Head Counterweight

Discussion in 'CNC Machines and Conversions' started by LSEW, Jul 20, 2013.

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  1. Jul 20, 2013 #1

    LSEW

    LSEW

    LSEW

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    Hi All, I an new to CNC, and have purchased an already converted X45 machine. It seems to work well, but lately I have noticed about .009 backlach in the Z.

    THe conversion uses ball screws and is driven by a stepper with a reduction and a Gates splined belt. For the Z, the whole head moves and there is a counterweight to balance the weight. I have begun taking the machine paart to get an indicator on the ball screw to find out where the backlash is originating.

    In doing so, I have noticed the counterweight for the head is not heavy enough to actually balance the head and the pulley at the top of the ball screw turns eaisly wwhen lowering the head, but much more difficult when raising it. In it's favor, the stepper seems to have no trouble raising the head. My question is how well should the counterweight balance the head?

    Any help on tracking the backlash would be helpful as well.

    Has anyone had an issue with the Gates belts stretching?

    maury
     
  2. Jul 29, 2013 #2

    starnovice

    starnovice

    starnovice

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    The counter weight just takes a load off of the z axis motor so it is not all that critical. The backlash is coming from the ball screw nut. You can repack the ball screw with larger balls to help take care of that. Here is a video on repacking the ballnuts [ame]http://youtu.be/8plIXSR5Nh8[/ame].

    Hope this helps.

    Pat W
     
  3. Jul 30, 2013 #3

    jwcnc1911

    jwcnc1911

    jwcnc1911

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    Backlash can come from many factors. End supports are usually the culprit in my experience. If it is your ball nut you can buy a plastic ball nut that is basically pressed on and it will completely eliminate that issue (the last one I bought lasted a couple years and at $16 was worth the purchase). You can also have take up in the drive "gearing" - in you case belts and pullies. Check all key ways to be properly keyed. Slippage will cause backlash as well as eventually roll the key.

    Basically I'm saying keep in mind that any mechanical component in the drive can cause backlash.

    When Grob (Google Grob Machine tools) set up our last new machine, the tech doing the job told me they design their counterweights to take about 90% of the load off of the motor.

    Hope this helps, enjoy finding the culprit - that's the fun part to me.
     
  4. Jul 30, 2013 #4

    starnovice

    starnovice

    starnovice

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    JW you are right I took a too simplistic approach. He should consider the other factors especially if the backlash tends to get worse over time.
     
  5. Jul 30, 2013 #5

    LSEW

    LSEW

    LSEW

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    Guys, thanks so much for the input. I watched the video and may do the ball replacement thing if I determine that is the problem. I'm in the process of removing parts from the head so I can get an indicator on the ball screw ends and on the vert. slide. I'm working alone and I need to find a way to support the counterweight as I can't handle it by myself.

    I did notice when turning the ball screw pulley that it feels a bit spongy for about 15 deg before the head moves.

    A 3 phase motor and VFD are ordered, will be converting to variable speed with the backlash repair. THe motor is lighter than the original, so that should help the counterweight issue.

    maury
     
  6. Aug 1, 2013 #6

    LSEW

    LSEW

    LSEW

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    Guys, digging into the head of my x45 I noticed the spindle bearings feel a little gritty in places. Had anyone changes the bearings in their spindle? If so, where did you find them. Talking with a machinist friend he says I should look for Class 5 bearings, poking around on Google has nor come up with a whole lot.

    maury
     

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