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  1. #1
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    Mar 2014
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    1029

    Another counter weight discussion

    Hi,

    I finished the conversion of my BF20L mill. Haven't cut anything yet because I don't want to run the machine without the counter weight on the column.

    Stroke of the Z axis is around 260mm. I haven't measured the spindle head but it is something like 30kgs. I have a 2005 double ball nut on the Z.

    So, a pair of air strut is easy to install however I have concerns about reliability of these because when they fail the machine will continue to work as it does and I won't know that the strut doesn't work anymore.

    A simple pulley system sounds more reliable but I don't feel OK about doubling the weight on the column. However adding the third pulley will half the weight so it is a little more better.

    I have no idea how the hydraulic counter weigh systems works on commercial machines. And I don't know if they are available for smaller machines.

    I would like to hear your opinions about this so I can decide.

    Best,
    Suat

  2. #2
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    Jul 2010
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    121

    Re: Another counter weight discussion

    The bf20 is a feather weight, the 2005 is more than capable to handle the weight without struts, I wouldn't bother.

    Sent from my SM-G781B using Tapatalk

  3. #3
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    Sep 2008
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    229
    Quote Originally Posted by Azalin View Post
    Hi,

    I finished the conversion I don't want to run the machine without the counter weight on the column.

    Stroke of the Z axis is around 260mm. measured the spindle head but it is something like 30kgs. I have a 2005 double ball nut on the Z.
    FWIW I have been running my moving-knee Harrison without a counterweight for 10 years without issues.

    However, if I wanted a counterbalance I would use a pneumatic cylinder fed by a _relieving_ regulator. That should provide constant force and you can monitor the pressure to be sure it works.

    Dead-weight counterbalances double the inertia off the system and may not be ideal for dynamic machines.

  4. #4
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    Join Date
    Nov 2020
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    203

    Re: Another counter weight discussion

    Quote Originally Posted by Azalin View Post
    Hi,

    I finished the conversion of my BF20L mill. Haven't cut anything yet because I don't want to run the machine without the counter weight on the column.

    Stroke of the Z axis is around 260mm. I haven't measured the spindle head but it is something like 30kgs. I have a 2005 double ball nut on the Z.

    So, a pair of air strut is easy to install however I have concerns about reliability of these because when they fail the machine will continue to work as it does and I won't know that the strut doesn't work anymore.

    A simple pulley system sounds more reliable but I don't feel OK about doubling the weight on the column. However adding the third pulley will half the weight so it is a little more better.

    I have no idea how the hydraulic counter weigh systems works on commercial machines. And I don't know if they are available for smaller machines.

    I would like to hear your opinions about this so I can decide.

    Best,
    Suat
    The big machines work with an air-over-oil system. I'm attaching a paper that has diagrams of system components. In your case probably a regular pneumatic system is in order and probably you only need one cylinder one one side. I don't know what this systems total components are, aside from the cylinder and a buffer tank.
    Attached Files Attached Files

  5. #5

    Re: Another counter weight discussion

    It's a pneumatic system to hold pressure and a hydraulic system to act on pistons with restricted orifices to prevent the pistons from moving too fast and act as low pass filters for vibration.

    You can build this any number of ways, but either a sealed volume or air is in the tank and you pump oil in to pressurize the system OR you pressurize with a top of tank connection to a pneumatic system.

    The valves are hydraulic valves not pneumatic, the pistons are hydraulic, and the oil has to flow both ways through fixed or adjustable restrictions to act as a shock damper (like car shocks).

    The simplist way you may have seen something like the restriction and piston setup is on a horizontal band saw, to control the drop rate. There is a one way restricted flow with a needle in an opening that you adjust with the little twist knob. In the lift direction a one-way valve let's you lift it quickly.

    In a CNC the restriction is for vibration damping and to prevent a sudden drop if the pneumatic pressure is lost. It would though also resist fast dynamics so you would need to size everything appropriately for the desired behavior.

    It's still a piston system, but unlike pneumatics, the hydraulics are incompressible so the restriction damps out "bouncing" behavior almost entirely.

  6. #6
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    Nov 2013
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    1343

    Re: Another counter weight discussion

    Hi,
    I don't think you need a counterweight or struts or anything.

    Lets imagine that your Z axis weighs 100kg.......much more than your estimate but it won't hurt.

    The mechanical advantage of a 20mm diameter screw of 5mm pitch is:
    MechAdv=circumference/pitch
    =20 x PI / 5
    =12.6

    Thus the 100kg, or 1000N force is reduced to a force of 1000/12.6=79.4N at a radius of (20/1000)/2=0.01m for a torque of 0.79Nm.

    The torque required to hold a 100kg axis weight against gravity with a 20mm diameter screw of 5mm pitch is about 0.8Nm or 150 oz.in. Even baby stepper can do that!

    What is the holding torque of your Z axis stepper? If its greater than 150 oz.in it will hold up the Z axis no troubles.

    Craig

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