Maintain position after power off?

[MFchief]

Hi All,

I have kind of an off the wall question, I've managed to get myself half way thru a thread milling job, and need to change the cutter to finish up. I have a new cutter on the way and it will take a few days to get here. My plan is basically to let the machine (PCNC 1100) sit until it arrives, change it out, being careful to set the tool height correctly (it's the same cutter as the old one, just not too dull to work), then finish the job.

My issue is, due to some re-siding work being done, I may be forced to power down the mill. I'm wondering, if I set the cutter to X0,Y0 prior to shutdown, then when restarting, zero the X & Y DROs, don't do a reference, will I be able to pickup where I left off and finish the thread?

Thanks
Terry

[zephyr9900]

Terry, I think you'll be OK. The Tormach has 5tpi leadscrews, thus .200" per revolution. The stepper motors are 200 full steps per revolution, with a 4-step cycle of ++, +-, --, and -+ of current in the stepper coils. The drivers are microstepped at 10 microsteps per physical motor step to give .0001" per microstep.

Stepper drivers, when they are powered up, come up in a hardwired configuration of phases being energized, but always on a specific full step of the four-step cycle. In my experience, this may vary from controller to controller, but at most you'll be two full steps off where you left off (which was more than likely on a microstep and thus partway between full steps).

So, if my understanding of stepper motor driving is as good as I think it is you'll be at the most +/- .002" from where you think you are. 1/4-20 you'll more than likely be OK. Finer threads, YMMV...

Randy

[MFchief]

Thanks Randy,

I'm hoping of course, that they don't shut off my power, at least until I can finish this piece. It's a pretty big one, M50 1.5, and of course due to Murphy's Law, it's the very last operation of quite a bit of work. I'm attempting to make an ER 40 collet fixture to hold small work and mandrels, similar to the fixtures they sell for 5C collets. I'll let you know how it turns out.

Terry

[dbrija]

If you do get powered down, you should be able to just re-reference the machine and be where you were. You could even re-find your original zero (unless you milled it away already). I know my Z-axis drops slightly at powerdown as the brake kicks in.

On my 1100, I have edited the RefAll routine to do a two step homing, one fast, and a slower re-touch. Repeats extremely well.

[MFchief]

dbrija,

I suppose I could attempt to re-zero after re-starting, since I used a probe to center the bore of the piece I'm threading, but I'm just afraid of introducing more sources of repeatability error than I have to. I'm pretty sure I can zero the Z well enough to pick up the thread because I was careful when I originally set it, and since the tool is in a TTS holder, I can set the new tool to the same length with the tool fixture. The tool is a double V cutter, so the reference for tool height in the fixture would be the flat bottom, which has no wear. If the X & Y are only off a small amount as Randy mentioned, I think it might work.

Terry

[zephyr9900]

On my 1100, I have edited the RefAll routine to do a two step homing, one fast, and a slower re-touch.

Same here, dbrija, but I don't get homing repeatability better than .002-.003". That's on par with just repowering and I think I'd take my chances with the statistics of being less than 2 full steps off on the repowering.

Randy

[TXFred]

I have a dumb question. Can't you re-indicate the part after the new tool arrives? It might take time, but if you go to the same X, Y and Z, you should be fine.

[apeman88]

For me... once I power off and on, it will maintain the same coordinates but always double confirm by setting it at X0 Y0 Z1 just to make sure after reboot, it is the same. However, after reboot, I cannot ref all. If I do, it will be off by around .005 to .01 on every axis. So... as long as you don't ref all, you should be fine.

Ken

[MFchief]

Fred, Ken,

Yes, I could do that, since the original zero position on the stock is still there. But the reason for the questions was to try to find the lowest source of error for after the tool change. It just seemed that doing the procedure I first asked about would be the best, compared with a total restart and zero, and I wondered what other folks thoughts or experience might be.

Ken's comment seems to echo what Randy said, and I believe is the way I will try it, assuming I do have to power down.
Thanks
Terry

[SteveKoerner]

This thread is particularly interesting to me. Although I haven't bothered to do the experiment -- I have always had my doubts about the likely accuracy of slamming against the switches to reestablish a reference. My doubts are confirmed by the reports here.

So given that it isn't very accurate, why bother at all? I seem to be doing fine without doing the recomended ref all. If you have to do an e-stop, isn't it always going to be quicker and a lot more accurate to rereference to the vise jaw or the part?

[zephyr9900]

Steve, ref all is for establishing the machine's working envelope to Mach. For one thing, M998 toolchange macro won't work unless the machine is referenced. Things like soft limits also won't work unless the machine is referenced. I run my lathe unreferenced because I (still...) don't have limit/home switches hooked up, but I do run into the physical hard stops from time to time with it.

Randy

[SteveKoerner]

Makes sense. I never use M998 - just something like g0 z3.

Soft limits sounds interesting; I don't recall seeing anything in the manual about that. Are soft limits available on the 1100?