G0704 table tight on ends


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Thread: G0704 table tight on ends

  1. #1
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    Default G0704 table tight on ends

    So the issue is pretty much exactly as the title says. I only noticed this as I was having issues cutting steel (MAJOR chatter even in light cuts) and I took apart the table to try and see if something is loose. Putting it back together and attempting to readjust the gib is where I noticed this.

    With the gib installed at it's loosest setting, the center of the table obviously has a fair amount of play and each end tightens up a good bit. Keep in mind I'm getting a feel for this with the ballscrew disconnected and sliding the table back and forth by hand. As I was re-assembling it I wanted to tighten the gib to remove as much play as possible while keeping movement smooth.

    If I had to guess, this may be the cause for my chatter issues (or likely a similar issue on the Z axis ways).

    Does this seem like an issue with the gib or the ways themselves?

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    Default Re: G0704 table tight on ends

    The table being tight at each end, may be gravity causing binding of the gib on the ends. You could try measuring the dovetail using a caliper and pins to see if the table has a problem.
    I would set the gib in the area you will be machining most of time, the center.



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    Default Re: G0704 table tight on ends

    Tightness on the ends is obviously wear in the middle. Short of a rebuild there's little you can do except set your gibs at a happy medium between a little loose in the middle and almost too tight on the ends.

    On a machine that light cutting steel, everything is going to matter as far as tooling, meaning cutting tool length to diameter ratios, work holding, feeds and speeds, you name it. Sometimes too slow speed and feed can cause chatter. Especially in the 45 degree quadrants of movement when both screws are turning at their slowest. This is where looseness and backlash can start acting up.

    On a machine like that I would tend to stick to HSS cutters. They're a lot more forgiving in steel on machines that lack mass and rigidity. Carbide doesn't give and pushes all the energy into the machine. HSS gives a little so will disperse some of that energy.



  4. #4

    Default Re: G0704 table tight on ends

    Lubrication is important as well, when I moved to oil lubrication I found I could run the ways much tighter than before and do so with less issues. I also bought a new table sometime in the last two years because I was tired of fighting the table in the on the ends. The new table more less fixed it, careful adjustment and oil did the rest. Grease I found to be terrible on these machines, they are too under power to drive through the grease and the hydraulic action of the grease is too high to be able to adjust the ways in any meaningful way. Chatter is pretty rare with my machine lately, at least in aluminum, and I can pretty run 3/8" tooling at specified limits.

    To adjust the ways I always send the table to an Full X+, then I twist it by hand, adjust the ways, send to to full X- and back to X+ and check again. Same process for Y. Once you get running well by hand, you give it a go under power. I generally count the turns out on the way adjuster on the thin end of the gib strip so that when I reassemble I get it back to where it was. With the X at the extreme I get very little rotation, I can just feel it move when pushing on it.

    Also, make sure the X ballnut isn't rubbing on the underside of the table. That was something I fought with a few years back.



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    Default Re: G0704 table tight on ends

    Over the last few days when it hasn't been ridiculously hot out I've been messing with it some more. For some context I have clearpath servos on all 3 axes so they have to be tuned/autotuned.

    What I did was loosen the gibs enough so the motor could turn without issue enough to autotune itself. Once it was re-tuned to work in a somewhat tighter environment that killed my issues with it stalling. Not sure if I somehow had a bad tune on it or what in the past but it's not stalling now. I made myself a program to run the table back and forth on a G0 at 200 IPM and slowly tightened the gib and add oil to the ways until I couldn't easily tell there was play in the center anymore. The good news is that it worked and I don't have issues with the motor moving, the bad news is that now I've introduced a fair bit of lost motion into the system. That's where I last left off. I don't THINK retuning the motor in this (even tighter) scenario would change anything, once I get back out in the heat I'll have to double check on whether the screw is in fact turning during the "lost motion phase" or not and also double check the preload on the opposite end of the ballscrew.

    As far as wear goes, this machine has relatively low running hours on it. I've definitely spent a fraction of the hours actually using it vs. fixing/changing/tweaking things on it. I originally bought it as what was a "CNC conversion prototype" about a year and a half ago so I don't think it was actually used much before I got it, and this has been an on and off project for me over the time I've had it. It does make sense though and I CAN tell too that lifting on either end while pushing near the tight spots did help to some degree (are we going to have to start making table counterweights here too? :P)

    For cutting steel, that's been the only material I've had issues with that haven't been spindle/motor related. I've never had chatter issues with cutting aluminum that I can remember. What trips me out is I've seen videos of the same machine cutting steel at a much more aggressive rate than I've tried without issue. Using bigger tools, smaller tools, more depth/width of cut, etc.



  6. #6

    Default Re: G0704 table tight on ends

    The only time I really fight steel is with hardened bearing that bore over size. Otherwise I can cut steel well it’s just slow.



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    Default Re: G0704 table tight on ends

    Which is relatively understandable, given it's hardened. I'm trying to cut 1045 steel which shouldn't be an issue at all given the recipes I've tried. I took some video of it cutting but it really doesn't do justice to the severity of the chatter and the noise of my coolant mister sort of overpowers it so I'm not sure it would be useful to upload and post.

    One video I keep going back to though is this, where Russ is using pretty respectable feeds and speeds.



    He was using a 1/4" 4 flute endmill and I've tried one of those, a 1/4" 5 flute, and 3/8" 5 flute taking less aggressive cuts than he is and I'm still wondering how I made it through my cut without snapping and end mill.



  8. #8

    Default Re: G0704 table tight on ends

    I’m not sure when my next steel project is but I’ll definitely reference those f/s. The face mill is chopping along. That 1/4” is certainly smooth.

    Once you get everything tight there only so much you can do, but I think you know that. These little machines tend to be extra picky I think.



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