Initial moves on CNC job varies from time to time

Hi,
I am using GRBL and UGS, and Fusion 360 for CAM, and i am making my very first chips as we speak.

I have not installed homing switches or Z probe switch yet. I just jog my way to roughly the XY position where i have put my origin in the model and GCODE, then I do the old paper slipping trick for setting the Z height. Afte that, I set all my axes to 0, and press start.

the funny thing is that sometimes, the spindle moves slightly up and over the workpiece before it moves in XY direction to where it will do its first cuts. This is what i prefer!
Sometimes though, and more often than not, the spindle start moving directly towards the location for the first cuts, without moving up and over the workpiece at all. It's snapped one of my precious little Proxxon end mills already. i can prepare for it though, because the visualizer in UGS shows me that the first move only involves XY movement.. that's when I zero the Z axis a little higher to avoid crashing. If the stock had any bumps or raised parts though, it would be unavoidable, i believe.

What i don't understand is what on earth am I doing differently from time to time to "achieve" this, and am I doing it in CAM or UGS, most likely??

Thanks for any response.

As I try only using open source software I'm not familiar with how Fusion 360 does things, but the "traverse height" (the minimum Z height to which the tool must rise before doing any non-milling movements) is a distinct concept any CAM processor keeps track of and should let you set as you wish. As such, Fusion 360's CAM (g-code output) processor should let you set it, and you should be mindful of what you set it to, for every project. In this particular case, there are pretty much only two possibilities: either you're unaware of this setting and some of your projects get generated with some default traverse height that is too low or zero, OR the UGS sender, for whatever reason, decides to rip the relevant lines from your g-code (not as outlandish as it sounds - senders routinely mess with the g-code before sending it to GRBL eg. to remove comments or replace arcs with many small linear segments etc. although in this case this removal would be a bug). The easiest way to decide where the problem is would be to load your g-code into a simulator BEFORE ever trying it with UGS, to see if the heights are correct or not. There are a number of these available for free (very useful to see what will happen even when bugs are not involved) - just search for "Basic CNC Viewer", "NCSim", "NC Corrector" or "CAMotics"...

blinklight is correct ..i would imagine its in your cam toolpath program and not UGS..UGS just reads the gcode and then sends that to your machine....you can also open your gcode file in notepad or word pad and see if it shows a Z movement at beginning or not..if its not there the issue is your cam software

if the z movement is there and your machine is not moving up in z then for some reason UGS is messed up/communication issue

here is a simple GRBL gcode file doing an engraving toolpath with z lift ups

T1
G17
G20
G0Z0.2500 lift up z .250 to start
G0X0.0000Y0.0000S16000M3
G0X2.9941Y4.8417Z0.2000
G1Z-0.0600F25.0
G1Y4.8800F50.0
G1X2.9910Y4.8592
G1X2.9855Y4.8298
G1X2.9734Y4.7875
G1X2.9625Y4.7599
G1X2.9487Y4.7300
G1X2.9421Y4.7176
G1X2.9238Y4.6875
G1X2.9159Y4.6761
G1X2.9434Y4.6811
G1X2.9657Y4.6871
G1X2.9755Y4.6905
G1X2.9819Y4.7115
G1X2.9872Y4.7379
G1X2.9914Y4.7727
G1X2.9938Y4.8155
G1X2.9941Y4.8417
G1X3.1129Y4.8409
G1Y5.7566
G1X2.8797
G1Y4.9468
G1X2.8788Y4.9190
G1X2.8755Y4.8882
G1X2.8697Y4.8571
G1X2.8608Y4.8257
G1X2.8484Y4.7945
G1X2.8322Y4.7639
G1X2.8121Y4.7349
G1X2.7882Y4.7082
G1X2.7613Y4.6848
G1X2.7321Y4.6651
G1X2.7016Y4.6493
G1X2.6704Y4.6372
G1X2.6392Y4.6286
G1X2.6082Y4.6229
G1X2.5775Y4.6198
G1X2.5481Y4.6189
G1X2.5118Y4.6195
G1X2.4838Y4.6212
G1Y4.5776
G1X2.5375Y4.5684
G1X2.5881Y4.5613
G1X2.6365Y4.5563
G1X2.6876Y4.5526
G1X2.7414Y4.5504
G1X2.7980Y4.5496
G1X2.8554Y4.5510
G1X2.9062Y4.5550
G1X2.9500Y4.5613
G1X2.9864Y4.5693
G1X3.0153Y4.5785
G1X3.0368Y4.5879
G1X3.0512Y4.5965
G1X3.0600Y4.6036
G1X3.0671Y4.6114
G1X3.0753Y4.6238
G1X3.0842Y4.6423
G1X3.0931Y4.6677
G1X3.1011Y4.7002
G1X3.1074Y4.7399
G1X3.1115Y4.7865
G1X3.1129Y4.8409
G1X3.2317Y4.8401
G1Y5.8755
G1X2.7609
G1Y4.9487
G1X2.7602Y4.9272
G1X2.7579Y4.9054
G1X2.7539Y4.8843
G1X2.7481Y4.8639
G1X2.7404Y4.8444
G1X2.7305Y4.8258
G1X2.7186Y4.8086
G1X2.7046Y4.7929
G1X2.6888Y4.7791
G1X2.6714Y4.7674
G1X2.6528Y4.7578
G1X2.6331Y4.7502
G1X2.6126Y4.7445
G1X2.5914Y4.7406
G1X2.5696Y4.7384
G1X2.5473Y4.7377
G1X2.5164Y4.7383
G1X2.4863Y4.7400
G1X2.4566Y4.7429
G1X2.4275Y4.7470
G1X2.3974Y4.7523
G1X2.3650Y4.7592
G1Y4.4802
G1X2.4586Y4.4613
G1X2.5193Y4.4510
G1X2.5737Y4.4434
G1X2.6261Y4.4379
G1X2.6809Y4.4340
G1X2.7381Y4.4316
G1X2.7979Y4.4308
G1X2.8303Y4.4312
G1X2.8614Y4.4324
G1X2.8910Y4.4343
G1X2.9192Y4.4369
G1X2.9459Y4.4403
G1X2.9712Y4.4444
G1X2.9949Y4.4491
G1X3.0171Y4.4545
G1X3.0379Y4.4606
G1X3.0571Y4.4672
G1X3.0748Y4.4744
G1X3.0911Y4.4821
G1X3.1058Y4.4903
G1X3.1192Y4.4989
G1X3.1312Y4.5079
G1X3.1419Y4.5173
G1X3.1518Y4.5275
G1X3.1612Y4.5387
G1X3.1701Y4.5512
G1X3.1785Y4.5650
G1X3.1865Y4.5801
G1X3.1939Y4.5966
G1X3.2008Y4.6145
G1X3.2071Y4.6339
G1X3.2127Y4.6546
G1X3.2176Y4.6768
G1X3.2219Y4.7005
G1X3.2254Y4.7255
G1X3.2281Y4.7520
G1X3.2301Y4.7798
G1X3.2313Y4.8091
G1X3.2317Y4.8401
G0Z0.2000 lifts up z .200 to go to next move