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Thread: increasing resolution

  1. #1
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    Default increasing resolution

    I have the 7.2 gear head steppers on my bot, and now I am looking to further increase my resolution. Does anybody have any ideas of the next step? I am seeing the resolution 'chatter' on aluminum parts cut on the angle (using both x and y axis to cut a vector). I would like to see those lines go away and for my parts to resemble things made on say a Haas or other metal specific CNC.
    Has anyone installed helical rack on their machine? I'm not sure if its the straight toothed racks and gears that are causing this chatter, or the stepper motor resolution.

  2. #2
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    Default

    jon...

    If you have the 7.2 gearmotors on your bot you are not suffering from a lack of resolution. Possible pinion lash or some gearbox lash inherent with all gearmotors that increases with use.

    "I would like to see those lines go away and for my parts to resemble things made on say a Haas or other metal specific CNC."

    You can reduce the edge chatter, but if you want "HAAS cut" looking parts, you must buy the HAAS. You cant expect a 20K+/- router table to cut like a 80K+/- machining center.

    You may be getting some chatter form the pinion/rack combo, but usually not noticeable enough to justify the cost of helical rack. I am not sure if the motors you have will work with helical rack, due to axial loads.

    You ask for the next step, but you do not say which model or year machine you own. PRT or PRS, standard or alpha?

    See picture below. This is about the best you can expect to get from a router.
    Attached Images Attached Images
    Last edited by Gary Campbell; 02-04-2017 at 03:10 PM.
    Gary Campbell
    GCnC Control
    GCnC411(at)gmail(dot)com
    ShopBot Controller Upgrades
    http://www.youtube.com/user/Islaww1


    "We can not solve our problems with the same level of thinking that created them"
    Albert Einstein


  3. #3
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    Default

    Gary, I have a 2005 PRT alpha upgraded with; 7.2 gear motors on all 3 axis, new control card, hsd spindle, and lots of bracing and extra bearings riding on BWC hardened rails. New pinions and the racks are not worn.

    I am in the process of building a single y axis gantry beam to further stiffen the chassis. I am hoping for a motor upgrade, (at the expense of speed is ok for me). Best case is using current drivers as well. Thinking of helical rack because I feel the vibration from it when I touch the machine as it runs. Backlash on the motors when I tug on them seems consistent and ok.

  4. #4
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    jon...
    I would suggest that you use a dial indicator and find the play in your machine. In most cases you will not be able to feel an amount that will return scalloped cuts in aluminum. I am attaching a doc that shows setups for a PRS, use it as a guide to check your PRT. Especially the lower Z rollers when the z is down near the table where you usually cut.
    Attached Files Attached Files
    Gary Campbell
    GCnC Control
    GCnC411(at)gmail(dot)com
    ShopBot Controller Upgrades
    http://www.youtube.com/user/Islaww1


    "We can not solve our problems with the same level of thinking that created them"
    Albert Einstein


  5. #5
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    Thanks for that Gary, I will look into that first. Here is a recent part cut from 0.5 inch material
    Attached Images Attached Images

  6. #6
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    jon...
    Make sure you define if or how much deflection in your machine and remove all on the mechanical side. I would be more than willing to offer you belt drive, motor and drive upgrades, but in most cases the OEM 7.2 alpha motors will not limit cut quality as much as the adjustments and the condition of your rails and rollers, especially if you have the 4 roller (vs 8) Z assembly.

    It does neither of us any good to throw hundreds (or thousands) of dollars at something and not improve the cuts

    Carefully inspect the pinion to rack interaction. In some cases the brackets that hold and pivot the motors are not orthogonal to the racks, allowing one end of the pinion to "dig in" causing a toothy sound.
    Gary Campbell
    GCnC Control
    GCnC411(at)gmail(dot)com
    ShopBot Controller Upgrades
    http://www.youtube.com/user/Islaww1


    "We can not solve our problems with the same level of thinking that created them"
    Albert Einstein


  7. #7
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    Yes this is the problem, trying to define where the slop is coming from. I know this will sound hokey but I installed rollerblade wheels on the outside of both the x and y axis cars gripping the frame. I guess my racks are getting old and the wear is starting to show there? I would like to make my gantry car unable to come out of square with the table using prismatic rail which would force it to travel in one direction only, something that is currently not happening with the BWC system. Anyways cut quality has improved dramatically with the addition of the blade wheels so I suppose they are compensating for the slop the racks and pinions are allowing or the rails.....

  8. #8
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    Here is a shot of my machine as it is currently. What you can see here that's different from a standard prt alpha is that I have had the x axis C beams milled flat on a huge logging specialty mill (40'by 8' work area, incredible machine). The bent sheet metal rails are mounted flush with the milled surface. I also installed aluminum darts along that rail to stiffen it. There is an under bed structure made from European birch plywood that also supports the rail along its length. There are BWC hardened rails mounted to the old rails. I have about an inch of clearance between my z axis absolute positive position and a sprinkler head, its a tight squeeze that made me get really creative with the dust extraction hose. I routed it along the cable carrier link belt on an aluminum tray mounted to the rear y axis extrusion beam. One of the oldest mods is the z axis stiffener angles, which also incorporates a fine tune adjustment for vertical z axis alignment. Then lastly I installed these roller blade wheels to see if I could tighten things up and that made a significant improvement to my cut quality and accuracy, but still not to where I would like the machine to be.
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  9. #9
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    this will be the bearing and rail system for my z axis I'm currently making, it will free up the 4 BWC v grooves currently on it for the other axis.
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  10. #10
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    I have some small amount of play at the rear of my y axis, and can force all of my motors out of position with moderate force

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