Re: [CAD_CAM_EDM_DRO] Re: Sherline Spindle Pulse disk for threading

Steve Blackmore wrote:

>On Sat, 10 Nov 2007 14:17:51 +0100, you wrote:
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>
>>If I understand correctly, you need quite some pulses when you want to
>>do threading on the lathe.
>>
>>
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>Not true.
>
>Only one pulse is needed by Mach3, EMC supports more and has feedback.
>
>

You could use only one pulse with EMC2, and it should work just as well
as Mach. We don't recommend it because it seems like it shouldn't work
well :) It may require a very simple spindle speed component to be
written, which provides a faked spindle orientation output.

>It depends how the controller works, Mach3 is quite capable of
>producing accurate multi-start threads just using one pulse per rev. The
>drawback is if the spindle slows, there's no correction for that pass.
>
>

Multi-start is relatively easy with only an index pulse - the cutter
hasn't started cutting when you're waiting for the next start to come
around. The comment about the spindle slowing is the exact reason why
threading with one pulse per rev "shouldn't work well".

>In reality this is not a problem, inertia and the low cutting forces
>involved in threading, shouldn't affect it.
>
>

I'd rephrase that like so "if you don't have spindle feedback, then you
need to use very light cuts to get acceptable threads". This is the
same thing you have to do when using steppers instead of servos - you
need to make sure that you never try to accelerate or move faster than
the steppers can go in the worst case, because you won't see if there's
a problem. In the threading case, you need to make sure that the cut
depth you use is low enough that the spindle power and inertia will keep
the speed constant enough for the thread quality you want. (that was a
mouthful ;) )

>Mach does this by reading the index pulse, and adjusting the feed to
>suit then locks those for that pass.
>
>

I sure hope it keeps adjusting speed over the threading pass (more like
"locks until it sees another index pulse").

>I believe that in EMC, if the spindle slows, the feed slows too, if that
>is correct it's not the best solution. The better option is to speed the
>spindle back up.
>
>

EMC2 does both. It tracks actual spindle speed so it can adjust XY
feed, but it can also control the spindle speed. The same feedback is
used by the spindle speed PID and the motion controller. You can unplug
the spindle motor and turn the spindle by hand, and EMC2 will continue
to track correctly. Unfortunately it won't track if you spin the
spindle backwards, so be sure to always spin forwards if you use this
method ;)

- Steve

>Being able to control and set the spindle speed is essential using
>either method.
>
>BTW - I thread at 500-1200 rpm, never any less :)
>
>Steve Blackmore
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