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IndustryArena Forum > MetalWorking Machines > Shopmaster/Shoptask > MILL TURN Z AXIS RAPIDS UPDATE
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  1. #1
    Join Date
    Jan 2008
    Posts
    458

    MILL TURN Z AXIS RAPIDS UPDATE

    There was some discussion about this, but it was on another thread about an older machine, so I thought I would put it out here for Mill Turn owners only.
    The standard configuration for the Mill Turn is a 4-1 reduction from the motor to the drive pulleys, so the speeds are set at about 15 IPM. That is more to do with the RPM of the motor than the torque, because steppers reach a point in RPM where the torque drops off very quickly. I figured I would try to find that sweet spot where the torque is at its best and try to increase the rapids. With some measuring, I found that I could replace the 18 tooth motor drive pulley with a 24 tooth and still use the original mount and belt, so just a pulley swap was all that was necessary. Going to a 24 tooth pulley changes the ratio to 3-1, so theoretically it increases the speed by a factor of 1.33, boosting the speed from the 15 range up to about 20. But as a practical matter, because the motor speed is reduced, it seems closer to that max torque point, and I could actually get 28 IPM. That was the limit for the upstroke lifting the mill head casting , motor etc. which probably weighs 150 #. On the down stroke, the IPM could run at 60 IPM easily until it reached the RPM limit, so I figured that with some lift assistance on the weight of the mill head, the rates could probably be somewhere in between 28 and 60. I had a pair of old gas struts that came off a hatchback and they had a 12" stroke. For a super scientific rating, I just placed one end on a bathroom scale and pushed down on the other watching the scale until the strut started to compress. That was at about 80#, so I figured 2 of them would give me 160# of uplift on the mill head. Just to test my idea, I simply propped each strut up under the mill head on each side of the spindle cartridge. Right away I could see an improvement- I kept upping the speed in the motor tuning file until I reached about 45 IPM where it began to stall on the uplift, on the downstroke it would go a bit higher. So I set my speed to 40 IPM and ran it through a bunch of cycles with no stalling. Mounting the struts took some figuring, because you can't mount them in the center of the mill head, and on each end you cannot put them in the center either. Finally I decided to try one strut on the tailstock end just behind the metal shield and the other on the headstock end coming up from between the lathe motor and casting. This gave me equal pressure on both ends and diagonally across. The struts have nice male ball joins on each end with 8mm threads, so I just had to drill and tap the casting for the lower mounts and then made some simple angle iron brackets for the mill head mounting. Its all up and working, and 40 IPM is a big improvement for the rapids. I think that I could go to even a bigger motor pulley like a 36 and get to a 2-1 ratio and maybe get even better speed, but that would require changing the motor mount, cutting away the shield cover and a longer belt.

  2. #2
    Join Date
    Sep 2008
    Posts
    140

    Re: MILL TURN Z AXIS RAPIDS UPDATE

    I added a gas strut to my mill, McMaster-Carr has a great assortment of struts and mounting hardware.
    McMaster-Carr

  3. #3
    Join Date
    Aug 2016
    Posts
    61

    Re: MILL TURN Z AXIS RAPIDS UPDATE

    Hey guys- I just found out that the struts I used were from a Dodge Magnum station wagon- rear door lift.

  4. #4
    Join Date
    Jun 2015
    Posts
    85

    Re: MILL TURN Z AXIS RAPIDS UPDATE

    I'd love to see some pictures of your setups. I'm still indecisive about where exactly to mount the struts and I'd like to add a power-draw-bar which is going to add even more weight (if I can find an option that's reasonable and still in production).

  5. #5
    Join Date
    Aug 2016
    Posts
    61

    Re: MILL TURN Z AXIS RAPIDS UPDATE

    Here are a couple shots. It's not possible to mount the struts exactly centered on each end. so I went for a diagonal. The tailstock strut was easy, just needing an angle iron bracket on the mill head casting and a tapped hole in the lower mount. With the sheet metal cover on, it is completely hidden. The headstock end was a bit more complicated because of the lathe motor mount and the DRO scale etc., but there was just enough room to fit it between the motor and bed. Once again, a simple angle iron bracket on the mill head and a tapped hole in the main casting. The casting hole required a tap extension for the hole. It sits at a bit of an angle, but not enough to affect the efficiency. The struts were the perfect length so when the head was all the way down they were fully compressed and they have ball joints on each end for easy alignment. The ball joints have male threads- 8MM X 1.25. The struts were used, but I just put them on a bathroom scale and watched the reading as I compressed them and came up with a backyard rating of 80# each, giving me a 160 # total lift. I'm guessing the mill head and motor is probably somewhere in the 200# plus range, because I could still get faster rapids down than up indicating that the struts did not fully neutralize the weight of the mill head.
    Attachment 330158

    Attachment 330160

  6. #6
    Join Date
    Jun 2015
    Posts
    85

    Re: MILL TURN Z AXIS RAPIDS UPDATE

    Thank-you! That's really helpful, I was scratching my head on where to mount that one by the lathe motor.
    I was looking at some options on McMaster. I might just go with one of the cheaper options (~$25/ea) at 100lbs extension force as I can easily swap them out for the same model in 150lbs if/when I add the extra weight of a power drawbar. There's also a nice corrosion-resistant/stainless-steel option that has 20lb extension force increments for ~$85/ea.

  7. #7
    Join Date
    Dec 2007
    Posts
    77

    Re: MILL TURN Z AXIS RAPIDS UPDATE

    Quote Originally Posted by BIGBLOCK1965 View Post
    Here are a couple shots. It's not possible to mount the struts exactly centered on each end. so I went for a diagonal. The tailstock strut was easy, just needing an angle iron bracket on the mill head casting and a tapped hole in the lower mount. With the sheet metal cover on, it is completely hidden. The headstock end was a bit more complicated because of the lathe motor mount and the DRO scale etc., but there was just enough room to fit it between the motor and bed. Once again, a simple angle iron bracket on the mill head and a tapped hole in the main casting. The casting hole required a tap extension for the hole. It sits at a bit of an angle, but not enough to affect the efficiency. The struts were the perfect length so when the head was all the way down they were fully compressed and they have ball joints on each end for easy alignment. The ball joints have male threads- 8MM X 1.25. The struts were used, but I just put them on a bathroom scale and watched the reading as I compressed them and came up with a backyard rating of 80# each, giving me a 160 # total lift. I'm guessing the mill head and motor is probably somewhere in the 200# plus range, because I could still get faster rapids down than up indicating that the struts did not fully neutralize the weight of the mill head.
    Attachment 330158

    Attachment 330160
    Bigblock,
    Just FYI, we have sent your photos to the factory and they are now testing a unit with struts.

  8. #8
    Join Date
    Sep 2015
    Posts
    38

    Re: MILL TURN Z AXIS RAPIDS UPDATE

    Quote Originally Posted by BIGBLOCK1965 View Post
    Here are a couple shots. It's not possible to mount the struts exactly centered on each end. so I went for a diagonal. The tailstock strut was easy, just needing an angle iron bracket on the mill head casting and a tapped hole in the lower mount. With the sheet metal cover on, it is completely hidden. The headstock end was a bit more complicated because of the lathe motor mount and the DRO scale etc., but there was just enough room to fit it between the motor and bed. Once again, a simple angle iron bracket on the mill head and a tapped hole in the main casting. The casting hole required a tap extension for the hole. It sits at a bit of an angle, but not enough to affect the efficiency. The struts were the perfect length so when the head was all the way down they were fully compressed and they have ball joints on each end for easy alignment. The ball joints have male threads- 8MM X 1.25. The struts were used, but I just put them on a bathroom scale and watched the reading as I compressed them and came up with a backyard rating of 80# each, giving me a 160 # total lift. I'm guessing the mill head and motor is probably somewhere in the 200# plus range, because I could still get faster rapids down than up indicating that the struts did not fully neutralize the weight of the mill head.
    Attachment 330158

    Attachment 330160
    Just a note. many of these "Tail gate Struts" are not just gas springs but also have one way dampening that limits the speed of opening and not closing. This may be contributing to your difference.

  9. #9
    Join Date
    Jun 2015
    Posts
    85

    Re: MILL TURN Z AXIS RAPIDS UPDATE

    Quote Originally Posted by Norton_BMW View Post
    Just a note. many of these "Tail gate Struts" are not just gas springs but also have one way dampening that limits the speed of opening and not closing. This may be contributing to your difference.
    Liftgate struts for cars usually have some heavy damping. The McMaster units I linked are sold as gas springs so I'm not sure they're the same thing. I've noticed a few CNC's, like the Skyfire units, come with gas assist from the factory. To be fair, they're feed rates are a full order of magnitude faster than the shopmaster. I think the z-axis runs at 120IPM vs. the 12IPM others had settled for before modifying the Mill-Turn. One of my buddies converted his Grizzly mill using double-nut ground ball-screws and some decent sized steppers, now he never stops giving me crap about the Mill-Yurn. He's running ~240ipm on all 3 axis while holding 1-thou tolerances :P You'd think a little machine like that wouldn't be able to do real work, but by using the high-speed-machining toolpaths it's a beast.

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