Where to install a tachometer on a knee mill

Pcmaker

Pcmaker

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I have a PM835S mill, which is a Bridgeport clone. Step pulley model. I'll be replacing the motor with a 3 phase so I can put a VFD on.

I'm trying to see how I can install a tachometer and an rpm display. I'm having a hard time trying to figure out where I can put the magnet
 
Try on the bottom of the ring nut of the spindle. Then install the proximity switch on the rear of the spindle as to not impede your view. The switch will have to be connected to the spindle so it travels with the spindle. Another option maybe on the bottom of the stepped pulley.
 
Pcmaker said:
I have a PM835S mill, which is a Bridgeport clone. Step pulley model. I'll be replacing the motor with a 3 phase so I can put a VFD on.

I'm trying to see how I can install a tachometer and an rpm display. I'm having a hard time trying to figure out where I can put the magnet
Click to expand...

Why does the VFD not already have a tachometer on it ?

Also note: if you remove the capacitors (start-and-run, or start, and run) from the 1-phse motor you (instantly) have a 3-phase motor !!
 
Mitch Alsup said:
Why does the VFD not already have a tachometer on it ?
Click to expand...
Unfortunately, there are stepped pulleys and an engage/disengage back gear between the motor and the spindle. With the 835S, that would mean using a "fudge factor" table with 8 different entries.
 
I installed mine on the top of the spindle step pulley and tapped the housing for the pickup. There was enough range between the magnet and pickup so that it works even with the height change of the pulley when going into/out of back gear. Just had to recess the magnet into the pulley so it would clear the housing.
 
@Pcmaker - See post #3695 at https://www.hobby-machinist.com/thr...in-your-shop-today.67833/page-370#post-794802
If you don't add a ring light like I did, it's a lot simpler installation ... just need to mount a diffuse-reflective photosensor to the bottom ring nut. Use an inexpensive digital tachometer, such as:
www.ebay.com

Digital Red LED Tachometer 10-9999 RPM Meter Gauge + Proximity Switch Sensor USA 7284067157751 | eBay

Find many great new & used options and get the best deals for Digital Red LED Tachometer 10-9999 RPM Meter Gauge + Proximity Switch Sensor USA at the best online prices at eBay! Free shipping for many products!
www.ebay.com
These come with magnetic proximity sensors. You wouldn't want to glue a magnet to the spindle, so you should substitute a diffuse reflective photosensor, like
Note that this one is available as either NPN or PNP. NC or NO is of no consequence. You'd have to figure out which one works with your tach, then substitute it for the prox. If you need help figuring out the electronics, feel free to PM me.

See the last photo of my previous post. I attached a ring of paper with dark and light segments to the spindle. I took a strip of paper a bit longer than the circumference, wrapped it around the spindle, marked the length at the overlap, cut off the excess, then folded it end-to-end to determine where to do the black/white transitions. Blackened the alternate segments between the folds with a Sharpie, applied double-stick to the back of the paper strip, stuck it on, then wrapped clear tape on the outside to protect it. My Mach Tach has a setting for how many segments per revolution. Don't know if the eBay has this feature, so you may have to have only one black/white transition on the paper strip.

If you need additional photos of my installation, please feel free to ask.
 
Jackle1312 said:
I installed mine on the top of the spindle step pulley and tapped the housing for the pickup. There was enough range between the magnet and pickup so that it works even with the height change of the pulley when going into/out of back gear. Just had to recess the magnet into the pulley so it would clear the housing.
Click to expand...
I'd like to have done that, but I'd installed a power drawbar, so the top of the housing was obscured. But it's good to know that the pickup has the necessary range! Thanks for the info.
 
I recessed the magnet into the pulley and used a little 2 part epoxy to hold it. The top of the magnet is flush with the pulley so there are no clearance issues
 
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