Issue milling a circular contour with CNC – 3018 Pro

Question

I have a 3018 Pro CNC and being trying cutting a contour of a simple circular part:

G-code:

(TestKnobContour)
(T1  D=1 CR=0 - ZMIN=-3 - flat end mill)
G90 G94
G17
G21
G90    
(2D Contour1)
Z15
S5000 M3
G54
G0 X10.8 Y0.1
Z15
G1 Z5 F10.0
Z1 F10.0
Z-2.9
X10.792 Z-2.938 F10.0
X10.771 Z-2.971
X10.738 Z-2.992
X10.7 Z-3
X10.6
X10.562 Y0.092
X10.529 Y0.071
X10.508 Y0.038
X10.5 Y0
G2 X9.851 Y-3.634 I-10.5 J0
G1 Z-2.75
G2 X8.983 Y-5.436 I-9.851 J3.634
G1 Z-3 F10.0
G2 X3.301 Y-9.968 I-8.983 J5.436 F10.0
G1 Z-2.75
G2 X1.351 Y-10.413 I-3.301 J9.968
G1 Z-3 F10.0
G2 X-5.735 Y-8.795 I-1.351 J10.413 F10.0
G1 Z-2.75
G2 X-7.299 Y-7.548 I5.735 J8.795
G1 Z-3 F10.0
G2 X-10.452 Y-1 I7.299 J7.548 F10.0
G1 Z-2.75
G2 X-10.452 Y1 I10.452 J1
G1 Z-3 F10.0
G2 X-7.299 Y7.548 I10.452 J-1 F10.0
G1 Z-2.75
G2 X-5.735 Y8.795 I7.299 J-7.548
G1 Z-3 F10.0
G2 X1.351 Y10.413 I5.735 J-8.795 F10.0
G1 Z-2.75
G2 X3.301 Y9.968 I-1.351 J-10.413
G1 Z-3 F10.0
G2 X8.983 Y5.436 I-3.301 J-9.968 F10.0
G1 Z-2.75
G2 X9.851 Y3.634 I-8.983 J-5.436
G1 Z-3 F10.0
G2 X10.5 Y0 I-9.851 J-3.634 F10.0
G1 X10.508 Y-0.038
X10.529 Y-0.071
X10.562 Y-0.092
X10.6 Y-0.1
X10.7
X10.738 Z-2.992
X10.771 Z-2.971
X10.792 Z-2.938
X10.8 Z-2.9
G0 Z15
M5
X0 Y0 Z0
M30

Candle shows that everything is fine for this G-code:

However, I am getting weird results (see top right):

What can I do for troubleshooting?

Answer 1

It is hard to be sure from the picture. When I wrote this answer, material you were cutting looked like clear plastic. When I reviewed it now, the material looks more like aluminum. It doesn't really matter, since the melting point of aluminum is well within the range of both high speed steel (HSS) and carbide.

What follows is a list of possibilities and things to check.

1. check the mounting of the spindle. Does it hang rigidly as you push the tooltip in x, y, and any point in the circle, or does it tip more in one direction than another? This could be caused by a loose mounting screw.

2. Are you actually cutting, or are you melting the material? It is very easy for a tool, especially a tight spiral 4-flute tool, to heat above the melting/softening point, and to then push through the material rather than removing it as chips. Using a tool with fewer flutes at a slower speed can sometimes help. You may also want to cool it with some water -- not enough to make a mess of the machine, but enough to help cool the tool. This happens easily with plastic, and also with aluminum. I have several times had to peel bits of solidified aluminum from inside the flutes of a carbide cutter.

3. It doesn't look like backlash. The 45-degree angle is curious. To see if it varies with or without load, try putting a marking pen in the collet and draw similar circles. It doesn't look like a primary software or hardware problem, since the inner circles were round. Only the outermost circle has the flat spot.

4. If the departure from a circle is only under load, reduce the load. Step the tool into the work by 1/2 of what you are doing now. How does that change the result?

5. Chips remaining near the cut can join in the melt. Try blowing compressed air on the cut while it is cutting, both for cooling, and to remove chips.

Some things it is unlikely to be, if the shape was intended to be a complete circle, and if the outer pass is the last pass performed:

• Skipping steps due to insufficient stepper motor torque
• Belt slipping (or lead screws skipping)
• Loose belts or backlash in mechanical shaft connections.

Answer 2

I've ran the code on my own CNC machine. I slightly adapted the code as my machine doesn't understand the movement without the instruction code:

Z1 F10.0
Z-2.9
X10.792 Z-2.938 F10.0
X10.771 Z-2.971
X10.738 Z-2.992
X10.7 Z-3
X10.6
X10.562 Y0.092
X10.529 Y0.071
X10.508 Y0.038
X10.5 Y0

is changed to

G1 Z1 F10.0
G1 Z-2.9
G1 X10.792 Z-2.938 F10.0
G1 X10.771 Z-2.971
G1 X10.738 Z-2.992
G1 X10.7 Z-3
G1 X10.6
G1 X10.562 Y0.092
G1 X10.529 Y0.071
G1 X10.508 Y0.038
G1 X10.5 Y0
etc...

As I used an engraver bit, I made sure the depth was touching the wood (engraving) when running at the lowest depth. The contour it drew was a perfect circle.

The code is therefor working as it should (carve a circle, in the photo above, the circle started at the hole in the top left and followed a clockwise path), the result from your milling exercise shows that the final segment of the circle is not giving you a circle segment, instead the milling path is sort of straight. I've seen such paths where the steppers are not powerful enough to mill through the material. As a result they skip steps, and in this case it results in a sort of straight path. You should try running a dry run (in air), or in softer material (this will determine if the code is producing a cirlce in your machine as well), and add more passes to milling the knob (for the final product).

Answer 3

Parts of the circle are at different heights. When I rendered your 30 or so actual G1 and G2 lines, here's what I got. It looks like a circle from above, but form the sde you can see depth changes. What are those coordinate lines with no G0 ot G1 or G2 or G3 command for?

=============================

Okay, here's a simple gcode I just made by hand do to something similar. Granted, I used all nice round numbers, but this is an example of what I think you're looking for. You'll have to change it as needed; perhaps more paths if your tool is less wide; more iterations at deeper depths, or even different coordinates. Enjoy.

G0 X30 Y30 ; move to start XY
M3 T12 S50 ; spindle on 50%
G1 Z-0.5 E1 ; down to depth1
G2 X50 Y50 J20.0 E1 ; arc
G1 X30 Y30 E1 ; corner
G1 X30 Y35 E1 ; move inward
G2 X45 Y50 J15 E1 ; arc
G1 X30 Y35 E1 ; corner
G1 X30 Y40 E1 ; move inward
G2 X40 Y50 J10 E1 ; arc
G1 X30 Y40 ; corner
G1 X30 Y45 ; move inward
G2 X35 Y50 J5 E1 ; arc
G1 X30 Y45 E1 ; corner
G1 X30 Y50 E1 ; move inward
G0 Z 10 ; move up to clear
M3 T12 S0 ; spindle off

=================================

Or, if I've made a mistake and you want simple circles:

G0 X30 Y30 ; move to start XY
M3 T12 S50 ; spindle on 50%
G1 Z-0.5 E1 ; down to depth1
G2 X30 Y30 J20.0 E1 ; arc
G1 X30 Y35 E1 ; move inward
G2 X30 Y35 J15 E1 ; arc
G1 X30 Y40 E1 ; move inward
G2 X30 Y40 J10 E1 ; arc
G1 X30 Y45 ; move inward
G2 X30 Y45 J5 E1 ; arc
G1 X30 Y50 E1 ; move inward
G0 Z 10 ; move up to clear
M3 T12 S0 ; spindle off