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Thread: Movie About the Z-axis Skipping

  1. #1
    Join Date
    Oct 2013
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    Default Movie About the Z-axis Skipping

    I dabbled over the last few weeks to no real effect. However, today when I went to check it out, I managed to get the problem occurring on video. Towards the end of the video, you can even hear the stepper skipping.

    Any thought please?

    Chazz

    PS,
    I had to send it to YouTube as it is too large for here even though it is only 31 seconds... It is unlisted so here is the link:

    https://youtu.be/uLLdmX5ZCSA

    Thoughts?

  2. #2
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    Default

    Here is the toolpath file up to (and slightly past) the point that it skips.


    TestFinish_025bn_ZB_Diag.txt

  3. #3
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    Default

    I would rethink hold down method and use a shorter bit, reduce vector boundary to stop unwanted z retract. Reply may not be related to z problem.
    Vector Studio 22

  4. #4
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    Quote Originally Posted by gc3 View Post
    I would rethink hold down method and use a shorter bit, reduce vector boundary to stop unwanted z retract. Reply may not be related to z problem.
    In general, I would agree. This started out as a test cut of a part of the pattern which had steep (sheer) drop-offs.

    In this particular case, it skipped while it was not even over the test material. In the end, that was material it was removing to help make sure the collet didn't smack into the material. That is why it ramps up towards that end of the run. However, in this case, I caught it being unable to lift the router. I just re-checked and the spring is attached and free from binding.

    This all started after installing my 5.5 upgrade. I am fairly sure that must have something to do with it; but I am at a loss as to what. I ran a toolpath file to run it up and down over and over for about 20 minutes and no skipping. In this case, it was while it was moving with an increasing X. I don't know what that might have to do with it; but I am thinking of a test to try to see if I can do it reliably in air (without cutting material).

    Thank you for your thoughts though,
    Chazz

  5. #5
    Join Date
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    Default

    Chazz, try dropping the Z axis feed rate from 2 to 1.8 and try again. If it still does it, drop it a bit more and try again. You could also try adjusting the ramp values (VR) to soften 3D ramping a bit with the 3D ramp threshold (try something above 100. 150 is good for the larger Bots). Lowering the slow corner speed can help too. A high 3D ramp threshold can make the Z axis ramping be harsh and could cause loss of steps with 3D files. Just a few thoughts.
    Scott

    If guns kill people, I guess pencils misspell words, cars drive drunk and spoons make people fat.

    "Those who hammer their guns into plows, will plow for those who do not" - Thomas Jefferson




  6. #6
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    Default

    So check me on these modifications please.

    For the z-axis feed rate, I could change the:

    MS,2.0,2.0

    into

    MS,2.0,1.8 (or less, I would be OK with 1.0 for now)

    After that, either try it first alone or group the cures together, try to add in:

    VR,,,,,,,,,,,150,,,,,

    or

    VR,0.4,0.4,0.4,0.4,0.4,0.4,0.4,0.4,0.2,0.2,150,0.1 5,65,,,0.2

    (Not sure if I can just skip the values to leave them unchanged or reset to defaults.)

    As for cornering speed, while perhaps a generally good idea, does it apply in this case? It was skipping while not changing Y at all. Does the cornering speed also effect X-Z plane corners?

  7. #7
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    Yup, change the MS in your file and it's easier/better to do the VR right in the fill-in sheet that comes up with the VR keystroke combination (blank values, when placed in a file, does leave the parameter unchanged). Cornering speed can add a little cushion to the change in direction at the end of raster moves and if the Z happens to be in motion at the same time as a raster change (like a steep relief wall), it might affect the Z, not sure, but it does soften XY direction changes which if too abrupt, can cause loss of steps.

    I wouldn't go lower than 2,1 on the MS as the XY axis will be forced to slow down while the Z axis is trying to catch up. I'd start with a 3D threshold of 150 and run it. If that doesn't stop it, drop the speed to 1.8 and then 1.6..... Also, try setting the minimum distance to check to 0.08. Too soften the raster direction changes try around 40%.

    A desktop unit is going to behave differently than a full size machine and a standard differently than an Alpha, so settings will vary. Once you find some that work for you, you can save those settings out to a custom file (US) and load them when needed. Just remember to also save your preferred default values so you can return to them with out having to write them down or do a full reset. I have saved, a set of values for default, 3D, and v-carving and just set them up as custom cut #'s to load at will.

    If anyone wants to read up on VR settings and what they apply to, Brady's 'excellent adventure' is here. This should really be a 'Sticky'! Admin, we should have a few 'Sticky' threads/posts for such things as it's a little hard to find these in the blog part of the website.
    Scott

    If guns kill people, I guess pencils misspell words, cars drive drunk and spoons make people fat.

    "Those who hammer their guns into plows, will plow for those who do not" - Thomas Jefferson




  8. #8
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    Thanks, lots to try.

    Is there a test toolpath file that can push the various machines to their limits as a check? I am looking at trying to create such a toolpath file like the one that I did to run to close to max-Z, close to min-Z, over and over for about 20 minutes. That one seemed to work out just fine. If I can add some to it, then it might become the start of a diagnostic checkout.

    In that effort, I will need to find a way to verify the physical height from time to time so, if it is skipping, it could be detected and stop. I was figuring on checking out the z-zeroing script as a basis and just leave the clip on while it was running through the test.

    In the end, I didn't seem to have this problem when I had the Z-limit of 4" -- as in I don't EVER recall it happening. It was only after that when this issue showed up. I wish there were some measurements I could make to verify my installation job.

    In doing this, to your understanding, causing the stepper to skip doesn't actually harm anything does it?

    Thank you

  9. #9
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    Oct 2013
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    MD
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    Default More Data...

    While I checked this before (from the bottom), when I re-checked the spring from the top, I found that it is rubbing up against the parts which travel along the rails.

    View from the top:
    0802151917_resized.jpg

    View from in front:
    0802151918_resized.jpg

    The front view shows a slight bowing of the spring.

    I think my overall problem is a concert of small things -- no one of which could give me this grief.

    The router is fairly heavy and while its weight has not changed, the plate which travels up and down with it has gotten larger. I don't think that the spring ever neutralized the weight of the router and plate; but instead was just meant to offset it "enough". Folks who have had both can help me out here on if the router weighs appreciably more than the spindle. If that is the case, the spring is probably fine for the spindle but just not enough for the router.

    There are a lot of various torques and forces during the router movement. In this case, the router bit was not in contact with any material; however, it was spinning. I don't think that spin had anything to do with this issue as the angle of the shaft was not being adjusted; but I am not sure.

    I don't know just exactly what triggered the skips; but it is in a zone where the spring rubbing against its obstruction would be at about its worst.


    Possible cures.... I am tempted to bend the upper spring mount just enough to see space past it and run the old file to see if it skips at all. It seems that it might work a little but I also might weaken the spring mount too much.

    gonna sleep on it and try it tomorrow provided I can get the time.

  10. #10
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    Default

    I'm not seeing anything unusual in the video that you posted. The biggest thing that stands out is the inadequate hold-down. It appears to me that the material is warping up after it's been roughed out - which is not that uncommon. If the material isn't dead flat, you're going to get some gouging.

    Regarding the Z axis itself, I would leave the router OFF and use keypad to move the Z up and down, while pushing down with reasonable force while commanding it to go up, and pushing it up while commanding it down. While moving and applying force, observe/feel for any tight spots in the travel or hiccups that may occur. This will emulate the Z being used under load. You will have to judge what is reasonable force before the motor stalls and compare that to cutting forces as if you were using a hand router.

    There is an outside chance that there is something flaky with the motor driver itself - which may only show up when things get hot. So if you encounter issues while machining, then stop and run the up/down test to see if it happens when hot compared to cold.

    Max machining speed on the DT is 4 IPS. In reality, you aren't going to run the Z faster than 2 IPS unless you are machining foam. I rarely run mine faster than 1 IPS especially in hardwoods. I would run the file you show at 2,1 and see how it goes. The faster you go, the less torque the motor is able to deliver. As Scott pointed out, you will want to tighten up your machining boundary vector to stop the Z from lifting at the end of each raster pass. This eats up a lot of time and causes other issues.

    -B
    High Definition 3D Laser Scanning Services - Advanced ShopBot CNC Training and Consultation - Vectric Custom Video Training IBILD.com

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