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Scaled down build - calibration - set chain lengths - issue

Hi all!
Back at my scaled down build using an old adjustable drawing board as the frame. Cut area is based on using 600*1200mm ply. All built and mostly calibrated until the set chain lengths part where it pays out what would seem to be too much chain - it would put my sled off the bottom of the board and leave too much on the inside of the gears for me to take up with the pulley and bungee arrangement.
Trying to understand the algorithm and figure out what I need to tweak. Any hints or direction to others that have travelled this path would be great!

Specs -
Calibration:
1758mm between motors.
400mm between motor gear and top of work area.
150mm from Chain at ring to centre of router bit.
Hardware:
Running from a Mac laptop (Yosemite).
Image below with actual chain lengths payed out. (Had to ‘tie’ them for the shot). Sled removed.

You will probably have to shorten the chains somewhat to make it work, but I think that’s a minor issue. The real question is why is it extending so far.

It’s been a while since I looked at that bit of code, but I believe that it extends a fixed length instead of trying to center the router. What happens if you press the “home” button? do the chains retract to the correct length?

Hey Bar!
Thanks for the reply.
Homed it and lengths seemed reasonable so hooked up the sled, positioning it in the centre of my work area ended up with about 200mm of chain after the ring links.
Didn’t go for the test cuts because was worried it would try to cut somewhere far out of my work area and found the “settings” tab where you can set the work area height and width.
Will try the calibration process again with these settings and report back.

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There is a chain extension setting that is at 1651. Change that number to something that approximates the distance from your motor to the center (minus the sled radius) that is divisible by 6.35 so the initial extension will give you a length that is useable. If you thread up the chains under the sprocket, you can use small pulleys and weights and not have to shorten your chains. My system is 4’x4’ and uses full length chains but I want the option for full size later, so I won’t be chopping them off, but you could if you want.

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Thanks Orob!
Nice set up.
Found the dims settings and re calibrated.
All was looking ok but someone didn’t keep an eye on the chain tension…
The first cut tells the story. Skip…skip…ohsh…t!
Have had to take the frame down to relocate but will be back at it on the weekend. May deploy a tension setup similar to yours!

Cheers,
Lofty

skipping is caused more by the chains not being parallel to the workpiece than
tension

make sure the attachment of the chains to the sled is set so that the sled hangs
straight, then adjust the top beam (or put spacers behind the workpiece) to make
the chains parallel to the workpiece.

David Lang