Digital tachometer on a BP clone

[Overland]

I've got my BP clone set up with an inverter duty motor, Fuji VFD and fixed single belt drive, and I'm very pleased with the results.
I want to add a digital tach. I've found some on ebay, but of course no real details.
Does anyone have any experience and recommendations they could share please ?
It seams you attach a magnet to the spindle - how and where to stop it flying off at 2,000+ rpm ?
Do they need a separate power supply, some seem to indicate 18 volts, some don't even mention it.
Or maybe I should just use a handheld one ?
Appreciate any help.
Bob

 

[Pathogen]

On Amazon or EBBay a cheap one, attach the magnet with JB Qwik or JB Weld.

Mount the display out of your way but visible.

They work terrific

 

[rons]

It would be a easy software change to display the VFD frequency scaled by whatever factor so that RPM is displayed.

Maybe if those spying ChiComms read this they will incorporate what we want in their next release...

 

[Overland]

Pathogen,
What about a power supply, do they include it, or do you need to find one ?
Thanks,
Bob

Rons,
Are you saying to program the VFD so it shows rpm instead of Hz on its screen ?
Thanks
Bob

 

[CarbideBob]

..
I want to add a digital tach.

Why? Not trusting the VFD and pulley sizes?
Is there a difference between say 1250 and 1310 RPM in use?
Not understating the benefit for the amount of work involved. Where does the "right" rpm come from?
Please do not tell me speed and feed charts, calculators or the such. These are ballparks.

 

[neanderthal mach]

The OP seems to have chosen a single pulley and adjusting speeds only with the VFD. Sorry, on a mill that's a mistake that will sooner or later bite you on the ass. Without further mechanical methods of getting lower rpms and higher levels of torque, it just ain't going to work. Then there's high torque/low rpm/low cooling fan speeds. There's also pretty damn good reasons the real BP's and there clones almost always have a back gear as well. With a single pulley and belt position then whatever measured diameters the two pulley's have, it's a simple ratio if there not 1-1. So motor indicated rpm on the VFD X your ratio at the spindle should be a simple & close enough fast mental calculation. With the step pulley's like mine has, then yeah that cheap rpm display is a help so I'm not doing different ratios every time I change the belt position. Not absolutely required just like Carbidebob mentioned, just helpful imo. There's dozens/maybe hundreds of forum posts out there about how those digital tachs work, getting the magnet or reflective strip in line with the pick up, mounting and powering them etc. Google even works to find everything I'd just have to repeat here.

 

[Overland]

Well Neo-man,
I've obviously still got the back gear, and use it, certainly for tapping.
I've set the pulley ration so the spindle is 45% of motor. so 1,700 rpm gives 800 at the spindle, great for a facemill, etc.
The motor is a Baldor Vector duty motor, with a max rpm of 6,000. It's designed to run at very low, up to very high revs.
Yes I can read the Hz on the VFD, and convert to speeds - just wanted to be able to read rpm's directly.
Bob

 

[rons]

It seams you attach a magnet to the spindle - how and where to stop it flying off at 2,000+ rpm ?

It makes more sense to use a small custom made clamp on a pulley. The clamp holds a extension that can be used for counting revs.

 

[motion guru]

https://www.practicalmachinist.com/...ately-display-180196/?highlight=Lagun+spindle

still working . . . can’t believe that was 11 years ago

 

[mksj]

Manual factory VFD on knee mills typically are single speed with a back gear. They then use the VFD scaled output to drive a scaled volt meter for the 2 speed ranges, typical output range will be 0-10V. You add a small back gear switch that switches the voltmeter scale or you can use a resistor divider to scale the range. I also install a back gear switch which flips the VFD FOR/REV outputs so the spindle direction remains the same. Alternate (simple) approach is to mount a small ring on the spindle with a a magnet and then add a pickup tach sensor, I often do that and also incorporate a LED light ring. Almost all generic tachs. off of eBay/Amazon run off of ~12VDC @ 50 mA, just add a small power supply or wall wart. Alternative is using a digital display 0-10VDC voltmeter with scaled outputs if you want to use the VFD speed output. Use a small limit switch or proximity sensor for a back gear sensor.

 

[rons]

Rons,
Are you saying to program the VFD so it shows rpm instead of Hz on its screen ?

It would be nice if the VFD is "programmed" internally by the firmware/micro-controller code to display a scaled value.

But usually those VFD's have small lcds or small segment displays. An external digital voltmeter with large segments and inputs for 0 - 2.5v is going to work.
Use the display for 0 to 2500 rpm. Use linear output voltage from VFD and scale it for 0 to 2.5 volts. Those digital voltmeters are cheap.