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Power required for R8 milling spindle

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  • Power required for R8 milling spindle

    I was looking at the South Bend badged machines in their brochure, and was surprised to see that the biggest mill in the series, the SB1028 10 X 54 which is similar to a Bridgeport has a 5hp motor. But the spindle fitting is R8, I would have expected BT30 at least, or BT40. I thought that Bridgeports with R8 normally had 2hp motors.

  • #2
    My 2 R8 CNC mills both have 3 HP spindles. In most cases, 3 HP is about all you can make use of with an R8 tool. I have used BP mills with 1 HP some, but mostly 2 HP. 1 HP leave you wishing for more a lot of times, 2 HP is fine for most work, 3 HP cruises through about everything with relative ease. More than that is not a bad thing, but it might be easy to spin a tool shank in the spindle with 5 HP. Not having first hand experience with over 3 HP on an R8, can't say for sure. I have run other machines with a lot more HP, but they were either CAT40 or 50 or big engine lathe.
    Kansas City area

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    • #3
      Hi, For the tooling that will fit inside an R8 collet, anything much over 2hp is wasted. But, there is tooling like face mills that have integral R8 shanks. And those can use more than 2hp at times. 5hp is a bit over kill perhaps, but maybe the manufacturer has a deal with a motor company for 5hp motors or they offer different spindles on the same machine like a BT-30/40. From an inventory control view, why stock different motors?
      If you think you understand what is going on, you haven't been paying attention.

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      • #4
        Bridgeports were traditionally 2hp for the variable speed 2J head and 1hp for the step belt J head. More than enough for the R8 spindle. Most of the extra hp on the 2J is consumed in the variable speed drive. I've found that loading up an end mill to the motor's capacity is a bit of a risk when relying on an R8 collet, and when switching to a solid tool holder for more security the end mill is now far enough out that the spindle bearings are being asked to do more than they should. Putting a 5hp motor to good use would definitely be cause for having a 40 taper spindle.

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        • #5
          While anything over 2 or perhaps 3 HP is over kill for an R8 spindle that machine is electric variable speed which is why the would have choosen the 5 HP motor. Its a 3 phase motor running on a variable frequency drive.
          Mike
          Central Ohio, USA

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          • #6
            5 chinese hp = 3 actual max.
            Southwest Utah

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            • #7
              And a 5HP motor run at half speed with a VFD is closer to 2HP, the motor torque can stay close to constant but you lose the torque multiplication you get from gears/pulley speed reduction.

              Dave H. (the other one)
              Rules are for the obedience of fools, and the guidance of wise men.

              Holbrook Model C Number 13 lathe, Testa 2U universal mill, bikes and tools

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              • #8
                I had 3hp in the South Bend Chipmaster 2 EVS in the shop I ran. At times I used a lot of the available power but never stalled it. I It was 2 axis cnc (Acurite millpwr control) so R8 spindle was ok.

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                • #9
                  My R8 spindle on my Taig is getting a 400 watt brushless DC motor. Fine for Taig sized tools.
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                  • #10
                    I use two mills at the museum, a drill mill with a 1 1/2hp single phase and the other with a 6 pole three phase 1hp motor. The are both R8, and seem to have equal power. I would worry about the R8 taper being unable to transmit 5hp safely. I could buy a 120mm diameter shell to fit on an R8 arbor, but 80mm seems plenty for me, and that with light cuts.

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