HELP! Inconsistent Y-Axis travel???????


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Thread: HELP! Inconsistent Y-Axis travel???????

  1. #1
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    Default HELP! Inconsistent Y-Axis travel???????

    I discovered a bit of a problem with my machine this week (CNCRP Pro 4x2 - V-rails version).

    Long story short, I've been trying to make a guitar rosette that is elliptical in shape. I machined the rosette inlay out of a solid piece of 1/8" thick walnut. Then I machined the inlay pocket for it (derived from the same geometry in F360). I made a test cut in some MDF to check for fit and it was fine. Then when I ran the same program in the actual guitar top, I found the pocket was undersized and the inlay would no longer fit. Long story short, I did a ton of handwork to make it fit, ran into some other problems, was able to program in a solution to that... and then lost the top due to sanding too far... but I digress....

    So round two, I ordered a new top, this time I did extensive testing on the programs. I had thought I found the problem... not select "use radius" in post processing on the rosette inlay part. So I made a test piece in high quality ply, and an matching pocket in the same ply. The parts fit very well. Then I made a new rosette, this time in four pieces cut from scrap ---- I didn't have any larger pieces to do it as a single part. I tested this against the pocket where the plywood inlay fit so well... and found the new pieces were a total of about 3 mm too long. Ok, I reasoned that maybe if I had a LITTLE accuracy error when rolling around the corners and going from radiused to straight surfaces, that it would add up around the four joints and accumulate.

    So I just took 3 mm of length out of one of the pieces where it would be under the fretboard (pretty standard in rosettes, actually).

    Then when I was ready to process the new soundboard, I decided to make one more test pocket. This time incrementing depth to make sure I went the right depth (not a lot of extra for sanding on those soundboards!). The new test pocket fit perfect, but I went way to deep. So, I relocated the work offset to the right by about 6 inches to cut a new test pocket. Afterwards what did I find? My rosette pieces were no TOO SHORT by.... 3 mm. The amount I had hand removed was now just what I needed to fit perfect in the pocket.

    So I decided to cut another test. This time I moved the Work offset back to the left where it was before. This pocket fit perfect to the shortened rosette pieces. WTF? As I recall, the rosettes (both of them ) have been cut further to the right on the table, whereas the pockets are typically on the left (just how I tend to set things up).

    So my conclusion is that the machine is cutting differently at different location of Y axis!

    I ran a test.... in the "left" position, I laid out a 100 mm line, the positioned a pointed bit over the end of it. I used MDI to tell it to move 100 mm to the right... it came up about 1 mm short! Then I did the same with it over to the right by about 6" (150 mm). Sure enough, a "G0 Y+100" moved it EXACTLY 100 mm.

    So HOW IS IT POSSIBLE for the Y axis travel to be inconsistent in different locations? I also laid out 1000 mm marks from far left to far right and moved the carriage manually by 1000 mm.... I got about 1001 mm of travel out of it.

    I should note that I am using the default tuning configs that CNCRP provided me with.

    Pictures below to demonstrate what I've just tried to explain. I really need to figure out the problem as I need to be able to do these accurate inlay things and not worry about whether parts are being cut in consistent locations.

    Ideas:
    1) inconsistent tension on V-rails/wheels is causing more resistance to movement in some places? (not sure this can happen on a stepper motor with R&P)
    2) motor tuning is just a little off? That wouldn't explain inconsistent travel though would it?
    3) undetected concentration of dark matter causing space/time to warp at one point on my table..... (ok, maybe that's too far fetched).

    Anybody got suggestions?

    Pictures below, will annotate them.

    First cut that fits with the shortened rosette pieces

    HELP! Inconsistent Y-Axis travel???????-img_2708-jpg

    Second test pocket cut, same program just with different axial "stock to leave" setting (deeper), 6" to the right. Note the gap now in the pieces.

    HELP! Inconsistent Y-Axis travel???????-img_2707-jpg

    Relative location of the two test pockets.... note the writing in the center which is the measured width of the inner ellipse on both... just under 2 mm difference!

    HELP! Inconsistent Y-Axis travel???????-img_2709-jpg

    Third test pocket, left handed Y position, further back on X-... this matches the first on in the same Y location. Perfect fit to shortened rosette pieces

    HELP! Inconsistent Y-Axis travel???????-img_2710-jpg

    Here are the test pockets along with the test plywood rosetted inlay I made, it matches the pocket on the left, which IIRC was cut in the same location.
    HELP! Inconsistent Y-Axis travel???????-img_2712-jpg

    Three pictures of the rosette in the new soundboard. I made sure the location was in the left most Y position. It turned out perfect, but man I don't need that kind of stress. note that the MOP inlays all fit perfect to their pockets. The machine seems to be accurate, but at about 12mm in size, I guess error would be a lot smaller if it was there.

    HELP! Inconsistent Y-Axis travel???????-img_2713-jpg

    HELP! Inconsistent Y-Axis travel???????-img_2714-jpg

    HELP! Inconsistent Y-Axis travel???????-fullsizerender-jpg

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  2. #2
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    Default Re: HELP! Inconsistent Y-Axis travel???????

    Assuming your machine is "square"? If you're not sure check it. If everything is square and your "steps per" are correct on all axis, check all your mechanical connections really well. Something as simple as set screws being loose on a coupler may cause either consistent or inconsistent errors. It can be software related but the majority of lost steps (especially on one axis and when it was previously cutting correctly and you made no software changes) are due to play somewhere.



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    Default Re: HELP! Inconsistent Y-Axis travel???????

    Quote Originally Posted by coherent View Post
    Assuming your machine is "square"? If you're not sure check it. If everything is square and your "steps per" are correct on all axis, check all your mechanical connections really well. Something as simple as set screws being loose on a coupler may cause either consistent or inconsistent errors. It can be software related but the majority of lost steps (especially on one axis and when it was previously cutting correctly and you made no software changes) are due to play somewhere.
    It was square last time I checked - maybe six months ago.



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    Default Re: HELP! Inconsistent Y-Axis travel???????

    I had some inconsistent parts and I found out that I could twist one of the pinion gears and rack the gantry on that one side. Then I found that the set screw I hadn't used loc tite on had fallen out. Oops.



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    Default Re: HELP! Inconsistent Y-Axis travel???????

    Finally got around to working on this today. I removed the Y-axis motor from the carriage and re-did the motor coupling. It did seem to have loctite on it, and I don't think it was loose. But I put it on snug and also re-adjusted the carriage wheels to make sure they were always tight at all points.

    Then I did another test and found I still have the same problem. Manual command to move Y+100 mm moves it 101, test to move y+400 moves it about 400.5

    So, I guess I just need to tune that motor? Using the default CNCRP setup, but maybe motors can vary?



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    Default Re: HELP! Inconsistent Y-Axis travel???????

    No, motors don't vary. One revolution is always one revolution.
    Are you sure that there's no play in the rack and pinion?
    Motor tuning isn't going to help when it gets longer in one section and shorter in another.

    Gerry

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    Default Re: HELP! Inconsistent Y-Axis travel???????

    Quote Originally Posted by ger21 View Post
    No, motors don't vary. One revolution is always one revolution.
    Are you sure that there's no play in the rack and pinion?
    Motor tuning isn't going to help when it gets longer in one section and shorter in another.
    From yesterdays tests, seems to be linearly consistent. Ie, the longer the move, the more it was off and too long. Will need to do more tests, but everything seems tight.



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    Default Re: HELP! Inconsistent Y-Axis travel???????

    Kosh,
    You might want to check for any kind of binding. In your situation with a motor running on a cog rail the most likely cause for inconsistent travel on Y axis would probably be missed steps due to binding. Take the motor off the table and put a piece of tape of the gear and get the motor to spin exact so many revolutions back and forth and ensure it is doing exactly as requested. This will help pin point the issue. If it is doing exactly one revolution when commanded and then say 10 revolutions when commanded then that suggests the steps for one revolution are correct, which suggests the cog is slightly mis-aligned with the rail. If the gantry is wide you could have some racking taking place that is causing the missing steps.

    Russ



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    Default Re: HELP! Inconsistent Y-Axis travel???????

    From yesterdays tests, seems to be linearly consistent. Ie, the longer the move, the more it was off and too long
    If that's the case, then steps/unit should fix it.

    But you said it a was off 1 in 100, and 0.5 in 400. That's not linear.

    Gerry

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    Default Re: HELP! Inconsistent Y-Axis travel???????

    Kosh, maybe related or not to your problem... I usually do my rosette channels by hand, using a circle cutting jig on my Dremel or laminate trimmer. There are quite a few reasons why your circles could be off, some related to and some not related to what's already mentioned. One thing is storing your rosettes and top in a climate-controlled area, taking them out only to do machining, then go back to that climate controlled area for fitting/installing. I would route a rosette channel, install the rosette, then inlay purfling afterwards to ensure the fit is perfect. Even a matter of a few minutes will cause that rosette channel to deform in wood just from wood movement.



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    Default Re: HELP! Inconsistent Y-Axis travel???????

    Quote Originally Posted by ger21 View Post
    If that's the case, then steps/unit should fix it.

    But you said it a was off 1 in 100, and 0.5 in 400. That's not linear.
    I can't measure exactly less than 1mm... it was close to .5 mm, could have been .4. Approx linear.



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    Default Re: HELP! Inconsistent Y-Axis travel???????

    If it was linear, it would be 1 in 100, and 4 in 400?

    Gerry

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    Default Re: HELP! Inconsistent Y-Axis travel???????

    Quote Originally Posted by ger21 View Post
    If it was linear, it would be 1 in 100, and 4 in 400?
    Typo , 1000



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    Default Re: HELP! Inconsistent Y-Axis travel???????

    That makes more sense.

    Gerry

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    Default Re: HELP! Inconsistent Y-Axis travel???????

    Hi Kosh - I hope you fix your problem soon. A tip for marquetry. Always use the same tool to cut the pocket and the inlay. I always make the inlay first. Then I run the pocket but tell the program that the tool is oversize say 0.5mm. This makes the pocket smaller than it should be. Then I try to fit the inlay and it's usually too big. Then run the program again telling the machine it has a 0.25mm over size and trial fit again then continue at say 0.2mm until it drops in. Different timbers never cut the same due to moisture and temp. Especially if you do one one day and another on another day. Cheers Peter

    Attached Thumbnails Attached Thumbnails HELP! Inconsistent Y-Axis travel???????-rocket-2-jpg   HELP! Inconsistent Y-Axis travel???????-rocket-jpg  


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    Default Re: HELP! Inconsistent Y-Axis travel???????

    Quote Originally Posted by peteeng View Post
    Hi Kosh - I hope you fix your problem soon.
    Fixed two years ago with an upgrade of to the linear bearings.

    A tip for marquetry. Always use the same tool to cut the pocket and the inlay.
    Yes, common sense. Thx



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HELP! Inconsistent Y-Axis travel???????

HELP! Inconsistent Y-Axis travel???????