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Discussion Starter · #1 ·
Hi all,

I have a basic 3018 Mini CNC Router that I have been using to cut earrings out of 2mm HDPE plastic sheets that I have melted from bottle tops.

I have started to look at making my next item, a reusable coffee cup carrier from 4mm plastic (which I have copied from a Youtube video). But I have started to encounter a problem with the CNC router suddenly lowering on the Z axis when there is no call in the code to do so. And when I mean lowering, it isn't just dropping a mm or so, it starts to lower like it is on a declining ramp. Eventually it will cut through the plastic, the waste plastic that I have sitting underneath the plastic I am trying to cut and jams into the metal tray underneath (or the bit breaks, whichever comes first).

In the latest instance of this occurring, it dropped approx 15mm in the space of approx 80mm. The item that was being cut out was the bottom flat piece that the coffee cups would sit on. It is kind of like an oval with long flat sections on the long edge. The router started in the bottom left corner, cutting across the bottom section left to right, cut the half circle at the right hand end and then on the right to left path it started to lower on the Z. When I look at the waste plastic it looks like it started lowering 3/4 of the way through the half circle.

The software that I use to create the tool path is Carbide Create and the software that I use to send the code to the CNC machine is grblControl version 0.8.1. I have included the tool path code below. There are only two Z commands, the first to lower the bit 4.2mm at the beginning and the second to raise it 2mm at the end.

Has anyone encountered this type of behaviour before? Does anyone have any suggestions as to how I can stop this from happening?

Any suggestions will be greatly appreciated.

Thanks in advance

Simon


Code:
G90
G21
(Top)
M05
M0 ;T102
M03S10000
G0X16.97Y16.66Z2.00
G1Z-4.20F203.2
X17.84Y15.78F508.0
X19.72Y14.04
X20.70Y13.19
X22.66Y11.62
X24.71Y10.13
X25.80Y9.41
X27.94Y8.08
X30.15Y6.87
X31.32Y6.29
X33.60Y5.25
X35.96Y4.33
X37.20Y3.90
X39.67Y3.15
X42.14Y2.54
X43.45Y2.27
X45.99Y1.86
X48.62Y1.57
X49.96Y1.48
X52.55Y1.41
X175.69Y1.42
X176.99Y1.46
X179.51Y1.65
X182.05Y1.98
X184.53Y2.44
X186.98Y3.04
X189.37Y3.77
X191.71Y4.61
X194.02Y5.59
X195.19Y6.14
X197.36Y7.27
X199.53Y8.55
X201.56Y9.88
X202.59Y10.62
X204.58Y12.15
X206.48Y13.78
X208.28Y15.47
X210.06Y17.31
X211.73Y19.23
X213.28Y21.18
X214.04Y22.20
X215.47Y24.30
X216.83Y26.49
X218.07Y28.75
X218.68Y29.93
X219.78Y32.30
X220.77Y34.71
X221.65Y37.17
X222.05Y38.42
X222.78Y41.02
X223.37Y43.59
X223.86Y46.27
X224.06Y47.65
X224.36Y50.36
X224.47Y51.79
X224.57Y54.53
X224.58Y55.99
X224.55Y57.40
X224.40Y60.15
X224.12Y62.91
X223.72Y65.57
X223.47Y66.91
X222.88Y69.58
X222.17Y72.16
X221.38Y74.63
X220.91Y75.92
X219.94Y78.35
X218.87Y80.68
X217.68Y82.99
X217.05Y84.12
X215.70Y86.35
X214.28Y88.45
X212.77Y90.48
X211.14Y92.47
X209.47Y94.31
X207.73Y96.06
X206.77Y96.96
X204.90Y98.58
X202.95Y100.11
X200.92Y101.55
X199.84Y102.26
X197.75Y103.51
X195.55Y104.68
X193.31Y105.72
X192.15Y106.22
X189.79Y107.09
X187.39Y107.84
X185.00Y108.45
X183.74Y108.72
X181.22Y109.14
X178.70Y109.42
X176.12Y109.57
X174.83Y109.59
X51.68Y109.58
X50.34Y109.54
X47.75Y109.35
X45.17Y109.02
X42.58Y108.55
X40.08Y107.96
X37.60Y107.23
X35.17Y106.37
X32.82Y105.40
X30.57Y104.34
X28.26Y103.10
X26.10Y101.79
X24.03Y100.39
X22.02Y98.88
X20.04Y97.25
X18.17Y95.53
X17.23Y94.61
X15.50Y92.78
X13.86Y90.86
X12.26Y88.81
X10.77Y86.69
X9.40Y84.54
X8.11Y82.27
X6.91Y79.92
X5.83Y77.54
X4.88Y75.12
X4.00Y72.56
X3.26Y69.96
X2.64Y67.34
X2.15Y64.73
X1.77Y62.00
X1.52Y59.23
X1.41Y56.47
X1.44Y53.62
X1.59Y50.85
X1.88Y48.08
X2.30Y45.36
X2.84Y42.71
X3.49Y40.16
X4.29Y37.55
X5.18Y35.08
X6.18Y32.66
X7.30Y30.28
X7.90Y29.10
X9.15Y26.88
X10.54Y24.65
X11.26Y23.59
X12.77Y21.51
X13.57Y20.49
X15.22Y18.54
X16.97Y16.66
Z2.00
M05
M02
 

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The first Z-axis cutting move:

G1Z-4.20F203.2

Does not move the z BY -4.20, it moves it TO Z = -4.2, see the G90 command at the beginning. Since the prior “rapid” move has The tip of the bit at Z=2. (The line G0X16.97Y16.66Z2.00 ), it will move a total of 6.2 mm down.

That is what the g-code commands. Where that ends up depends on where you set Z=0, top or bottom of stock.
 

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Mike
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3,940 Posts
You are using a really old version of grblControl so you might want to download a newer version and also check to see if there is a firmware update for your equipment. Another thing would be to make sure the axis cables are shielded from crosstalk.
 
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Discussion Starter · #5 ·
That happened to me yesterday on the same machine, and it was the spindle actually slid down in the mount. After repositioning the spindle and tightening the spindle mount, it worked fine.
This might be where the problem was. The mount for my spindle has cracked where the bold it so I am unable to tighten it. I have used a hose clamp instead and I will see if that prevents the problem from recurring.
 
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