Re: [Emc-users] Helical interpolation not quite round, WTF?

[Gene H. <ghe...@wd...>]

On Tuesday 25 September 2012 23:23:36 Igor Chudov did opine:

> I have a routine that lets me mill a deep circle in several passes,
> typically to cutout circles from plates.
> 
> Something strange is going on and the resulting shape is not exactly
> round. It can be best described as a circle with humps.
> 
> The routine is below. It is basically a while loop, milling
> progressively deeper until Z reaches the required depth. Every loop
> consists of two half circles milled helically, with the end mill going
> down along a spiral.
> 
> Any idea why the shape may not end up being circular?
> 
> Thanks
> 
> Routine attached.
> 
> (Copyright - Igor Chudov, released under GNU Public License V3)
> 
> O<circulargroove> sub (circular groove, good for milling through holes
> too) #<xc>     = #1 (X center)
>   #<yc>     = #2 (Y center)
>   #<safez>  = #3 (safe height)
>   #<depth>  = #4 (depth of milling)
>   #<radius> = #5 (outside radius)
>   #<milld>  = #6 (end mill diameter)
>   #<frate>  = #7 (feed rate)
>   #<zstep>  = #8 (z step, per circle, optional)
> 
>   O<if> if [ #<frate> NE 0 ]
>     F#<frate>
>   O<if> endif
> 
>   O<if> if [ #<zstep> EQ 0 ]
>     #<zstep> = [#<milld>/3]
>   O<if> endif
> 
>   #<z> = #<safez>
>   #<r> = [#<radius> - #<milld>/2]
> 
>   G0 Z#<safez>
>   G4 P0
>   G0 X[#<xc> + #<r>] Y#<yc>
> 
>   O<loop> while [ 1 ]
> 
>     O<if> if [ #<z> - #<zstep> LT #<depth>]
>       #<zstep> = [#<z> - #<depth>]
>     O<if> endif
> 
>     G2 X[#<xc> - #<r>] Y#<yc> Z[#<z> - #<zstep>/2] R#<r>
>     G2 X[#<xc> + #<r>] Y#<yc> Z[#<z> - #<zstep>] R#<r>
> 
>     #<z> = [#<z> - #<zstep>]
> 
>     O<if> if [ #<z> LE #<depth> ]
>       O<loop> break
>     O<if> endif
> 
>   O<loop> endwhile
> 
>   G2 X[#<xc> - #<r>] Y#<yc> R#<r>
>   G2 X[#<xc> + #<r>] Y#<yc> R#<r>
> 
>   G0 Z#<safez>
> 
> O<circulargroove> endsub
> 
> M2

The first thing I'd check would be uncompensated backlash.  Then it would 
go to damaged screws, like a highly used area wobbled out.  Third would be 
scaling error on one axis, but thats fairly easy to check.

Cheers, Gene
-- 
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
My web page: <http://coyoteden.dyndns-free.com:85/gene> is up!
A penny saved has not been spent.