Minimum encoder count for spindle tapping.

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Minimum encoder count for spindle tapping.

Postby Derek » Mon Dec 10, 2018 3:19 pm

I'm building a new spindle for my Bridgeport that will have a 10,000 RPM max speed. I'm moving away from the AMT encoders as I've had a few issues in the past. I have a few spare US digital encoders but the wheels are all too high a count. I need to order a wheel and it seems like the smaller the count the better for pulse width.

Any ideas?

Derek
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Re: Minimum encoder count for spindle tapping.

Postby Robertspark » Mon Dec 10, 2018 3:23 pm

10,000 rpm = 166.6r rev / sec.

http://shop.cncdrive.com/index.php?productID=767

@ 500 ppr >>> 166.6 x 500 = 83.333r kHz....

I'd look at 500 ppr / similar "low" count encoders... nice chunky digital signal

sure others will have a view.
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Re: Minimum encoder count for spindle tapping.

Postby Robertspark » Mon Dec 10, 2018 3:24 pm

these also always look interesting and useful...
http://shop.cncdrive.com/index.php?productID=346


Resolution can be set, with the build-in 4 microswitches.

Selectable resolutions:48, 96, 100, 125, 196, 200, 250,256, 384, 400, 500, 512, 800, 1000, 1024, 2048 PPR.
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Re: Minimum encoder count for spindle tapping.

Postby cncdrive » Mon Dec 10, 2018 3:45 pm

Derek is right, for servo position control the AMT very high resolutions are not optimal, it can have issues because of the interpolation speed of the AMT's capacitive sensor. The problem is that the signal can lag a bit which is a problem for fast servo position loops, but that's not really a problem for thread cutting and tapping, because that error from the lag is so small and for thread cutting there is no need for so high resolution.
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Re: Minimum encoder count for spindle tapping.

Postby cncdrive » Mon Dec 10, 2018 3:46 pm

Rob,

And that is the base resolution. For thread cutting and tapping the controller reads all signal edges, so the resolution range is 48*4=192 to 2048*4=8192.
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Re: Minimum encoder count for spindle tapping.

Postby spumco » Tue Dec 11, 2018 1:31 am

My 200-line encoder works just fine for rigid peck tapping 4-40 to 1/2-13 threads (high speed & very low speed).

Using the UCCNC PID feature, spindle RPMis stable within 5RPM up to my max of 8kRPM, both reported by UCCNC and with a separate laser tachometer.
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Re: Minimum encoder count for spindle tapping.

Postby Robertspark » Tue Dec 11, 2018 6:59 am

cncdrive wrote:Rob,

And that is the base resolution. For thread cutting and tapping the controller reads all signal edges, so the resolution range is 48*4=192 to 2048*4=8192.


That I did not know, is it in the manual and I've not read it?
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Re: Minimum encoder count for spindle tapping.

Postby cncdrive » Tue Dec 11, 2018 12:04 pm

Well, 4x encoder decoding mode is the usual way of decoding the encoder signals and that is how the UCCNC is also doing it.
And the AMT datasheet tells you the pulses per revolution which is one encoder channel's pulse counts per revolution.
So, the resolution of the encoder comes from the A and B channels' both the rising and the falling edges counted and this is why it is called 4x decoding mode, because it gives 4 times the counts as how many pulses one channel per turn has.
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Re: Minimum encoder count for spindle tapping.

Postby Derek » Tue Dec 11, 2018 5:24 pm

cncdrive wrote:Well, 4x encoder decoding mode is the usual way of decoding the encoder signals and that is how the UCCNC is also doing it.
And the AMT datasheet tells you the pulses per revolution which is one encoder channel's pulse counts per revolution.
So, the resolution of the encoder comes from the A and B channels' both the rising and the falling edges counted and this is why it is called 4x decoding mode, because it gives 4 times the counts as how many pulses one channel per turn has.


Just so I'm clear on this. An AMT encoder with the dip switches set to 400 is the same as a 100 count wheel in an optical encoder that outputs in quadrature. Or do the dip switches on the AMT equal line count on an optical wheel.
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Re: Minimum encoder count for spindle tapping.

Postby Battwell » Tue Dec 11, 2018 8:39 pm

most manufacturers seem to express these as lines or counts
lines being the raw channel amount and count being the quadrature
eg my fanuc encoders are 2500 line which equals 10000 counts
it can get pretty confusing when shopping!
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