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Bridgeport Servo Retrofit (Picture Heavy)

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  • Bridgeport Servo Retrofit (Picture Heavy)

    In the thread here I posted some research and calculations I did for retrofitting a Bridgeport J-Head with a 1.5 KW (~2 HP) Mitsubishi AC brushless servo drive:

    Poly-V drive calculations for Bridgeport drive pulley

    Most AC brushless servos have a Metric mounting pattern, with a square face and a round inset. Since I needed to mill mounting tabs into the plate, this all had to be done with the rotab -- fun exercise. I finally got some time in the shop this afternoon and was able to finish-up the servo plate, and thought you guys might be interested in a pictorial build log:



    I started with a 3/4" piece of 6061, and band-sawed it to approximately 10.3" (the OD of the Bridgeport motor plate is 10.25"). I scribed the approximate center of the sawn plate, and drilled a 1" hole (the servo has a 24 mm shaft). So how do you mount a 10" round plate on an 8" rotab with only a single hole? My solution was to take a cheap Chinese MT-3 threaded arbor, saw off the tang, clean it up on the lathe, and then drill and tap the arbor for a 3/8-24 drawbar. Then you put the oversized plate on the rotab, drop the MT3 adapter through, and draw it in with a 3/8" bolt from under the rotab. The MT3 taper centers the plate as you tighten the drawbar:


    "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

  • #2
    With the plate centered, the next step was the mill the circular depression. These are all action shots by the way (tool spinning). I just grab my Wife's camera and snap as I go -- she complains about me getting cutting oil on the camera...



    After I milled the circular pocket and the square motor mounting face, I dropped the 8 mm mounting holes. Then I needed to clean-up underneath the MT3 arbor mount. So I made some goofy 1/2-13 to 8 mm threaded studs, and mounted the plate through the motor mount holes. This meant that I needed to re-center the plate.

    So out comes the Blake Co-Axial indicator, first to center the hole in the rotab, and then again to center the hole in the servo plate on the rotab. The next several steps were going to require a lot of tool changes, and swapping the Blake in and out, so I switched to an R8->ER-40 collet chuck:



    We were discussing how much the ER chucks overhang, you can see that here, where I'm cleaning up the flash that was under the MT3 arbor:



    One of the really nice things about these ER speed collets is that you can have multiple collet nuts on deck, so swapping tools is really fast. These are two other ER-40 collet nuts with the collets installed, in addition to the one on the spindle:

    Last edited by lazlo; 11-16-2008, 10:16 PM.
    "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

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    • #3
      After checking that the servo motor fits the two pockets and the hole pattern correctly , it's time to cut the tabs. The normal Bridgeport motor plate has a 1/2" swivel bolt on one end, and a 2" radial slot on the opposite end to set the belt tension.

      The easiest way I found to do this on the rotab was to calculate the radial section of the tabs starting at the bottom left, and feeding clockwise so I wasn't climb milling. In other words, the bigger tab needs to be 2.85", so you calculate how many degrees of rotab travel that is: 10.25" OD * Pi = 32.2" circumference = 0.0895 inches per degree of rotab travel. So the top tab is 31.8°, half of that is 15.9°, subtracted from a 90° arc is 74.1° of rotab travel to cut the tab. Repeat for the bottom tab comes up with 77.4°.

      So starting at the bottom left of the smaller tab, you need to travel 77.4 + 74.1 = 151.5° to cut the left tab:



      Lots and lots of swarf

      Last edited by lazlo; 11-16-2008, 10:51 PM.
      "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

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      • #4
        Finally, the Bridgeport bolt holes are on a 9.2" circle, so drill the half inch pivot hole (I blued it for peace of mind, but the hole was located with the rotab), and then, to insure alignment, I chain drilled the 2" long radial bolt slot on the opposite tab:



        Then I switched to a 1/2" slot drill to clean it up:



        I've got some cosmetic cleanup to do (chamfer the edges and bolt holes), and hopefully I'll have my first live test with the servo mounted on the Bridgeport tomorrow.

        Stay tuned
        "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

        Comment


        • #5
          Stupid Question:

          What are the advantages of a servo drive spindle on a manual machine? Or are you converting the mill to CNC at a later time?

          I must have missed some earlier threads on the topic

          Oh, BTW - Nice job! I like the picture with the big pile of swarf and a few tools poking out from under it.

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          • #6
            Servo drives have a near flat to flat torque curve. That means it has the same torque at 1 RPM as it does at 2000RPM. The acceleration and deceleration times are also incredible. With a drive set to velocity follow the drive will do its best to maintain the commanded RPM. Most drives have a -10-0-10v input that controls speed and direction.

            With a VFD you loose torque as you drop the speed unless you have an encoder setup.

            Here is my 10EE after I installed the servo drive in it:



            And here it is drilling a 1.25" hole in aluminum. Not even in backgear!



            The plan is to use the 2kw motor thats in the lathe now in the spindle of my CNC lathe. I have a 3.5KW motor to replace the 2.0KW motor in the lathe. But I have to use a generic drive so I need to mount a new encoder to the servo motor.
            Last edited by macona; 11-16-2008, 11:21 PM.

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            • #7
              This is a really nice pictorial. Keep it coming.

              Rob

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              • #8
                this is great!

                Comment


                • #9
                  Well it is about time you showed us something you built Lazlo. Be watching your mail toward the end of the week. Nice it look good show us some more .
                  Every Mans Work Is A Portrait of Him Self
                  http://sites.google.com/site/machinistsite/TWO-BUDDIES
                  http://s178.photobucket.com/user/lan...?sort=3&page=1

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                  • #10
                    awesome work. can you tell us what the source for the servo motor and controller are? i would like to do this upgrade myself. are you finding that 2kw motor is sufficient or should i go right to the 3.5KW?

                    thanks

                    jerry

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                    • #11
                      Looks good so far,so what are we using to drive it?Since servo motors are popping up all over for less than a 2hp 3~ motor there should be a lot of applications for them.Servo driven disc sander?Servo ceiling fan???
                      I just need one more tool,just one!

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                      • #12
                        Seems like a good answer to the metal/wood cutting band saw dilemma.

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                        • #13
                          Originally posted by wierdscience
                          Since servo motors are popping up all over for less than a 2hp 3~ motor
                          This looks pretty slick, where are you guys finding these servo motors, Ebay or ?

                          Paul T.

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                          • #14
                            Originally posted by SDconcepts
                            awesome work. can you tell us what the source for the servo motor and controller are? i would like to do this upgrade myself. are you finding that 2kw motor is sufficient or should i go right to the 3.5KW?
                            The motor is a Mitsubishi HR-SF152, which is a medium inertia, 1.5 Kw AC brushless servo with a 16,384 pulse per revolution absolute encoder. I'm driving the servo with a Mits MR-J2-200A (2 KW) microprocessor-controlled servo controller. This controller does constant-torque, constant-velocity, and position control.



                            The servo inertia is important: servos come in light, medium, and (rarely) heavy rotor inertia. You want to match the servo inertia to the mass of what you're going to be spinning. So on a lightweight axis controller where the servo needs high agility, you want a light rotor intertia.

                            But for a machine tool, where you're going to be turning heavy loads, you want a medium or high inertia. Unfortunately, the price of the servo goes up exponentially with the rotor inertia. but you know how Ebay is: you can find all kinds of amazing deals...

                            Originally posted by Fasttrack
                            What are the advantages of a servo drive spindle on a manual machine?
                            I found a Bridgeport J-Head with great bearings for $400. The catch was that it didn't have a motor or the drive pulley stack.

                            At the time, I was cobbling together the various bits I needed for a servo-driven gear hobber, and I was trying to figure a clean way to mount a shaft encoder to the mill head so I could synchronize the hobber spindle. Since a servo motor already has an high-resolution encoder, and the Mitsubishi servo drives are built for Master/Slave (follower) mode, it seemed like a really clean way to add a high performance motor with the big side-benefit that I can literally plug-in an electronic gearing system for gear hobbing, or cutting helix's on the mill.

                            As far as the power, I really think 1500 Watts (2 HP) is going to be overkill. The stock Bridgeport head is 1 1/2 HP (the Varispeed is 2 HP because of the power loss from the Reeves Drive). AC brushless servos are also around 99% efficient. and like Macona says, servos have an immense amount of torque, and an almost flat torque profile all the way down to 0 RPM.

                            For a machine tool, a closed-loop spindle is especially nice -- these high-end controllers (which are 6 - 8 years old and considered obsolete) are constantly monitoring the actual position of the rotor and tuning the motor for maximum performance. So where an induction motor would bog-down in a heavy cut, a servo controller will keep tweaking the motor torque, 1.6 million times/second, to keep the motor as smooth as possible.

                            When you see those videos of a Mazak turning a mirror surface on hardened tool steel, a lot of that is coming from their servo-driven spindles (Mazak uses Mitsubishi servos and controllers).

                            So maybe putting a high-end servo on a Bridgeport J-head is putting lipstick on a pig, but it's about the same price as buying a pancake motor, I don't think it's been done before, and it's a lot of fun too
                            "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

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