# Where to install a tachometer on a knee mill



## Pcmaker (Dec 24, 2020)

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


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## aliva (Dec 24, 2020)

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.


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## Mitch Alsup (Dec 24, 2020)

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



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 !!


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## hman (Dec 24, 2020)

Mitch Alsup said:


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


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.


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## Jackle1312 (Dec 24, 2020)

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.


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## hman (Dec 24, 2020)

@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: 








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

Proximity output current: 20mA(For the sensor). Hall proximity model:NJK-5002C.  1 x NPN hall proximity sensor. Red LED Specifications Display:5 x 2cm red LED. Sensor wiring:brown(Power+),black(Test+),blue(signal).



					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 


			https://www.amazon.com/Taiss-photoelectric-NC（Normally-Adjustable-E3F-DS30B2/dp/B073WRGGTG/ref=zg_bs_306933011_6?_encoding=UTF8&psc=1&refRID=WZ2ZYNDFP0SYKSZN11JF
		

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.


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## hman (Dec 24, 2020)

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.


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.


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## Jackle1312 (Dec 24, 2020)

It should fit with a power drawbar


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## Pcmaker (Dec 24, 2020)

Jackle1312 said:


> It should fit with a power drawbar



Did you just glue the magnet on the top of the pulley?


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## Jackle1312 (Dec 25, 2020)

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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## Pcmaker (Dec 25, 2020)

Jackle1312 said:


> 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




Did you drill on the top of the pulley and just pressed and glued the magnet? How close does it have to be on the sensor?


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## mksj (Dec 25, 2020)

One question would be if mounting the magnet on the headstock pulley will work in back gear? Another approach which I have done on several mills is to mount it to the ram and read off the bottom of the spindle. This also integrates a LED Halo ring light which helps reduce work shadows. See revised drawing which is for the PM935. On the ones I made previously there was less of a gap between the inner magnet ring and the outer sendor ring, but it needs to be increased because the spinning magnet next to aluminum was inducing a magnetic field effecting the reading.


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## Danegineering (Dec 25, 2020)

I recently completed a DC to 3-Phase conversion on a Syil X6. I used a 3D printed collar with heat seated threaded inserts. This is installed on the motor shaft. We will see if it lasts, but it was printed in PetG with 6 perimeters so it’s very sturdy. If you don’t have a printer or know someone who does, and want to try this, DM me and I can probably help you out. Of course the trick here is this mill has a 1:1 on the drive pulley. 
	

	
	
		
		

		
			






Sent from my iPhone using Tapatalk


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## Jackle1312 (Dec 25, 2020)

I drilled the top of the pulley for the magnet and just used epoxy to hold it. The sensor had enough range that it works in and out of back gear with out any issues.


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## hman (Dec 25, 2020)

Expanding on my earlier post ... and especially for those of you who are not too big on electronics ... here's an explanation (and memory aid) about the "alphabet soup" designations for NPN/PNP and NO/NC sensors.

NPN - When activated, the sense lead (black in the example below) connects to the 0 volt (or negative) lead.  When the sensor is not activated, the sense lead floats.  See diagram below.  Easy to remember if you think of the sense lead "going Negative" on activation because there are two N's in NPN.  



PNP - When activated, the sense lead (black in the example below) connects to the +V (or positive) lead.  When not activated, the lead floats.  Easy to remember if you think of the sense lead "going Positive" on activation because there are two P's in PNP.  



NOTE - The colors of the leads may vary.  Check the documentation for what color goes with what function, and what voltage the sensor likes to eat for breakfast.  The documentation will also tell you the maximum load you can place between the sense lead and the appropriate power lead.  If you're going to something electronic (like a tachometer) the load will be tiny, so no worries.

NO (normally open) - Sense lead floats when sensor is not activated, connects (to either +V or 0V) when sensor is activated (just like an NO switch)
NC (normally connected) - Sense lead connected to +V or 0V when sensor is not activated, floats when sensor is activated (just like an NC switch)


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## hman (Dec 25, 2020)

@mksj - WOW!  That's a very nice light-and-tach installation!  Wish I'd seen yours before I built mine.  I just couldn't figure out how to safely attach a magnet to the spindle.  Didn't think of an additional ring.  You rock!


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## Ken226 (May 26, 2021)

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.



Sorry to bring back an old thread,  but Im finishing up a 3 phase vfd conversion ony PM935ts, and have a relevant question.

I measured the movement of the spindle pulley when switching between the hi and low gear positions, and found that the pulley moves up 0.22" when shifted into back gear.  

I set up some aluminum spacers of various thickness and measured the maximum distance my tachs sensor would read a 5mm x 6mm neodymium magnet to be 0.26"

I drilled the pulley, pressed in the magnet,  and tapped the head cover for the sensor.  Everything is connected, and it works fine, in both gear positions.

It reads correctly in hi gear,  but obviously not in back-gear.

On my pm935, the back gear ratio appears to be 0.12, which makes setting speeds  pretty easy.   Just use 0.12,  or its inverse proportional as a factor. 

Indicated rpm x .12 = actual rpm.   Or,  desired rpm x 8.33 = indicated rpm.

So,  how did you secure and route the sensor wire.  I prefer not to drill/tap holes for cable ties if I don't have too.   Something with double sided tape?

Edited:

I just ordered a pack of these to try for 7$,  but I wonder if they'll fall off the first time oils splashes on one.


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## Janderso (May 26, 2021)

Amazon.com: AGPtek® Professional Digital Laser Photo Tachometer Non Contact RPM Tach: Automotive
					

Buy AGPtek® Professional Digital Laser Photo Tachometer Non Contact RPM Tach: Tachometers - Amazon.com ✓ FREE DELIVERY possible on eligible purchases



					www.amazon.com
				




As an option = cheap


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## Ken226 (May 26, 2021)

Janderso said:


> Amazon.com: AGPtek® Professional Digital Laser Photo Tachometer Non Contact RPM Tach: Automotive
> 
> 
> Buy AGPtek® Professional Digital Laser Photo Tachometer Non Contact RPM Tach: Tachometers - Amazon.com ✓ FREE DELIVERY possible on eligible purchases
> ...




To verify that the integrated tach I've already installed is accurate?  Or did you have something else in mind?

Or, was you replying to the OP?  If so,  this is a year old necro-thread,  I brought back because one of the respondents used a similar installation method to me.  I was just wondering how he routed his sensor cable.


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## Janderso (May 26, 2021)

Ken226 said:


> To verify that the integrated tach I've already installed is accurate? Or did you have something else in mind?


My apologies sir,
I didn't realize this was an older thread. 
Sometimes I need to listen harder.
Just another way of determining RPM. Some may not be aware of their existence.


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## Diecutter (May 26, 2021)

Janderso,  That is what I ended up using and it functions fine.  Bonus points: I can check the rpm's on all my other machines with it, plus the mower, car, fan, etc. The one I got was from Ebay for about $15.


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## Ken226 (May 27, 2021)

Those little wire clips from Amazon got the job done.


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## hman (May 28, 2021)

Ken226 said:


> I just ordered a pack of these to try for 7$,  but I wonder if they'll fall off the first time oils splashes on one.


I'd suggest the following:

1. Before attaching the clip, clean the machine surface with isopropyl alcohol; let it dry thoroughly.
2. Open a fresh mini-tube of super glue (I like the itty bitty ones from Harbor Freight). 
3. Remove the liner from the double-stick tape on the bottom of the mount.
4. Put just a few drops, widely spaced, on the sticky surface.
5. Carefully position and press the mount down on the machine surface.

The stickum on the tape will hold the mount in position while the super glue "kicks."  Just be sure you put it in the correct position to begin with!

Removal of the mount is best done by slicing through the double-stick, then either scraping the residue off the machine's surface or dissolving it with mineral spirits or acetone.


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