Problems with straight lines on Scara/Polar table

Hi everyone,

Hoping somebody might be able to help. I just completed a build of a scara machine utilizing some of the designs posted on this sight. I have the GRBL setup per instructions supplied throughout the site and the machine works really well with one peculiar exception. If I use designs with circles or arcs, it works perfectly, but any designs with long straight lines (ie the wipe/eraser design from sandify) each line will have a small curve inward towards the end (about 3/4 to the end) of the line. It is consistent and reproducible. The designs still look cool, but it kind of bugs me that the straight lines aren’t fully straight. Any ideas from anybody who has built these tables or used sandify for your gcode?


p.s. - I can post some video and/or pics if that would be helpful.

Pics would be great. Vids are great too, but you’ll have to post them elsewhere (like YouTube, or anywhere) and then post a link.

I am curious where the lines are. My first idea is some backlash, but I think that all the time. I’ll guess if I see the picture. Maybe someone else with a SCARA machine will recognize the issue.

I am assuming you are using the SCARA gcode output. I don’t have a good way to validate that. It would be nice if I could just paste it in a website like ncviewer and see that the turns aren’t in gcode.

Thanks for the quick response Jeff! I knew you would be the right man for the question… :slight_smile:

Here is the pic of the pattern in sandify, and then how it turned out:

I’ll work on a video when I have a second. Let me know what other information I can post. I am using the Scara GCode output, Max rho of 1 and units per circle of 6. Her is the Gcode file if that is helpful, but sounds like it is difficult to validate…

square.gcode (253.8 KB)

Anyway, thanks for taking a crack at it!


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OK, here is a video. This is pattern imported from the JSisyphus example tracks into Sandify, then exported:

Here is a still of the closeup of a triangle that should, in theory have straight sides:

Here is the video of it in action:

Around 1:00 in is a great example.

Strange stuff. Thanks again!


I’m convinced I have it figured out. It took me a minute to find it.

In those lines, the “elbow” is pulling in (making the radius smaller) and that is a straight line. Then it flattens out and starts pushing out the elbow.

Right when it changes, the ball seems to follow a constant radius arc. You can see it in the video. The ball goes from a line to an arc, to a different line.

Then the ball starts tracking out again.

Your elbow joint has some backlash. When it is pulling in, it is hard against one edge of the backlash. When it starts reversing, it isn’t moving, just picking up that slack. Then it gets taught and starts moving again.

If you’re using GT2 pulleys, maybe the grub screw is loose on the flat of the motor shaft. If you’re using meshing gears, they might just be that loose. You can probably jog the elbow joint and see that when it changes directions, it doesn’t move immediately. If you can watch or feel it when it does that, you should be able to see where the slack is. Another approach is to just enable the motors (so they hold still) and wiggle the elbow to see where it is slipping.

It is really interesting to see how freaking sharp it is. You can see it in the images even. A straight line, and arc (on a constant radius), and a straight line.

Really nice build. Holy cow is that a clean looking build and almost no border.


You are incredible! I had some slack in my belts and tightened them up nice and snug, and wouldn’t you know it got rid of 90% of the issue:

I’m going to futz around with the pulleys and make sure there is no slack anywhere. Thank you SO MUCH for the feedback. You are a genius!

Also, thanks for the feedback on the table. Here is a look under the hood to anybody interested:

Thanks again Jeff. You are a sand table God!



That is a slick, compact assembly under there. Thanks for showing it off.

Are the files available somewhere?

Glad it is working better. I am sure you can make it nice and tight. Be sure to check the grub screws keeping the pulleys on the motor shafts. Those are the least obvious place for slack. They are commonly cause for issues on the CNC machines.

Thanks again!

All of the credit for the table goes to these forums. There are a couple of users that I pulled ideas from, but I used the files (from @woobag) at:

as my basis and then customized the parts (arm length, etc) to fit the build. I used a planter bowl off amazon as the container and it worked pretty nice.

-Arduino+CNC shield+gcodesender on a Pro Micro (@karltinsly) (github) - with your helpful additions
-Using WLED as my LED controller.

Now onto building my MPCNC!

Thanks again -


You’re using the gcode sender?! Nice!