by bartdring, published

midTbot by bartdring Oct 16, 2017
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3938Views 542Downloads Found in Robotics


This is a simple pen drawing bot. It is like a T-Bot (AxiDraw), but I moved the motors to the middle. This has the advantage of making the machine a little smaller and way cooler, in my opinion. See the video.

I made mine really small, so it can fit in my backpack, but it is easily scaled up by using longer rods and a longer belt. With that said, the X axis rods will start to get springy above 200mm long.

The firmware is a special version of Grbl. The only change is the spindle PWM is used to raise and lower the pen. It also has the CoreXY features of Grbl enabled.

You can use just about any CNC controller that can run 2 steppers and a servo, but I made a custom controller that sits in the center and makes the wiring super clean. A limited qty are for sale on Tindie.

See my blog post blog post on this for further details.
Follow me on Twitter for announcement.

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The Pen carriage requires support


Besides the 3D printed parts you will need these parts


Part Qty Source
NEMA 14 x 20mm lg Stepper Motor 2 Amazon, eBay, omc-stepperonline.com
Pulley GT2 x 16 tooth 2 Amazon, eBay
Smooth Idler 3mm Bore 6 Amazon, eBay
Linear Bearing 6mm Double (LM6LUU) 4 Amazon, eBay
Linear Bearing 3mm (LM3UU) 4 Amazon, eBay
Linera Bearing Shaft 6mm (length 150mm-300mm) 4 Amazon, eBay
Linera Bearing Shaft 3mm dia. x 40mm 2 Amazon, eBay
Micro Hobby Servo 1 Amazon, eBay
GT2 Belt 6mm wide length * W * 2 + some Amazon, eBay
CNC Controller 1 Tindie, Amazon, eBay
Screw M3 x 8 6 McMaster
Screw M3 x 18 2 McMaster
Screw M3 x 25 4 McMaster
Screw M5 x 10 1 McMaster
Rubber Feet 4 Home Depot


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Hi, I have a stupid question, Is X on horizontal? Is Y with the PEN? When I home it, X,Y goes to the switches without problem. But Y endstop is max? X is min? Do I need to set anything in GRBL?

You can change the homing directions with setting $23 in the $$ men

Each axis is a bit value in a byte in this format (00000ZYX). Changing the Y would be binary 00000010 or 2 decimal. Send $23=2 to set that.

Can you remix for those smaller 28byj-48 motors?

I'd like to use the 28byg motors too.
Can you share the cad files, maybe i can modify them.

I have not used that motor. From what I can find from searches, it does not appear to be an ideal motor. I think it has a speed reduction gear and the wiring will not work well without some hacking.

that's true, but i have some spare 28byj-48 (hacked) and L293D motor shield unused...

I can post any files you like. What type are you looking for. The design was done in PTC CREO 4.0.

Another issue is the Grbl firmware outputs step and direction signals. I don't think those motor drivers support that.

I can use solidworks (2016), i don't know which file type can be shared beetween the two programs, i'll try to open any file you'll share and then we'll see.

And, yes, i'm also aware that grbl can't manage the motor's coil directly. :)

Are you planning on restocking the boards on tindie? Looks like a fun little bot to make.

can you share the code ?