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Cut of the final sled is going awry

As the final step of the setup, I need to cut the final sled.
The sled is being cut as an oval instead of a circle as displayed in the webcontrol console.
While cutting the top of the sled, the temp sled went off the board during the first pass and came back on its own

however, on the 2nd pass, it went of the board at a different point and could not come back down onto the board.

Also as can be seen all the holse seem to be ovals instead of circles :

So a couple of questions

  1. is there a way to correlate the image in WebControl UI to the Line number so the next time I start I can ask it to start at a different point ?
  2. Is there more callibration that needs to be done to get it to cut circles instead of ovals ? (something I might have missed?)

attached the gcode for the final sled (courtesy @EastBaySource ) (74.6 KB)

help is appreciated as usual



a Follow up Question:
When the CNC was doing its thing, the slack on the chain and the bngee cord was quite a bit. How much slack should I be expected to see ? I wonder if this has something to do with my problem

that third picture looks like chain wrap around the motor sprocket that can be from excessive slack catching around the motor. your basic measurements for setup should be accurate enough to get nearly circles.

The measurements I had to take :

  1. Distance between left and right motors
  2. Distance from Motor to the cutting surface

I have a 10ft top beam. I am curious if the top bean has to be 12ft (some of the other posts mention that this could perhaps alleviate some of the issues (based on these instructions from @EastBaySource : Assembly Gude )

Other than those I just retained the defaults

@EastBaySource : Per diagrams and instructions (on the Assembly and Calibration step), the bungee cord is not under as much tension as show in the picture. Should the top bean be switched to a 12 ft bean instead of a 10 ft beam ?

the problems that a 12’ top beam solve are much more sublte and generally only
happen in the bottom corners. So while it’s a worthwhile thing to look at,
that’s not causing your big problem.

David Lang

Since your Arduino got damaged during installation and you’re experiencing random issues i would suggest to get a new Arduino - they are fairly inexpensive.

The ovoid shape is most likely related to calibration - check your distance between motors, it should be around 3007mm or 120in

The slack can be fixed by shortening the bungee cord, or you can go the counterweight route if you like. Just put a nail in the middle of the top beam (front face) and route the cord on top of it, then down and behind the spoil board, do the same for both sides - as other seller have suggested, you can use soda bottles as weights.

About the wider top beam, it’s true a wider top beam will help the chains to have more sideways pull force on the sled while at the bottom corners, but it will also put a lot more stress on the motors while cutting high up on the board, therefore if you go wide, you also need to go higher to avoid this - Unfortunately in doing so you will be introducing more sag and whipping effect into the system which will play against accuracy (especially on the bottom corners).

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just shorten the bungee

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a few clarifications and an intentional oversight :

  1. The top beam in my setup is 12 ft wide and not 10 ft as I originally thought /mentioned

  2. The distance betwene the outer edges of the motor is 119.5", the distance from center of gear to top of cuting board/surface : 17.5" (close to within a few mm)
    Current configuration is :

  3. Ring configuration with rotational radius of : 138.4 mm

  4. Chain configuration of the top to the sled (I am not sure this is right )

  5. Distance between motors : 2982.4 mm (this may need to be corrected to 2994.9 mm)

  6. Height of motors above workspace : 444.5 mm

All of the movements from WebControl to the cutting board seem to be mirrored (left → right ) Switching the left and right motors did not really do anything So I am thinking that the Chain configuration is wrong). But with the Chain configuration set as bottom instead of Top, caused the motor gears to move clockwise instead of counter clockwise. With the Chain configuration as it is, (top), gears move in the counter clock wise direction.

So with all of the images and clarifications in place, can someone please tell me what might be wrong ?



Facing the frame and cutting board looks like so :

Facing the frame, the left is :

and a slighgtly closer look :

Facing the frame the right is :

and a slightly closer look :

a “selfie” of the aurduino board (facing the frame and mounted to the back of the top beam )

Per @Orob 's recommendation, I have “shortened” the bungee as well.

This is pretty close to 10 feet not 12’ - (10’ = 120in)

It’s wrong - should be “bottom” feed - this might explain why your cutting directions are reversed.

Should be closer to 3007mm - (your picture shows a 10tf beam)

Irrelevant - it will move the center up/down a bit - will not affect calibration

Edit - in your 4th & 6th pictures it looks your left/right motor cables are reversed as well

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@2cents Thanks fo rthe response. the Holiday has addled my math skills :slight_smile: Yes It is indeed 10 ft. and not 12 ft. the number 2994.9 was the distance between the out edges of the motors (119.5 " = 3035.3 mm) - 40.4 mm

I will reverse the motor cables as well as the Top to bottom. Hopefully that will fix the mirror image issue I am having.



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the measurement is between the center shafts of the gears (and since this is
hard to measure, between the left edge of the gear on the left side and the left
edge of the gear on the right side)

What software are you using? it sounds like you are using the old calibration
method, which is known to be weak.

David Lang

I am using web control. And following docentation posted fromy earlier posts for help. If the way I am doing is old and not as accurate I would appreciate pointers to the new instructions



Webcontrol has an option to use the ‘holey calibration’ firmware, we’ve found it
to be more accurate (but it takes a lot more measurements to do the calibration)

David Lang

Would this mean “updating”/flashing the Holey calibrationfirmware on the board? or is this not required and I can continue to use the Eastbaysource Firmware. (the last time I tried to update the firmware, I ended up with the left and right motors continuoslyy turning as son as they were powered up)

this is why. see this post

in there is a link to download the holey firmware. Instructions here on how to get webcontrol to flash it.

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one thing to check is the possible contact of the lower chains to the chain guides (see photo).


Cant tell for sure but it looks like if your sled gets towards the top of your work piece the bottom chains may be coming into contact with the nylon chain guides on each side. try to manually move your sled to the location where your cut went off the rails and see that your chains going from the motors to the sled arent rubbing against anything.

Your issues at this point are not calibration or firmware

I would strongly suggest you to connect your shield right and set chains feed to “bottom” before do anything else.

Having shield connections reversed and set chain feed to top to compensate for it will yield to the issues you’re having

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I will redo the connections and chain set up and try again.
Thanks much for all the help.

top vs bottom is more than just the direction the gears move, it also affects
the calculations of where it thinks the sled is.

David Lang