What am I getting this effect?...

[cnczoner]

Hi all,

This was a finishing pass taking 0.010" off the perimeter of this part with a 0.5" dia M42 cobalt endmill at 1500 RPM and 5 IPM. What's very odd is that the straight sides have a very nice finish, but around the rounded sections it looks like scallops or scales and also looks like the curve was interpolated with a low number of straight edges. You can ignore the "dent" at the cardinal point as I need to fix my backlash compensation settings.

Thoughts?

BTW, I'm using the G02 circle code in Mach 3 and wondering now if there is some setting the makes it interpolate a circle with a certain number of divisions. I'll go investigate that in the meanwhile.

Cheers,
-Neil.

[cnczoner]

Surely someone here knows...

[Ken_Shea]

Neil,
Well I guess "knowing" is a relative term, however, if that were on my machine that little divot in the center would be very disturbing, perhaps excessive backlash in the ball screws, something loose ???, I believe Mach III has software compensation for backlash. The little cutting segments leading up to that could be from too low a tolerance setting.

Ken

[HuFlungDung]

It does look like an interpolated arc, but if your gcode used a G02 then it cannot be the fault of the code.

Is this a stepper driven system? How many steps/rev are you using? A coarse setting there will likely show up as a facetted circle.

What quality is your axis drive system? Is it a ballscrew?

[Mitsui Seiki]

Post a copy of the program.

[cnczoner]

Ken, I started tweaking the machine and seem to have reduced the backlash, but had not decreased the backlash compensation in Mach 3 yet, so I'm not worried about the divot because I know what it is.

Hu, it's an acme screw, and it has a backlash adjustment screw, but I haven't properly eliminated the backlash yet (until I make an extension for the allen key so I can get in there). See this photo... http://narwani.net/neil/mech/X-Axis_Screw_Nut.jpg

Stepper driven, with many thousands of steps per inch, because the belt drive has a 1:10 reduction IIRC (so 10 stepper steps makes one acme-screw step). I need to check on that machine tomorrow for the actual numbers. Yes, G02 and G03, but perhaps some setting I have in Mach 3

Mitsui, it's hand-written g-code. Here's the finishing pass snippet...

(----- Profile outside -- 0.1" steps -----) G01 Z2 ( Clear over top ) G01 X0 Y-2 F30 ( Clear of material ) M6 ( Load 0.5-inch 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 Z-0.1 F1 ( Plunge 0.1 inch ) G02 X6.375 Y-0.25 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 Z-0.2 F1 ( Plunge 0.2 inch ) G02 X6.375 Y-0.25 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 Z-0.3 F1 ( Plunge 0.3 inch ) G02 X6.375 Y-0.25 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 Z-0.4 F1 ( Plunge 0.4 inch ) G02 X6.375 Y-0.25 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 Z-0.5 F1 ( Plunge 0.5 inch ) G02 X6.375 Y-0.25 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 Z-0.6 F1 ( Plunge 0.6 inch ) G02 X6.375 Y-0.25 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 Z-0.65 F1 ( Plunge 0.65 inch, through complete depth ) G01 X7.24 Y0.625 F5 ( Cut in a bit ) G02 X6.375 Y-0.24 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

[Mitsui Seiki]

I suggest you change your start point to the lower straight part of the piece instead of in the middle of the curve.

[cnczoner]

Stefan, I did that so that it would break off closest to where it was clamped. I'm guessing you're recommending that for the divot though, right? Even if I did a complete circle, I get that segmented/interpolated effect.

[HuFlungDung]

We don't know what type or quality of machine this is. It is possible for drive screw inaccuracies and/or drive screw bearing eccentricity (in the bearings) or intermittent stick/slip of the screw or slides to combine with excessive backlash and produce a wavy surface. The prime indicator of this would be waviness that is some multiple of the lead screw pitch.

[Mitsui Seiki]

Originally Posted by cnczoner I'm guessing you're recommending that for the divot though, right? Even if I did a complete circle, I get that segmented/interpolated effect.

Yes,that's the reason. And you have tried to do a complete circle and the same result?
Have you tried different feeds and speeds?

[cnczoner]

Originally Posted by HuFlungDung We don't know what type or quality of machine this is. It is possible for drive screw inaccuracies and/or drive screw bearing eccentricity (in the bearings) or intermittent stick/slip of the screw or slides to combine with excessive backlash and produce a wavy surface. The prime indicator of this would be waviness that is some multiple of the lead screw pitch.

Okay, I can see that. But I'm thinking about a straight diagonal line. I should see this effect then too, unless both axes have the same problem and in sync with each other. I can run a test of that easily tomorrow. But with the backlash reduced/eliminated, you're suggesting I should not see this effect, right?

The mill is a converted (by someone else) ... HF geared-head mill. It's a bit larger than an X3. Conversion really being only the addition of stepper mounting plates and drive pullies/belts. I was planning to do a ballscrew conversion with some Roton screws, but with all my other electronic and automotive projects, I decided to just adjust this, reduce/eliminate backlash and run as is for now. Meanwhile, I keep my eyes peeled on ebay for an appropriate ballscrew. But I've barely begun to tweak it.

[dapoling]

I have seen this before and it was bad ball screws.
As to the program using a G2 as others have said is not the problem but I would change your tool path to enter and exit part before feeding in Z.
Your feeds and speeds are fine, depending on the machine can be increased.

good luck