1. #1
    Hi All,
    .
    I need some advice on a couple of niggles I have with my gantry type router:
    .
    First up is two sided machining - I am getting a step of 0.5mm between the two sides in the Y axis, even though I am using an L shaped jig (aligned to within ~0.02mm to the machine), with the part zero in the centre of the block and flipping about the X axis. Stuff I have already tried/checked:
    .
    Checked the G-Code in case there is a reason for the problem (can't see any).
    Checked repeatability using my touch probe from a known point on the machine to the jig on the Y axis and its acceptable at <0.05mm or less.
    Checked the same with the touch probe between the jaws of my digital callipers - same accuracy.
    Run several two sided jobs and its consistent (which I guess is a blessing).
    .
    Here's where things get a little weird: after drawing a blank I decided to have a go at a part and using a simple straight line slotting cut back and forth along the x axis trimmed a 30mm ali block down to the size I need to do the job. I then noticed that whilst the slot was nicely 6mm wide (6mm end mill) the slot looked like it was ~0.25mm further towards Y+ from the Y zero point, which could explain why my two sided jobs are coming out stepped i.e. the job is offset by +0.25mm from zero in the Y on one side and when flipped the other side is the same, which would come out at a ~0.5mm step in the work.
    .
    I thought "hmmm might be backlash?" but its in one direction only and the dimensions of the completed job are accurate enough.
    .
    Hopefully that all made sense and I'm now wondering if I have made a cock-up in MACH3 that is offsetting every job by +0.25mm from zero in the Y axis? I will use a "kludge" for now by offsetting the Y zero by -0.25mm, so the job starts cutting in the right place.
    .
    Second niggle (which has occurred after the first so not necessarily related), which I think I know the answer to anyway:
    .
    My vertical axis seems to have become a little "slack" i.e. I can push the bottom of the Z axis plate in the y axis direction and I have some play (not much, but enough to cause chatter when cutting in the Y axis direction) and it seems to be getting progressively worse.
    .
    I've run round all the bolts on the "carriage/z-axis" and all is tight, so I am suspecting I may have some Y axis bearings going on the fritz, but also another reason maybe that if you look at my machine on my build thread the 120x60mm top gantry beam may have "slipped" down the gantry sides and is causing some slack, but I'm no expert in whether the bearing blocks would allow this and/or whether any slippage would allow play considering there "shouldn't" be any to start with unless the bearings are worn/wearing.
    .
    As usual opinions/advise welcome.
    .
    Cheers
    .
    .
    Chris

  2. #2
    Are you sure your motor steps per is correct and the machines moving exactly what you tell it to move.? What's the motor tuning at.? Vel & accel.

    Have you checked for End float on the ballscrew.?

    Only thing I can see that would cause this if no offset is applied could be those 2 things and I'd go for the Steps per and motor tuning first.!

    Edit: No actually for what it takes I'd grab the screws for end float first but then I'd go for Steps per before anything else.!
    Last edited by JAZZCNC; 16-04-2014 at 01:08 PM.

  3. #3
    Thanks Dean,
    .
    I'm just running a bunch of jobs in pine, which I don't give a monkey's about the stuff above ;) but will check the ballscrew and steps/accel afterwards.
    .
    Chris

  4. #4
    Update guys after getting some time to dismantle my Z and Y axis (Z needed some maintenance anyway).
    .
    Dean, you were spot on - the Y axis ballscrew locking nut must have slipped a little bit and was allowing a bit of movement - I've had this problem before and thought I had cured it by using a beefier set screw, so I may end up drilling out the copper/brass pad in the locking nut and trying a direct clamp from the set screw this time (with Loctite).
    .
    Of more concern is the Y axis chatter, which was probably only partially the fault of the ballscrew, as I have examined the bearings for lateral play (see photos) and I am able to "roll" the z-axis in the direction of the arrows (shown in red) and I can see about 0.5mm of "light" between the bearings and the rail. I know from other threads that round rail and bearings are prone to wear and probably in exactly this sort of manner.
    .
    So I am thinking to fix it temporarily I'll loosen and raise the main 120x60mm cross beam to take some of the slack out (shown in green), but also look at replacing with profile rails and bearings (20mm possibly) - I'll either use Chai if he does these, or Zapp/Gary for convenience, if I can work out which bearing blocks I should go with and whether the whole assembly will fit in the same space? Hopefully they will fit in place of the existing rail/bearing units, but the top and bottom carriage extrusions will likely need replacing with ali plate custom cut affairs.
    .
    Question - I take it the profile rail/bearings are an order of magnitude "stiffer"?
    .
    I also have the option for the top bearings to make the carriage plate wider by ~60mm each side, which I am thinking will be even more stable as the bearings will be further apart? (shown in blue)
    .
    As always opinions/advice welcome.
    .
    Cheers
    .
    .
    Chris
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    Last edited by Washout; 23-04-2014 at 04:57 PM.

  5. #5
    Quote Originally Posted by Washout View Post
    .
    So I am thinking to fix it temporarily I'll loosen and raise the main 120x60mm cross beam to take some of the slack out (shown in green), but also look at replacing with profile rails and bearings (20mm possibly) - I'll either use Chai if he does these, or Zapp/Gary for convenience, if I can work out which bearing blocks I should go with and whether the whole assembly will fit in the same space?
    .
    Question - I take it the profile rail/bearings are an order of magnitude "stiffer"?
    Yes 20mm and the wide type for Y axis if they'll fit but you'll probably use slim type for Z axis to allow room.

    Chai does sell these.

    Yes profiled linear guides and carriages( Guides not rails, carriages not bearings To please Jonathan.! . Lol) are in a differant league to round type bearings.

    Do it sooner rather than later you won't regret it. Oh don't be surprised when you get them if they feel stiff or even slightly notchy as that is normal and just the preload on bearings.!

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