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formula for slotted disc
« on: September 26, 2012, 09:32:42 AM »
Hi Guys,
I currently have a slotted disc with a opto sensor to measure the rpm of a spindle on a mill.
As of now the rpm is measureing correctly, however It reads rpm up to 2340 . How do i calculate what size disc and slot I would need to measure up to 5000 rpm. I currently am using a 4 inch disk with a .375 slot. What are the limiting factors, formula for calculating the range of the rpm being sensored with respect to disc diameter and slot width. Would I be giving up the low range to accomplish a higher sensored rpm.

Thanks Joe
Re: formula for slotted disc
« Reply #1 on: September 26, 2012, 10:34:54 AM »
Not sure if I am missing something here. But disk diameter cannot have any effect over the RPM figure. I mean, if the motor is turning at a true 1000 rpm and the disk is 2" or 4" or what ever then it will still be going the same RPM. However, if your using a timer in the slot then the slot length is the only thing that will effect the value. If the opto just sences the slot and does no slit length / time count function then the slot length will have no effect either. Is it possible that your returned value of 2340 RPM is indeed correct?

Mike.
Re: formula for slotted disc
« Reply #2 on: September 26, 2012, 10:37:17 AM »
Thanks Mike

Yes it was checked with a tachometer
Re: formula for slotted disc
« Reply #3 on: September 26, 2012, 10:49:13 AM »
Look in the MACH3 Turn manual - Page 4-12

"For reliable running Mach3 needs a pulse of at least 200 microseconds from the slots. You
can calculate your slot angle or width as follows:
Suppose the maximum speed of your spindle is N (rpm).
The slot angle required alpha (degrees) is given by
alpha = 0.0012 x N
If the diameter of the disc is D (inches or mm) then the slot width W (in same units) will be approximately
W = 0.0088 x alpha x D
So for example if maximum spindle speed is 3000 RPM and disc diameter is 110 mm then:
Alpha = 0.0012 x 3000 = 3.6o
Slot width = 0.0088 x 3.6 x 110 = 3.48 mm
Big slot needs to be 50% wider = 3.48 x 1.5 = 5.22 mm
The example in figure 4.11 has three slots at about 4 mm and one about 8 mm wide.
You will see that you cannot use a disc with many slots or holes in it or use a conventional
encoder. The pulse would be too fast. The original index sensing arrangements can usually
be simplified when retrofitting an old lathe . These often had multiple slots for speed
sensing together with a single index slot.
As well as for timing threading operations, Mach3 also uses the index pulse to display the
actual speed of the spindle and can implement a feed-per-revolution, rather than feed-perminute,
based on it."
« Last Edit: September 26, 2012, 10:50:55 AM by 762x51 »
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Re: formula for slotted disc
« Reply #4 on: September 26, 2012, 11:34:13 AM »
Thanks 762x51
I checked my math and it would call for a .316 wide slot assuming i make it 50 percent bigger . so I have the slot oversized by .059 . It seems like something else is limiting the sensor .

Offline RICH

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Re: formula for slotted disc
« Reply #5 on: September 26, 2012, 07:43:38 PM »
Multi-slotted  discs ( timing) are no longer supported for the lathe. Single slot was found to be just as accurate, in fact it is very accurate and more so than one can measure with common devices. Just a comment I  felt worth mentioning.

RICH

Re: formula for slotted disc
« Reply #6 on: September 26, 2012, 08:12:57 PM »
Just curious,
what speed range is anybody else getting when it comes to measuring rpm?

Offline Hood

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Re: formula for slotted disc
« Reply #7 on: September 27, 2012, 01:36:29 AM »
I use encoders now as I dont use the parallel port but when I was using the opto I could read to 4500rpm no problem, that was the max I ran the spindle at.
Is your opto fast enough?

Hood
Re: formula for slotted disc
« Reply #8 on: September 27, 2012, 01:46:52 AM »
Hood,
Not Quite sure what your looking for .
But it is a Pmdx 170 optical sensor
•Response time of less than 10 uS
Max pulse rate 25 Khz
« Last Edit: September 27, 2012, 01:54:52 AM by jab_814 »

Offline Hood

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Re: formula for slotted disc
« Reply #9 on: September 27, 2012, 04:18:05 AM »
That should be fast enough, what Dia of disc do you have? If it was too small then possibly the time between On/Off might come into play.
Hood