# Strange Behavior During Cut

And I should mention this, in case anyone was curious as to what is on top of my top beam, holding the nail that holds the chain, it just got 2 6" pieces of 2" x 2" and predrilled 3 holes in it, then used some 2 1/2" deck screws to attach it to my top beam. This allowed me to get the chain high enough out of the way, so my rollers could be moved to where the nail used to be, closer to the top of the beam. Now I don’t have to worry about the chains rubbing when it’s in the far corners of the workspace. Any questions, feel free to ask.

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I hear you. However, if you start on top of a known block (how exactly we get “on top” of it when the sled is level to the work piece is TBD I realize) and we know the size of the block, we can reliably find it’s edges. Once we have that and we transit across the board to the other corner in a straight line, we can reasonably continue until we touch the block, couldn’t we?

That is to say, let’s say we have touch plates in all four corners, as proposed. And we start in contact with the upper left touch plate (you lower the bit down from above to get your Z and then, because the block size is known, you can fairly accurately move out past the maximum edge distance (from the Z test location) and then move back in to touch and confirm edge locations.

Any reason why we couldn’t slowly progress in what we believe is a straight horizontal line near the top edge until we bump the inside of the upper right touch block (whose size is also known)? Then rinse and repeat heading down vertically and then back across horizontally and then back up vertically? Have there been calibration situations where an expected horizontal lines travels materially diagonally across the workpiece over 8’?

If we could do this, it would allow collecting data points at each known dimension block and we would be able to impute the arrival landing position after we grab our X,Y with the sensor on arrival (and therefore assess the accuracy of the real path traveled relative to expected path traveled between the previous and current block)?

Pardon the low res video quality, but I’m thinking something like this video from the link I included above about my XYZ block for the Shopbot. The Youtube video from the originator is here: https://www.youtube.com/watch?v=f4mFNMzBV2k

It just strikes me that as long as the user has accurate dimensions for their wasteboard or workpiece, it would be great if we could have the system measure the movement through sensors and do that portion of the calibration itself.

It also occurs to me that having basic sensor blocks in the design may be able to help in other areas of initial setup and calibration when deployed in alternate locations or orientations. E.g. the block could be sized so combining more than one yields ideal spacer sizes for various things during frame construction, and likewise deployed for initial chain or other setup. I don’t have all the answers, just pulling a thread to see where it might go…

-Jeff

Right, this is where CNC milled aluminum calibration blocks can come in. It seems like the missing piece. The blocks would ensure a known reference piece in the end users hands that could be built on programatically to collect the data points and interpolate.

-Jeff

Out of curiosity, how long did it take you to run through the calibration after your changes and now that you knew what to expect?

-Jeff

If I had to guess… Actual time working on the calibration… 45 minutes give or take. To me the longest part of the calibration is the chains. Not because it’s difficult, but because they feed out so dang slow haha But, I’ve learned, when you’re at the Extend Chain portion, for my machine, I change the value to 3030mm and attach a hammer with a piece of coat hanger to the end of the chain that hooks to the right motor, so I don’t have to babysit it for 3030mm of slow moving action.

no, because you do not know how to move in a straight line. If the other block
is symmetrical, then you have a pretty good shot at finding it, but if it’s
somewhere in between, you may be moving on a curve.

if the chains match, and the top beam is truely horizontal, the movement will be
completely symmetrical and you will get no value from checking both left and
right sides of the machine. If the machine isn’t symmetrical, then you don’t
know how to find the other block because what you think is a horizontal movement
isn’t.

David Lang

the problem is in positioning these blocks.

If you have a block that is 6’ wide, that’s one thing. but if you have two
blocks that are 1" square, how do you position them exactly 6’ apart? (with
sub-mm accuracy)

David Lang

David, thanks for the response. I’m not quite sure I’m parsing your response correctly. When you say “if the other block is symmetrical”, do you mean the other calibration block or it’s location relative to the first block or both? I was envisioning an identical known dimension block just oriented to index to the opposing corner.

I was trying to get a sense of what the range of non-symmetrical error might be, as that would seem to constrain the ideal size for the calibration blocks. Obviously it could be enormous but practically speaking I’m guessing it’s within a few inches in practice given the nature of the design? And is likely much smaller for a square and well assembled example of the frame?

By checking both left and right sides you would get known absolute positions being detectable using the blocks, and creating that dataset for calibration, wouldn’t you? And if you land somewhere other than expected, you have some offset information to help explain where you thought you would land and where you actually do [that is assuming you land on the block but just not where expected]. Maybe I’m not understanding the optical calibration discussion but it seems to me like we’re in the same church, maybe I’m just sitting in my own pew.

-Jeff

You design them as right angles of known dimensions and you index them to the edges of the workpiece used for calibration. Ideally a piece factory cut square, or squarely on the home goods panel saw (if available) or squared at home if necessary. So with CNC milled squares in the corners of a square sheet, we should be able to set edge positions at least. Center positions are another matter, of course. But how far can we get with multiple data sets with great accuracy from each corner using this method?

-Jeff

[Edit: I don’t know if Ron Olson had a particular reason for using this dimension, but the block I had milled is 3.5" x 3.5" for use on the 4x8 table CNC, for example]

@dlang There is a picture of the block from Fusion 360 in this thread that might help explain it better than I have: CNC Milling a ShopBot XYZ Finder via Xometry Service - Review

David, thanks for the response. I’m not quite sure I’m parsing your response
correctly. When you say “if the other block is symmetrical”, do you mean the
other calibration block or it’s location relative to the first block or both?
I was envisioning an identical known dimension block just oriented to index to
the opposing corner.

I was meaning location

I was trying to get a sense of what the range of non-symmetrical error might
be, as that would seem to constrain the ideal size for the calibration blocks.
Obviously it could be enormous but practically speaking I’m guessing it’s
within a few inches in practice given the nature of the design? And is likely
much smaller for a square and well assembled example of the frame?

By checking both left and right sides you would get known absolute positions
being detectable using the blocks

1. only absolute to the accuracy of the location of the blocks
2. finding the corners is not good enough, you need to find many locations
around the workpiece to map out what the curves are, and since the size and
direction of the curve depend on your particular machine, the system cannot know
how to move and find the blocks.

now, you could manually move it to find the next block and have it measure where
it thinks it is. But how accurately can you position these blocks?

it doesn’t matter if you identify locations by a camera or by detecting a touch
(if you look for posts by me, I describe a way to do this pretty early on)

The issue is having enough points to measure, and knowing where those points are
accurately enough.

your idea of a block at each corner isn’t enough points and they aren’t
positioned accurately enough (because the dimensions of the sheet are not that
accurate)

David Lang

This works for the shopbot because it has very precise mechanics that allow you
to move in the X direction independently from the Y direction, so you don’t need
to figure out how to move in a straight line, you only need to detect where to
start.

since X and Y are reliable, you can even put blocks on each corner and detect
that you don’t have the board straight and have software compensate for it.

but if your movement down the X direction isn’t a straight line, but is instead
a curve, you need to map that curve, and since that curve may miss a block that
you put beteen it, this isn’t something that can be automated

You design them as right angles of known dimensions and you index them to the
edges of the workpiece used for calibration. Ideally a piece factory cut
square, or squarely on the home goods panel saw (if available) or squared at
home if necessary.

first off, factory cut parts are not known dimensions.

cuts done at the home center or by you may or may not actually be square and are
even less likely to be of known dimensions at this distance.

So with CNC milled squares in the corners of a square
sheet, we should be able to set edge positions at least.

yes you could, but not without you guiding the machine to find the blocks. The
system doesn’t know how to find them until it’s calibrated.

Center positions are
another matter, of course. But how far can we get with multiple data sets
with great accuracy from each corner using this method?

not very far.

David Lang
k

Gotcha. Out of curiosity, will a four motor maslow get closer to a straight line in this initial calibration area or is issue similar?

-Jeff

True, but you can measure the material you purchase which is no better or worse than the current measuring method. And while materials can be any size, I have found, quite often, that they are actually dead accurate when I check them in the big box store. No guarantee, I know, but it’s not quite the problem it sometimes is made out to be.

Ok so what if instead of sending it off from block A to block B, we move it by hand (via repositioning through the software) over the second block and have it calibrate against that new block, but at the new corner? And repeat for all four? so now we know where they are in the coordinates of the work area, and depending on how we execute the relocation, assuming the human makes an attempt to get it there as straight horizontally as possible (block A to block B) then we can also learn something about the directional compensation the operator has to do to reach block B, which would help us understand sage in that direction?

-Jeff

probably not.

David Lang

True, but you can measure the material you purchase which is no better or
worse than the current measuring method. And while materials can be any size,
I have found, quite often, that they are actually dead accurate when I check
them in the big box store. No guarantee, I know, but it’s not quite the
problem it sometimes is made out to be.

when you are doing press fits of slots and tabs, you need to measure the
thickness of each board, because they vary enough to not fit tightly otherwise.
I just spent over \$3k on plywood sheets and they varied enough to affect the fit
in all dimensions.

Ok so what if instead of sending it off from block A to block B, we move it by
hand (via repositioning through the software) over the second block and have
it calibrate against that new block, but at the new corner? And repeat for
all four? so now we know where they are in the coordinates of the work area,
and depending on how we execute the relocation, assuming the human makes an
attempt to get it there as straight horizontally as possible (block A to block
B) then we can also learn something about the directional compensation the
operator has to do to reach block B, which would help us understand sage in
that direction?

the user is going to tell the machine to move X and then correct in Y. Also, the
user isn’t going to care if the actual movement of the router is a straight line
of a curve.

David Lang

I was unlucky to build my frame under that assumption only to find out the 3/4-inch plywood I used for the backboard was 96&3/8-inch wide.

I might be wrong, but if you use the directional buttons to get to the top left block, the machine probably would be able to find the top right block on its own because of the symmetry. It will always take a curved path to get there (the amount of curve being minimized if its well calibrated). Finding the bottom block right block from the top right block, however, likely won’t happen because its not symmetrical in that direction. The user would have to use directional buttons to get it there.

I know cost is always a consideration, and touch sensors are another way to go, but with regards to a more traditional setup block, my first article samples from my hobby project to test out on-demand manufacturing just arrived and they look pretty good. I posted a photo over here: CNC Milling a ShopBot XYZ Finder via Xometry Service - Review and will update with more details in the next day or so.

I think some CNC milled calibration blocks or tools might be closer to being viable than I certainly would have thought a few months ago. I know having a known perfect reference of some type is a challenge we’ve kicked around lots…

-Jeff