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IndustryArena Forum > MetalWorking Machines > Fadal > Axis movements are jerky
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  1. #1
    Join Date
    May 2012
    Posts
    7

    Axis movements are jerky

    I was machining some oval shaped pieces and noticed the axis movements are jerky and create little straight lines, like the machine is stopping for a moment at each direction change. The stepping movement matches the monitor display when each line of code changes. This is a Fadal 4020 running off a shop link USB drive. I changed baud rate to 38400 with no noticed affect. Now that I am paying attention the problem is with all direction changes not just these ovals. Is there something in my programing causing this affect? Also is there a way to start in the middle of a program on the shop link USB drive? Thanks for any input... Terrible Terry

  2. #2
    Join Date
    Jan 2009
    Posts
    39

    Re: Axis movements are jerky

    Two places I'd look are:
    1) make sure you don't have "exact stop" active, or the machine will accelerate and decelerate for every line segment, and
    2) see if you're using lines to approximate curves instead of arcs to define curves.

    I'm not familiar with shop-link and haven't ever used your Fadal machine, so I don't know if they permit using arcs, but most CNCs do.

    Lloyd

  3. #3
    Join Date
    Oct 2009
    Posts
    26

    Re: Axis movements are jerky

    If you are drip feeding off of the shoplink and using high feedrates, the control has a hard time keeping up with the axis movements. I believe "exact stop" is the default setting in most post processors for the fadal.

  4. #4

    Re: Axis movements are jerky

    My machines do that if I don't add G8 (no feed ramps)

  5. #5
    Join Date
    Jan 2015
    Posts
    417

    Re: Axis movements are jerky

    G8

  6. #6
    Join Date
    Jan 2004
    Posts
    3154

    Re: Axis movements are jerky

    I bet it is your code, not the machine.
    Convert your code to circles (G02-G03) instead of line movements (G1)
    www.integratedmechanical.ca

  7. #7
    Join Date
    Dec 2008
    Posts
    2
    Quote Originally Posted by DareBee View Post
    I bet it is your code, not the machine.
    Convert your code to circles (G02-G03) instead of line movements (G1)
    My Fadal is jerky in format 1 when I use fanuc code from Mastercam I have to switch to format 2

  8. #8
    Join Date
    Dec 2008
    Posts
    2

    Re: Axis movements are jerky

    Try format 2 if not useing Fadal programming

  9. #9
    Join Date
    Jun 2012
    Posts
    516

    Re: Axis movements are jerky

    Quote Originally Posted by Terrible Terry View Post
    I was machining some oval shaped pieces and noticed the axis movements are jerky and create little straight lines, like the machine is stopping for a moment at each direction change. The stepping movement matches the monitor display when each line of code changes. This is a Fadal 4020 running off a shop link USB drive. I changed baud rate to 38400 with no noticed affect. Now that I am paying attention the problem is with all direction changes not just these ovals. Is there something in my programing causing this affect? Also is there a way to start in the middle of a program on the shop link USB drive? Thanks for any input... Terrible Terry
    its for sure your post processor. maybe it is outputting every 0.003" in a G1 moves and you'll get plenty of digs and flats. this kind of machining is OK for roughing but no good for finishing. The other guys mentioned G8 feed ramps. Also you need to look into the gain setting on your machine. You can do that in the SETP menu. high gain for high feed, low gain for low feed is how it would be ideally. set it all back to default by reading the fadal manuals. RTFM on this one.. will help you a lot.

    in my post I have it output gain changes prior to a tool path using M-codes where it will set the gain to high if the feedrate is bigger than 70 ipm. then it sets it back to normal if the feed is lower on the next tool path. I'd say this is just optional and it won't affect the jerky axis

    about drip feed. the 'buffer' is displayed and you will know if you're running too fast if the buffer emptys out. usually it will just read 256 the whole time. when the buffer gets down to 2 you will be jerking really bad. this can happen when you run like 80+ ipm for a thousand lines or so of G1 moves. that can happen in some of these newer roughing routines like volumill, adaptive roughing, imachining, etc. it can also happen (more rare) on 3D machining. on a fadal I'd stick to 70 ipm or lower to avoid gouging on 3D paths.

    fadal also did an option called Advanced Feed Forward. this is a way to change the gain, and also the position tolerance. in other words you can rough at 100ipm, clip the corners by 12 thou (just make sure your wall offset is like 15thou or more). then you can set it up dynamically for finishing where the gain is set appropriately and the TOL could be like 0.001" to where the machine will not proceed unless it hits within the TOL from one block of code to the next.

    its a great feature in concept, in reality it is unclear to me if it helps a lot. I have that on my 4020 and use it for these roughing paths with only G1 moves. it runs smoother than trying to use arc moves for those new roughing type paths.

    for finishing i run pretty slow feeds (70 or less) and I use arcs




    at any rate, seems like you have some post writing in your future. best of luck.

    also someone said format 2. you better get on that also.

  10. #10
    Join Date
    Feb 2008
    Posts
    86

    Re: Axis movements are jerky

    On a DC Fadal resolvers with bad bearings or some other issue I still can't comprehend can cause jerky motion. I've had loose wires to the tach also create such a scenario and its always a good idea to check and likely replace the brushing in DC motors and blow out all the carbon that's been accumulating for the last zillion years. All provided this isn't a software speed issue of course.

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