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Help Determining Accurate Rpm On Bridgeport With Vfd

Discussion in 'The Tool Shed' started by truckee4x4, Oct 2, 2021.

  1. Oct 2, 2021
    truckee4x4

    truckee4x4 Grant Kaye 2022 Sponsor 2021 Sponsor 2020 Sponsor

    Truckee CA
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    We have an old Bridgeport mill in the makerspace metal shop that I help set up and teach at (Truckee Roundhouse).

    Since the building does not have 3 phase 220V power, at some point the Bridgeport had a VFD added to it. As such the mechanical dial on the mill does not solely control the speed. One must turn on the VFD to "run" and then while the spindle is spinning, dial the mill to a certain RPM value and then use the potentiometer on the VFD to set it to the digitally displayed RPM.

    The person that set this all up added a sticker to the mill that said "KEEP MECHANICAL DIAL SET TO XXXX RPM" at some point, but the sticker has disappeared. It was my understanding that in order for the VFD digital dial to read accurately, the machine speed dial had to be set to a certain RPM and stay there.

    How can I go about figuring out what this should be? Is there a way to do it mathematically via the power, data plate on the Bridgeport, and VFD manual, or do I need to rent a tachometer from O'Reilly and get some reflective tape and run some experiments?
     
  2. Oct 2, 2021
    Howard Eisenhauer

    Howard Eisenhauer Super Moderator Staff Member

    Tantallon, Nova...
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    The tach is reading motor rpm? So what you want is to know what the rpm of the original motor was, unless the po changed pulley sizes inside the head that's what you would set the spindle speed control to the get a 1:1 relationship between the motor & spindle. If the pulleys were changed then you'll need the tach.

    Photo tachs can be had cheap, handy things to have.
     
  3. Oct 2, 2021
    truckee4x4

    truckee4x4 Grant Kaye 2022 Sponsor 2021 Sponsor 2020 Sponsor

    Truckee CA
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    Thanks Howard. I don't think the guy that donated the mill changed the pulleys.

    I will post some pics and video a little later when I am over there at the shop.

    While I have you here Howard - do you have a good reference link you could share to cutting speeds for 2 and 4 flute HSS end mills in mild steel? I never know what to trust when I google these, and I'd like to print and laminate a reliable chart and post by the machine.
     
  4. Oct 2, 2021
    Howard Eisenhauer

    Howard Eisenhauer Super Moderator Staff Member

    Tantallon, Nova...
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    I got a cardboard sliderule dealy fom Niagra Cutters a couple of years ago but it looks like they've gone all 21st century-

    Niagara Cutter
     
  5. Oct 3, 2021
    truckee4x4

    truckee4x4 Grant Kaye 2022 Sponsor 2021 Sponsor 2020 Sponsor

    Truckee CA
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    Here is the VFD and the mill data plate. When the VFD display is set to "freq" to range is 0-60.
    UNADJUSTEDNONRAW_thumb_8112.jpg UNADJUSTEDNONRAW_thumb_811b.jpg
     
  6. Oct 3, 2021
    Howard Eisenhauer

    Howard Eisenhauer Super Moderator Staff Member

    Tantallon, Nova...
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    So what rpm does the display indicate at 60 cycles?
     
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