# Bridgeport clone spindle too fast



## ed_h (Apr 19, 2012)

I have a Jet JTM-4VS, a Bridgeport clone with a variable speed head. When I checked the spinddle speed with an optical tachometer, I got a speed more than double what the dial on the head indicated. There is an adjustment screw on the top of the head in front of the motor, but I can screw it in all the way, and it only reduces the speed to a little less than double the indicated speed.

Anyone else have this problem? What did you do about it?


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## gaston (Apr 19, 2012)

Most "clones" will run about 18% fast if they have 3 phase motors made to run on 50hz power and are running on 60hz. At least my Chinese Comet does.
I adjusted the speed readout plate on the vari drive head to reflect the true speed ( as close as I could) useing a good digital tach.


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## Tom Griffin (Apr 19, 2012)

What voltage are you running and what voltage is the motor set up for?

Tom


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## ed_h (Apr 19, 2012)

It is the original 220 VAC, 60 Hz single phase motor.


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## Starlight Tools (Apr 19, 2012)

Ed H

Have you checked tha calibration of the tach, best way I know of to do that is to aim the tach at a flourescent bulb, they actually have a known strobe effect and that can be measured.  If you are in North america or a country with 60 Hz power, the tach will read 7200 RPM, if in a country with 50 Hz power, the tach will read 6000 RPM.

Next is what are you aiming at to take the reading, is it possible that you are picking up an extra reflective spot.  If there are more than one spot on the rotating item that reflects light, the tach will pick that up and double, or triple or even quadruple the reading depending on how many "reflective" spots it sees.

Walter


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## Tom Griffin (Apr 19, 2012)

Make sure the motor is actually set up for 220v, they can be set up for either voltage. Not sure if a motor set up for 110v would actually run but that might explain yours running 2X.

Tom


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## Starlight Tools (Apr 19, 2012)

Most of these motors are induction motors and they run at an RPM based on the input frequency, and number of windings,  2 windings, 3600 RPM (3450), 4 windings 1800 RPM (1725), 6 windings 1200 RPM (1150)  the numbers in brackets is the actually RPM after losses and friction are acounted for.

If the motor is wired for 110 and you have 220V input, you will get the same speed for a short moment, then lots of smoke.

If the motor is wired for 220V and you have 110V input, the motor may or may not operate, depending on the load.  If lightly loaded, it should come up to speed, and will run at full or close to full RPM, but will not have any power and will quickly bog down if loaded.

Walter


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## ScrapMetal (Apr 19, 2012)

I'm with Walter on this one.  Check your tach and it's settings as well as look for other reflective spots.

-Ron


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## jgedde (Apr 19, 2012)

ScrapMetal said:


> I'm with Walter on this one.  Check your tach and it's settings as well as look for other reflective spots.
> 
> -Ron



I think Ron is spot on.  You're likely double triggering the tach from a second reflective spot.  Try wrapping the shaft being being measured  with black tape and add a single reflective dot - either the tape they come with or a spot of foil and try again.

Either that or Jet put in a 3600 RPM motor instead of a 1800 RPM motor - but, I doubt it.

John


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## ed_h (Apr 19, 2012)

Thanks for the input, guys.

I checked cal on the tach with my belt-drive drill press.  According to the belt chart, it should be 540 RPM.  Tach reads 510 +/- 2 RPM.  About a 5% error--within expectation.

My tach has a bright red illuminator, and a red filter on the detector.  This filters out most ambient light.  It came wit some stick-on highly reflective strips, kind of like what highway signs have on them.  To eliminate extraneous false reflections, I wrapped the spindle with black electrical tape and put the reflective pad on the tape.  Readout on the head says 500, tach reads a pretty steady 960.

I also checked to make sure the dial on the head readout hadn't slipped.  It hadn't--The speed adjust is turned all the way to its stop on the slow side, and the dial reads 500, just what it should.


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## jgedde (Apr 20, 2012)

ed_h said:


> Thanks for the input, guys.
> 
> I checked cal on the tach with my belt-drive drill press.  According to the belt chart, it should be 540 RPM.  Tach reads 510 +/- 2 RPM.  About a 5% error--within expectation.
> 
> ...



Is it a PITA to pull the motor off?  I'm guessing at this point you'll find it's running 2X as fast as it should for the reduction in your mill...

John


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## ed_h (Apr 20, 2012)

John--

It's the original motor, 60 Hz, 220 VAC, 1725 RPM.  Since an induction motor's speed is tied to the line frequency, I can't think of what could cause it to run fast.

I'm thinking now that the adjustment mechanism just isn't allowing the full range.


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