I was facing a pice of wood with my 1.5 TTS and set off the -X limit switch. I was using the pendant so I just backed off of the limit switch and started in the -Y direction with the pendant (without re-refrencing). I had only gone an inch or so when I noticed that the Z was dropping! I hit the E stop (good thing it was wood) and noticed that the DRO still said Z zero even though the tool was at least 0.150 down.
I brought the Z up and rebooted the computer and everything seems to work ok now.
3.5 year old series II all stock.
Damn, there goes another generalization.
No the Z was being driven down. The center of the plunge was scorched by the non cutting part of the face mill I am lucky it was wood else I would most likely have killed a set of inserts.
I obviously still make mistakes but if it had occurred to me to use wood when I first got my machine I would have saved a fortune in tooling just learning cnc.
I've heard of the Z dropping before, but not sure what the cause was (maybe the stepper brake). As a test I placed an indicator under the nose and testing this overnight. Might be worth a try....
I've had a single Z drop on my Series 2. I don't think it's related to what happened to David's mill, but it doesn't hurt to chronicle it here.
I powered up the mill and computer, and walked away while the computer was booting. I heard the sound of a stepper running and then a thump, and ran back into the garage to find that the head had dropped.
Luckily, I had an ER collet in the spindle holding a .375" chamfer mill. Previously I had been working on some .375" stock in the vise, with the origin set to the top center. Since I had removed the last piece of stock, the vise jaws were now open to about .385", and the tool was centered over the opening. So the cutter wound up between the vise jaws, and the ER collet's face was resting on top of the jaws. There was no damage at all.
Mach hadn't loaded when this happened, so I attribute it to the parallel port sending odd signals during bootup. I don't know how the mill managed to ignore the lack of a charge pump signal. And I'm not sure if the stepper was actually feeding, or just freewheeling as the head fell under gravity. Possibly the parallel port just sent a signal to release the brake, letting the head drop.
I have changed my bootup strategy. Now I bring Mach online, then power up the mill.
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I have one of the old old Series 1 units without the Z-brake. When the gibs weren't super-tight, the head would drop when power was removed. It really sounded like it was being driven, but wasn't. I got real good at sticking a chunk of 2x4 under the head in a hurry.
I installed the Series 3 upgrade since it came with a free brake (really, it was only about 2x the cost of just a Z-motor-with-brake) and now cringe for just a second every time I power down without my 2x4 handy, but it holds every time.
Just to add to the fray:
There was this one time (only) when I was using a Series 1 mill. I forgot the startup sequence, but I got to a point where the Z was very slowly stepping downwards, at maybe 1 step every second. The Z display on Mach wasn't changing. I at first wasn't sure what was going on coz I was hearing a light thunk thunk thunk sound every second. It was only where I put my finger on the Z gib did I realize that the head was very slowly moving down. Thru some sequence of using the E stop and reset in Mach I was eventually able to get it to stop doing that.
I have had a series I (no brake) for years and never had the Z axis fall.