MR - 4 : Robotic Tank

by timmiclark Nov 21, 2016
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Hi Guys,
Only a pair of motors, of any style.
Setting mine up for steppers now.
BTW - Any servo can be modified for continuous rotation by cracking the cases, and cutting the pin off. You'll see it.
Mine is 100% (density) PLA and really tough. Takes 5 hours for one tread.
Ender 3 and CURA, brim no support
One addition,
any pair of motors working on one track, would buck each other, (stretching and contracting the track) unless syncro'd together. Really high quality steppers MIGHT work, but any other type of two - on one track will not.

The video below of the core-less drone motor running at about 60 miles per hour is impressive.

Also M2 X 40mm allen key cap screws, drill the outer holes at 2mm and the middle hole in the track at 2.5. drop of superglue under the head to retain the screw, and the graphite lube, or some kind of lube is a must. (remember - real tanks use grease. :D ) Bearings may be a future mod.

Photos when finished...


Hi, may i know with type of motor you use for the inner track?

First off Thanks! been wanting to print this for some time...

ok so all parts printed some custom parts I can upload as a remix...but 26 tracks is very tight like I have to really edge them on....and they don't really turn....but 27 tracks is pretty loose...guess I need to just wire it up and see.

I have 4 motors on this do you think the motor will compete? I can easily take two out fo the equation....

Hi, thanks for uploading these very nice designs!
Since I like MR4 more then MR6, I've some questions about this model.

I wonder why:

  • you did not continue the flat tracks in MR6?
  • you changed from servo back to dc-motor? (more power? easy control?)
  • are the track pieces the same?

I guess, I just have to make one of these... :)

Not Flat Tracks: Because the ridges on the MR6 track provide more grip on the surfaces that I have in my classroom.
Not Servo: I teach coding and I have more interesting code problems with DC motors

The MR4 has more problems that the MR5(not publicly available) or the MR6 - MR4 not as easy to print, not as easy to maintain, has track catching issues

What size bolts were used?

Hey! Please, could you tell me if I can adapt the tank to support a 5V DC Motor??

Please, answe as fast as you can! Thanks!

If you have the technical ability- then please, go right ahead!

Be great if you could publish what you come up with on thingiverse as a remix!

I don't know the wiring I don't know how the sketch and the bill of materials please I really want to make this project somebody help me!
I printed all the parts !! Please help me!

There is a sample wiring diagram in the images for this file.

Essentially you will need:

2 x standard size continuous rotation servos
1 battery (I have found two or three 18650 batteries in a clip work really well)
An on/off switch
A power converter to drop the battery voltage to 6V to power they servos
A second power converter (if needed) to drop the battery voltage to the controller power input
You'll get less controller restarts if you put a 470 micro farad capacitor across the battery (after the on/off switch)
A controller (Arduino or Raspberry Pi etc)
A cup full of patience (or red wine - which ever is cheaper)
A teaspoon of perseverance

There is a sample wiring diagram in the images for this file.

Essentially you will need:

2 x standard size continuous rotation servos
1 battery (I have found two or three 18650 batteries in a clip work really well)
An on/off switch
A power converter to drop the battery voltage to 6V to power they servos
A second power converter (if needed) to drop the battery voltage to the controller power input
You'll get less controller restarts if you put a 470 micro farad capacitor across the battery (after the on/off switch)
A controller (Arduino or Raspberry Pi etc)
A cup full of patience (or red wine - which ever is cheaper)
A teaspoon of perseverance

Does anyone know if there body part that holds the pi camera similar to the early version?

Uploading one right now!

Thank you! I downloaded the files again but didn't see the new piece.

I just did it again - must have forgot to press save

Thank you again!!

While printing tracks with Flex medium I have the same question. Knowing it could help not to print parts one by one and can save much time.
Thank you

May 31, 2017
Is there a print list for this?
I figure:
(2) body
(52) Tracks
(4) Drive Cog
(1) Arduino holder
(4) Inner Track Frame
(4) Outer Track Frame
(2) Center Track Frame
(?) Spindles
(?) Track [Also which version 2_2 or 2_4]

Any help would be appreciated

Hi There,

I have done some experiments with printing the tracks in flex. They present a few problems.

(1) This particular model (and my earlier tank models) rely on the reduced friction of ABS on ABS (or PLA) - when I did a set of tracks in flex, the friction was rather high, and the motors were put under strain.

(2) The stretch in the tracks means that they do not last very long. The flex tracks deform over time.

I have been experimenting on printing the first 4mm of track in flex and the switching to PLA, but my printer that can print in flex can only do a few tracks at a time like this and it takes too long.

Apart from the flex tracks being slightly quieter on wooden floors, there is only minimal traction advantage, and once again, only on smooth surfaces.

I am really, sorry, but am no longer developing this model as it was designed in software that is no longer available. I have upgraded computers several times and moved house twice since I created this model & I have lost the part list :(

Hello !
What is the electric motor used ?
Thank you, sincerly

Does someone have a BOM list for the electrical components? and also the arduino code? i'm quite a noob in the electrical compartment.

Hi Max,

This design is intended to allow people to experiment with a range of electronics, as such there is no BOM.

You need: 2 x "standard" continuous rotation servos; A buck step down voltage regulator (to provide the required power to the servos); A battery that has higher voltage than the servos require; a controller (e.g. arduino) to run it; and what ever sensors you feel you need.

Sorry I can't be more specific - I have moved away from development of the servo motor model in favour of geared DC motors.

Hi, once again great project, I'm building one. Have you already made a new design to substitute the servo motor with geared DC motors or still in development?

heres the flat cover i made to fit my mr-4


mr-4 cover blank
by igotit

Awesome project! I am trying to print and build now with PLA print.

Thanks for the build, I'm currently working on modding it for 2 2212 brushless outrunners, will post files when done, and if it works XD

im very impressed with this platform.
my heng long is twice the size and struggles in my yard on wet grass.
very well designed platform.


ive made one
tried uploading it,but having issues.i also designed a flat cover for it,cant get it to upload without errors.
this is a well thoughout kit,everything fit together well,sliight track issue .used all 2.2 pads,and the v4 sprockets.
took a lil over 70 hours in my cr-10,petg 0.2 res.
i used 4d x 1 1/2 finishing nails for track pins.
heres a short test vid using rc gear i had on hand.

id be intrested in a body half to house a tx03 fpv camera instead of sensors.


Nice! Looks great all in white!

I had thought about working on on a camera for this one ... by I have been side tracked by other projects!

no rush,life is more important.
im just happy for what it is.
probably be good with circuit board style camera,eitherway its a fun versitile tank.
once i learn how to cad,i kinda wanna modify it to have alittle more ground clerance and maybe some simple tread tracks,nothing fancy.
i even though about stretching her 3 inches.....but by that point its a whole different tank....lol.
thanx for this design

Thanks for getting back to me from the other file, this one is definitely going to work better, not that the other was bad at all. I think the servos will give better control. Very much interested in the camera setup as well. I downloaded those files as well. This going to be a great build for the kids.

Is there a print list for this?
I figure:
(2) body
(52) Tracks
(4) Drive Cog
(1) Arduino holder
(4) Inner Track Frame
(4) Outer Track Frame
(2) Center Track Frame
(?) Spindles
(?) Track [Also which version 2_2 or 2_4]

Any help would be appreciated

Can you share STP files?

There is a link to the source files in the description - that is the best I can do.

i've found that 15GA. 1-5/8th" serated nails work perfectly for joining the tracks

So what was the intent for the servo mounting? The holes fit 1.75 mm filament - was this the idea?

Thank you for the models, but giving us an idea of what you used to put everything together is also helpful. Cheers!

Any idea about what I can do to get the track to fit correctly? I have them fitted and there does seem some slack, but I cannot fit with one less track. My problem is that when running the track rides up the non driver cog and it becomes too hard for the servo to drive so it stalls.

A few questions:
What did you print out of (both cogs and tracks)?
Which printer do you use?

A few suggestions:
PLA is not as good as ABS for mechanical devices (I have found) - it is not as flexible and has sharper edges
File the cogs so that they have no sharp edges.
Ensure that you have around 10mm of sag when the tracks are suspended in mid air.

The hard edge of the cog "teeth" from PLA is a possible reason - I might put a 1mm chamfer on them to see if that helps.

Iam printing some that i put a chamfer onto.

It is almost as though the front cog is too big, have a look at the video you can see the track riding up the cog so that it is amost riding between tracks but by then the slack has gone and so the servo stalls.


Have uploaded a new version of the cog (Version 4) to resolve this issue.

A couple of questions.

What switch did you use? I have got myself a 16mm push button switch but it fouls against the servo, you must have a very short one there.

Can you think of a way of increasing the ground clearance?

I had to look around for a 16mm switch that didn't do that - or for my second one I put the servos on the other end to the switch.

I would have to remodel the whole thing to increase the clearance properly - the cheat way would be to make the tracks thicker

Re tracks, I thought of that :-)

Putting the servos the other end to the switch means there is less room in there for buck convertors etc. I could always make it longer I suppose. Would you be willing to release the 123d files for the sides of the robot? You have released the rest but those parts are not there.

Sorry my mistake they are there but not made in 123d. Are they too difficult in 123d?

The accuracy required was too tricky to get right in 123D - it had no model to sketch constraints capability

@timmiclark I'd like to try the treads on my dual material printer, but I think I'd like to modify them so the pinned parts are a second material. Could you please post the CAD file for the tread?

I think that you normally use these buck converters (https://www.aliexpress.com/item/DC-DC-Adjustable-LM2596-Step-Down-Power-Converter-Module-LED-Voltmeter-USB-Port-Y102/32702087523.html?spm=2114.13010608.0.0.BFNfuG) are you using this one in this?

If so how are you mounting it? I am trying to mount mine now and there is easy way to do so whilst leaving enough room for the Micro USB cable etc.

Have you any pics?

Hi Jenny,

With my first tank (in the video) I used a smaller power converter which I just placed inside the body of the tank - I am yet to model anything for the larger power converter from the link you supplied - but I do intend to some time soon.

I might have a solution, if so I will upload later.

Have you got a BOM for this ie bolts and length needed? I have got half way through the print and it occurs that this may need some very long bolts!

Me again, in the description you said it is designed to be bolted together, what type of screw did you use?

I think I use 65mm bolts with a diameter of <4mm (M4) - I then shear off the excess

I think they are M3 holes but I am going to drill out to 4mm as M3 > 50mm are like hens teeth.

Once again a great design and your instructional videos have guided my year 7 (11 year olds) to build their own robots from scratch this year so I, and they cannot thank you enough.

Could you please tell me what the different versions of the main body are designed to take sensor wise?

Oh and how did you do the fab colour scheme?

BTW trying to use this design in the upcoming PiWars (http://piwars.org/) hope that you do not mind.

Hi Jenny,

At this stage it has one body part that takes two sensors:

  1. a HC-SR04

  2. an adafruit colour sensor

Both of them work with a Raspberry Pi - the ultrasonic sensor works best if you put it in its own thread as the further away an object the longer it takes to get a reading. The colour sensor is brilliant and needs a library - the instructions for getting it going on the Pi is on the Adafruit website.

The colour scheme comes from the fact that I had access to a full colour 3D printer for a time. That tank is a little heavy and brittle because of the print material I used - so when my normal printers are repaired I'll replace it with boring one colour ABS.

Good luck with the PiWars - let me know how it pans out!

Do these sensors also work with the Arduino Nano?

Thanks for that.

I will alter the body design to fit my batteries etc and upload them for others when done, if that is OK with you.

Need to fit line followers and Time or Flight or Ultrasonic sensors for automous navigation around a line course and a maze.

This is extremely cool and well thought-out. Do you have your arduino sketches available, or is it assumed to be simple enough to not include. Thanks this is great.

what are all the needed things i need to buy (motors, suspension etc.)?

and does it need Servos AND motors? Or just servos?

Any solutions? The spindle in the servo unscrew/ itself as it tries to move under any sort of pressure or load.

You may need to re-work the spindle to better match your servo - they are all a bit different in their sizes.

Reprinted the part, lower quality but added supports. Thanks.

Comments deleted.

Despite the interesting difficulties, still REALLY loving this thing. I'm assuming the entire frame is printed in ABS, or another less-rigid-than-PLA plastic?

Despite modifying the cogs, tracks still will not fit properly on PLA printed parts - I've modified the drive aseembly frame to subtract distance between the gears. 2mm is just barely not enough..it does roll, but I'd like to not need a huge-torque servo to drive it, and not run the risk of the plastic melting from friction at a slow trot xD

Currently working on 3mm shorter parts...center frame (only on one side, marked by notches), and inner and outer frames for the "Blank" cog side. The drive cog side is unchanged. Screw hole and guidance peg/hole positions are also unchanged on both. IF I can get that to work, I'll go ahead and modify the center case parts to fit the shorter drive assemblies. Sadly, all I have to print with is PLA, but i have tons of it. :D

I am a bit curious as to the "compressed" dimensions of the flexible materials. Does the entire drive assembly compress somewhat when you have the tread track fitted? Do you have to "work" the center compartment a bit to get it to fit into the profile of the drive assemblies? did you do the track in ABS only, and the rest in a more rigid material? Or is my printer (or myself) just somehow screwing things up, and i'm totally off on the wrong track here?

Finished my build ^-^

Everything fits, with my modifications, and the thing will actually move without too much work for the servos. I'll try and get some top covers whipped up, one with a raspberry pi mount point.

hi, what did you use for the print, and do you need Servos AND motors or just servos?

Regular old, generic PLA. for whatever reason, my print came out either too big, or too stiff, so I had to mod the blank drive assembly parts to be 3mm shorter. that gave me enough space to get the track closed, and everything to run without melting plastic xD If you have a tight fit for your belt, i recommend powdered graphite lubricant - works SUPER well.

just 2 standard sized (CONTINUOUS ROTATION) servos (the mini ones won't work).

Awsome! Though it was a ABS print only, almost freaked Out lol.

It may very well be - you may need to do some modifications to your parts, to make it work with PLA. Or it may be myself, or my machine, are just a good bit out of calibration. ^-^

I am looking for continous rotation motors but it's first time to use it and I cannot judge if type and size are correct for this project. Can you give some link for motors as reference? It would be very helpful.

(the link below littlebirdelectronics seem to not work...)

Hey! Cool model. Could you detail the BOM for screws and bearings?


Comments deleted.

Looks awesome Tim. Print is well underway. I too am an Australian teacher from the Gold Coast my area is senior and middle years Computer Science as I have a software engineering background. I only have limited experience in 3D CAD design though which is often a barrier of turning our ideas into reality (when using 3D printers anyway). I would love to touch base with you and see if we could get a collaboration project going, if you're interested let me know? Also what CAD program would you recommend to introduce to high school kids? I have dabbled in Inventor and Revit but they can be pretty heavy.

Hi Daniel,

Collaboration sounds fun. I'd really like to know how to teach students to write a function to get the tank to rotate "a" degrees from a current heading "b" using an onboard electronic magnetic compass or gyro!

I teach 123D Design to all of year 7 - then I work with Fusion 360 after that (from year 9 on more complex stuff) - most of this project was done in 123D Design.

Thanks for the reply. I will give it a crack after I finish printing and constructing the model. It's printing as we speak (will probably take a few day though to print off the components. I was planning to use an Intel 101 board which has a built in Gyro with Bluetooth LE so I will give the function a crack. I might have to wait for the post though as I am not sure if I have a spare H-bridge lying around (unless I pull one out of another project). I'll give 123D a crack as well. Thanks for the advice.


Awesome - I am using the 101 with my year 10's who are building this:
Which from a software point of view is one and the same as the green camo tank from the video

By the way - you shouldn't need a dual H-Bridge - this tank uses Continuous Rotation servos (which I power off a buck, step down power converter) - I'll put an image up of the circuit diagram, but connected to an Arduino Nano.

MR - 3 : 9G Servo Driven Robot

This looks awesome! How many track sections do you print to complete one track?

Sorry I didn't put this down! There are 26 tracks per side.

Thanks for putting the number of tracks in the description. I was having a little trouble counting the tracks in the pictures :) I plan on printing some extra anyway. Did you melt the 1.75 mm PLA (or glue it) to keep it from sliding out of the tracks? I have not completed mine all the way yet, but I was wondering if the PLA that holds the tracks together works it's way out if you don't secure it.

I have found, that because the filament has come off a reel and has a natural bow, the bow tends to keep the filament in place. Alternatively you could give it a jab with an old soldering iron tip.

Pretty sure it's 26 per track. He does have a earlier version this has has been based on that mentions 26 per each track. See http://www.thingiverse.com/thing:824979

Raspberry Pi Camera part for Drogerdy Tank Bot

Counted off of the half-track picture above...got 28, so I'd say either 26, 27, or 28 xD I'll print up 30 just to be on the safe side

Printing this up piece by piece...will be able to confirm tomorrow, as I'm printing up this sides last outer track frame, and the rest of the tread pieces tonight.

Gotta say, well done on this - even with my failprinter, and the poor quality print, the half of this side i've printed and assembled spins like a dream. can't wait to get this thing built xD

Yes! I built this one with the "failprinter" brand in mind! - it has a lot more tolerance than the previous models - and has eliminated the track alignment issues that many were having.

Be sure to print the tracks with NO SUPPORTS! - Otherwise you have to drill them out! I would also print a few extra - good to have spares.

I use MatterHackers' MatterControl to do the slicing/printing. The default option is for support to build platform only, so I don't have to worry about "Supported Holes" xD

Had a lot of sagging, since i don't have a part cooling fan (yet). The supports really help, and as long as i don't build the support interface layer, they're super easy to remove. The first 7 or so I printed without supports, though, sagged towards the tread face, which is a direction the belt never flexes anyways...so I guess i'm using them for aesthetic purposes.

Had to print out 27 track sections to complete the track.

There are a few spots where the drive/drift cogs don't hit the track correctly. I'm totally willing to blame that on the very poor print quality that I did tho - will reprint both, install, and see if that fixes :D

That is odd - perhaps a warped part(s)?

The mathematics of it all implies that there should always be an even number.

No warp. Measurements are looking good. Printed out with PLA, which might be the problem. I know ABS has some flex. PLA really doesn't.

As a work around, I'll try a bit of sanding on the cog and track-cylinder surfaces, and perhaps adding a second bevel to the gear teeth (to the edge that forms the top, which isn't beveled). Will report back on success/failure.

OK, modified the Blank Cog file (THANK YOU for providing it!). dropped the height of the top face of the toothed part by 0.5mm. added a 0.5mmx3mm flange to keep it from sliding in the socket. beveled the un-beveled edge of the teeth, and moved the face of the teeth in by 0.5mm.

Tested by rolling 12 track segments around the cog, and linking it up - PERFECT FIT! I can NOT wrap 12 track segments around the unmodified cog - there's about 1mm or so between the holes, and they cannot be linked.

I've made the same modifications to the drive cog, and am currently printing - if this provides a working PLA printed drive system, I'll go ahead and...uhm...upload the files to google drive and link here? @[email protected] I dunno.

I experienced the same problem as MrWisski had where the tracks did not sit into the cogs very well causing the 26 count to not be enough to complete the track. I am really new to 3D printing so I am sure it is something that I have done wrong. I am reprinting a few track sections with a finer quality setting and some support structure to see if that corrects the issue. I also noticed that the cog seemed a little too thick so I also plan on machining them down a little bit. I should also be able to machine down the track sections some as well if needed.

Question: What type of printer? What X & Y accuracy do you have? My Up! Plus printed this OK and it has 0.2mm accuracy in the X & Y

Check Measurements

Should be:


  • Cog diameter: 60mm +/- 1.5mm
  • Cog thickness: 10mm +/- 0.5mm


  • Distance from pin hole centre to pin hole centre: 15.7mm +/- 0.4mm
  • Diameter of rounds: 9mm +/- 0.5mm

All these measurements are pretty important! - The tolerances I have build in are as high as I could go without causing issues - most printers work within 0.2mm in the X/Y planes. The big exception in my experience is the "cube pro" which can be out as much as 1mm in some cases!

Check that your print be is perfectly level - in four directions - (1) side to side, (2) back to front, (3 & 4) both diagonals. This will cause issues - print a beam with 4mm holes every 10mm along the whole length of your printer - you may find that they get further apart towards one end if it is not perfectly level.

The Up! printers level the bed by printing it level - using a raft. Try printing onto a raft.

I had to use 27 at first to as 26 wasn't long enough, then I realized that the track wasn't sitting snug on the cog and it wasn't grabbing as effectively as it should. So I took the track off and stretched it by hand (pulling on the ends) removed a link and the magic 26 links worked like a charm. Best part is that it was now sitting better on the cogs.

Cool project!!! I have some ideas and wanted to know if you can post up the main files so i can tweak them without starting from scratch.

Apparently Thingiverse wont take ZIP files - or freecad files

You can get them here: https://drive.google.com/open?id=0B0oR8i_t-zjyVnRWb1hxeUlPRlE

I thought I had put the 123D files up in a zipped folder - will check into it.

Got a picture posted. Thanks.

I wanted to post a picture of the finished print. Operation pending receipt of electronics. "Made" tab appears to be closed.

Thanks. Printing well underway and will send photos of finished project.

George S.

How do the drive servos connect to the drive wheels? I'm in the process of printing now. Thanks
George S.

Hi George,

There is an STL in the Thing Files called Spindle2 - this screws onto the servo's spindle and fits into the drive cog (the one with the square hole in it)

You may need to modify this spindle for your servos if they are a different brand from mine. (Mine are "Parallax" brand)

Good luck printing!