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Turn | Is eccentric turning possible with Mach4?
« on: October 06, 2019, 03:02:28 PM »
Hi,

I have a 2 axis lathe with encoder on the spindle. I use SolidWorks/SolidCAM to generate g-codes. No problems at all. Only thing that I wasn't been able to get SolidCAM to post a simple eccentric turning program. So I guess SC doesn't support eccentric turning at all.

Question is how can I write a program to cut a piece like in the attached image?

Thanks,
Suat

Offline smurph

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Re: Turn | Is eccentric turning possible with Mach4?
« Reply #1 on: October 06, 2019, 04:34:51 PM »
There is nothing in standard G code that specifically does this.  Hence why SolidCAM would not post a program to do it.  There may be machine that have custom G codes for this type of operation though.  And it would have a specific post processor for SolidCAM to use it. 

If your motion controller is capable of real time gearing of the X axis motor to the encoder on the spindle, you could do something like this.  For instance, I know a Galil could do this. 

Otherwise, to me, this would be more of a 4th axis milling type of operation. 

Steve
Re: Turn | Is eccentric turning possible with Mach4?
« Reply #2 on: October 06, 2019, 06:41:07 PM »
Hi,
one other alternative is to approximate the shape in Gcode.

I, some time ago, generated some Gcode that allowed a lathe to cut a hex by moving the X axis in and out in
synchronicity with the C axis (spindle). You could do the same thing but instead of a hex make it an eccentric.

Craig
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Offline Graham Waterworth

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Re: Turn | Is eccentric turning possible with Mach4?
« Reply #3 on: October 06, 2019, 08:21:47 PM »
You also need a Y axis to keep the tool on the centre line, you are in effect producing a circle with the tool in the XY plane.

Without engineers the world stops
Re: Turn | Is eccentric turning possible with Mach4?
« Reply #4 on: October 06, 2019, 09:15:44 PM »
Hi,
I seem to recall NOT having to have a variable Y axis when 'turning' a hex, and I likewise cant imagine that you would need a variable
Y axis to turn an eccentric.

I'll try to find the post that had the hex code.

Craig
My wife left with my best friend...
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Re: Turn | Is eccentric turning possible with Mach4?
« Reply #5 on: October 07, 2019, 05:21:45 PM »
Hi,
stick with me here...this may take a couple of posts to flesh out.

I have attached a pic of an eccentric circle of radius r with its centre displaced from the rotational centre of the lathe
by a distance d. As the job rotates (increasing theta) the centre of the eccentric describes a circle with components:
x'=d cos(theta)
y'=d sin(theta)

According to the diagram we require the distance r' from the lathe tip to the lathe centre as a function of theta.

r'=(r2-d2sin2(theta))1/2 -d cos(theta)

All that is neccessary is to generate gcode moves at sufficiently fine intervals of increasing theta.

Will come back with some details.

Craig
« Last Edit: October 07, 2019, 05:23:30 PM by joeaverage »
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Re: Turn | Is eccentric turning possible with Mach4?
« Reply #6 on: October 07, 2019, 05:30:36 PM »
Hi,
whoops....my attached pic is upside down....this attachment may be easier to read.

Craig
My wife left with my best friend...
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Re: Turn | Is eccentric turning possible with Mach4?
« Reply #7 on: October 07, 2019, 05:51:44 PM »
Thanks Craig
Re: Turn | Is eccentric turning possible with Mach4?
« Reply #8 on: October 07, 2019, 06:38:56 PM »
Hi Azalin,
as an example if the eccentric diameter is 50mm and its is displaced 10mm from the lathe centre:
r=50
d=10

With the spindle at 30 degrees theta =300
r'=(502-102sin230)1/2-10.cos(30)
   =(2500-100x0.25)1/2 -10x0.8660
   =49.749-8.660
   =41.089mm

Now if the lathe spindle rotates another degree so theta=31
r'=(2500-100x0.2652)1/2-10X0.8571
    =41.165mm

The Gcode that would link these two steps is:
g1 x41.089 c30
g1 x41.165 c31

This is clearly too painfully slow to calculate by hand so that's what Lua is for. Note I would calculate (using a Lua script) every 0.1 degree
or maybe even finer, thus I would use a macro as a sort of Wizard.

Craig
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Re: Turn | Is eccentric turning possible with Mach4?
« Reply #9 on: October 07, 2019, 06:44:52 PM »
Hi,
this approach will, as Graham suggested, put the lathe tool either under or over the centre line throughout the toolpath.
Depending on the material that may be a problem which could only be cured by having the Y height
move  synchronously.

Craig
My wife left with my best friend...
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