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Author Topic: Spindle Index RPM not Matching Actual RPMs HELP  (Read 4253 times)

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Spindle Index RPM not Matching Actual RPMs HELP
« on: May 26, 2009, 07:12:45 PM »
So I've finally got my 9x20 lathe CNC conversion done. I set up the spindle index but it doesn't regist the True RPMs. At 200 RPMs it reads 200 RPMS, at 2000 it reads 270 or so. What is the problem.

I'm using a 2 7/8" diameter disc with a .125" wide slot. Is the slot too wide or too narrow, could that be the problem? Any help would be apprecaited, thanks.
Re: Spindle Index RPM not Matching Actual RPMs HELP
« Reply #1 on: May 26, 2009, 07:49:37 PM »
Hi,

It sounds like the slot is too small and the pulse is no long enough to be measured by Mach. It could also be that the sensor is too slow to respond to the short time the slot is exposed at higher rpms,

Cheers,

Peter.
« Last Edit: May 26, 2009, 07:54:51 PM by Peter Homann »
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Homann Designs
http://www.homanndesigns.com
email: peter at homanndesigns.com
Re: Spindle Index RPM not Matching Actual RPMs HELP
« Reply #2 on: May 26, 2009, 07:58:18 PM »
The sensor i'm using is the C3 Pulse Index Board from CNC4PC which is made for this, so I hope it's not the latter. If you're not positive about the slot size then I don't want to make it bigger without more help, but thanks so far.
Re: Spindle Index RPM not Matching Actual RPMs HELP
« Reply #3 on: May 26, 2009, 08:41:47 PM »
2000 rpm = 33.33 Hz

therefore 1 rev takes 1/33.33   or 30ms

At the diameter you are using, the circumference is 2.875" X PI = 9.03"

your slot is .125"  = .125/9.03  x100  = 1.38% of the circumference.

So your slot is visible for .0138 x 30ms = 0.414ms or 400uS


You need to ask CNC4PC what is the rise and fall times of their index board, not just the opto sensor.

Then, for Mach3 to be able to see the slot it needs to be sampling at around twice the frequency. 1/400uS  x 2 = 5000Hz

So with mach at the lowest kernel speed of 25KZ, sampling the slot is not the issue. My assessment is that the slot is not wide enough and that the pulses exiting a the index board are non existent or too narrow.

You could make the slot wider to solve the problem or ask the manufacturer why it doesn't work.

Peter.
----------------------------------------------------
Homann Designs
http://www.homanndesigns.com
email: peter at homanndesigns.com
Re: Spindle Index RPM not Matching Actual RPMs HELP
« Reply #4 on: May 26, 2009, 08:46:35 PM »
Ok, I'm not sure I understand the math, but I appreciate the help. Peter what should I widen it to?

Offline RICH

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Re: Spindle Index RPM not Matching Actual RPMs HELP
« Reply #5 on: May 26, 2009, 08:54:42 PM »
Try .2" even .25" it can only help. My disc dia is 3" and slot was increased from around that, even have some wider and they work fine.
RICH
Re: Spindle Index RPM not Matching Actual RPMs HELP
« Reply #6 on: May 26, 2009, 09:05:53 PM »
Ok, I'm not sure I understand the math, but I appreciate the help. Peter what should I widen it to?

Basically a wide slot is good. A 1/2" slot seems reasonable. As Rich suggested, try 1/4"  There is no use trying to find the minimum you can get away with because it may fail if you go to 2200rpm, etc

In the end Mach3 looks for the transition edge, once if detects that the rest of the pulse is unused, so to Mach the width of the pulse is not important, it just needs to be long enough for it to be detected.

If you are worried about the balance of the disk, you could drill a hole opposite the slot (But not where the sensor will see it)

Cheers,

Peter.
----------------------------------------------------
Homann Designs
http://www.homanndesigns.com
email: peter at homanndesigns.com
Re: Spindle Index RPM not Matching Actual RPMs HELP
« Reply #7 on: May 26, 2009, 09:12:52 PM »
Ok I increased the slot to 1/2" and at 2000 RPM it still only reads 670 RPM. At that rate the slot would need to be 1" or bigger. I think it must be a setting thing.
Re: Spindle Index RPM not Matching Actual RPMs HELP
« Reply #8 on: May 26, 2009, 09:15:28 PM »
I am running the same c3 board on my 9x20 with a 1/4" slot in a 2 3/4" diameter disk and it seems to be working just fine although I have never run it over 1100 rpms.
Re: Spindle Index RPM not Matching Actual RPMs HELP
« Reply #9 on: May 26, 2009, 11:16:04 PM »
I reread the short paper on installation and it mentions the debounce setting. When you goto the Gen. Config page there is a separate setting for Index Debounce. It was originally set at 100 but I kept knocking it down until it worked. Mine worked from 10 to 30, I left it at 30 now it reads correct. Thanks for all the help.