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Thread: Double Sided Coin Project - XYZ Zeroing

  1. #1
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    Double Sided Coin Project - XYZ Zeroing

    After terribly threadjacking Bills thread on doing the Z-Zero, I thought I would start one to document my experiments with making 2 sided coins. I figure this is a good use of time, as it could help me with making 2 sided circuit boards.

    Some of this will be redundant from the other threads I've done on the coins and my threadjacking.

    The key in doing this I think is that you need to have a reliable and repeatable XY starting point, especially if you want to make more than one item.

    So here's an example what I'm working on.
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    So, After a number of attempts I started thinking through the process and what I could do to ensure that I could get the front and back images placed correctly.

    Mistakes I had been making were:
    1) Just using screws to hold the material to the spoil board. It did not provide enough support for working the thin metal. I would get chatter and bad cuts.
    2) I had been trying to register the position of the workpiece by using the lower left corner. Since I was not ensuring the workpiece was 'in line' with either the x or y dimension, well, This was problematic, not to mention I was not getting what I would call an accurate xy-0,0 with my eyeball.
    Also, it's not super easy to get an xy zero by eyeball, at least that is my experience.

    Around this time I started using a touchplate to establish the Z-0. I quite liked it and it was giving me a much more accurate position on the z axis.

    So I decided I needed to do 2 things
    1) Come up with a better way to hold the workpiece, and have that method let me be able to do multiples, if I so desired.
    2) Come up with a better way to establish the XYZ-000.

    I had also found some information on being able to probe for the x and y zeros using the same command that is used for zeroing the Z. Bill has done a great job of documenting that in his post, so I'll not go over the commands here.

    So I designed a jig that would hold the workpiece in place and provide a a way to hold an x/y touch plate a known, fixed distances from where the project origin should be.

    The jig consists of a 'pocket exactly the same height of the aluminum bar stock (1/8" thick, so this I made the pocket 3/16" deep). I also included channels along the edges to be able to make sure any swarf would not interfere with the placement of the stock.

    I made the touch plate pocket a square, because I originally thought I would use a piece of 1" square al tube, but I had some al angle stock laying around I used instead.

    Click image for larger version. 

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    And here's what it looks like IRL...

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    Now I could just run the commands by hand to do the zeroing, but http://chilipeppr.com/ is in my opinion a really nice little environment for running gcode. It's got a 3d viewer, command window, jogging controls, and all kinds of widgets to do all kinds of things. They have a version for grbl, a version for tinyG, and a few others I don't even know what they do. It's also open source, based off of javascript and they make it pretty darn easy to extend. You can literally create your own version of the interface by forking it, and then fork any of the components in it and make whatever changes you want if you have some kind of need or idea that isn't already implemented.

    So that's what I did, I forked their 'Touch Plate' widget which lets you zero out your z axis using a touch plate. I added the ability for it to work in either MM or Inches and and the ability to zero the X, Y, and Z dimensions.

    Here's a screen shot of the probe XYZ widget I put together on top of the 3d representation of the gcode to cut my spoil board. You can see that the zero point is where the stylus is sitting on the workpiece.

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    So, My thought process is
    • I have a nice jig to hold the workpiece solidly.
    • I have a place to put an x,y touchplate a known distance from the 0,0 point.
    • When I placed the spoil board on the CNC, I used a piece of MDF I cut to have parallel sides. I can take this coin board on and off and as long as I use that board to ensure the spoil board is parallel to the X axis, I should be good


    So Here's my latest results and I'm pretty close. This is in reverse order of the cut. The front, and the profile cut to cut it out are done last. You can see it all looks 'ok'. I've got some issues I'm working on with quality of the cut, but for this, I'm concerned with placement.

    The back side of the coin (which was cut first) is off vertically by about 1mm.

    The X dimension though, looks pretty good though, so I'm encouraged.

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    I believe that difference is due to two factors.


    1. The offset factor I enter into the probexyz widget
    2. My machine could use some calibration. I had some issues with the control board and just got everything up and running last week, and I have done a little dis and re assembly. I had only ever done a 'rough' calibration on it to start with any way.


    So, That's my story and I'm sticking with it.

    What do you all think?
    Programmer - An organism that turns coffee into software.
    If all your friends are exactly like you, What an un-interesting life it must be.
    "A door is what a dog is perpetually on the wrong side of" Ogden Nash


  2. #2
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    looks to me you're on about the same track I would be, no ideas other than keep refining the approach

  3. #3
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    The joys of fixturing!

    Well documented, brent. Thank you!

    There are two sides to every coin -- ha ha ha -- there's the machine zero point and there's the part flipping part.

    It may or may not be useful, but one of the things I tend to do with just about all my 2-sided machining is to build it in a way that the project is self-aligning. What I mean by that is having some vectors in your model that the machine cuts as part of the first process (or as a separate first process altogether). This goes a long way toward placing the parts properly when you flip them.

    For example -- I'll draw in a couple of holes out in waste-land on a part - in your case, I'd pick two corners of the square stock. I would lock those vectors so they're always located such that the part is aligned about the same center point.

    I take those vectors and generate a toolpath to cut those holes and usually a pair of 2nd holes (more on that in a sec) into my fixture and press pins into those holes - these will be what the part will locate on for the 2nd op. This is also where i'll save the X and Y zero into a G5x work offset (since I always home my machine with the limit switches, this makes it super easy to return to EXACTLY the same spot every time). My fixtures are also done with the 2nd set of locating pin holes into my table surface to guarantee I place them back where they go every time. I have a few "fixturing holes" already machined into my table surface so I can reuse them for many fixtures - with a vector drawing of all of them. When I make a new fixture, i start with that file and locate the fixture geometry such that it aligns with some of those holes. This is the reason my table surface isn't my spoil board - it's more critical than a spoil board. But my spoil boards i mount to the machine use those fixturing holes, too.

    After the 2nd op fixture is made, I'd cut a 2nd pocket somewhere that allows me to hold the work for the first op. The first op is to cut the locating holes and whatever is needed for side one. You don't need to really care where you're starting as long as it fits within the rough stock because the locating holes will be cut by the machine and therefor consistent with the ones in the fixture. This way the part is self-centering - and you don't have to worry about probing or any trouble with the workpiece being accurately sized or any of that. The xy zero are saved once and your practices ensure the fixture is always in the same place.

    This is just how i developed a workflow in the absence of probes -- i still want probes - especially for Z zeroing on the part - that'd be handy.
    Jason Beam
    Sacramento, CA

  4. #4
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    My first attempt on these was to use locating pins and holes on the fixture with holes drilled by the CNC on all 4 corners.

    Maybe I need to revisit that method. The problem I was having with that was keeping the aluminum held firmly enough to prevent chatter.

    Hmmm, 4 pins, 4 holes.

    I did check the calibration on the machine last night and I am off. I've got a good idea on how to check and correct that, so will work on that later today.
    Programmer - An organism that turns coffee into software.
    If all your friends are exactly like you, What an un-interesting life it must be.
    "A door is what a dog is perpetually on the wrong side of" Ogden Nash


  5. #5
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    Looking at your finished photos, there's a difference in the thickness of the material remaining on the edges. That makes it appear that the center point of your material isn't calibrated correctly.

    I've had a couple of projects on which the first cuts I made were drilling alignment holes that went through the material and into the backer board (not the waste board). Since those holes were on the centerline in my drawing, the were dead center on my carve. In my case, I had the holes drilled on the vertical centerline of my project, so I flipped the material side-to-side after doing the back. The front was then properly aligned since the pins were located in the same place referenced to both sides.
    Bill Arnold
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  6. #6
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    Quote Originally Posted by Brent Dowell View Post
    Maybe I need to revisit that method. The problem I was having with that was keeping the aluminum held firmly enough to prevent chatter.
    Bolt it down. I can think of several ways, but the one that seems most likely to minimize sideways torque would be put screw in inserts on either side of the piece between the coin spacing and then manufacture pressure bars to hold the stock down and in place. If you face the pressure bars with something like 70-80 durometer neoprene maybe 1/32 or so thick (http://www.amazon.com/dp/B00CCGWN2S/ref=biss_dp_sa2) it should be dense enough to minimize flex and still allow it to "stick" in place pretty well. if you make the pressure bars out of steel you can use a pretty narrow piece vertically and it should be pretty rigid.

  7. #7
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    Quote Originally Posted by Brent Dowell View Post
    My first attempt on these was to use locating pins and holes on the fixture with holes drilled by the CNC on all 4 corners.

    Maybe I need to revisit that method. The problem I was having with that was keeping the aluminum held firmly enough to prevent chatter.

    Hmmm, 4 pins, 4 holes.

    I did check the calibration on the machine last night and I am off. I've got a good idea on how to check and correct that, so will work on that later today.

    this is where having a metal lathe is super handy ... I would turn pins with threaded ends - make the pins protrude about .030" below the surface of the material and screw the part down.

    Fixturing is as much an art as it is a science. I have to balance the fixturing effort with the project all the time. If you're gonna cut a bunch, it makes sense to make up a good fixture with good workholding. I would avoid more than 2 pins if you can help it - in most cases, more pins don't really improve results and can make things slower to produce.

    If i were doing your coins, I'd have my blanks cut to a consistent size, slip them into a pocket on the fixture and run operation 1 -- this would be all the toolpaths that create the first face and cuts the two alignment pin holes (diagonal corners in the blank, just outside the diameter of the coin's profile cut). Then I'd free the part from the fixture's pocket (screws, ds tape, what have you), flip the part over and drop the blank onto the pins to align them and then screw the part down to the fixture. Change your fixture offset and go to town on the 2nd side. The only variable is that fixture offset so you can tweak one setting to get the centering perfect if you need to (though usually the toolpath that cuts the fixture's pin holes is the one that establishes the offset zero points)
    Jason Beam
    Sacramento, CA

  8. #8
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    The big thing I think right now is that I just need to calibrate the machine correctly. It's definitely off. On the one hand, that makes for a very tight fit for the bar in the slot right now.

    I think we are pretty much on the same page.

    Assuming I get the machine calibrated, I have faith that this fixture should work pretty well.

    It's quick and easy to make, nearly disposable.
    It holds the workpiece very well, and I could run multiples at the same time, if I want to.
    With the calibration points set on the fixture at the same time the holding channel is cut, I'm assured of a repeatable zero between runs.

    So I think I'm going to head down this road a little farther.
    Programmer - An organism that turns coffee into software.
    If all your friends are exactly like you, What an un-interesting life it must be.
    "A door is what a dog is perpetually on the wrong side of" Ogden Nash


  9. #9
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    Well, This test on some thin ply seems to indicate a fairly well centered xyz 000.

    Now to go put it to work on a piece of aluminum.

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    Programmer - An organism that turns coffee into software.
    If all your friends are exactly like you, What an un-interesting life it must be.
    "A door is what a dog is perpetually on the wrong side of" Ogden Nash


  10. #10
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    Ok, I think I have at least the fixture and zeroing working to a decent degree.

    Need to work on the quality of the design and engraving.

    It's always something.

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    Programmer - An organism that turns coffee into software.
    If all your friends are exactly like you, What an un-interesting life it must be.
    "A door is what a dog is perpetually on the wrong side of" Ogden Nash


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