ways, and I would assume the same about ball screws. I need to open one of my screws to check the ball size. I may want to go with over sized balls in the future.

You should be able to determine the actual pitch of the screws. My first ball screws were 13/64" pitch on a 5/8" screw. A little difficult to figure, but once you get the exact decimal value, it is accurate. My second set of screws were a little easier. 15 mm screws with 10 mm lead. I measured this on the screw itself. Count how many groves you see in an inch or CM. Convert either to a decimal value and you have something to work with. The value for my 13/64" screws @ 10 microsteps turns out to be accurate @ 9846.153846 steps per inch. Odd values like that are fine if that is what the math works out to. My other screws are 5080 steps per inch.

If you are driving your gantry or table with a single screw for a long axis, you may be experiencing flex or skewing/racking. This will surely throw the gantry out of square. I built my router using steel and was therefore able to preload the bearing trucks enough to get rid of racking and square the gantry to the table, but less rigid machines might only be able to get rid of some of the racking. Driving this axis with two screws would be the best way to insure that you won't have trouble with racking and unsquare cuts.

If you are getting perfect circles and they are only off by dimension, then I think you should refine the screw lead calibration. This is assuming your spindle and Z axis is solid with little play or flex.

If you have your axes set for 100 IPM, then when the machine travels rapid from cut to cut, it should run @ 100 IPM. To increase the speed during the cut, this is the feedrate, it should be done in your cam program. It may have a preset feed speed for a particular tool you choose in the Cam, but you should be able to adjust to your liking.

On your screen shot, I like to use constant velocity rather than exact stop. It just seems to work smoother. If you have a lot of intricate corners though, exact stop might be better and give more accurate results. It would be slower though. CV doesn't slow down for direction changes much, but exact stop is just what it says it is.
I liken CV to highway driving or Nascar and ES to city stop and go traffic.

[Khalid]

Not real sure I am understanding everything correctly, but I'll give it a whirl.

Missing balls is not good, but not totally bad. If it is only a couple, I wouldn't expect much backlash. It will wear quicker I would think. There is some space between balls in linear ways, and I would assume the same about ball screws. I need to open one of my screws to check the ball size. I may want to go with over sized balls in the future.

You should be able to determine the actual pitch of the screws. My first ball screws were 13/64" pitch on a 5/8" screw. A little difficult to figure, but once you get the exact decimal value, it is accurate. My second set of screws were a little easier. 15 mm screws with 10 mm lead. I measured this on the screw itself. Count how many groves you see in an inch or CM. Convert either to a decimal value and you have something to work with. The value for my 13/64" screws @ 10 microsteps turns out to be accurate @ 9846.153846 steps per inch. Odd values like that are fine if that is what the math works out to. My other screws are 5080 steps per inch.

If you are driving your gantry or table with a single screw for a long axis, you may be experiencing flex or skewing/racking. This will surely throw the gantry out of square. I built my router using steel and was therefore able to preload the bearing trucks enough to get rid of racking and square the gantry to the table, but less rigid machines might only be able to get rid of some of the racking. Driving this axis with two screws would be the best way to insure that you won't have trouble with racking and unsquare cuts.

If you are getting perfect circles and they are only off by dimension, then I think you should refine the screw lead calibration. This is assuming your spindle and Z axis is solid with little play or flex.

If you have your axes set for 100 IPM, then when the machine travels rapid from cut to cut, it should run @ 100 IPM. To increase the speed during the cut, this is the feedrate, it should be done in your cam program. It may have a preset feed speed for a particular tool you choose in the Cam, but you should be able to adjust to your liking.

On your screen shot, I like to use constant velocity rather than exact stop. It just seems to work smoother. If you have a lot of intricate corners though, exact stop might be better and give more accurate results. It would be slower though. CV doesn't slow down for direction changes much, but exact stop is just what it says it is.
I liken CV to highway driving or Nascar and ES to city stop and go traffic.

Thank You very much LEEWAy for detailed reply. I have following to say:

a)My machine runs rapid at 100 IPM. I set my CAM (i use Vcarve Pro) in tool setting 100IPM too...currently i m using ES not CV.. I will try CV now...

b) What is the optimum value of acceleration in Mach3?

c)How can i increase the feedrate in Vcarving??

[LeeWay]

Optimum accel. will be entirely different for every machine. This is why it's called motor tuning. You have to tune each axis. I have different rates on all my axes. I turn up my IPM and my accel. until I start missing steps. Then I back off by about 10 to 20%. Then when the machine is cutting under load, if you miss no steps, fine. If you miss steps, back off both settings some more until you miss no steps.

My settings on my mill are about 180 IPM with about 12 Accel. There about anyway. My Z axis is a little slower, but it's a lot heavier. Something like 120 IPM and 7 or 8 on accel. This isn't max, but what I am comfortable with running on a fairly small mill. If I was doing some intricate 3D work, I could turn up these settings to about 300 IPM on X and Y.

You will have to tune the motors differently when you change from CV to ES to get best performance.

I am not yet familiar with Vcarve, so someone else will have to help out with that aspect. You could post some of your Gcode. That may help pinpoint why it is feeding so slowly.

[Khalid]

Oh...one thing more in CV mode what CV FEEDRATE u put in??..currently i have default +1 as shown...

by increasing the acceleration value what we get...???

Best Regards

[Khalid]

here are the VCarve image of the design i m working on and attached is also the gcode file of it

[ger21]

1- Whenever i am machining , my X-axis shifting one side. If it is shifting left side then it contineously shift left side with small increments till the completion of job and i m getting diagonal machine part. Is that the affect of missing balls? The longer the job the greater the accumalitive shift value

No, you're losing or gaining steps somewhere if it continually drifts in one direction only.

3- When i draw circles. The smaller circle has little inaccuracy. As the radius of circle increased i m getting smaller the diameter of circle. i.e If the circle has 20mm dia i am getting 19mm. If circle has 40mm dia i m getting 38mm. as the size increases the greater the difference.The circle i get is complete circle with no distortions.
3- I have no backlash in my ball screws.
4- 2040.81634 is that correct??/ If u have something nearer value then please tell me...
Please tell me what to do. And if this is the problem of missing balls??? what will be the affect if some balls missing in ball screw????

These are not caused by missing balls. If the circle gets smaller as the size gets bigger, it sounds like your steps/unit is incorrect. If you make a series of suares, do they get smaller as the size gets bigger? If a series of squares does the same thing, then your steps per unit is wrong.


And as for CV mode, make sure you have the latest version of Mach3. There have been some fixes to CV recently.

[Khalid]

Thank you GER21
I have Mach3 lockdown Ver Version R1.83.027...
Now as per Leeway instructions i changed to CV mode and changed the IJ mode to ABSOLUTE...and did machining of above part (Size is 320mm x 74mm x 10mm) in 36 minutes... I found no problem with it... I dont know but i will further machine some bigger parts then i will see what happeneding...

would you all tell me what CV FEEDRATE i put????

[Juka]

4- 2040.81634 is that correct??/ If u have something nearer value then please tell me...
Please tell me what to do. And if this is the problem of missing balls??? what will be the affect if some balls missing in ball screw????

Try to change dir and step pulse to 5 instead of 3, or have you try it yet ?

[Khalid]

Thank You JUKA for your time...I changed many time the Step and Dir to 5 but it automatically comes to 3 and some time comes to 4... some time it remains 5...
By the way increasing the Step and Dir value what we can get or what happen?