z height problem

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z height problem

Postby martin54 » Thu Aug 29, 2019 7:59 pm

Hoping one of the machine experts can point me in the right direction or at least suggest where to start looking, not sure if I should have put this in the hardware section or not??

So spent most of this moring cutting a large file, hadn't planned on leaving the workshop but best laid plans... So as I wasn't sure how long I was going to be away rather than just pause the job (which I now wish I had done) I made a note of the line number & then shut the machine down.

On my return the machine went back on & was homed. Gcode was loaded & cutting was resumed from where I had left off, you can see from the photo that there is a huge cliff where the machine started up again, not measured it but has to be at least 0.1mm, maybe a little more. the work offset is the bottom left corner & the machine was cutting the finish pass starting from the top right hand corner working back towards the machine starting position, the step is in the downwards direction.
I doubt this is any kind of wood movement, we don't really get many problems with humidity changes up here in Scotland. :lol: :lol:

z change.jpg
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Re: z height problem

Postby mtylerfl » Thu Aug 29, 2019 8:15 pm

Prior to restarting, did you reset your Z-zero at the exact same spot you had previously?
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Re: z height problem

Postby IslaWW » Thu Aug 29, 2019 9:07 pm

This is most likely wood movement. From stress relief as well as humidity. If not then the Z reference was inconsistent. In most cases where there is a Z limit/home (normal) then the z should not have to be reset once the machine is homed. You would have been better off to leave the controller on to ensure that this type of issue does not happen.
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Re: z height problem

Postby Leo » Thu Aug 29, 2019 9:51 pm

That line looks a bit too uniform to be wood movement. I could be wrong, but it just does not look that way to me.

I have done restarts before but I don't reset the "Z".

I am thinking a bit of inconsistency in the Z setting
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Re: z height problem

Postby martin54 » Fri Aug 30, 2019 3:12 am

Shouldn't be any need to reset the z axis Michael so no I did not :lol: :lol:

Gary I was going to leave power on the machine but I wasn't sure if I would be back at the workshop latter today I have not had any issues like this previously.

Leo, funny you should mention that because the first thing I said to myself was the line looks far to uniform for it to be a wood related issue :lol: :lol:

So it looks like it might be a bit of inconsistency on the z axis, any idea what might have caused that? Haven't had any problems that I can think of in the past relating to the z axis. Home switches are magnetic, don't know if that helps at all :lol: :lol:
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Re: z height problem

Postby Wayne Locke » Fri Aug 30, 2019 5:59 am

I would leave the control box on in this situation. The steppers or servos will lock the axis. For whatever reason if the axis drops at all this will happen. Whether it should or not is beside the point. I did a turning a few years ago with some complex resin and metal inlays and kept the box on for three straight days to guarantee that none of the axises moved during the various application and drying processes.
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Re: z height problem

Postby phill05 » Fri Aug 30, 2019 9:53 am

Martin,

Not sure how this happens but I found if I am cutting straight along X or Y axis if I hold or shut down machine for a while I get the very same mark, what I did was leave the controller on and change to cutting 22.5 degrees and now it does not happen, I think when one axis is on hold for a while when it is asked to move it jumps a step or loses a step but not sure if this is same as your problem.

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Re: z height problem

Postby martin54 » Fri Aug 30, 2019 11:54 am

Wayne Locke wrote:I would leave the control box on in this situation. The steppers or servos will lock the axis. For whatever reason if the axis drops at all this will happen. Whether it should or not is beside the point. I did a turning a few years ago with some complex resin and metal inlays and kept the box on for three straight days to guarantee that none of the axises moved during the various application and drying processes.


My workshop is a rented industrial unit so I always try to knock everything off when I leave, plus I am a bit tight & doing that uses extra electric :lol: :lol:
Seriously though even if the axis had drifted a little with the controller being powered off as soon as the machine was homed it should have corrected that. This wasn't the firsty time I have done this sort of thing, might be the last if it is going to give me problems like that though :lol: :lol: :lol:

Phill, how do you find cutting at an angle like that & why 22.5 degrees? I have thought about trying it myself on a few occassions especially on those jobs where you can't decide if it is cutting better with or across the grain but never seem to get round to it. Not really sure why because I can find plenty of time of other types of experiment :lol: :lol:
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Re: z height problem

Postby phill05 » Fri Aug 30, 2019 12:39 pm

Martin,

By chance I tried 22.5 degree to get past a real hard spot in some stone that fell on that line it cut really well, next I tried on metal and found it left a nice fine back ground, left it on 22.5 ever since as all three motors are working at the same time it never caused the line you are getting again, don't know why so just left it as is, try it might work for you.

22.5.jpg.jpg


22.jpg.jpg


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Re: z height problem

Postby TReischl » Fri Aug 30, 2019 1:04 pm

Hello Martin,

If you do not have industrial strength high accuracy limit switches then you may not repeat the z home position when you rehome the axis after shutdown. Do not know you machine, sooooo. . . .
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Re: z height problem

Postby scottp55 » Fri Aug 30, 2019 1:30 pm

Martin,
Just curious...how big was piece, and how exactly was it held down?
3Ds open up a lot of end grain, and it's not the humidity change in the air, as much as the humidity change for the interior of the wood at cut depth(PLUS stress relief).
As you know it's not much of an issue with my tiny stuff cut in one shot, but like I said curious.
(IF it had been me...I would have resumed with Z-zero set a shade high, and then snuck up on it....but only had to do that twice.)
Bummer! :(
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Re: z height problem

Postby garylmast » Fri Aug 30, 2019 1:47 pm

Because you shut the power off, when you turned it back on, you more than likely had a little backlash in the screw movement. The couple of penny's you're saving on power is less than the time and material of redoing or fixing it.

Just my two-cents which now I don't have to pay my power bill. :mrgreen: :mrgreen: :mrgreen:

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Re: z height problem

Postby dwilli9013 » Fri Aug 30, 2019 1:57 pm

Just my two cents Martin,
It appears that your stopping point was about midway through the cut. If you think about it your machine was getting quite a workout at that point and was well warmed up. So my guess all 3 axis would have grown a bit during the cut due to heat buildup and cutting operations.
Then when you come back to the cut some 24 hours later everything in the machine has had time to cool down and contract back to normal. When you came back to continue the cut my guess is you homed and went right back to the cutting where you left off. Thus the machine has not had the ample time to grow back to the parameters it was at on the end of the cut the prior day. Don't know your machine but this anomaly would be more evident on machines which are mostly aluminum construction.
In any event either your headed for a butt load of hand finishing or drop your z a bit and rerun the finishing pass. Beautiful piece by the way.

I also have switched most of my big carves to the 22.5 raster on all of my fish passes. Just seems to leave a finer finish for me. This holds especially true on purple heart which I do a fair amount of.

Like I said just my two cents. Also my theory goes completely out the window if the step went down on resuming the cut.... :lol: :lol: :lol:
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Re: z height problem

Postby garylmast » Tue Sep 03, 2019 3:49 pm

It was the end of the day yesterday so I didn't want to start machining, but I did set my material and zeroed all the axis. This morning my zero axis was off .3 mm. Apparently when the power is off, there's no pressure on the servo motor (stepper motor in some cases) and it backs off the screw slightly. It's probably a good idea to check the zero point if the power is turned off.

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Re: z height problem

Postby mtylerfl » Tue Sep 03, 2019 3:54 pm

Exactly.
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