CNC accuracy problems

[jtriggs]

I have a cnc router using gecko drivers, amtek dc servo motors, and
a 65 volt dc power supply. My software for design is a combination of
MasterCam Mill V.9 and AutoCad 2006 and my post processor is Mach 2.

The problem I'm having is getting straight cuts. In straight lines the
cutter arcs the cut slightly instead of a straight line.
In other instances the cut of a rectangle will be skewed so that the
the lower right corner and the upper left corner will be slightly off center.

I have checked the gantry and table and all is properly set up and square.

Any ideas will be helpfull.

[lakeside]

I would bolt a machine square to table indicate to square then do a program move to see if you have either a drift under power or maybe some play in your lead screw this does sound like it a mechanical issue and not a controller issue unless it a backlash comp issue

[ger21]

Does it do it in straight X only and Y only moves?

How long are the straight moves?

How fast is the PC?

[jtriggs]

X axis only. One cut is approx 3 inches long and the arc is about .0625 at the center
of the arc.
On the skewing, the cut is only about 3/4" long or less but I get about 1/8" of
skew at top left and lower right .
The machine is a Pentium 4 2ghz running on Windows 2000

[ger21]

Do you get the same skewing at different places on the machine?

[jtriggs]

I guess the answer would be yes.
I am cutting two pockets. They are easiest described as
rectangular shapes but actually they are routs for
Humbucker pickups so they are mainly rectaingular with smaller "ears"
cut on each side. It is in these ear cuts that I get the skewing.
The upper right and lower left corners are correct but the lower right
and upper left corners drift out about 1/8" each. The rest of the cut
is fine.
I do two of these cuts about 2 1/2" to 3" apart and both cuts have
the same problem.

[ger21]

Do you have backlash comp on and are you running constant velocity?

[jtriggs]

No backlash compensation has been set as far as I know.
I'm not in my shop right now but I have a copy of Mach2
on the computer I do my design work on and I have not
set the backlash comp on either computer.
My router uses ball screws and not lead screws if that makes a
difference.

[ynneb]

Is this the sort of thing you mean ?
If this is the case, have you tried exact stop mode?

[jtriggs]

Actally, it lokks more like this.
The skewing is mostly on the "ears" at
each side.

[ynneb]

That is not a perspective drawing? that is a view from directly above?
Post the cutting file in this thread and a few of use can test it out on our versions of Mach.
This will help us detemin if its a CAM problem or a Mach problem or a Machine problem.
EDIT: Also post the dxf too

[jtriggs]

No, this is as viewed from above.