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Mach Ballbar test?
« on: February 21, 2008, 10:53:15 AM »
Hello,
I was wondering if there would be a way to emulate a ballbar test using Mach3?
Thinking out loud here, If you connecteded a digitizing probe to the spindle and a rotary post of some sort, then commanded different arcs on the same idea as an official ballbar test. Could Mach output a plot or something that you could then use to tune the Machines drives or verify the synchronization of the drives? Maybe not a full-fledged Ballbar test but it could help for troubleshooting and fine tuning.

Thanks for any input/ideas,
Cutmore
Re: Mach Ballbar test?
« Reply #1 on: February 21, 2008, 12:23:58 PM »
Hey CUTMORE,
I have no answer for ya. Just want to thank you for introducing me to yet another aspect of CNCing that I'd never heard of.
Question: The Renishaw reads to a micron...When setting up on a mill for example, does the software compensate for the G coded arcs not having the EXACT same radius point as the sensor ? Seems like there would be some degree of concentricity in the setup. Especially at the micron level.
Thanks for the BOOT,
RC
Re: Mach Ballbar test?
« Reply #2 on: February 21, 2008, 12:33:06 PM »
Overloaded,
The software for the Ballbar test only records/displays the path the machinetool makes, It is not intended to be control software. What you would do is perform the ballbar test to determine where the machinetool errors are, (ie. x faster than y in 1st quadrant) and from that you can determine whether backlash or servo tuning etc... is the cause. You can perform arcs in different directions to check that all 3 axis move where they are told to go.

If this could be made inexpensively everyone with Mach could measure the accuracy of their machines.

Cutmore
Re: Mach Ballbar test?
« Reply #3 on: February 21, 2008, 01:04:34 PM »
Thanks CUTMORE, I understand more now.
RC
   
Re: Mach Ballbar test?
« Reply #4 on: May 01, 2009, 03:39:35 PM »
I know this is a very old post, but I wanted to bring it up as I have done a little pondering about building such a test tool.

Just for background, the Ballbar test is simple in concept:  it measures your CNC machine accuracy by connecting a special digital scale to your CNC.  One end of the scale attaches to your CNC head (router head, waterjet head, mill head) and the other end attaches to a fixed point on the table.  Then you program your CNC to do a circle around the fixed point.  The digital scale records, very accurately, the path taken then analyzes it and displays it on a computer screen with diagnostic information.

http://www.renishaw.com/en/6813.aspx



When I first saw this, I thought I could build a similar unit in the shop.  I would use a chinese micrometer scale hacked to send the data to a PC on the serial port (see http://www.shumatech.com/support/chinese_scales.htm ).  To handle the pivot on either end, use something similar to the ballbar: magnetic ball joints (see http://www.tmcmagnetics.com/ball_joint_assembly.html )

Finally it would be then an exercise in writing some software that would read the scalefrom the serial port and compare it against a known circle.

Thoughts? Comments?  Easier high accuracy ways to test accuracy of a CNC?

thanks


Offline RICH

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Re: Mach Ballbar test?
« Reply #5 on: May 01, 2009, 06:05:15 PM »
Hi All,
Never heard of that tester before and the link was interesting.
Sometimes just happy that the circle cut won't accept a square peg !  ;)
RICH

 
Re: Mach Ballbar test?
« Reply #6 on: May 04, 2009, 06:00:50 AM »
Hi all,
I too have been trying to come up with a cheaper Ballbar device than the Renishaw version. FYI there was a used one on Ebay for $6000.!

I have found a LVDT which will output a quadrature signal or an Rs232 signal.  You can download a free version of the Renishaw ballbar software.

I want to develop something which will work within Mach that woud allow eveyone to have the ability to quickly evaluate their machine's health/accuracy.

Cutmore
Re: Mach Ballbar test?
« Reply #7 on: February 14, 2011, 06:54:05 PM »
There is another type of ball bar test, it is an old method of checking a CMM that only requires a bar with a ball at either end (sometimes mounted on a pin 45 degrees from the bar) that and a spreadsheet.  It's a method of checking for position accuracy and squareness without requiring an expensive calibrated reference like a laser or the type of ball bar system mentioned previously.  The only part that needs to be accurate is the two balls have to be the same size - not a specific size, mind, just the same size – and a good sphere, I think for our purposes a couple of large ball-bearings, 3/4" or so, might do.  In industry, the part that is calibrated would be the balls being as close to a perfect sphere as possible and the same size.
It should be possible to probe one of the balls with a wizard to find the center of the ball, probe the second ball, find the center of the second ball and then compute the distance between centers.  If you did this with the bar at 45 degrees to two axis`s and then did it again a different 45 degrees to the axis`s and your results were the same for both, then that would mean your axis’s would be square and accurate at that area.  A complete check would be more complex, but just the same several times over.  It's similar to measuring corner-to-corner to check square.
Does anyone know the method of probing and finding the center of a sphere - my math stopped in high school 40 years ago. :o
BrianC
Re: Mach Ballbar test?
« Reply #8 on: November 03, 2015, 02:56:21 AM »
Hi All

I know this topic is rather old but I thought I'd make a belated post about this as I used to work for Renishaw calibrating ballbars (among other things)  so I have some knowledge of how they work if anyone is interested