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Thread: problems with arcs

  1. #1
    Join Date
    Jan 2018
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    Los Angeles
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    Question problems with arcs

    Hi All,

    I have a PRS Standard 5x12.

    Things run great X,Y,Z in a straight line. I've made some intricate cassette tape cabinets and the math on those all checked out when I was done.

    Today I went to make some large arcs connected with flat ends and the two identical shapes came out different.

    The shapes were drawn in Rhino and imported into Aspire 9. I drew a bounding box 48x96 so i could stay within the plywood area. I carefully rotated the shapes in Rhino so they were nested. Once I was done I exported the geometry as a DXF and opened it in Aspire where I made the tool paths.

    So even though the two shapes are copies of each other, they come out not matching. There are flat ends connecting the arcs and they aren't the same length, even the curves of the arcs are slightly off when you put the pieces face to face. The pieces are about 90" long.

    What is also strange is that the pieces are otherwise flawless. I used a downcut .375 roughing bit and only took .120" passes at 40ipm. There doesn't seem to be any play in the gantry and all the passes seem to be lining up with each other, so my thinking is that it's not slipping somewhere if it keeps making the same (wrong) shape over and over.

    My inclination is that it's software, but I'm wondering if anyone out there has any ideas.

    Also, is there a verifiable way to do the Circle test? Like, after you run a circle ( say 6") and let's say it's off.. then what? Asking for future me. (I'll test tomorrow morning)

    thanks in advance to everyone who has any ideas.

    all my best,
    JAMES

  2. #2
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    Don't know anything about Rhino but Aspire 9 is capable of drawing the shapes you describe. Could be something not reading right between the 2.

  3. #3
    Join Date
    May 2014
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    MA
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    Have you checked to make sure that your machine is square? Your gantry could be racked. I had that issue crop up when I was cutting a lot of parts that were mirror images of one another. When I glued them together they didn't line up. This is my method of checking for, and adjusting square:

    https://www.youtube.com/watch?v=s2z9tTbTdcY

  4. #4
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    ERIC!!!!!
    YES!! I found your youtube video yesterday while the shopbot forum was down and I was done smashing my face into the wall in frustration.

    Thanks for making that video, it truly saved me!

    I also drew some 30-40-50 triangles with an engraving bit before and then after your pin test just to make sure things were back to normal.

    Also the idea of using the stops as a way to have a 'reset' on the registration will be super helpful the next time my gantry gets bonked out of alignment.

    thanks again!

  5. #5
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    Jan 2018
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    Los Angeles
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    hi dmidkiff,
    Aspire is capable, but there was a ton of parts already drawn by someone else in Rhino. Rhino itself tends to be much more robust (for me at least) in terms of drawing a lot of corresponding and interconnecting parts. Being able to choose what part of the curves to snap makes it so much easier than Aspire. I tend to just use Aspire on the PC to draw all of the tool paths and output to the bot. If I could get / afford the RhinoCAM plug-in for Mac ( not sure if they even make it) i would do everything within Rhino because it's just so robust.

    I thought my problem might be a software issue, but it turns out the machine probably got bonked and I didn't notice until I tried to make big pieces that would fit together. Eric, below has a great video on how to make sure your system is aligned and happy.

  6. #6
    Join Date
    May 2014
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    Glad I could help! Remember that your gantry gets out of alignment every time you power off your machine. What you're supposed to do is:

    Figure out how much you're out of square. I do the pin-at-the-corners method because it's the absolute biggest thing you can get to square. If you can get that giant rectangle square, even if you're off a tiny amount, that error shrinks the smaller you cut. IE, most things you cut are likely going to be smaller than the table..

    Now, for what you need to do for this to not happen again is:

    Power the machine down, pull it back against the stop blocks so that both sides of the gantry are tight against the blocks. While holding it there power it up. Check for square. Then, move the blocks where you think you need them. Power off. Pull the machine against the blocks, power up. Check square again. Keep doing this until you get it square.

    Once you get it square EVERY TIME you power it up, hold it right against the stop blocks before you power it up, this way the gantry is squared the same way every time you power up so you get consistent results. Make sense?

    If you don't pull it tight against the blocks every time you power it off, the gantry could "land" in a different, random out of square spot every time you power off. Try clamping down one side of your gantry and pushing on the other side with your hand lightly, you'll see what I mean.

    If you want to get really nerdy you can use this sheet I made:

    https://docs.google.com/spreadsheets...it?usp=sharing

    Make a copy for yourself, and if you fill in the info correctly it can tell you exactly how much you need to move your stop blocks. Draw a diagram on paper so you don't move the wrong side.

    I have an autosquare mechanism on my machine and this is what I use to get the correct numbers.

  7. #7
    Join Date
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    +1 on this. Everyone with a gantry tool should be doing this every time they power up their tools!

    Quote Originally Posted by EricSchimel View Post
    Glad I could help!
    Remember that your gantry gets out of alignment every time you power off your machine. What you're supposed to do is:

    Figure out how much you're out of square. I do the pin-at-the-corners method because it's the absolute biggest thing you can get to square. If you can get that giant rectangle square, even if you're off a tiny amount, that error shrinks the smaller you cut. IE, most things you cut are likely going to be smaller than the table..

    Now, for what you need to do for this to not happen again is:

    Power the machine down, pull it back against the stop blocks so that both sides of the gantry are tight against the blocks. While holding it there power it up. Check for square. Then, move the blocks where you think you need them. Power off. Pull the machine against the blocks, power up. Check square again. Keep doing this until you get it square.

    Once you get it square EVERY TIME you power it up, hold it right against the stop blocks before you power it up, this way the gantry is squared the same way every time you power up so you get consistent results. Make sense?

    If you don't pull it tight against the blocks every time you power it off, the gantry could "land" in a different, random out of square spot every time you power off. Try clamping down one side of your gantry and pushing on the other side with your hand lightly, you'll see what I mean.

    If you want to get really nerdy you can use this sheet I made:

    https://docs.google.com/spreadsheets...it?usp=sharing

    Make a copy for yourself, and if you fill in the info correctly it can tell you exactly how much you need to move your stop blocks. Draw a diagram on paper so you don't move the wrong side.

    I have an autosquare mechanism on my machine and this is what I use to get the correct numbers.

  8. #8
    Join Date
    Jan 2018
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    EXACTLY.

    I'll make sure to do this as part of my routine from now on.

    also, Eric is awesome.

    -j


    Quote Originally Posted by bill.young View Post
    +1 on this. Everyone with a gantry tool should be doing this every time they power up their tools!

  9. #9
    Join Date
    May 2014
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    MA
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    Thanks! Just glad I could help.

    There's another procedure for doing this, one I cribbed from that's actually in ShopBot 3... If you go to "Help" and the to "Squaring Your X Car" (or something like that) it'll bring you to a PDF that shows how to this. I think my procedure is a little easier, but that's just another source of information if you want it.

    They go into loosening the gantry bolts and such, which I think is a little extreme. My machine was not that much out of square, I think only like .04" so holding against the stop blocks totally fixes the problem. Unless your machine is totally whacked out of square I think what I've outlined is plenty.

  10. #10
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    Hey guys, just the other day I had to cut a few sheets of 4x8 down to 95x47 for a customer. Aside from the regular calibration I do, I like to check parts from time to time just to make sure they're square. I cut these panels in 3 passes, the last pass being a very light one in reverse.

    I pulled a tape on them and they were perfectly square. On a 4x8 machine a 95x47 part is just about the biggest thing you can make in one shot, and if you're out of square at all, that's where you're going to see it.

    This is why I square using the very far corners of my machine as you saw in the video a few posts up.

    I also wanted to share this:

    https://youtu.be/RGEmztTTnIE

    This is my autosquare modification I did to my ShopBot. Basically, using an extra proximity sensor it racks the gantry back into square every time I power up my machine. Gary Campbell gave me the idea for this. It's been super reliable and allows me to dial in really finite adjustments to get my machine calibrated exactly where I want it.

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