Zero Z-Axis Failure

I'm pretty new to this forum, but I've learned a lot from it so far. I've been running a Shopbot PRTalpha (circa 2004) for a few months.

Within the last few days, something strange has been going on when I go to zero the z-axis with zero plate. The machine will plunge down to the zero plate (like normal), but when the bit hits the plate, it doesn't want to stop. It starts to make 'grunting' noises for about 5 seconds, then claims the 'stop' button was hit.

Then, the carriage will go straight up about 8 inches until it goes so high that the dust collection hits the top rails and forces the machine to stop. This is odd, since I hit the 'stop' button repeatedly, and the carriage ignores my pleas for help and keeps moving upwards.

Any ideas what could be going on? I've was able to zero the machine for the past 6 months with no problems up until now.

Any ideas will be greatly appreciated. Thanks.

-Matt

a forum search search will give you all the info you need plus more than you care to read...

Matthew...
Here is what is happening:
Z moves down, switch contact is NOT made
machine looses steps and position ( grunting noise)
with postion loss, it doesnt know how far to move up and hits hard stop.

That's what IS happening. There are a number of posts concerning this happening. Some software, some electrical, some machine settings. You need to figure out which one(s) the culprit.

Have you changed anything lately? Does the Z plate light Input 1?

Like Gene says, the answer is allready posted, but we cant troubleshoot it for you.

also make sure your bit is not painted on the bottom keeping it from making contact
I use a bit from one company and they are always painted and I have to sand off the paint to z-zero

Randy, I wonder if that is also true of some of the newer 'super coated' ones as well? I have been having an issue every once in a while and never gave that a thought. Hmmmmm! Russ

After a few simliar issues I always manually touch the plate on the bottom of the bit and check the input "light" on the software comes on before running the zero routine.

Becomes habit after the first few goes and has saved me a lot of hassle from bits that have picked up a bit of pitch.

Thanks to everyone for the tips. It turns out that when I touched the alligator clip to the zero plate, the Input 1 light was not coming on.

I sanded the zero plate with 220 grit to clean off the aluminum oxide that has collected over the past six years and now it works fine.

Great advice on touching the plate to the bit before running the zero procedure! I'll be doing that every time from now on.

This is off topic but does deal with zeroing and some of you are far more advanced than me. I began a 12 hour 3d finishing cutting when i realized a big thunder storm was moving in. i stopped the cutting, returned all axis to their zero position, with the z being at 1.0. Bam 5 minutes later the power went out. Nothing has even been close to my bot other than a nat during the power outage. Now that the power is back on should i assume that the zero is still intact or should i run a c3 command?

Quote:

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Originally Posted by cowboy1296

Now that the power is back on should i assume that the zero is still intact or should i run a c3 command?

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You should assume that your zero references will be close to the original in XY (as long as you didn't move it) and that your Z will need to be re-zeroed. You can re-run C3 as long as you feel confident that it is repeatable.

I run the stock prox switches on the little desktop, but not my big machine. Instead, when I run a critical or long file, I'll bury an 1/8" bit (or whatever I am using) about 1/16" into the spoilboard at an XY whole number coordinate - E.G. - 12,24. I will then write on the spoilboard in pencil next to the 'dink' I did 12,24. Now no matter what, I can always drive the tool to that location, bury the bit into the hole again and then VA, 12, 24 to get dead nuts on without any worries about prox switches being off. VA assigns the current resting location of the tool to the XYZAB you tell it.

The reason I said that the XY locations will be close and not perfect is because as you work the material (assuming wood), it gets heat soaked from the action of cutting, in addition to stress relieved where the material moves a small amount - indistinguishable when you are machining it...but apparent when you stop and restart a file - especially overnight. Temperature and moisture play a role in things moving around...So I have learned over the years to try not to stop in the middle of a 3D file unless I want to deal with a little ridge here or there.

It is possible to nudge the Z down a little at a time to feather out the transition on parts that have a huge disparity between day 1 and day 2, and sometimes you have to 'sneak up' on the Z by nudging it down and run it to see...maybe stop, nudge down and restart that line again until you get it butter smooth. Warpage isn't linear so you have to run the whole raster line to make sure you aren't dinking it down too much.

-B

thanks. neat idea about plunging into the spoil board. I think that i am going to do a c-3 and z zero and add .005 to my z to cut below what i started.