Getting spindle RPMs on a Bridgeport J head

[JimBuchanan]

Pulley or vari-speed will make it a challenge to get the spindle RPMs. The back gears or low speed make it even more an issue. I decided to use the 3/4" hex bolt at the top of the drawbar.

Here is how I did it, at least for now or until I want to look into power drawbars. The photo shows an aluminum disc with a turned down 12 point 19mm socket and press fit into the disc. I used 19mm instead of 3/4" because it gave a more snug fit or at least the runout was less as the hall effect sensor would be in close. So, regardless of what pulley or back gear in or out, the RPMs of the spindle are counted.

The disc has 6 magnets inset into the edge. The magnets are doughnut shaped with a whole in the center. I also observed the magnetic field was not as expected because I was getting 2 signals for each magnet. I had to configure "12" magnets in the TouchDRO settings in order to get in the correct RPM ballpark. I think the 2 signals have something to do with the doughnut shape of the magnets. Anyway, I readjusted the hall sensor position to give just one hopefully strong signal and changed the configuration setting back to "6" magnets.

Running the mill with VFD, I have the upper frequency at 60 cycles and my objective was to use the published speeds to know when I was getting proper RPMs. The 3rd highest pulley speed on the mill speed plate is 1115 RPM and the TouchDRO was measuring 1130-1150. So, 3% variance is acceptable.

 

[David2011]

This is something I’m planning on adding to my mill as well. My VFD is set to operate between 30 and 90 hertz but it usually stays at 60.

What is the reason for 6 magnets? Won’t one work?

 

[JimBuchanan]

As I understand it, more magnets gives more signals and faster updates in TouchDRO, especially at lower RPMs. This info is in the documentation.

Independently from this, I think I'm seeing a RPM resolution of +/- 10 RPM. In the above scenario, my RPM display in TouchDRO was alternating the 2 readings, 1130 and 1150 RPM. I wonder if even more magnets would give enough data to have a finer resolution?

 

[woodchucker]

did you buy magnets that have the correct N-S pole orientation? That could be the reason for the 2 hits per mag, but I would probably agree, the hole might be the cause.

edit: looks like your magnets are set at different depths.

 

[JimBuchanan]

Well, yes, they are in the correct orientation, and it really needs to be confirmed that a signal is generated. This was made easy as my hall sensor flashed a green LED on each pulse.

I think some metal was stuck to the underside of 1 or 2 of the magnets when I epoxied them in.

While these magnets used are super strong, I made an effort to reduce the distance to the sensor as short as possible, just to make sure. There is a little variance in the depth of the magnet insert, and there is also the runout of the drawbar and slop in the socket fit. I would say overall though that the clearance between the magnet and sensor is 1/8" =/-. I also believe the sensor placement being right in the middle of the magnet "sidewall" gives a strong signal. I hand cranked the spindle for quite a while and always got a bright green LED flash.

 

[woodchucker]

Well, yes, they are in the correct orientation, and it really needs to be confirmed that a signal is generated. This was made easy as my hall sensor flashed a green LED on each pulse.

I think some metal was stuck to the underside of 1 or 2 of the magnets when I epoxied them in.

While these magnets used are super strong, I made an effort to reduce the distance to the sensor as short as possible, just to make sure. There is a little variance in the depth of the magnet insert, and there is also the runout of the drawbar and slop in the socket fit. I would say overall though that the clearance between the magnet and sensor is 1/8" =/-. I also believe the sensor placement being right in the middle of the magnet "sidewall" gives a strong signal. I hand cranked the spindle for quite a while and always got a bright green LED flash.

when I was saying orientation I meant
some magnets are N-S on the flats, some are N-S on the edges.. you can buy them different ways.

 

[dabear3428]

As I understand it, more magnets gives more signals and faster updates in TouchDRO, especially at lower RPMs. This info is in the documentation.

Independently from this, I think I'm seeing a RPM resolution of +/- 10 RPM. In the above scenario, my RPM display in TouchDRO was alternating the 2 readings, 1130 and 1150 RPM. I wonder if even more magnets would give enough data to have a finer resolution?

some of your magnets may be giving intermittent false triggering causing the variance on RPMs, you moved the sensor but it may still be picking up some double hits some times. recommend replacing the magnets to some that will trigger correctly.

 

[Reddinr]

Hard to tell from the photo but is your sensor going right over the center of the magnets? The magnetic field likely is a little bit donut shaped because of the hole as you suspect. If you haven't yet, maybe try offsetting the sensor up or down as well as trying different distances.

 

[David2011]

As I understand it, more magnets gives more signals and faster updates in TouchDRO, especially at lower RPMs. This info is in the documentation.

Independently from this, I think I'm seeing a RPM resolution of +/- 10 RPM. In the above scenario, my RPM display in TouchDRO was alternating the 2 readings, 1130 and 1150 RPM. I wonder if even more magnets would give enough data to have a finer resolution?

OK, that makes sense. I'm relating it to my model airplane engines which use Hall effect sensors for the ignition. There's only one magnet but they work at higher rpms. For starting they need to see at least a 300 rpm rate and the engines idle around 1800-2000 rpm in the sizes I have.

 

[JimBuchanan]

some of your magnets may be giving intermittent false triggering causing the variance on RPMs, you moved the sensor but it may still be picking up some double hits some times. recommend replacing the magnets to some that will trigger correctly.

No false triggering. I'm getting strong signals and consistently the same readings. That's called precision. I confirmed the accuracy by matching the readout to the Bridgeport pulley speed plate. What I was trying to describe is if the resolution (accuracy down to 10's or farther down to 1's RPMs) is increased by using more magnets. Or, if the TouchDRO code calculated RPMs only to the 20 RPMs, as in 1130 then jumping to 1150 RPM, then back, as observed. That implies a resolution of down to 20 RPM, and 1140 RPM +/- 10 RPM.