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IndustryArena Forum > MetalWorking Machines > Bridgeport Machines > BP Clone Post-Conversion Observations
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  1. #1
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    BP Clone Post-Conversion Observations

    This post is intended to help those who are looking into converting their Bridgeport or Bridgeport clone manual mill to CNC. This topic has been discussed many many times in the past but I'd like to list my observations after completing my 2 axis conversion. First off, I bought a Taiwan-made BP clone, in mint condition, from a local community College. I recently decided to add 2 axis (X-Y) CNC controls to it so I could make more complicated parts at my home shop. Now, I work at a machine shop where I have access to both a Haas VF3 and VF4, both with 4 axis rotaries, but I like to come home and be able to work on my own projects. The technical specs of my conversion are as follows:
    -Hiwin ground ballscrews
    -Nema 34 1128 oz-in hybrid steppers
    -KL-8020 hybrid stepper motor drivers
    -Inductive proximity sensors for homing
    -Custom made yoke and motor brackets
    -Custom made belt and motor guards
    -Centroid Acorn controller
    -Centroid MPG pendant
    -Fusion 360 (free hobby use)

    First, I'd like to say that I'm very pleased with the performance and accuracy of the Hiwin ballscrews. They are preloaded so they have only a few tenths of backlash and they are silky smooth.

    Second, I initially thought I would still be able to use my mill as a manual machine after the conversion. I really didn't think this through because the only way I know that one could achieve this correctly is by removing the belt between the leadscrew and stepper motor. With the guards/shields in place, this is very difficult unless you have a tensioner that you can slack the belt with.

    Third, if you are thinking of converting a BP clone, you may have to make a custom yoke to mount your ballscrew nuts to. Some Taiwan mills use the same size yoke so you may get lucky and could use the old Elrod yokes now sold by Servo Products. I started out by making a bullet aluminum yoke for my machine. Once installed, I quickly realized that the force exerted by the y axis ballscrew can flex the yoke and cause significant lost motion....about 0.002" on my machine. Don't make the mistake I did, use steel if you have to machine your own. Speaking of making your own yoke, these are a precision component. The perpendicularity of the ballnut mounting surfaces is critical!

    Fourth, the cast aluminum bearing retainer mounts on the ends of the table and knee are not rigid enough for cnc use. The y axis retainer on the knee is probably okay but the x axis retainer will flex and cause loss of motion.....I've measured up to 0.0015" on my machine. This can be remedied by making the retainers out of steel and use thicker proportions where you are able.

    Fifth, be aware of the limitations of a Bridgeport type mill when deciding on a cnc conversion. These knee mills, especially those with R8 spindles, cannot be pushed hard or you run the risk of knocking the head out of tram. I've been using a 6 flute, insert, 2" diameter shell mill for facing for quite some time and I can easily push the cutter hard enough to overcome the tram setting. These mills are capable due to their versatility but only with a loss of rigidity. You can't "feel" how the machine is cutting with CNC control so sometimes it is difficult to know just how hard you are pushing the machine.

    Last but not least, as others have pointed out, some milling operations are just faster on a manual machine. The MPG pendant makes a world of difference when setting the work coordinates and jogging the machine around but at the end of the day, basic operations such as bringing a part to length on the side of a vice can, IMHO, be performed quicker manually.

    Hopes this helps anyone out there considering a cnc conversion on a BP or BP clone. It can be a fun project but it is a labor of love. You can achieve great results and have a fairly accurate machine after a conversion but you could also save yourself ALOT of work if you can find an older oem cnc that needs a control retrofit.

  2. #2
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    Re: BP Clone Post-Conversion Observations

    Quote Originally Posted by toyoman View Post
    This post is intended to help those who are looking into converting their Bridgeport or Bridgeport clone manual mill to CNC. This topic has been discussed many many times in the past but I'd like to list my observations after completing my 2 axis conversion. First off, I bought a Taiwan-made BP clone, in mint condition, from a local community College. I recently decided to add 2 axis (X-Y) CNC controls to it so I could make more complicated parts at my home shop. Now, I work at a machine shop where I have access to both a Haas VF3 and VF4, both with 4 axis rotaries, but I like to come home and be able to work on my own projects. The technical specs of my conversion are as follows:
    -Hiwin ground ballscrews
    -Nema 34 1128 oz-in hybrid steppers
    -KL-8020 hybrid stepper motor drivers
    -Inductive proximity sensors for homing
    -Custom made yoke and motor brackets
    -Custom made belt and motor guards
    -Centroid Acorn controller
    -Centroid MPG pendant
    -Fusion 360 (free hobby use)

    First, I'd like to say that I'm very pleased with the performance and accuracy of the Hiwin ballscrews. They are preloaded so they have only a few tenths of backlash and they are silky smooth.

    Second, I initially thought I would still be able to use my mill as a manual machine after the conversion. I really didn't think this through because the only way I know that one could achieve this correctly is by removing the belt between the leadscrew and stepper motor. With the guards/shields in place, this is very difficult unless you have a tensioner that you can slack the belt with.

    Third, if you are thinking of converting a BP clone, you may have to make a custom yoke to mount your ballscrew nuts to. Some Taiwan mills use the same size yoke so you may get lucky and could use the old Elrod yokes now sold by Servo Products. I started out by making a bullet aluminum yoke for my machine. Once installed, I quickly realized that the force exerted by the y axis ballscrew can flex the yoke and cause significant lost motion....about 0.002" on my machine. Don't make the mistake I did, use steel if you have to machine your own. Speaking of making your own yoke, these are a precision component. The perpendicularity of the ballnut mounting surfaces is critical!

    Fourth, the cast aluminum bearing retainer mounts on the ends of the table and knee are not rigid enough for cnc use. The y axis retainer on the knee is probably okay but the x axis retainer will flex and cause loss of motion.....I've measured up to 0.0015" on my machine. This can be remedied by making the retainers out of steel and use thicker proportions where you are able.

    Fifth, be aware of the limitations of a Bridgeport type mill when deciding on a cnc conversion. These knee mills, especially those with R8 spindles, cannot be pushed hard or you run the risk of knocking the head out of tram. I've been using a 6 flute, insert, 2" diameter shell mill for facing for quite some time and I can easily push the cutter hard enough to overcome the tram setting. These mills are capable due to their versatility but only with a loss of rigidity. You can't "feel" how the machine is cutting with CNC control so sometimes it is difficult to know just how hard you are pushing the machine.

    Last but not least, as others have pointed out, some milling operations are just faster on a manual machine. The MPG pendant makes a world of difference when setting the work coordinates and jogging the machine around but at the end of the day, basic operations such as bringing a part to length on the side of a vice can, IMHO, be performed quicker manually.

    Hopes this helps anyone out there considering a cnc conversion on a BP or BP clone. It can be a fun project but it is a labor of love. You can achieve great results and have a fairly accurate machine after a conversion but you could also save yourself ALOT of work if you can find an older oem cnc that needs a control retrofit.
    You said you can't use your mill manually, if the steppers you are using are not powered, then the handles should be free to turn, I have servos on one of my manual mill conversions and there is no problem to use it manually or under CNC control.

    Even with the Bridgeport conversions you have to add better locking to the knuckle joint, here is a snip of one I did, the center clamping bolt could hold the head in place by itself the 3 bolts and the center pivot pin where all custom-made parts to overcome the head movement.
    Mactec54

  3. #3
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    Re: BP Clone Post-Conversion Observations

    Quote Originally Posted by mactec54 View Post
    You said you can't use your mill manually, if the steppers you are using are not powered, then the handles should be free to turn, I have servos on one of my manual mill conversions and there is no problem to use it manually or under CNC control.

    Even with the Bridgeport conversions you have to add better locking to the knuckle joint, here is a snip of one I did, the center clamping bolt could hold the head in place by itself the 3 bolts and the center pivot pin where all custom-made parts to overcome the head movement.

    Maybe the stepper is with brake? I am thinking about doing the same with my BP clone from TW. Got Hiwin ballscrew and yoke set designed for BP convert. Waiting for the moment to pull that trigger. Appreciate all the useful information in this post. As I do expect use it manually after, would like to understand more about what is going on. If the hybrid stepper is not powered up, I can only think of pulley ratio or brake. Another odd scenario is somehow stepper wiring self connected but can't think of how.

  4. #4
    Community Moderator Jim Dawson's Avatar
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    Re: BP Clone Post-Conversion Observations

    Quote Originally Posted by softnado View Post
    Maybe the stepper is with brake? I am thinking about doing the same with my BP clone from TW. Got Hiwin ballscrew and yoke set designed for BP convert. Waiting for the moment to pull that trigger. Appreciate all the useful information in this post. As I do expect use it manually after, would like to understand more about what is going on. If the hybrid stepper is not powered up, I can only think of pulley ratio or brake. Another odd scenario is somehow stepper wiring self connected but can't think of how.
    It's no problem using the machine manually with servos or steppers installed. Mine is set up to switch between manual and 2, 3, or 4 axis CNC with a push of a button and flip a lever. Mechanically steppers tend to ''cog'' which would be a bit annoying in manual mode. I have brushed DC servos on my machine, silky smooth in manual mode. Steppers do become generators when manually driven, and send the power back to the drive and this acts as a brake, so adding a relay or switch to disconnect the stepper from the drive would be a big help. A little electrical logic would be required to make sure the stepper is never connected or disconnected when the drive is powered up.
    Jim Dawson
    Sandy, Oregon, USA

  5. #5
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    Re: BP Clone Post-Conversion Observations

    Quote Originally Posted by Jim Dawson View Post
    It's no problem using the machine manually with servos or steppers installed. Mine is set up to switch between manual and 2, 3, or 4 axis CNC with a push of a button and flip a lever. Mechanically steppers tend to ''cog'' which would be a bit annoying in manual mode. I have brushed DC servos on my machine, silky smooth in manual mode. Steppers do become generators when manually driven, and send the power back to the drive and this acts as a brake, so adding a relay or switch to disconnect the stepper from the drive would be a big help. A little electrical logic would be required to make sure the stepper is never connected or disconnected when the drive is powered up.
    That is some interesting thing to learn. Even with power off, the stepper driver make it behave like been connected? Never tested that before, only noticed difficult to turn by hand when stepper wires are self connected.

  6. #6
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    Re: BP Clone Post-Conversion Observations

    When cranking the handles by hand, with the steppers and drives powered down, you still have the resistance of the stepper fighting you. I have a 2:1 drive ratio on my machine, mainly to increase torque and smooth out the motion. So when moving the axis by hand, you are driving the stepper twice as fast and it just doesn't feel good. I don't have any experience with servos on a machine like this but I definitely can see how they would be as smooth as a true, manual-only machine. Mactec54, I like the bolt idea to increase the rigidity of the knuckle.

  7. #7
    Community Moderator Jim Dawson's Avatar
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    Re: BP Clone Post-Conversion Observations

    Quote Originally Posted by toyoman View Post
    When cranking the handles by hand, with the steppers and drives powered down, you still have the resistance of the stepper fighting you. I have a 2:1 drive ratio on my machine, mainly to increase torque and smooth out the motion. So when moving the axis by hand, you are driving the stepper twice as fast and it just doesn't feel good. I don't have any experience with servos on a machine like this but I definitely can see how they would be as smooth as a true, manual-only machine. Mactec54, I like the bolt idea to increase the rigidity of the knuckle.
    As an experiment, power down your machine. Disconnect the X stepper from the drive and turn the handle. I'm curious if that will make a big difference in the way it turns.
    Jim Dawson
    Sandy, Oregon, USA

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    Quote Originally Posted by Jim Dawson View Post
    As an experiment, power down your machine. Disconnect the X stepper from the drive and turn the handle. I'm curious it that will make a big difference in the way it turns.
    Jim, I will try this tomorrow, or should I say next year haha, and will report back. I'd be so happy to be able to use the machine manually again. I kept my DRO on the machine after the conversion so I could still achieve precision

  9. #9
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    Re: BP Clone Post-Conversion Observations

    Quote Originally Posted by toyoman View Post
    When cranking the handles by hand, with the steppers and drives powered down, you still have the resistance of the stepper fighting you. I have a 2:1 drive ratio on my machine, mainly to increase torque and smooth out the motion. So when moving the axis by hand, you are driving the stepper twice as fast and it just doesn't feel good. I don't have any experience with servos on a machine like this but I definitely can see how they would be as smooth as a true, manual-only machine. Mactec54, I like the bolt idea to increase the rigidity of the knuckle.
    Yes, with using a servo you don't know they are there, even with the stepper disconnected, you will still feel some cogging when turning the handles, just the nature of the beast, should still be ok to use in manual control though.
    Mactec54

  10. #10
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    Quote Originally Posted by Jim Dawson View Post
    As an experiment, power down your machine. Disconnect the X stepper from the drive and turn the handle. I'm curious if that will make a big difference in the way it turns.
    Okay, I disconnected the X and Y stepper motor wires with Acorn and the drives powered down. It made no difference in the drag felt when manipulating the axes by hand. Now, I did set up a drilling job and did everything manually. After completing the job, I felt better about using the mill manually again. You can feel the physical "steps" of the motor but I got used to it in time. One of these days, I may go with Clearpath servos and this should eliminate feeling the physical steps of the stepper motor.

  11. #11
    Community Moderator Jim Dawson's Avatar
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    Re: BP Clone Post-Conversion Observations

    Thank you for doing that. It makes a huge difference on my Z axis (quill) stepper, but that has a 15:1 reduction. I have a stepper on my quill, and brushed DC servos on the X and Y. I just flip a lever to mechanically disconnect the stepper from the quill drive gearing.

    If you decide to go with servos at some point, you might want to also look at DMM Tech servos https://store.dmm-tech.com/

    and the SureServo2 (Delta) line from Automation Direct. https://www.automationdirect.com/selectors/sureservo2

    I have used Clearpath, DMM Tech, and SureServo2 products in various projects, and have found the DMM Tech and SureServo2 to be very good values.
    Jim Dawson
    Sandy, Oregon, USA

  12. #12
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    Re: BP Clone Post-Conversion Observations

    I did a few calculations on the theoretical deflection of the X-Y yoke, one for aluminum and one for steel. Using the standard cantilever beam equation and using the following values:
    -distance between the centerline of the Y-axis ballnut mount and the flange surface where it mounts in the saddle as the "length" variable (3.3" in my case)
    -635 lbs of maximum force (1128 oz-in motor w/ 2:1 reduction, 0.200" lead ballscrew, 90% efficiency of ballscrew)
    -moment of inertia will vary according to your design (mine is 3.293 in^4)

    All aluminum deflection is 0.0004"
    All steel deflection is 0.00008"

    These numbers may be not reflect reality exactly but they provide insight as to how much more stiff the steel version is over the aluminum....5X more!

    FYI to anyone who is machining their own yoke...

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