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Feedrate for 4th-axis rotary table
« on: August 18, 2011, 06:19:32 PM »
Mach3 has worked great with my 4th-axis rotary table, rotating around the X-axis.  Now I have repositioned the rotary table to rotate around the Z-axis.  Then set Config->Toolpath->Axis of Rotation to "Z-axis".  However it is still using the Z-axis DRO to calculate the feedrate, when it should be using the X-axis DRO.  Has anyone gotten Mach3 to correctly calculate the feedrate for a Z-axis rotary table?

The calculation from IPM to DPM would be something like this:

Z-axis rotary feedrate in deg/min = (360 * IPM) / (2 * PI * (SQRT(X^2 + Y^2) - T))

where:
IPM is the desired feedrate in inches/min
X is the X-axis DRO
Y is the Y-axis DRO
T is the tool radius
PI is 3.14159...

Does my formula look correct?  Is it possible to input this formula into Mach3 anywhere?

Thanks, Rick

Offline BR549

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Re: Feedrate for 4th-axis rotary table
« Reply #1 on: August 18, 2011, 06:46:53 PM »
HUM I don't recall ever seeing a rotary table on the Z axis of a CNC mill.  The motion would just duplicate the interpolated X/Y moves.  I don't think you can sync the feedrate in that combination.

Maybe a picture or drawing  of the setup would help.

(;-) TP

Re: Feedrate for 4th-axis rotary table
« Reply #2 on: August 18, 2011, 07:32:57 PM »
I want to turn inner and outer diameters in a single CNC program.  In the attached photo I would be turning to a specific outer diameter.  The Y-axis DRO is always zero.  The Z-axis DRO is set to bottom of stock.  The A-axis rotates 360 degrees counterclockwise, then the X-axis DRO steps to the left 0.005", and the process continues until 0.02" stock has been removed.

If Mach3 cannot do it, I can just put the feedrates in the g-code when I generate it.  There is no interpolation to worry about.

Offline alenz

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Re: Feedrate for 4th-axis rotary table
« Reply #3 on: August 19, 2011, 02:59:49 AM »
Hi Rick, I looked into this a while back and no, the C-axis (rotary table axis parallel with the spindle axis) is not supported. I had written a little Gcode conversion app for a one time project (a 10 inch clock dial) back 9 or 10 years ago for CNCPro, (a DOS program that predates Mach). The feed rate solution was to calc a new F-word for each block. I wanted to convert the app to Mach but was stumped on the feed calc. Someone, I think from this group, suggested using G93, Inverse Time Mode and that did the trick. 
In response to TP's comment, the referenced clock dial was my reason to convert from rectilinear to polar.  My Sherline 5400 mill only has a 3 ½ inch throat clearance.  So that is the max diameter of a round part that I can cut using X-Y. Convert to polar and it doubles; I can cut anything that will fit inside a 7 inch circle. Or by offsetting the RT in the Y axis I can cut the outer 3 ½ inches of any diameter part (within the limit of the Y travel).  And then admittedly it turned into a challenging fun project that I may never use.  :D
Al
Re: Feedrate for 4th-axis rotary table
« Reply #4 on: August 19, 2011, 04:42:07 AM »
Al,

I thought it might be done with the C-axis too, however, in the latest version of Mach3, there's a place to specify the axis of rotation (X, Y, or Z) for the A-axis.  It's under Config->Toolpath->Axis of Rotation.  I don't think it is extra work for the Mach team, since they would simply plug in different axis DROs to the same feedrate formula, as follows:

X-axis rotary feedrate in deg/min = (360 * IPM) / (2 * PI * (SQRT(Z^2 + Y^2) - T))
Y-axis rotary feedrate in deg/min = (360 * IPM) / (2 * PI * (SQRT(Z^2 + X^2) - T))
Z-axis rotary feedrate in deg/min = (360 * IPM) / (2 * PI * (SQRT(X^2 + Y^2) - T))

OK, the following g-code is a example of "turning" the outer diameter of my part.  It will "plunge" sideways at 5 IPM, then rotate 360 degrees at 5 IPM, and repeat.  However, I must specify DPM (degrees per minute) for A-axis moves, instead of IPM (inches per minute).  The conversion is done with the formula (above).  It assumes Mach3 is zero'd to the center of rotary table.  I used a MS Excel app to generate the g-code.  However, Mach3 could very easily do this math, and probably should be doing it.

Code: [Select]
;TURN OUTER DIAMETER OF PART
;TOOL RADIUS IS 0.15625
N14 X0.469 F5
N15 A-360. F915.9997
N16 X0.4665 F5
N17 A-720. F923.5297
N18 X0.4639 F5
N19 A-1080. F931.1845
N20 X0.4614 F5
N21 A-1440. F938.9673
N22 X0.4588 F5
N23 A-1800. F946.8812
N24 X0.4563 F5
N25 A-2160. F954.9297
;FINISH
N27 A-2520. F954.9297


Rick

Offline BR549

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Re: Feedrate for 4th-axis rotary table
« Reply #5 on: August 19, 2011, 09:58:19 AM »
That particular process is rarely to never supported in the CNC world . That is why you don't find the feedrate sync provided. You can program the machine to cut the part without the aid of the Rotary table setup. Yes even LARGE parts.

The z rotary in that fashion is a hold over from the manual mill days where it was a very handy feature.

NOW the Rotary Z is used on some 6 axis machines but not in the fashion you have setup  AND  the CAM does all the programming .

(;-) TP

Offline alenz

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Re: Feedrate for 4th-axis rotary table
« Reply #6 on: August 19, 2011, 07:09:40 PM »
<snip>
You can program the machine to cut the part without the aid of the Rotary table setup. Yes even LARGE parts.
<snip>
(;-) TP

TP, I have a problem seeing how a circular part that is larger than the throat clearance can be cut in a single set up without using the rotary. For example take a 6 inch flywheel. When the work hits the column I’m only half way across the part and can’t go any farther (even with clever programming<grin>). With the rotary I only need 3 inches of throat travel for the whole job. Tell me what I’m missing? This is a 5400 Sherline with 3 ½ inch throat. This is the situation that prompted the use of the RT in the Z-axis.
Thanks,
Al 

Offline RICH

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Re: Feedrate for 4th-axis rotary table
« Reply #7 on: August 19, 2011, 07:30:54 PM »
Rough cut the square plate to a diameter or shape that fits the throat clearance. Then just use the rotary as an indexer doing  a few revolutions and machine full depth until it's round. Measure and then feed into the piece along the x axis till piece is to dimension. Just away....
RICH

Offline BR549

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Re: Feedrate for 4th-axis rotary table
« Reply #8 on: August 19, 2011, 07:45:12 PM »
I never said single setup(;-). You index the part on the table just like you would do the rotary BUT in a manual way. Machine each section then rotate the part. I have machined  MANY parts that would not fit the table as per say.

I did not mean it to sound like your approach was not a good idea I was trying to explain why you never see the Feedsync on that combination.

(;-) TP

Offline alenz

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Re: Feedrate for 4th-axis rotary table
« Reply #9 on: August 20, 2011, 12:08:20 AM »
Hi TP yeah my bad, all my comments pertained to a single set up.  And I agree my method is not widespread, perhaps the entire user community consists of one user (me).
But I don't understand why? Would you rather have (as in my example) a 6 inch flywheel that was made using my method with a single setup on a rotary and that's known to run true or one that was cut half way, then removed, rotated 180 deg, re-clamped and completed? To me it's a no-brainer. I am ready to defend my position too  :)
Al
« Last Edit: August 20, 2011, 12:23:57 AM by alenz »