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4th axis toolpath wanders
« on: April 09, 2011, 04:21:06 PM »
Can somebody help me understand.  Got a 4th axis setup and all I've seen is adjust motor calibration to whatever you can can stand.  What does this mean?  I put a part in and try to cut and the tool path wanders around no matter what the motor settings are for a-axis.  What do I have to do to tune 4th axis?
Re: 4th axis toolpath wanders
« Reply #1 on: April 09, 2011, 04:43:27 PM »
Have you set the A Axis as angular in general config.

In the setting page,ALT-6,  they're DRO for rotation radius, try to enter the radius of the part in the A DRO.


Jeff

Offline RICH

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Re: 4th axis toolpath wanders
« Reply #2 on: April 09, 2011, 05:06:55 PM »
Quote
What do I have to do to tune 4th axis?

I will assume it's driven by a stepper motor and if that is the case then you "tune" it the same way you would the other axis.
The A is still just getting pulses and it can be linear or angular in setup. So adjust velocity and acceleration the same way.
RICH

Offline Greolt

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Re: 4th axis toolpath wanders
« Reply #3 on: April 09, 2011, 08:49:08 PM »
I am guessing that "wanders" means slow.

Tune the axis to whatever it is reliably capable of.

All axis move in units per min.   With a rotary axis those units are degrees.  

So what is 60 ipm on the linear axis (desired speed of the tool in the work), is 60 degrees per min for the rotary.

That 60 degrees per min angular feedrate will make the tool move through the work at a speed dependant on the distance the tool is away from the centre of rotation. (in your case, very slowly)

So Mach has a feature to compensate the rotary axis feedrate, to accommodate differing radius that the tool is cutting at.

It is activated via the Toolpath Setup menu.   Check "Use Radius for Feedrate"  All the other settings in this box are to do with the toolpath display window.

On the Settings page there are three DROs labelled "Rotation Radius".  IMO they would be better labelled "Rotation Offset Radius"

They are to tell Mach the distance that the relevant axis origin (Z in this case) is offset from the centre of rotation.  (A axis in this case)

So if you are machining on the outer surface of a 10 unit diameter job and Z axis origin (zero) is set on that outer surface, then the correct value for the "Rotation Offset Radius" DRO is 5.  The distance that Z origin is OFFSET from centre of rotation.

If, on the other hand, the Z axis origin is at the centre of rotation (my preferred method for most jobs) then the correct value for "Rotation Offset Radius" DRO is zero.  The distance that Z origin is OFFSET from centre of rotation is zero.

Mach takes the Z axis DRO value and the "Rotation Offset Radius" DRO value and adds them together to ascertain at what radius the tool is cutting at any one time.  Then compensates the angular feedrate to have the tool move through the material at the desired speed.

Maximum velocity as set in motor tuning is honoured, so that will always be the upper feedrate limit.

Now there is one little "Gotcha".   A zero value in the "Rotation Offset Radius" DRO will automatically disable the entire feedrate compensation feature.  This is a known bug and is being addressed by Artsoft at this time.  Hopefully it will be fixed soon.

The workaround for this, is to use a very small value (eg. 0.001) in the "Rotation Offset Radius" DRO when zero is the correct and desired value.  Small enough to have no measurable effect on feedrate, but not zero.

Hope that all makes sense.  ;)

Greg
Re: 4th axis toolpath wanders
« Reply #4 on: April 09, 2011, 08:59:57 PM »
No, slow is not the problem,  I can live with slow relative to other axis movements.  This problems is it doesn't engrave in a straight line relative  to long (x) axis.  It starts a line of text crooked then the line below starts writing in the wong place, oftentimes overwriting due to crookedness of line above.  Seems like the stepper is slipping losing steps as it rotates.  Slowing down doesn't help , I thought slower speed would improve torque and eliminate slippage, wrong?

Offline Greolt

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Re: 4th axis toolpath wanders
« Reply #5 on: April 09, 2011, 10:59:55 PM »
No, slow is not the problem,  I can live with slow relative to other axis movements. 

Wrote all that for nothing.  ;D  ;D

Hope you get it sorted.

Greg

Offline RICH

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Re: 4th axis toolpath wanders
« Reply #6 on: April 10, 2011, 08:09:44 AM »
Quote
Wrote all that for nothing.
Not really Greg as it's a good description of setup.

cadwiz,
Maybe just try cutting a straight line for some length and repeating for a few rotations. Maybe your setup is offset is off center or there is runout on the piece.
Could also be how you used the software to generate the code and it is crooked to begin with.
FWIW,
RICH
Re: 4th axis toolpath wanders
« Reply #7 on: April 17, 2011, 02:54:48 PM »
Got it.  Did a simple test, 3 horizontal lines running along x-axis with some up/ down zig-zags. Corner to corner.  Simple code and it works perfect, lines end up going to corners and not outside horizontals.  Did some more searching and figured losing steps due to complexity of cutting characters (20,000+ lines of code).  Changed step/dir pulse direction in motor tuning from 2 to 5 and boom, letters engrave in straight line now and don't start overwriting.  Man, CNC tweaking is a PAI sometime, been battling this for several weeks now.
Cadwiz
Re: 4th axis toolpath wanders
« Reply #8 on: April 17, 2011, 03:49:45 PM »
Pulse duration from 2 to 5, not direction.  Stupid iPhone autocorrect