Getting spindle RPMs on a Bridgeport J head

[woodchucker]

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.

do you really need precision down below 10 rpm or even 20?

 

[JimBuchanan]

Nope, I'm happy with the way this turned out. I'm just commenting on the things I've learned on the build out. I need to re-focus on the Z axis scale now.

 

[ycroosh]

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.

View attachment 492214View attachment 492215

This is a neat idea. Too bad I can't "borrow" it for my mill

 

[PNW_Guy]

I'm confused, and clearly missing something, although I think it's great.

Doesn't the draw bar move up and down with the quill?

 

[mksj]

You can also put a tach sensor and LED ring light off the spindle. Example is for a PM-935 but I have also made these for the PM-949 with different dimensions. One thing that I did find out is that if the magnet is spinning next to an aluminum ring it will induce a magnetic field in it that can cause an erratic tach reading. So in subsequent versions I have more distance between the inner and outer ring. I also find that the generic 8 or 12 mm NPN hall sensors you can pick up are much easier to install/work then the IC hall sensor.

 

[woodchucker]

not knowing whether the balls in my bearings are SS or hardened steel, I am reluctant to put a magnetic holder on the quill by the output. I am worried about magnetising the balls and what effects it may have on them.

 

[JimBuchanan]

I'm confused, and clearly missing something, although I think it's great.

Doesn't the draw bar move up and down with the quill?

The aluminum puck with 6 magnets ride on the upper bearing locknut and bearing sleeve locknut of the top housing. It slips onto the hex nut of the drawbar. As the drawbar moves up and down, the magnet puck stays put vertically, but spins with the drawbar.

 

[PNW_Guy]

The aluminum puck with 6 magnets ride on the upper bearing locknut and bearing sleeve locknut of the top housing. It slips onto the hex nut of the drawbar. As the drawbar moves up and down, the magnet puck stays put vertically, but spins with the drawbar.

Ah, thanks

 

[JimBuchanan]

While I felt I had a good tach install, a bug appeared in use. Yes, there is a gotcha in the above build. It affects bridgeports and I'm sure most all clones. I wasn't getting rpm's when I engaged the low speed back gears. When the cam ring is switched to low speed, it raises the sliding spindle pulley hub about a 1/4" to engage the bull/back gears. That 1/4" rise moved the so called magnet puck up out of range of the hall effect sensor.

Earlier I described my use of magnets with holes and the resulting double counts. Well, I ordered 6 new solid magnets with a more broad magnetic field, adjusted the position to get counts in both low and high speeds, and that took care of that.

I tried to take photos to illustrate the cam ring moving up and down. The photos were taken from the right side, not facing from the front. But if you were standing in the front, 1st photo, cam ring handle straight ahead and spindle hub down, high speed clutch engaged for high speed. 2nd photo, cam ring handle off to the right and spindle hub up, high speed clutch dis-engaged so low/back gears are in the drive train.