Some updates on "speed and feed" and the mill chattering

One is a very highly embarrassing confession. Along the lines of Gunner's own trailer tire passing him by at the red light.
The mill, as I mentioned, was chattering under what I considered to be moderate cuts. I wondered how come it was chattering.
Now I know why: The head is attached to the rigid ram by means of four socket head cap screws. And when installing the head, in general excitement, I forgot to tighten them. So the head moved somewhat under loads in X direction. Very embarrassing and humbling.
With that fixed, the mill is no longer chattering.
I have one more weirdness: under heavier cuts, the mill triggers a limit switch error for a reason that I cannot understand. But if I do not push it quite as hard, it does not do so. I tend to think that it is something related to the hardware of limit switches and maybe some vibrations.
i
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Ignoramus29468 wrote:

Odd. Worn out switch that teeters on the edge of tripping? RFI/EMI from the VFD leads to the spindle motor too close to the limit switch leads and the interference gets stronger as you put some load on the motor? Limit switch connection somewhere vibrating loose?
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"Pete C." wrote:

Further on this, check "debounce interval" or equivalent settings for your limit switch inputs, adjusting that higher can give more noise immunity. You can also put some small bypass capacitors across the inputs, I had a similar issue and putting a 0.1uf cap across the line killed the noise.
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Pete, I am not yet sure how to think about it, but I will try to get to it. Right now all limits are wired in series. I may want to change them and wire them as separate inputs to EMC, so that at least I will know what tripped.
Putting a little cap and perhaps a resistor across the line would probably help if is was a noise issue.
I know that VFD is mounted next to the motor and the cable from VFD to motor, IIRC, does not touch any of the limit switch cables. But I will check this.
i
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On Thu, 26 Aug 2010 11:02:55 -0500, Ignoramus29468

I think we suggested that early on....<G>

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I have a sealed "RF filter". Would it make sense to connect it to the power input of the VFD? Just for cleaner operation.
i
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Depends on just what this "RF Filter" is. Any more info available?
Joe Gwinn
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Ignoramus29468 wrote:

No need to be embarrassed, it happens.

I'd be more likely to believe electrical noise, from either the spindle VFD or the servo drive. Both use PWM, and produce high-voltage pulses with sharp rise and fall times. These pulses may get stronger under greater mechanical load.
Are you sure it is a limit switch error and not a following error?
Jon
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I am positive that it is a limit switch error.
I sort of agree with your explanation about electrical noise.
I tend to blame the wires from servo amps. I do use twisted pairs, but maybe I did a bad job somewhere.
i The wires to
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On Thu, 26 Aug 2010 18:51:13 -0500, Ignoramus29468

Can i ask how its wired in EMC? The limits switches on my control use 24 volt DC and are wired NC to an opto isolator. This is a very noise resistant design. I'm curious if you can do the same in EMC.
Karl
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Karl Townsend wrote:

You can do ANYTHING in EMC. But, Igor is using my (Pico Systems) PPMC board set. It has an isolated 5 V power supply (DC-DC converter) on the digital I/O board, and 16 general-purpose opto-isolated inputs. There is a default assignment of these, with a + limit, -limit and home switch assigned for each axis. So, a 3-axis machine uses up 9 digital inputs. EMC2 provides different messages for each limit tripped condition (Machine is on + X limit switch). The default config file set will inhibit motion past a limit switch (except when homing and the HOME_IGNORE_LIMITS parameter is set for that axis).
Of course, you can wire up any different gear you want, and connect it to EMC's functions however you choose, in either HAL or ClassicLadder.
All my servo amps have L-C output filters, so I just don't have this sort of trouble with them.
Normally closed contacts are the best way to do things, as you say. The way I have the DIO board set up, and electronic sensor with NPN output can also be used, but the DC-DC converter doesn't have enough extra capacity to power the sensor itself, just to sense the output.
Jon
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On Thu, 26 Aug 2010 18:51:13 -0500, Ignoramus29468

So have you individualized them yet, Ig? If not, do so and try the same cut. Let us know if it still errors out.

Twisted pairs will certainly limit noise transmittal.
I'm waaaay behind you. My sister got the -other- book for me on the re-purchase, and I think I'll be happier with it. I'm just getting into it now. The info is very basic (so far), but that reinforces info I've learned here. It does not follow the programming style "cookbook". http://fwd4.me/b47 _The CNC Cookbook_, E Hess.
I'll be getting Overby's book when it releases, too. http://fwd4.me/b46 _CNC Machining Handbook: Building, Programming, and Implementation_, Alan Overby (of Custom CNC, Inc. fame, high-perf ShopBot controllers)
-- Not merely an absence of noise, Real Silence begins when a reasonable being withdraws from the noise in order to find peace and order in his inner sanctuary. -- Peter Minard
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Not yet, no. I was making some chips today (a lot of)

I am not an electrical engineer, unfortunately, but I wonder if a capacitor and a small resistor could help with noise suppression.

You know, I am a computer programmer, used to coding.
I have a simple task, how to face (remove top of) a part. IO, I want to mill this profile
-/~~~\__/|__/~~--~~\ ,,./ ~~~~\---,,,...
To become this profile:
_____________________________________
In several layers and several passes per layer. Pretty simple, huh?
(I need it because I want to square off some scrap cutoffs).
And yet I have been writing, rewriting, finessing, debugging subroutines to do this for days.
Why? Because there is so much that I learn works "in math theory", but does not work in practice. For example, I learned that climb milling works a lot better than conventional milling (does not make nearly as much noise). So I had to redo it to only do climb milling.
I will be rewriting the subroutine tomorrow again, to mill in a spiral fashion, to avoid useless passes when I am not milling.
But at least I could get something done today. i
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The following statement sounds like the prelude to many more posts regarding problems (real or imagined) that might've been eliminated by following some knowledgeable advice of adding a capacitor to the signal lines to see if a problem might be noise-related.
*** "I am not an electrical engineer, unfortunately, but I wonder if a capacitor and a small resistor could help with noise suppression." ***
IIRC, the advice was to use a small-value capacitor.. with the resistor being interjected by the OP.
--
WB
.........


"Ignoramus29468" <ignoramus29468@NOSPAM.29468.invalid> wrote in message
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    [ ... ]

    They could -- depending on the value which the EMC interface senses as "switch open".
    Electrical noise control is a tricky job.
    Did you also twist the wires to the switches, or are they a single big loop? A loop is asking for noise pickup.
    Start with a pair at the most distant switch (assuming that you don't re-wire individual inputs for each switch). Twist until you reach the next switch, and open one side of the twisted pair and insert the new switch and continue twisting past (and including) each switch until you finally reach the interface at the computer.
    Best would be to find a small gauge shielded twisted pair, and ground the shield at the computer end only.
    The opto-isolater (from another followup) is a good suggestion too.
    [ ... ]

    Personally -- I would simply use the "pocket" canned cycle, making the pocket the whole size of the workpiece -- or a little larger if it sticks above the vise. It has the milling pattern you want, it starts at a given height, and goes down in reasonable sized steps.
    Yes -- it will be cutting air part of the time -- but unless you want to write a custom program detailing all the projections on the workpiece, I think that the pocket makes sense.
    Hmm ... even if it does not stick above the pocket, clamp some sacrificial material between the vise jaws and the workpiece, and size the pocket so it does not reach all the way to the jaws.

    Pocket routines are rectangular spirals, which seems to be what you want -- unless your workpiece is round. You didn't say. :-)
    And -- you might want to pocket down to about 0.020" above the intended size, and then cut via zig-zags a bit longer than the full workpiece length to get a slightly nicer appearance of parallel milling lines.

    Good.
    Enjoy,         DoN.
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On Thu, 26 Aug 2010 23:33:14 -0500, Ignoramus29468

Fun! Have you figured out storage and recycling for same, yet?

First eliminate the known possible causes, then scope it to find out if a nasty noise is actually present. Then ask an EE for help.

I think that's why many routines are circular (well, rectangular) in nature, presenting the cuttable material to the flutes in the proper direction.

Yeah, that's what I meant above.

Chips is chips, eh? Carry on!
-- If we attend continually and promptly to the little that we can do, we shall ere long be surprised to find how little remains that we cannot do. -- Samuel Butler
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On Sat, 28 Aug 2010 09:32:09 -0700, Larry Jaques

Having a proper ocilloscope helps a lot too.
Gunner
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wrote:

So what model are you going to be selling to him, Gunner? ;)
-- If we attend continually and promptly to the little that we can do, we shall ere long be surprised to find how little remains that we cannot do. -- Samuel Butler
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On Sat, 28 Aug 2010 18:17:52 -0700, Larry Jaques

Since Ive got 5-7 or so...if he wants one..after he learns what he needs, Id simply send him one for shipping and maybe a swap for something.
But I dont have any storage scopes, which may be what is needed.
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Ignoramus29468 wrote:

Twisted pairs won't work on a PWM drive, where there are 80 V transitions in a couple hundred ns on one wire. The PWM transitions are not truly differential, so they don't cancel out. The dv/dt is huge on these things, so they act as serious radio transmitters. My own servo amps have L-C output filters to drastically cut down on the EMI problem, I am just amazed that so few other commercial units do this.
You could put ferrite cores over the wires, or use shielded cable either on the motor wires or the limit switch wires. I'm kind of surprised there is a problem, as the digital inputs on the PPMC DIO board are optoisolated, and those isolators are pretty slow.
Are you using normally open switch contacts? In that case, you have open-circuit wires acting as antennas. It would have been better for a few reasons to use normally closed contacts, so a dirty switch contact or broken wire would cause a fail-safe condition. It would also get rid of any floating-wire effects. Probably some capacitors to ground would solve the antenna problem.
Jon
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