Counter balancing Z axis..


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    Default Counter balancing Z axis..

    My Z axis is relatively heavy and I notice that when carving with a lot of rapid Z moves over time my Z will lose steps, so I've been considering a counter weight to help alleviate this problem.

    How would I go about measuring/calculating for a spring or a gas strut? I'm at a total loss on how to figure this out...

    Adam,

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    G540, Rack and Pinion Drives-X/Y axis, 1/2-Ball Screw-Z Axis w/THK HSR 25 Linear Slides, Steppers KL23H2100-35-4B, Power Supply-KL-600-48


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    Community Moderator Jim Dawson's Avatar
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    Default Re: Counter balancing Z axis..

    The quick way would be to disconnect your Z motor, then use a spring scale (fish scale) to weigh the Z slide assembly. Look at the force required to pull it up. That will get you in the ballpark.

    If you don't have a lot of movement in your Z axis, a coil spring might work. But the force of a coil spring is not linear, so having a really long spring relative to the travel would be in order. Not the best system

    Gas struts are generally hydraulically damped, so do not work well where you have a lot of fast movements required.

    The best way to do it is to use an adjustable air spring. This consists of an air cylinder, a small air tank, and a regulator. 5 gal portable air tanks are available at your local Harbor Freight or auto parts store. Air cylinders are available from Automation Direct, McMaster, Grainger, etc. as are regulators. You just size the air cylinder so you can run it at 30 or 40 PSI to get the amount of lift needed. Supply regulated air to the tank, and Tee the air cylinder off the tank side of the regulator. The tank is to maintain a constant pressure on the air cylinder, just the regulator alone is not adequate, it won't react fast enough. I'm using this system on my machine and it works great!



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    Default Re: Counter balancing Z axis..

    I would recommend under balancing slightly. This is a reason the air cylinder is a good idea because dialing it in to how you want it is as simple as a slight pressure change.



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    Default Re: Counter balancing Z axis..

    Quote Originally Posted by 109jb View Post
    I would recommend under balancing slightly. This is a reason the air cylinder is a good idea because dialing it in to how you want it is as simple as a slight pressure change.
    I'm building a new CNC. The part of my Z that moves up and down will weigh just over 150 lbs I estimate. I was planning on two gas struts. Do you think I can use an air cylinder without a tank attached to it? What do you do, just set the output pressure from a compressor to the pressure you want, fill it up, then close a valve?



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    Community Moderator Jim Dawson's Avatar
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    Default Re: Counter balancing Z axis..

    Quote Originally Posted by NIC 77 View Post
    I'm building a new CNC. The part of my Z that moves up and down will weigh just over 150 lbs I estimate. I was planning on two gas struts. Do you think I can use an air cylinder without a tank attached to it? What do you do, just set the output pressure from a compressor to the pressure you want, fill it up, then close a valve?
    See my post below:

    Quote Originally Posted by Jim Dawson View Post

    Gas struts are generally hydraulically damped, so do not work well where you have a lot of fast movements required.

    The best way to do it is to use an adjustable air spring. This consists of an air cylinder, a small air tank, and a regulator. 5 gal portable air tanks are available at your local Harbor Freight or auto parts store. Air cylinders are available from Automation Direct, McMaster, Grainger, etc. as are regulators. You just size the air cylinder so you can run it at 30 or 40 PSI to get the amount of lift needed. Supply regulated air to the tank, and Tee the air cylinder off the tank side of the regulator. The tank is to maintain a constant pressure on the air cylinder, just the regulator alone is not adequate, it won't react fast enough. I'm using this system on my machine and it works great!




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    Default Re: Counter balancing Z axis..

    You need a reservoir with a high volume relative to the cylinder volume so that the pressure is pretty much constant regardless of the position of the piston in the cylinder. That is why Jim mentioned the 5 gallon air tank. The formula for this is P1 * V1 = P2 * V2

    With a 5 gallon air tank you have 1100 cu inches in just the air tank. Lets say you have a 3 square inch piston and it moves 10 inches. That is a 30 cu inch change. Also assume that the fully compressed volume is 3 cu inches and the charge at this point is 45 psi. So, 45 * 1103 = P2 * 1133, solve that and you get 43.8 psi when extended 10 inches.

    Without the air tank it is 45 * 3 = P2 * 33. solving that you get P2 = 4.1 psi at 10 inch extension.



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    Default Re: Counter balancing Z axis..

    Quote Originally Posted by Jim Dawson View Post
    See my post below:
    I don't know what the internal differences are between air cylinders and traditional gas struts. I've never actually looked into it. That's what I'm trying to determine. The air cylinders seem to have a larger diameter. I'm wondering which is better to use IF you are not going to use a tank.

    It sounds like you are saying that you absolutely need a tank to use air cylinders. So in essence, that answers my question, and means that they are different in the way that they work internally.

    I think I will be trying the gas springs, unless I can be convinced otherwise. If that fails to provide the results I need, then I would have to implement the more costly but ideal option you suggest.

    I wonder if there is a type of gas strut that is better for CNC machines? Perhaps without so much dampening? Does anyone know?

    Also, have you ever tried using gas struts on a build, or did you go directly to the best solution?

    adam_m, I hope this talk is helpful to your question. It's on the same topic so I didn't think you would mind as we are all gathered here to discuss this topic.

    I'm interested to know in regards to your problem, what motor you are using, what the lead of your z ball or lead screw is, what IPM are your Z rapids, and what acceleration settings you have. That would probably help us to get a better understanding.



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    Default Re: Counter balancing Z axis..

    Quote Originally Posted by 109jb View Post
    You need a reservoir with a high volume relative to the cylinder volume so that the pressure is pretty much constant regardless of the position of the piston in the cylinder. That is why Jim mentioned the 5 gallon air tank. The formula for this is P1 * V1 = P2 * V2

    With a 5 gallon air tank you have 1100 cu inches in just the air tank. Lets say you have a 3 square inch piston and it moves 10 inches. That is a 30 cu inch change. Also assume that the fully compressed volume is 3 cu inches and the charge at this point is 45 psi. So, 45 * 1103 = P2 * 1133, solve that and you get 43.8 psi when extended 10 inches.

    Without the air tank it is 45 * 3 = P2 * 33. solving that you get P2 = 4.1 psi at 10 inch extension.
    Ahhh, I see. It's just a straight tube with a piston. Thanks for clarifying.



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    Default Re: Counter balancing Z axis..

    Gas struts and air cylinder are similar in construction, but the gas strut piston has a hole in it whereas the air cylinder piston doesn't. So the force from a gas strut is equal to the pressure charge times the area of the rod, not the whole piston. The hole in the piston allows the charge volume to only change by the volume of the rod instead of by the volume of the piston. So it is working in essentially the same way as an air cylinder and large tank to keep the charge volume similar. The difference is that since the gas strut effective force is based on the rod diameter, which is usually about 3/8" it has to have a really high nitrogen charge. a 40 pound force strut has about 370 PSI charge pressure. Essentially the piston diameter is only to dictate the volume of the outer cylinder.



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    Default Re: Counter balancing Z axis..

    The benefit of the air cylinder is that you can change the output force simply by adjusting pressure. If you go with gas struts, and need more or less force, your only recourse is to buy different spec struts.



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    Default Re: Counter balancing Z axis..

    Thanks for all the posts and yes I'm open to any discussion on the subject Nic77, hopefully it will help any who are interested.

    My current setup is using a 1/2 ballscrew and Keeling KL23H2100-35-4B motor usually running 50-70ipm and my Z has about 8" travel, maybe 50-70ipm is too fast which is part of my problem?

    I'm a bit hesitant on the air setup since I'm not wanting to have to run the compressor all the time in order to facilitate the machine to work properly, I was thinking more in terms of a gas strut and or spring type of solution. The air solution seems to be the most efficient and precise and maybe I'll consider it as an alternative if nothing else surfaces.

    You just size the air cylinder so you can run it at 30 or 40 PSI to get the amount of lift needed.
    Just how exactly would you size an air spring, do they come based on weight it can handle similar to a gas strut since the air pressure is adjustable it seems like it might have a large range of weight it could handle.

    The discussion of the gas strut vs air is interesting, would the gas strut actually prevent the Z axis from being able to move if the rapids are too quick, because it can't recover fast enough?

    Assuming I try a gas strut, would it make sense to "Assist" and not totally try to compensate for 100% of the weight. Say the Z axis is 30lb and I pick up a strut rated for 15lb - 20lb, thoughts?

    Do they make some type of coiled spring like a window shade spring would use that might be an option? Although in that case I guess it would also not be linear in force...

    Adam,

    Attached Thumbnails Attached Thumbnails Counter balancing Z axis..-snag-0083-jpg  
    G540, Rack and Pinion Drives-X/Y axis, 1/2-Ball Screw-Z Axis w/THK HSR 25 Linear Slides, Steppers KL23H2100-35-4B, Power Supply-KL-600-48


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    Default Re: Counter balancing Z axis..

    What do you have your acceleration set at? Usually, reducing the acceleration will help if you're losing steps.
    When I first setup my machine, it worked fine on 2D cut's but lost position on 3D cuts. After reducing the acceleration, it's been fine for about 8 years now.

    Gerry

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    Default Re: Counter balancing Z axis..

    Gerry,

    The acceleration is at 15, I think its in the picture attached.

    The strange thing about the loss of steps was that it happened 30-40 minutes into the part but it happened. I've checked the axis for binding thinking that might have something to do with it but the axis practically falls to the table with the servo unbolted. Manually turning the axis you can tell there is some weight to move...

    I've switched out the stepper and am going to give it a shot thinking possibly there is something wrong with the other but I think my Z could be aided with not having to move 25lb-30lb around

    Adam,

    Attached Thumbnails Attached Thumbnails Counter balancing Z axis..-2017-05-06_14-38-11_430-jpg  
    G540, Rack and Pinion Drives-X/Y axis, 1/2-Ball Screw-Z Axis w/THK HSR 25 Linear Slides, Steppers KL23H2100-35-4B, Power Supply-KL-600-48


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    Default Re: Counter balancing Z axis..

    I'd try reducing the accel to 10.

    Gerry

    UCCNC 2017 Screenset
    [URL]http://www.thecncwoodworker.com/2017.html[/URL]

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    JointCAM - CNC Dovetails & Box Joints
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    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)


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    Default Re: Counter balancing Z axis..

    have you tried lowering your microstep resolution on the drive? Have the motor current set to its max?



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    Default Re: Counter balancing Z axis..

    ger21, I'll give that a try.

    louieatienza, isn't the microstep tied to the ball screw pitch? Showing my ignorance here, if I change the microstep from the 10166, won't that impact the accuracy of the "Z" axis?

    I think I understand the Velocity and Acceleration but I "Assumed" the microstep was fixed to thread pitch.

    I found a quill spring that functionally might work (1-3/4" x 5/8" Quill Return Spring Assembly Fits Many 8" Bench-Top Drill Presses | eBay) as an alternative idea?

    Adam,

    G540, Rack and Pinion Drives-X/Y axis, 1/2-Ball Screw-Z Axis w/THK HSR 25 Linear Slides, Steppers KL23H2100-35-4B, Power Supply-KL-600-48


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    Default

    Quote Originally Posted by adam_m View Post
    ger21, I'll give that a try.

    louieatienza, isn't the microstep tied to the ball screw pitch? Showing my ignorance here, if I change the microstep from the 10166, won't that impact the accuracy of the "Z" axis?

    I think I understand the Velocity and Acceleration but I "Assumed" the microstep was fixed to thread pitch.

    I found a quill spring that functionally might work (1-3/4" x 5/8" Quill Return Spring Assembly Fits Many 8" Bench-Top Drill Presses | eBay) as an alternative idea?

    Adam,
    You have to change
    it on the drive itself also



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    Default Re: Counter balancing Z axis..

    You could try a constant force spring, which winds up like a tape measure. You can get them at McMaster-Carr.

    Your confusing steps/unit with microstepping.
    You change the microstepping at the drive, and then adjust the steps/unit to match your microstepping and screw pitch.

    Gerry

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    JointCAM - CNC Dovetails & Box Joints
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    Default Re: Counter balancing Z axis..

    Quote Originally Posted by ger21 View Post
    You could try a constant force spring, which winds up like a tape measure. You can get them at McMaster-Carr.

    Your confusing steps/unit with microstepping.
    You change the microstepping at the drive, and then adjust the steps/unit to match your microstepping and screw pitch.
    Does a G540 and KL23H2100-35-4B have the ability to change the microstepping? I'm pretty sure there is nothing on the motor so it would have to be on the G540 if possible...


    Any thoughts then on a constant force spring? IS there some type of sizing guide, etc.?

    Adam,

    G540, Rack and Pinion Drives-X/Y axis, 1/2-Ball Screw-Z Axis w/THK HSR 25 Linear Slides, Steppers KL23H2100-35-4B, Power Supply-KL-600-48


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    Default Re: Counter balancing Z axis..

    No, you can't change the microstepping on a G540, so scratch that idea.


    Any thoughts then on a constant force spring? IS there some type of sizing guide, etc.?
    They are measured by force, so you want one close to or just under the weight that you're lifting.

    Gerry

    UCCNC 2017 Screenset
    [URL]http://www.thecncwoodworker.com/2017.html[/URL]

    Mach3 2010 Screenset
    [URL]http://www.thecncwoodworker.com/2010.html[/URL]

    JointCAM - CNC Dovetails & Box Joints
    [URL]http://www.g-forcecnc.com/jointcam.html[/URL]

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)


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