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  1. #13
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    Default pneumatic vs dead weight

    Hi Dave, greetings from a fellow Texan with an IHCNC mill. Sorry if my posting is a bit off, it is a Saturday night and I discovered "The Kraken" this evening with wonderful results...

    I have been dealing with z-axis problems, and I have been considering a counterbalance as well (even though I agree with Bob Warfield that I am probably experiencing 'sticktion' (I'm a motorcycle guy, and I know about 'sticktion'). If I make a few up and down movements to break 'sticktion', my z-axis does not fault. Nonetheless, I suspect that relieving some of the effects of gravity, on such a large head as the IHCNC with gears incorporates, will have positive affects on my z-axos problems. I talked with Gino (marvelous guy to have had the privilege to talk too, and I love the whole IHCNC family), but he naysayed the whole counterbalance solution: "the servos are plenty strong enough".

    Nonetheless, I still struggle to believe that counterbalances aren't a good thing. I can trivially add counterbalancing to my turnkey, but I favor the simplicity of pulleys and weights. I would leave a little positive weight on the head to mitigate backlash issues (obviating Gino's major concern), but I am curious as to why you chose pneumatics over dead weights. I know this has been discussed before, but I'm curious about 'your' motivations (and 'modern' concerns from other IH fans as well). I am currently considering adding counterbalancing to my mill too.

    Please, anybody else feel free to chime in...



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    Default Please ignore my Saturday night posts...

    That stuff put a beast in my belly. Awesome flavor though...

    I think I will experiment with both types (pneumatic & dead weight). I'll make sure my bracket design can accommodate either, and I'll try dead weights first, getting a feel for the acceleration issues. I like the idea of a pneumatic solution with all regards except maintenance. I'm usually shy of introducing maintenance issues: KISS because I am stupid, as I prove time and time again when I implement overly complicated solutions.



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    I look forward to your findings. I have been struggling with the "best" solution to counter the excessive head weight, also. I'm going to start with static weight on chains and hope I don't need a plan B. I need to weigh the entire head assy. to see how much lead I'll need (6.55 ounces per cubic inch, I'm gonna need a bunch). Good luck and I hope your project works out.
    Ron E.



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    Member neilw20's Avatar
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    One thing I note with counterbalancing.
    Originally my SX3 had 60KG strut and 45KG head.
    This meant it was bouncing around on the strut, and the screw was pulling it down, and erratic.

    Since I replace it with a (longer) 40KG one, it is very repeatable now.

    Like having a trailer on your car with the load too far to the back.
    The car bounces all over the place.

    http://www.cnczone.com/forums/syil_p...trut_x3_-.html

    [SIZE=1][SIZE=2]Super X3. 3600rpm. Sheridan 6"x24" Lathe + more. Three ways to fix things:[/SIZE] The [SIZE=2]right way[/SIZE], the [SIZE=2]other way[/SIZE], and maybe [I][SIZE=2]your way[/SIZE][/I], which is possibly a [B][I]faster wrong way[/I][/B].[/SIZE]


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    Quote Originally Posted by veggiedog View Post
    ... but I am curious as to why you chose pneumatics over dead weights.

    Please, anybody else feel free to chime in...
    You opened the door for anyone, so I will add my two cents.

    I have given this much thought as I will be trying to counter balance the knee of a machine similar to Chich's. Please, lets not allow this thread to digress into a "knee vs. quill" thread as it is not the appropriate forum.

    First is the thought that air does not add mass to what needs to be moved so acceleration issues -- which are going to be there already simply based on the mass of the knee or the head -- are not exasperated.

    The second is that with a suitable air regulator the system can be easily adjusted to null out the weight of whatever is put on the table. True, the same could be said for a granular selection of counterweights but the thought that I could simply turn a knob to set the appropriate pressure seems easier than going behind the machine to add weight. I am assuming this is not an issue for an RF-45 clone as the weight of the head never changes other than minor differences based on tooling.

    The down sides of air appear to be the cost and complexity of an air system and the constant need for air. I dismiss the former out of a naïve sense that once it is set up and running it should be fine until I pass on and I dismiss the second because I already have a robust air system in the shop so it is not a big deal. However in your case the added complexity only mitigates the mass issue as you do not have a 'changing weight' issue.

    In summary I think you are going in the correct direction to use a counterweight system and then try to resolve the acceleration issue within the software controling the system.

    The only modifications I will be exploring from the implementation presented by Dave would be the use of bicycle chain and sprockets rather than cable and pulleys.

    Arvid



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    Default How much does counterbalancing tweak the column to bed alignment?

    I'm considering transferring the weight of the head to the mill stand rather than the column. I could build extensions up from the back two legs of the stand (I have a stand similar to the one IHCNC sells), run a rotating shaft between the two extensions, just over the column. Plus add forward extending arms to both those extensions, with a rotating shaft between those extensions above the max height of the head. Put pulleys on both shafts for the cable, thus transferring the weight of the head to the stand. I'll try to pull on the head at its balance point to minimize tilting forces on the column.

    I wonder how much that will tweak the column's alignment with the bed: it currently has a big head cantilevered off the front, and I'm thinking about taking that weight off the column. I guess I could lower the head onto a temporary support extending up from stand, and measure with a dial indicator attached to the table with the tip indicating near the top of the column way.

    Simple never is as simple as I initially think it is...Off I go to measure...



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    Well, setting the head down onto supports on the stand by hand turning the z-axis until I feel resistance tweaks the column back 7 thousandths measured approximately 24" up from the base of the column (top of the column flange) using a test indicator with the tip on the column way and the base on the table.

    Any thoughts from someone more knowledgeable than me (I'm just a hobbiest...)? Shimming the column to base flange seems to be a potential solution, if warranted, but I would seek out a square cylinder to "truely" true up the mill before I would muck around with changing anything. However, I did place a small shim at the base of the head to head swivel when I first got the mill to square up the spindle to the table. Never did measure the column squareness to anything of significance (just to a square held to the table, but it was only 6 or 8 inches tall, and I don't recollect finding much deviation over that distance).



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    I'm embarking on this myself and definitely going with a pneumatic counterbalance. I deal with a lot of pneumatics in my profession so I've ordered up some pistons that I believe will work great. They're 25mm diameter, but I plan to just connect this to a regulated air supply in constant connection with my air system. A reservoir would work as well, but would need to have about 30-50psi stored in it to really take the load off the motor.

    I like the idea of a constant force regardless of the Z position and the smaller diameter should be easy to fit behind the column.

    If anyone is interesting in one of these, they're Bosch Rexroth 25x700mm pistons and I'd sell them at my cost of $125 each.

    Ken



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    Davo, I know this post is old and you may no longer be around so taking my chances here. I’m working up a pneumatic setup myself with a 550 cylinder 35/60 and utilizing 35 roller chain and sprockets. I have an old propane tank for a reservoir and I am down to figuring out what needs to be done for air control. This is where I am confused and I have struggled finding information online hence the old post. I have no previous pneumatic experience. My mind tells me if I push 40lbs of air in the lower and upper cylinder I will lock it and defeat the purpose I am after. How do we achieve the motion necessary to counter the head and appropriately differentiate the air pressure from one port to another? Do you have a diagram and or suggestion of pneumatic valves I would need to make this happen? Obviously I need a regulator. Is there a solenoid to either side that is turning on and off by the motion control outputs as direction changes or is this less complicated than that? My logic isn’t clear here however my mechanical abilities are certainly fine given the proper information. Hopefully you or someone else see’s this post and can help. Thank you.



    Quote Originally Posted by davo727 View Post
    When I had the gearbox apart I installed brackets to the top about halfway between the motor and spindle for counterbalance cables to attach to. Made up brackets to mount the pulleys to. They are 2 1/2 in steel from grainger. The cables are vinyl coated 1/8 in. The cylinder is 3.25 in bore, 21 in stroke. New unit off ebay $50.00 to the house Hose runs from a 9 gal air tank to the rod end side of the cylinder. 40 psi in the tank seems to work great, gives about 240 lb of pull on the cables. Im going to add one more cross brace to the pulley mount and then i will pull it off and paint it. I will also mount the tank up under the stand.Dave




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    Smile Re: My pneumatic counterbalance

    It is pretty much all here.
    You need to read the thread from the start.
    Air goes into one side of the cylinder only, at some fixed pressure.
    If the volume of the tank is large in comparison to the volume of the cylinder, then once charged to a fixed pressure to balance it, nothing else is needed.
    The smaller the tank, the greater the pressure change from one end of travel to the other. Just have the tank volume a lot larger than the cylinder swept volume.
    If the air temperature is much lower than when charged, the pressure will become lower. eg. hot summer, to freezing winter.
    If you want it to operate over a wide range of temperatures, with no manual adjustments, then a regulator would solve this.
    Alternatively, just use a counterweight, and no air complications are needed. Just a bucket of concrete.

    [SIZE=1][SIZE=2]Super X3. 3600rpm. Sheridan 6"x24" Lathe + more. Three ways to fix things:[/SIZE] The [SIZE=2]right way[/SIZE], the [SIZE=2]other way[/SIZE], and maybe [I][SIZE=2]your way[/SIZE][/I], which is possibly a [B][I]faster wrong way[/I][/B].[/SIZE]


  11. #23
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    Default Re: My pneumatic counterbalance

    Quote Originally Posted by neilw20 View Post
    It is pretty much all here.
    You need to read the thread from the start.
    Air goes into one side of the cylinder only, at some fixed pressure.
    If the volume of the tank is large in comparison to the volume of the cylinder, then once charged to a fixed pressure to balance it, nothing else is needed.
    The smaller the tank, the greater the pressure change from one end of travel to the other. Just have the tank volume a lot larger than the cylinder swept volume.
    If the air temperature is much lower than when charged, the pressure will become lower. eg. hot summer, to freezing winter.
    If you want it to operate over a wide range of temperatures, with no manual adjustments, then a regulator would solve this.
    Alternatively, just use a counterweight, and no air complications are needed. Just a bucket of concrete.
    Yup, what neilw20 said. That's the way mine works. The bucket of concrete works too.

    Jim Dawson
    Sandy, Oregon, USA


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