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Thread: Z axis rebound

  1. #1
    Join Date
    Dec 2017
    Posts
    33

    Default Z axis rebound

    Ok, I have been cutting some knotty pine. My problem is that the z axis will rebound about .125 when a v bit or a endmill hits a knot. Not every time but often. 12,000rpm, 80 inches per minute. I was running 100 and slowed it down last week. Because I thought it would help with this issue. Today I noticed the bit would spin in the collet with the collet tight. So, I am ordering a new collet but was wondering if there is some sort of mechanical tension adjustment on the z axis of a Shopbot? Any advice you can give would be appreciated.

    " Life is hard when your dumb" John Wayne

  2. #2
    Join Date
    Mar 2013
    Location
    Memphis TN
    Posts
    719

    Default

    Is the Z Axis actually moving or is the bit being driven deeper into the collet?

    It is almost impossible to overtighten an ER25 collet using the wrenches supplied by Shopbot (unless you are an ironworker or a WWE professional).

    You should replace your collets when they cannot hold a bit properly.
    ShopBot Details:
    PRS Alpha 96x60x12
    4hp Spindle
    12" indexer
    Aspire
    Rhino

  3. #3
    Join Date
    Dec 2017
    Posts
    33

    Default

    No the bit stays the same depth. The z axis assemble rebounds. Like if you lower the z axis too low.
    I have ordered a new collet. I have a Porter Cable router.
    Just wanted to know if there is a tension adjustment I might not be aware of.

  4. #4
    Join Date
    Mar 2013
    Location
    Memphis TN
    Posts
    719

    Default

    It's always best to precisely describe your equipment, including model, year, software version, feeds, speeds, anything pertinent.
    ShopBot Details:
    PRS Alpha 96x60x12
    4hp Spindle
    12" indexer
    Aspire
    Rhino

  5. #5
    Join Date
    Apr 1999
    Location
    Rock Hill SC
    Posts
    475

    Default

    A lot of vendors will send out 6MM and 1/4 inch bits interchangeably. There is a difference and that might be what is causing your problem. Mike your bit and see just what it is. Keep in mind that collets and nuts are wear items and need to be replaced when worn.

  6. #6
    Join Date
    Dec 2007
    Posts
    2,310

    Default

    I have bought .25 inch bits that said .24 on them and that is what they were.

  7. #7
    Join Date
    Dec 2017
    Posts
    33

    Default

    Quote Originally Posted by jerry_stanek View Post
    I have bought .25 inch bits that said .24 on them and that is what they were.
    Well, I ran two different programs this afternoon, I changed the vbit. Tightened is as much as possible. Holding the router from turning the bit did not slip as before. No issues with running either program today. Very possible that the bit was just enough undersized to allow it to slip.

  8. #8
    Join Date
    Dec 2017
    Posts
    33

    Default

    Quote Originally Posted by coryatjohn View Post
    It's always best to precisely describe your equipment, including model, year, software version, feeds, speeds, anything pertinent.
    Shopbot, Alpha, PRT 4G, Mach 3, V Carve Pro 9.5, Porter Cable 7518, 13,000 rpm, 80 inches per minute.

    It is a older used machine that has several items that are not original to the machine. Still trying to figure out if the rebound that i am getting is normal for these machines. I did improve the cutting today with the bit change. But still leaves me not knowing if i have a secondary issue or the z axis should be well from now on.

  9. #9
    Join Date
    Jan 2004
    Location
    iBILD Solutions - Southern NJ
    Posts
    7,962

    Default

    Quote Originally Posted by Bank Walker View Post
    Shopbot, Alpha, PRT 4G, Mach 3
    FYI - There's no such animal...the machine is either an PRT Alpha or it is a PRT Standard 4th Generation (4G). An Alpha machine will have violet/black motor label, whereas a Standard will have a red/black motor label.

    In the case where your machine is an Alpha - the early PRT Alphas used direct drive AS911 motors. These had the tendency to be 'springy or spongy' when the control box is on and you attempt to move an axis by hand with enough force to overpower the motor. It will very much act like a spring - and spring back as the internal encoder and driver attempt to resolve the original position. If this is what you are referring to, it is considered 'normal' for that vintage machine. Plunge speeds should be carefully set so you don't get that deviation in position while the motor is in 'alpha mode' trying to correct itself. It is simply a matter of asking more from the motor than it is capable of delivering.

    If running a Standard, the Z will simply lose position and never correct itself. Since you didn't mention position loss, I would guess you have an Alpha.

    The length of time that the motor is allowed to self-correct is factory set at 4 seconds. After 4 seconds, if it was not able to return to the commanded position, the driver will alarm out and power to the motor is shut down for safety - just in case your arm is in there.

    From a machining standpoint, the AS911 motors in direct drive format suck. However, if you were to buy a 5:1 planetary gearbox for each, this would result in incredible performance/torque that rivals even the newest Alpha machine with none of that 'spring' in any axis.
    High Definition 3D Laser Scanning Services - Advanced ShopBot CNC Training and Consultation - Vectric Custom Video Training IBILD.com

  10. #10
    Join Date
    Dec 2017
    Posts
    33

    Default

    Quote Originally Posted by Brady Watson View Post
    FYI - There's no such animal...the machine is either an PRT Alpha or it is a PRT Standard 4th Generation (4G). An Alpha machine will have violet/black motor label, whereas a Standard will have a red/black motor label.

    In the case where your machine is an Alpha - the early PRT Alphas used direct drive AS911 motors. These had the tendency to be 'springy or spongy' when the control box is on and you attempt to move an axis by hand with enough force to overpower the motor. It will very much act like a spring - and spring back as the internal encoder and driver attempt to resolve the original position. If this is what you are referring to, it is considered 'normal' for that vintage machine. Plunge speeds should be carefully set so you don't get that deviation in position while the motor is in 'alpha mode' trying to correct itself. It is simply a matter of asking more from the motor than it is capable of delivering.

    If running a Standard, the Z will simply lose position and never correct itself. Since you didn't mention position loss, I would guess you have an Alpha.

    The length of time that the motor is allowed to self-correct is factory set at 4 seconds. After 4 seconds, if it was not able to return to the commanded position, the driver will alarm out and power to the motor is shut down for safety - just in case your arm is in there.

    From a machining standpoint, the AS911 motors in direct drive format suck. However, if you were to buy a 5:1 planetary gearbox for each, this would result in incredible performance/torque that rivals even the newest Alpha machine with none of that 'spring' in any axis.
    Ok, thanks for the help.The machine has had so many modifications. Lord only knows the true original model or year it was made. The man I got it from bought it in SC years ago. He said it was as is then and he has moved 8 or 9 times himself. I only said it was an Alpha based on size. I think you called it PRT 4G based on the pictures I posted last year. I honestly do not know enough to be trying to describe it to professional cnc people. Might be why i was kinda hesitating on posting machine info. Lack of knowledge. I did recieve a thumb drive with it that was supposed to have the machines info but when it tried to view it the drive was not formatted.

    Great to hear the rebound I am getting is normal. I do have a new spare z axis assembly that came with the unit. Still wrapped in original plastic. If needed i could swap it out but it appears there are newer upgrades that can be made that would get me better performance.

    When the rebound occurs, say up .125, it maintains that new level as being correct. I do loose the original depth. That being the case the machine is probably a PRT 4G like you thought it was last fall when I bought it.

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