Machsupport Forum
Mach Discussion => General Mach Discussion => Topic started by: mach_dude on January 27, 2008, 05:15:44 PM

Hi all,
I am using Mach 3 with a converted HF gearedhead mill (acme screws). I am getting a really odd effect around curves (using G02 command) that makes it look like it's being estimated or interpolated to something like a 3540 sided polygon. The photo below is 6061 milled with an M42 cobalt 1/2" endmill at 1500 rpm, 5 ipm and this a 0.010" finishing pass. You'll notice that the straight sides have a very clean consistent finish, but as it continues around the curve, the finish gets nasty  mostly due to the polygon effect. Ignore the incorrect backlash compensation which is visible at the cardinal point.
I'm not at the mill now, but the setup has something like 50,000 steps per inch so I'm sure it's not a resolution problem.
The question is ... are circles really being interpolated to a polygon of this resolution in Mach 3? If so, is there a setting that controls this? I do have backlash compensation on, and IIRC I had to change the CV settings and acceleration for that but I'll post those values when I get back to the mill later.
Cheers,
Neil.
(http://narwani.net/neil/mech/alum_scallops3.jpg)

Hi Neil,
What did you use to create the shape?
post the gcode that did this, and some sizes of the part
Graham.

are the gibs tight, sure looks like the problem I had on my clapped out old Bridgeport which had loads of play.
Hood

Graham,
I used hand written gcode. That part is 1.25" wide x 0.625" tall. Here's that code snippet, but I'm eliminating any resolution/interpolation as the problem at this point after some tests today. See next post below.
( Profile outside  0.1" steps )
G01 Z2 ( Clear over top )
G01 X0 Y2 F30 ( Clear of material )
M6 ( Load 0.5inch finishing endmill )
T03 ( For CNC simulator )
G01 Z0.1 F30 ( Over top of material )
G01 X7.25 Y0.625 F30 ( Start point on R.H.S. )
G01 Z0.1 F1 ( Plunge 0.1 inch )
G02 X6.375 Y0.25 R0.875 F5
G01 X0.625 F5
G02 X0.625 Y1.5 R0.875 F5
G01 X6.375 F5
G02 X7.25 Y0.625 R0.875 F5 ( End on center of R.H.S. )
G01 Z0.2 F1 ( Plunge 0.2 inch )
G02 X6.375 Y0.25 R0.875 F5
G01 X0.625 F5
G02 X0.625 Y1.5 R0.875 F5
G01 X6.375 F5
G02 X7.25 Y0.625 R0.875 F5 ( End on center of R.H.S. )
G01 Z0.3 F1 ( Plunge 0.3 inch )
G02 X6.375 Y0.25 R0.875 F5
G01 X0.625 F5
G02 X0.625 Y1.5 R0.875 F5
G01 X6.375 F5
G02 X7.25 Y0.625 R0.875 F5 ( End on center of R.H.S. )
G01 Z0.4 F1 ( Plunge 0.4 inch )
G02 X6.375 Y0.25 R0.875 F5
G01 X0.625 F5
G02 X0.625 Y1.5 R0.875 F5
G01 X6.375 F5
G02 X7.25 Y0.625 R0.875 F5 ( End on center of R.H.S. )
G01 Z0.5 F1 ( Plunge 0.5 inch )
G02 X6.375 Y0.25 R0.875 F5
G01 X0.625 F5
G02 X0.625 Y1.5 R0.875 F5
G01 X6.375 F5
G02 X7.25 Y0.625 R0.875 F5 ( End on center of R.H.S. )
G01 Z0.6 F1 ( Plunge 0.6 inch )
G02 X6.375 Y0.25 R0.875 F5
G01 X0.625 F5
G02 X0.625 Y1.5 R0.875 F5
G01 X6.375 F5
G02 X7.25 Y0.625 R0.875 F5 ( End on center of R.H.S. )
( Full depth at once finishing pass, 0.010 inch into side )
G01 Z0.65 F1 ( Plunge 0.65 inch, through complete depth )
G01 X7.24 Y0.625 F5 ( Cut in a bit )
G02 X6.375 Y0.24 R0.865 F5
G01 X0.625 F5
G02 X0.625 Y1.49 R0.865 F5
G01 X6.375 F5
G02 X7.24 Y0.625 R0.865 F5 ( End on center of R.H.S. )
(Return to home)
G01 X7.25 F10 ( Clear away )
G01 Z2 F30
G01 X0 Y2 F30

Hood,
No I'm not sure the gibs are setup properly. I recently picked up this mill from someone else and have been learning to machine on it (I'm not even familiar with nonCNC machining) so I've not tweaked the mill yet. I need to adjust out the backlash too.
But I ran a number of tests today. I do get that same problem (periodic "waves") with a straight diagonal line though. This is 0.375" thick...
(http://narwani.net/neil/mech/alum_scallops10.jpg)
I then adjusted out (in software) the notch/divot at the cardinal point due to the backlash compensation stepper reversal, and reducing the backlash speed from 50% of max to 10% seems to have worked beautifully.
I also played with different feeds around an arc and the repetitive waves stays mostly the same, though a little less pronounced at lower feeds. I also noticed that the finish is much better at lower feeds. Here's the best I got at the end of the day, which is a 0.010" finish pass at 0.5ipm and 1500 rpm. The material is still 6061T6, approx 2" wide x 0.375" thick. This photo makes it look a bit better than it actually is, but still it's much better than yesterday. :)
(http://narwani.net/neil/mech/alum_scallops11.jpg)
Still a lot of tuning and tweaking to go, but on the mechanicals of the machine. We're leaving the scope of this forum now, so I'll go research that quietly ;)
Cheers,
Neil.