4th Axis Weight and Size limitations?

[Dylan]

Hey there, so cnc user here.  I just made my first mostly (there were all kinds of novice user errors made ) successful project with my Mega V XL.  So now I'm ready to sink my teeth into an ambitious project that I need to get done for a costumer. (Got to pay the bills right?)  What I'm making is a pair of legs for a kitchen island overhang, they are kind of like corbels, scrolly with acanthus leaf on one side.  Anyway the piece that I want to do with the cnc would be made from a piece of cherry 28" long by about 8 6/8" by 8 1/5".  I'm estimating that a piece like that will weigh about 40 lbs.  I had been thinking to use Millright's rotary axis that I purchase along with the mega v to do this.  Here's my question, do you think the 4th axis can handle a piece of wood that large and heavy without issues?  Or would I be smarter to get Vectric's Cut3D and mill it from each side?

And yes, I know that I will need to do some modifying to actually put a piece of wood that large on Millright's Mega V 4th Axis. 

Thanks in advance for any advice.
Dylan

[bLouChip]

Dylan,

I believe the answer is safely yes, that you can use the MegaV A axis for that 40 lb x approx 8.5 inch dia rotational work load. I calculated the torque to be 64 oz inch force necessary to spin up the work load from 0 RPM to an effective 2100 mm/min feedrate (or 3.1 RPM) and sustain it. A significant factor of the necessary force is the acceleration of the A axis;  you need to reduce the factory value ($123) of 4000 deg/sec/sec to a value of 750. This value translates to 1400 mm/sec/sec acceleration (for 8.5 inch dia) which is same as the XY axes factory value. The MegaV stepper motors are rated at 270 oz inch of available torque.

Related... there is a bug in MegaV grbl controller which yields an incorrect and significantly slower feedrate when gcode motion of G1 XA statements are used and the motion mode is G94 Distance Time mode, which is the normal/default and usual motion mode. The bug does not manifest itself in G93 Inverse Time motion mode, so use that in your CAM post processor if possible to avoid the bug. If your CAM post proc does not offer G93, then I have a fix for the grbl controller or you can get it from MillRight Support.  I corresponded with MR Support on this and provided the fix earlier this year in Jan-Feb time frame. More info on the bug and fix is here: millrightcnc.proboards.com/post/25719/thread

here is a link to the torque solution: docs.google.com/spreadsheets/d/1V5kBrP5P3NUUXZMAOPrUZX7nbjM268Co/edit?usp=sharing&ouid=113908666165449996050&rtpof=true&sd=true

Good luck with the project

Cheers,

Lou

[Dylan]

Hey Lou,
That information is very helpful to know and I really appreciate you taking the time to work it out. I only just today started playing around with the fourth axis and I'll be doing some test projects for sure before trying anything the size of the island legs. Due to software, machine size limitations and how long I want the machine to run each day I'm still debating the best method to tackle the island legs. So much to learn and consider.

I will have to check see if the bug is an issue for me, I am using the post processer that Vcarve downloaded for the Mega V when I installed Vcarve. In reading your comments about the bug I see you mentioned that UGS 2.0.12 supports 4th axis control. Would you recommend using that over Millright's version of UGSplatform?

[bLouChip]

update: I misunderstood my son's correction to the solution, xls has been updated again.  Answer remains similar, there should be enough torque to get the job done on that 40lb work load, but adjust A axis acceleration to 500 deg/sec^2.  I also added an estimated torque allowance for chip load.

Dylan, you're welcome, glad to help.  I've been needing to do the same calc for torque needed to rotate a steel pipe for a plasma cut coming up later this year.

I just had my MechEng son review my torque solution and he found a couple of mistakes, so I adjusted the xls in the reference link. The new torque value to your work load is 129 oz-in, still within the specs of MegaV motor.  A good test would be to give 100 or so alternating positive and negative rotational commands of at least 180 degs each, at 2100 mm/min or 1140 deg/min, using your 40 lb 8.5 in dia work load.  Then make sure the chuck returns to the home position with accuracy. If it doesn't, you're likely loosing steps during acceleration, so reduce A axis acceleration until your return to zero is accurate.

Thus far I have only used my 4thAxis on laser etching of powder coated tumblers, which is how I discovered the G94 G1 XA feedrate bug; I didn't have the option to use G93 motion mode, LightBurn does not support that mode.

re. UGS latest ver vs. MR-UGS,  I was using UGS from github on a CarveKing machine for a year or so before I got my MegaV, so I just continued with what I had since it was already supporting 4thAxis.  The general rule of thumb - if it ain't broke, don't fix it; applies to software as much or more than anything   I do recall recently I was helping a fellow MegaV user get through an issue where the grbl configuration needed adjustment, and we discovered that the MR-UGS has an MR custom setting control which locks-out the ability to change grbl config settings.  See this post to unlock: millrightcnc.proboards.com/post/25786/thread

I have found that UGS does a good job of keeping configuration data separated by version, so exploring multiple versions should not be a problem.