How do I set up BOTTOM FEED in the calibration?

Hi Community,

I am in the middle of the calibration process. I have just finished measuring the distance between the motors. It has asked me if I am going to “bottom feed” or “top feed” to the sled. I have selected TOP FEED. When I go to extend the chain to attach it to the sled, it extends it over the top, not under the the bottom. Any ideas on how to resolve this?


IMG_2707 IMG_2708

Hi @zachbliss. Sounds like you needed bottom feed? I could be wrong but I think you will have to recalibrate after you change it. Sorry.

Did you see the
wiki page for calibration


I second @WoodCutter4. If you chose Top Feed, then your setup should look like the left hand picture and the slack chain (extra chain) is to the left and right of the work area for the left and right motors respectively. If you choose bottom feed then the chain slack ends up over the work area. Also note that those pictures are of the left side motors.

Either way, I believe that for most that have implemented them, weights work better than bungies. I plan to swap my bungies out when I can find some time.

Also, the second picture appears to be a bottom feed picture on the left (assuming that’s the left side motor).

@zachbliss, here is a link to some (mostly old) pics of my setup (EDIT: see picture details for description/comments) that show a bottom feed machine with slack across the top bar. I also used 2.5 lb bricks instead of bungees for the slack (same bricks on the sled). As you have probably already read, bungees are not ideal.

Where do things stand now? Did you just pause the calibration at the extend chain step?

wow! your frame looks slick. i would love to eventually setup a metal frame. beautiful!

if i were to continue with the wooden frame (10’ 2x4 across the top) as built, without access to the pulleys you have so cleverly assembled, would you recommend top or bottom feed. i am leaning towards just setting it up with the bottom feed as with the bungee setup that was sent with the kit. after setup is complete and i have a few cuts under my belt, I would love to know the components you used to make your dream machine come to life.

so, am i smart to go with bottom feed?

sorry… just reading other comments. i am going with the bottom feed.

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Honestly, I’m not sure if there is any accuracy improvement for top or bottom feed but I think that bottom feed gives better slack management options. There is definitely an advantage to using a fixed weight system as opposed to bungees. There are lots of threads on this issue, like this one:

Theoretically there is a tiny advantage in width with bottom feed :slight_smile:
Since i saw this picture on GitHub - ldocull/MaslowDue: Maslow CNC, second generation based on GRBL and Arduino Due i am wondering why we are entering 12 o’clock measurements… perhaps the different compensation happens in the Firmware.

How much grams you are gaining with adding like 3/4 of both sprockets to the width, David’s spreadsheet can answer. Not much i guess.

Bungees are weakest when needed and to strong when they should not. Depending on your sled weight and how tight you set the rubbers up, it is possible that you can introduce a backlash on the motor sprockets.
I moved from having the chains on the sides and from rubber to water.
2 Plastic jugs with 1 GAL (~3.78 L) capacity, each filled with 2 L water, ~2kg (~4.40 pound) is enough.
Constant pull on the slack side. On 0 budget the original bearings can be used.

The chains are out of the way and out of dust, plus the bungee-headache is gone. Could not be happier.

Kind regards, Gero

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There’s not an accuracy difference between top and bottom feed, but top feed
significantly reduces chain skip because the chains line up with the sprockets
much better (with top feed, the slack is pulled towards vertical instead of the
15 degree angle of the sprockets, so is far more likely to skip)

David Lang


the reason that we set the 2 o’clock and reference everything from there is that the point of departure from the sprocket changes and the chain angle changes (from 10 degrees to 80 degrees, almost a quarter wrap)

it’s also much easier to set the sprocket to 12 o’clock than a side tooth to a particular location

the stock software does compensate for the wrap and departure angle (I don’t think the due version does)

I spent a couple bucks for pipe sweeps and let the weights go down the side of the machine, these weights are also around 2Kg (random hunks of metal I had around). I also replaced the sprockets with the pulleys, Gero replaces the s-hooks with something that holds the sprocket better.


Are you advocating for bottom feed here?

I think so, and just a transposed word.

If advocating for bottom feed, I think you are correct that the slack chain management is easier, but it not much harder to place the pulleys on a top feed system out at the same z-height as the sprockets to prevent the torquing that happens if they are left to be pulled vertically.
I don’t have a great closeup of my chain slack setup, but I think you can see the 2x4 piece that extends the bungy pulley out to the same z-height as the sprocket at the bottom right of this shot:
Google Photos
My plan is to replace the bungy with a cord that goes back up to the top of the work area, where the bungy is currently fastened, and have a second pulley that leads to a weight. Just need to find the time to install the parts I have for it.

Are you advocating for bottom feed here?

yes, bottom feed, slack on the top)

I think so, and just a transposed word.

If advocating for bottom feed, I think you are correct that the slack chain
management is easier, but it not much harder to place the pulleys on a top
feed system out at the same z-height as the sprockets to prevent the torquing
that happens if they are left to be pulled vertically.

even if you do that, there is a lot of chain that bowes back, and as the bungee
gets loose, everything swings back.

We talked about adding wings like you have when we were designing the new frame,
but ended up deciding that it was easier to have the slack across the top.

in your shop, it’s hard to get a side view of the chain to see how much it’s

David Lang

I agree that the bungee needs replacing. It’s on the list of tasking that I haven’t had time to get to yet. But as for bowing, I think that with the right weight, bowing should be minimal. And with my current set up, I have encountered exactly zero chain skips, which I believe is thanks to the roller that holds the chain firmly to the sprocket in concert with the bungee pulley being at the same z-height as the sprocket.

Here are better shots showing the extra piece of 2x4?:
Google Photos

at about 1/3 the way down, there is no perceptible bow in the chain. The separation of the chains as they go back up is due to the terminal connection point being slightly behind the sprocket
Google Photos

Google Photos

how much weight? too much (more than the tension on the sled side) and you have
backlash error)

David Lang

yeah, it’s a optimization problem I will likely never have time to solve. I am thinking just enough to keep the slack in control. Maybe a couple pounds.
As cool as sub millimeter accuracy is, I don’t need it on this scale for the projects I have planned. For the most part 1/8" or sometimes even 1/4" is fine for most of what I have in mind.