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Installing a digital tachometer

Dusty

Dusty

(Bill)
Premium Member
My BB 10" x18" Craftex lathe didn't come with a digital tachometer so I picked up a kit to do that. With everything being mapped out I have made the bracket to hold the 'Digiten' brand sensor in the gear change compartment. Plan is place the magnet(s) on my spindle outer nut edge then line up with the sensor. Unfortunately said outer nut edge is syndical. The not so strong 12mm diameter x 2mm thick magnet that came with my kit is placed on the outer spindle nut so I spin the spindle at top speed and the magnet stayed in place and didn't move.

So where am I going with this, my only concern is over time the magnet weakens and centrifugal force throws the magnet into the gearing. I can go to a much stronger 10mm diameter x 3mm thick magnet which is a much better fit on the nut edge. I doubt the sensor will balk at my using a smaller diameter magnet.

Can also add a drop of lock tight to the bottom of the magnet which should help?

Then what about spindle balance should I use an apposing magnet on the outer nut for balance?

The tachometer box mount is a portable micro plug-in type connecting with the sensor as far as wiring goes.

Suggestions welcome.
 
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Lee Valley - https://www.leevalley.com/en-ca/sho...disc/73479-adhesive-backed-rare-earth-magnets

The adhesive on these is tougher than the magnets. Trying to remove one will break the magnet before the adhesive gives up.

Only snag is the sensors are polarity sensitive so if the adhesive is on the wrong side it won't trigger the sensor.

If the tach you bought is the same as my $15 Amazon one then the polarity is correct.

I'd be happy to send you a couple of 3/8" ones from my inventory...

IMG_2183.jpg
 
YotaBota said:
Is this the one you have? https://www.amazon.ca/DIGITEN-Digital-Tachometer-Proximity-Switch/dp/B01FQMI18Y/ref=sr_1_14?crid=3RI2YR4O2XNJF&keywords=digiten&qid=1653257383&sprefix=digiten+,aps,166&sr=8-14
This is the same one I have and mounted on the back side of the spindle, no balance issues and epoxied into the sleeve. I made the bracket for the sensor and the threaded hole was already in the headstock.
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@YotaBota hey Mike yes my tach kit came via amazon shortly after I started into my right knee surgeries so this upgrade project has been on hold for some several years. Popped out to the shop and took this photo, as you can see my unfinished bracket which is also held in place by an existing bolt. My bracket is made from aircraft grade stainless steel, what else? Unfortunately my spindle doesn't stick out except for the two locking nuts. Not a whole lot of meat there for magnets to hang on to and stay away from migrating towards the head stock, that is why I suggested using lock tight. Good to hear you have no balance issues using a single magnet. I have both a red and green digital screen, much will depend on lighting interfering with readout.
 

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YotaBota said:
Is this the one you have? https://www.amazon.ca/DIGITEN-Digital-Tachometer-Proximity-Switch/dp/B01FQMI18Y/ref=sr_1_14?crid=3RI2YR4O2XNJF&keywords=digiten&qid=1653257383&sprefix=digiten+,aps,166&sr=8-14
This is the same one I have and mounted on the back side of the spindle, no balance issues and epoxied into the sleeve. I made the bracket for the sensor and the threaded hole was already in the headstock.
Click to expand...

This solution is about as easy as it gets. And it's a direct output solution. If one wanted to, you could make a ring collar that Clamps to the rear side of the spindle and add additional magnets with the appropriate division factor to improve resolution.

Balance is a zero issue. How can that tiny magnet compare to an unbalanced work item in the chuck jaws.

Love it.

When the time comes for me to do my lathe, you guys will have worked out all the best solutions for me.
 
Dusty said:
@YotaBota hey Mike yes my tach kit came via amazon shortly after I started into my right knee surgeries so this upgrade project has been on hold for some several years. Popped out to the shop and took this photo, as you can see my unfinished bracket is also held in place by an existing bolt. Unfortunately my spindle doesn't stick out except for the two locking nuts. Not a whole lot of meat there for magnets to hang on to and stay away from migrating towards the head stock, that is why I suggested using lock tight. Good to hear you have no balance issues using a single magnet. I have both a red and green digital screen, much will depend on lighting interfering with readout.
Click to expand...

You could glue a small magnet into one of the square wrench notches.

Better yet, make a rear spider that you can use for other things and build the magnet holder into that. The spider could be clamped from inside or on the narrow lip, or screws drilled and tapped on the end, or drilled and tapped onto the rear most nut. Obviously it works with the existing wrench recesses so it will work just as well with some threaded holes or even lock screws that engage the wrench recesses. Lots of options. Looking GREAT!

Edit - 10 votes for a rear spider combo solution. You will never know how you managed without one!
 
@whydontu said 'Only snag is the sensors are polarity sensitive so if the adhesive is on the wrong side it won't trigger the sensor.'

Can you expand on this as my instructions say nothing about polarity.
 
Susquatch said:
You could glue a small magnet into one of the square wrench notches.

Better yet, make a rear spider that you can use for other things and build the magnet holder into that. The spider could be clamped from inside or on the narrow lip, or screws drilled and tapped on the end, or drilled and tapped onto the rear most nut. Obviously it works with the existing wrench recesses so it will work just as well with some threaded holes or even lock screws that engage the wrench recesses. Lots of options. Looking GREAT!

Edit - 10 votes for a rear spider combo solution. You will never know how you managed without one!
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@Susquatch Yes thought about using a wrench recesses however that demands a tiny diameter magnet, likely wouldn't work. Guessing with that!

What in heavens name is a rear spider set up? Pictures please!
 
Not all sensors are polarized. Some work both ways.

Small very powerful magnets exist. Search for neodymium magnets.

Here is a rear spider I swiped off the net. I'm not home or I'd take a photo of mine.

Screenshot_20220522-201506_Chrome.jpg
 
Same model here this is what I did, temporarily I used a glue gun and its become more permanent for now, same bolt for the mount.
Remember that when you do threading you can use a 70 or 80 gear in the "A" position just ensure there is clearance for those times.
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Since your spindle does not stick out of the rear of your lathe, the easiest way to attach one is from the inside. Basically, you are adding a small chuck to the backside of the lathe. Its almost as easy to add it from the inside of the spindle as it is from the outside.
 
Dusty said:
@whydontu said 'Only snag is the sensors are polarity sensitive so if the adhesive is on the wrong side it won't trigger the sensor.'

Can you expand on this as my instructions say nothing about polarity.
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The mag sensor works best when it is sensing the N pole. If you move the sensor towards the magnet, on one face of the magnet it won’t trip, on the other face (N) it will trip about 6mm away.
 
combustable herbage said:
Same model here this is what I did, temporarily I used a glue gun and its become more permanent for now, same bolt for the mount.
Remember that when you do threading you can use a 70 or 80 gear in the "A" position just ensure there is clearance for those times.
View attachment 24046

View attachment 24047
Click to expand...

Excellent suggestion and I do have a glue gun, built a cedar strip canoe with it in the mid 1970's. Dating myself again. LOL
 
I love the little square magnet. That rings the bell!

Does your E-box allow for more than one sensor with a divider circuit? If so, use 2 magnets or even 4 to smooth out the display at low speeds.

I'd be using @whydontu 's square magnets before I'd be making a spider. But I'd still be making the spider for its own sake down the road. A rear spider is a very useful addition to any lathe.
 
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