MultiBot CNC v2

A low cost 3D printed CNC that can be built with minimal tools yet is capable of great things.

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I set out to make a large format, low cost 3D printed CNC machine that is capable of handling hard woods and aluminum. The goal was to make it as accessible as possible, so it uses off the shelf parts, and does not need any heavy tools to assemble it.

I have been interested in building a CNC machine for years, even going as far as building one out of old CD Rom drivers, and another from an old 3D printer.  A year ago I stumbled onto Nikodem Bartnik's DIY 3D Printed Dremel CNC project and found the inspiration I was looking for.  I took his excellent ideas and built my own version of his machine. That machine was great, but was lacking in several ways so I sat down for this second redesign and think I finally hit upon a great combination.

The project goal was to make a machine that had a working size of at least 11"x17", the final working volume ended up being 365 mm x 465 mm x 75 mm (14"x18"x3").  To make it as inexpensively as possible.  And to make it accessible so that anyone with a 3D printer and a few simple hand tools could put it together.  All while being capable of doing serious milling of hard woods and aluminum.

Here is a full parts and price list for the machine.

I'm using 12 mm smooth rods and LM12LUU bearings for the linear motion because they are readily available and very low cost.  However at the size were working with the 12 mm rod is not stiff enough.  After running tests with different configurations of rods I settled on doubling up the rods, that is almost half the cost of using 16 mm rods while still being significantly stiffer than a single 12 mm rod.  You can see from the graph below that deflection at the tool in the X and Y direction is greatly reduced with doubled up rods.

I'm using 8mm Acme trapezoidal lead screws for the drive system.  In the original design I was getting a lot of backlash.  I experimented with several different configurations but finally landed on combining a brass nut with a Delrin nut in compression to eliminate backlash.  This is easier to adjust than using two brass nuts, safer than using Delrin nuts, and much stiffer than using spring based backlash nuts that you can find on Amazon and AlliExpress.  The Delrin nuts are just flexible enough that you can have a fine control over tension making it easy to dial out backlash without adding a lot of resistance to the setup.

In my original design I was relying on the bearings in the stepper motors to take all the load from the lead screw.  This was not a good arrangement and on some of my axis I was able to deflect the drive shaft by several mm.  I worked around this by adding skateboard bearings on each end of the lead screw and using 3D printed nuts to tension the lead screw between the bearings.  This provides a lot of strength and completely decouples the stepper motors from the system.  I also added standoffs to the stepper motors to make maintenance easier and to ensure I can use every inch of my linear rod for motion.

I struggled for a long time with the 3D printed parts cracking under load. I have gone through many design iterations trying to come up with a clamping and screwing system that can handle the forces of a milling machine without sacrificing the convenience of a 3D printed part.  Eventually I hit on the idea of using heat set brass inserts to strengthen the screw holes and using separate clamps to hold my rods in place.  This has proven to be quite robust and requires a minimal amount of effort to put together.  You do need some sort of a heat source to set the brass inserts.  This has the added benefit of making the machine very serviceable, it is easy to take it apart and put it back together now without worrying about damaging the 3D printed parts.  Well worth the small increase in cost and complexity in my mind.

As much as possible I have tried to make the parts symmetrical for easy reuse, and have carefully designed all parts to be printed as designed without needing any supports or brims.  The goal is to take out the complexity as much as possible so a...

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  • Drum sander

    David Tucker01/08/2021 at 01:09 0 comments

    Vacation is over and life is getting busy again but I found the time to make a small project.  I made an insert for my drill press table that allows a drum sander with a bearing to be recessed in it.  My drill press has a press fit chuck and I don't trust it to handle a lot of side torque, so this little insert takes the load.  

    It is working really good and I was even able to use it with the fence to sand a board to a uniform thickness. I'm not going to publish the plans because both the table and sander have been discontinued from Harbor Freight so it would be difficult to replicate this build.  But there are similar tables and drum sanders out there and it would not be hard to replicate this on your own if your interested.

    Note that this does not compare to a $200 oscillating drum sander, but then again I think I put $6 into the build.  It will leave some lines on your parts, but it is a great way to quickly rough in a shape.

  • Wrap it up

    David Tucker01/04/2021 at 06:21 0 comments

    Ok I cut a 22 cm diameter disk and used it in place of the Thien baffle and that seems to work just as well.  At least with only 2" of dust in the bottom of my separator.  I will need to wait on more dust before I can evaluate if the lip is really needed or not.  For now I'm going to leave the disk in place and leave the dust bag off my shop vac so I can better evaluate this over the long term.

    So for now the final design is 3 rings of 1/4" plywood at 33 cm, 28.5 cm and 22 cm.  Glue the top two together and use 10 cm standoffs for the baffle.  It seems to be working well, with only a bit of dust making it through to the shop vac.  I'm also dropping the diffuser for now, it does not seem to help much and it is easy to plug up.

    The wood lid and Thien baffle are not needed but they do make things nicer to deal with and have some benefits to the strength and dust collection.  I think the vacuum release valve is the big win, it saves the shop-vac motor when you get a plug and stops the system from getting stressed.  I'm surprised there is not one built into the vacuum.

    On a side note my 1-1/4" shop-vac hose is really not up to the job. It clogs on the smallest pieces and overall is just a real pain.  On the other hand most of my tools don't need a 2" hose, and it was free with the vacuum.  Eventually I should break down and pick up a 1-7/8" hose, it would probably be a better compromise between suction, flexibility and clogging.

  • Validating Thien

    David Tucker01/02/2021 at 19:47 0 comments

    I have been trying to come up with a way to peek inside Dusty to see how well it is working.  I had been thinking of ways to hide a camera in there, but this seemed like a doomed idea, either the camera will get hurt, or it will not capture the image, or just cause a disturbance in the air flow.  Finally I stumbled on this great video on reddit.  They had the bright idea to put a strong light behind the bucket to cause a shadow to be cast by the dust.

    I only have an orange bucket, and nothing close to a strong light, but it finally hit me that I have a car and the headlights are really bright.  A quick setup in a dark garage and a box to line it all up and it worked great!

    I was able to see the dust swirling in with and without my Thien baffle, and also using my diffuser.  The results with the straight outlet and diffuser were about the same.  The dust flowed in a nice swirl but if I plugged up the inlet I could see dust cavitating back up from the bottom and upon inspection I could see a hole was drilled in the center of the dust.  

    Adding in the Thien baffle seems to have eliminated this cavitation, when blocking the inlet there is no evidence that the dirt in the bottom is being disturbed. And there is no hole in the center of the dirt either.  I'm changing my view of the Thien baffle, it is not just that it reduces the air speed at the edge of the baffle, I think it also provides a more direct route between the inlet and outlet when the vacuum is increased by restricting the inlet and that in turn keeps the air below the baffle smoother and reduces cavitation of the dust.

    Next is to cut a new baffle that lacks the lip, and probably one more that is 1/2 the diameter to see if the lip is important and if the diameter is a big deal.  I also need to compare the straight and diffused outlet with the Thien baffle.  I tested with only the diffuser and noticed that there was a pile of dirt under the diffuser.  If  the diffuser is not effective then it would be better to remove it, it adds complication and material on printing and can be a pinch point where material can clog up the vacuum.

  • Baffled

    David Tucker01/01/2021 at 03:08 0 comments

    It occurred to me that while I can't properly test the Thien baffle without enough sawdust to fill up the bucket I can test my own baffle simply by lowering it into my bucket.  I did just that, lowering the old outlet port and the new baffle into a bucket of sawdust and other junk while measuring the distance from the bottom of each and the surface of the dirt.   

    The regular open outlet port started to suck up light particles of Styrofoam at around 10  cm and started sucking up sawdust at 3 cm.  My new baffle did not suck up the Styrofoam till 2 cm above the surface and even when pushing it into the sawdust it did not suck up any dust.  Now my new outlet is 5 cm deeper than the old outlet, so that is only a 5 cm gain on the Styrofoam.

    There is almost no air movement at the bottom of my baffle, I suspect I could drop the bottom two layers of holes to shrink it up without greatly reducing its effectiveness.  I did notice that it appeared to be disturbing the air around it much less than without the baffle, I got movement on my Styrofoam balls almost immediately when using no baffle.

    Anyway I may publish the baffle, it appears to be working well, well enough to give it a proper try at least.

  • Diffusing the situation

    David Tucker01/01/2021 at 02:06 0 comments

    I have been thinking a lot about my dust separator and trying to visualize how it all works. I decided to do a writeup on my belief, and hopefully someone smarter than me can point out the flaws in my thinking.

    For starters the Thien separator, why does it work (or does it work at all).  There are two ways to look at this one.  The first is that somehow the particles enter the chamber, spin around the wall and slowly drop, that much is true.  Then the Thien baffle would act as a knife blade, separating the dropped particles from the return air flow.  However I don't think this is a valid way to look at it, in fact I suspect the small tab on the separator is not essential for this to work (whether or not it has a small benefit).  Rather my thought is that the separator is reducing the speed of the return air by spreading the flow over a very broad area before allowing it to drop into the bucket.  The idea in my head is that there is a concentrated flow of suction near the return nozzle and if the sawdust piled up high enough in the bucket then this stronger suction would be able to pick up particles and send them on to the shop-vac.  The separator forces the air to spread out and that in turn causes  a reduction in suction near the edge of the baffle that allows the sawdust to rise much higher in the bucket before getting sucked into the shop-vac.  I suspect that a much smaller diameter baffle that was perfectly circular would work just as well.

    My idea was if we are just trying to slow down the air, ie divert the suction so it is oriented horizontally rather than vertically, then why not just use a baffle (diffuser?).  I created the baffle below with a series of holes that are more and more restricted as you go down in depth.  That causes a strong suction at the top of the baffle and a slow suctions at the bottom edge with no suction on the bottom itself (it is fully blocked).  That in turn causes a series of air currents that steer the air out sideways from the baffle rather than downward and hopefully that has the same impact as the Thien separator without the added complexity and space of the extra baffle disk.

    All of this is independent of what is going on with the inlet side of the separator.  On that side particles are thrown into the bucket wall at a high speed and hopefully they loose momentum quickly as they are asked to rapidly change direction and drop out of the air flow.  In theory the goal is to keep these two systems separated, hopefully the outlet suction is diffuse enough that it has little to no influence on the inlet vortex.

    Anyway I have a lack of sawdust at the moment so I can't effectively test these right now.  I may try to test them anyway just to make sure they don't make things worse, but we won't know how good they work till I have 3/4 of a barrel of sawdust to play with.  I guess I need to come up with some new ideas on what to make and stop trying to improve my tools.

  • The rise of Thienos

    David Tucker12/31/2020 at 01:30 0 comments

    I have wanted to make a wooden lid for Dusty from the beginning but I lacked the skills to fabricate it.  Now that I have a bandsaw that can cut circles I decided to make one out of two layers of 1/4" plywood glued together.  

    This is working great, it no longer warps under a vacuum and it is trivial to remove (maybe too easy in fact).  The only issue in making this is that I used a jig saw to make the inner holes.  The problem is my normal blade would not turn tight enough to make the cut, while my thin blade warped all over the place.  In hindsight I should have broken down and got a circle cutter.

    I did have to extend the threads so they could fit through the thicker top. I have updated my models on thingiverse to match.

    I went ahead and tried making a Thien baffle for this while I was at it.  My first attempt turned out to be a bit of a dud.  When I did back to back tests with and without the baffle the baffle made things worse. The big problem appears to be that the baffle was to close to my lid, allowing the dust to get trapped under the inlet nozzle.  That in turn keeps the dust circulating at the top and eventually getting sucked back out the outlet.

    I did not get a great shot of this all together but you can see it from the side in the shot below.  These are not perfect shots but in the one above there is no new dust in the shop-vac, the inlet is blocking you from seeing that.  in the second there is quite a bit of dust and when I took the lid off Dusty a lot of dirt came out of the top.

    I'm printing new standoffs that will extend the baffle down to see if I can make it work better.  I will make a new post once I have a chance to test it.  I also had an idea for making a diffuser on the outlet to try and broaden the return air path without using a baffle.  I'm printing it out now and will let you know how it compares.

    Anyway overall this worked great.  I would probably use a higher quality and thinner board for the lid next time, this is really course stuff, but it is all I had laying around.  Also I have a vague idea of using the bucket handle as a latch to hold the lid on.  If I added small tabs to the lid that the handle would lock into I think it would hold together well even if this all tipped over.

  • Band Saw

    David Tucker12/29/2020 at 03:00 0 comments

    My lovely wife got me a bandsaw for Christmas.  It has been high on my wish list for years but I was nervous about getting a small one that could fit in my limited shop space.  I got a WEN 3939T 2.8 amp 9" bandsaw from Amazon. It seemed like a reasonable compromise between size and performance.  It is just light enough that I can move it around without much of a struggle and yet just powerful enough (barely) to actually be useful.

    So far I'm really liking it.  It cuts well and just has enough power to resaw a 2x4 the long way.  I'm not sure how well it will hold up to 4" thick hardwoods but still it is nice to have a saw that can make quick and reliable cuts that does not make me nervous when using it.  I have a skill saw, a chop saw, and a jig saw and all either scare me or cut rather rough.

    One of my first projects was to cut out some circles from some scrap plywood I had laying about.  At first I started to research circle cutting jigs, but I finally hit on the trick of just nailing my target to a piece of scrap wood and clamping that to the table to create a pivot.  I found that you need to move the pivot point an inch or so in front of the blade to allow it to bite in and keep it from wandering too much.  Anyway works a treat and costs nothing!  A proper jig is probably a great idea if you do this all the time but for a one off I can't complain.

    I noticed the table insert was a mm to shallow for the bed, causing small material to catch on the lip so I made a zero clearance insert.  However I noticed there was a lot more dust on top of the table after adding in this insert.  The dust collection works ok but it produces almost no vacuum at the base of the blade and I think it needs that large opening in the insert to allow the chips to fall into the collector.  I'm thinking of redesigning the whole thing at some point, but for now the built in dust collector work ok, and better when left stock (so far).

    I also made a dust boot adapter for my smaller shop vac hose.

    Anyway over all I'm really pleased with this little machine.  It is certainly not something you would want if you had a proper sized shop, in that case spend the money on a decent sized 14" bandsaw with a built in base, but it is great for my small space and needs.  In summary if you don't expect this to be a full size bandsaw you will be very happy with it.

  • State of the Union

    David Tucker12/24/2020 at 02:05 0 comments

    I have been working on my CNC journey for a while now and thought it would be a good time to take stock of where I have been and how it is going.  I have wanted a CNC machine basically for ever, but 4-5 years ago I started down the road.  

    First I build a hacked up machine from old CD rom drives just to see if I could do it. That was a great learning experience but it did not result in a very functional device.  I did wire it up to a 3D pen and managed to print out some small objects along with making a few drawings on post-it notes.

    Next came a XYZ da Vinci miniMaker from my wife for Christmas.  I worked with that for about a year, getting comfortable with simple cad work and learning the basics.  This is a great machine if you have no interest in electronics.  There is basically nothing you can change on the machine, it works great for what it does but you can't make it better (or worse) by tinkering.  I actually reversed engineered the protocol for communicating with the printer, but was never able to get things to a point where my code was simpler to use than XYZ's software.  Again this is a great printer if you want to make miniatures but don't know anything about computers, it does what it does well, but can't go any farther than that.

    After a year with the miniMaker I picked up a Creality Ender 3 printer for a great price.  This has proven to be an excellent, and yet frustrating, little workhorse. When it works great it is very reliable and dependable and produces parts with good finish and accuracy.  When it goes bad you feel like your in an alternate universe because nothing seems to make it better.

    I have made only a few modifications to my ender 3:

    • Latest Marline, with manual mesh bed leveling enabled.  This is the number one upgrade everyone should make.  If you don't know about arduinos just buy a dongle from someone with the latest firmware on it. I believe TH3D sells a kit but I could be wrong on that.  Anyway this is free and you will get much better quality out of your printer over the crusty old stock firmware.
    • BuildTack print sheet. I can't stress this enough, a quality print bed surface makes a big difference.  I experimented with a lot but settled on the BuildTack shetes.  Partly because they worked so good with my XYZ miniMaker and partly because they just hold up really well.  Get a thin paint spatula to go with it and throw out the scarry paint scraper, your build surface and fingers will last longer.
    • Dual gear extruder, extrusion has always been my big nemesis with the ender 3, I tried several extruders but have been really happy with there dual gear extruder.  The price is right and it is quite functional and easy to install.  Use your old extruder tension spring rather than the yellow one that came with it, it is too strong.
    • Octoprint, I am very happy with this piece of tech.  With a tiny bit of effort you can monitor and print from anywhere, allowing your machine to live in a closet rather than needing to be out.

    I wish we could get an ender 3 style machine with a few simple upgrades, for something close to the same cost:

    • Up to date firmware, come on everyone this is a no brainer.
    • Simple bed leveling built in.  The miniMaker has a push button you can manually extend when leveling.  It probably cost $0.10 to add to the printer and is 90% as good as a fancy BLTouch.
    • Quieter stepper drivers and fans that shut off when not in use.  This is not critical but it can make the rest of your house happier about your hobbies.
    • A hot end with a captive heat break liner like on the E3D V6.  Probably the biggest failing of all Creality printers is that they run the bowden tube all the way down the hot end. This sounds great on paper but in practice you end up having to jam everything together really tightly to keep it from leaking.

    The newer machines from Creality try to solve...

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  • Downcut bit

    David Tucker12/24/2020 at 00:33 0 comments

    In an effort to improve the finish on my cuts I picked up a Hozly 1/8" two fluted down cut bit from Amazon. They came in today so I made a few sample 30 mm boxes to test them out on from a piece of poplar I had on the machine.

    From left to right is using a piece of double sided tape, triangular tabs with a finishing pass, and square tabs with no finishing pass (the fastest). 

    • The double sided tape was a complete failure, it broke loose before the part was finished and left crud on the bit.  It also put a gouge in the side when things came loose.
    • The square tabs were a huge problem still causing shudders when the machine plunged. And you can't see it well in the picture but the surface finish is much rougher as well.  
    • Adding in a 0.5mm finishing pass and using 10x6 mm triangular tabs had the best effect, with a nice finish everywhere that would need minimal sanding. These bits were able to cut the triangular tabs with no noticeable shuddering, much better than the single fluted 1/8" upcut bit.

    These bits need to be run faster, I ran them at 800 mm/min and 12000 rpm but I could probably have run it even faster (or slowed down the rpm's.  It cut well but the chips were very thin, maybe a mm in width but the full height of the cut.  It may be worth while to run these really fast and add in two finishing passes, at a slower speed so there is plenty of room if the bit wanders a bit on the fast moves.

    Anyway they are an improvement over the single flute upcut bits I have been using, but not a huge one.  I will probably keep these as my primary bit for now, but I would still recommend starting on the others.  They are more forgiving, and the cost is so low you can break one a day without worry.

    I'm not super eager to revisit double sided tape.  It did not stick well enough and it was really a pain to put down, tending to bunch up at the cut line (probably part of the problem).  Maybe the blue tape and crazy glue idea would work better.  Anyway I would only consider using it for things like circuit boards that need to be very flat, and for much larger pieces where you can keep the tape far away from the cutter.

  • Stress test

    David Tucker12/23/2020 at 19:15 0 comments

    I'm always on the hunt for a new way to measure the stiffness of my setup.  I find that testing the flex of individual parts does not translate that well to the overall flex in the system. It captures the upper bound but the end result is usually stiffer. 

    Here is my latest attempt to visualize it.  I blocked the end of my spindle against a block of wood and zeroed out the system, then moved the axis against the block till the steppers started to slip.  That point should be around 12 kg of force on the axis.  I can tell you that is well more than the force needed to snap a 1/8 bit!

    Anyway by taking a photo before and after the motion and creating a gif we can visualize the flex in the system.  On the X axis you can see there is some flex on the Y rails, and it appears most of the flex is in the bearings on the X axis.  In the Y direction you can see that most of the flex is between the Z rails and the motor mount.

    In both directions the motors stepped 2 mm before skipping a step.  That should be fairly accurate, one step should work out to 0.04 mm if my calculations are correct, and we should not be able to get more than one step off before the motor fails.  So the max flex in the system under loads well beyond what my normal bit can handle is less than 2mm.  That is still quite a bit of flex but I don't know that it is too far beyond what a shapoko or other hobby machine can handle. 

    I may work up the nerve to sacrifice a router bit to see what the flex is at the limit right before the bit breaks.  That would be a more realistic test, and it would be nice to know what real world forces the bit can handle.  However I'm not in a hurry to break a bit...

    Interestingly enough the right most Y stepper failed long before the left stepper.  I could continue to jog the left axis another mm after the right would not move any more.  I suspect there is a power imbalance on the motors, or it is possible the z carriage is shifted just a bit to the right.  Anyway something to think about.  I suspect it does not matter that much.  As long as the machine is strong enough to rapidly decelerate without skipping steps it is strong enough.  The actual cutting force is small in the grand scheme of things.

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Kosma wrote 12/22/2020 at 10:31 point

How You create a gcode in linux? For example i create a 3d object in blender, how make a gcode from object? stl etc?

  Are you sure? yes | no

David Tucker wrote 12/22/2020 at 22:36 point

I'm not sure how to generate gcode from a Linux tool path.  You will need to do some research on that.  I did a small writeup on how I do it using Fusion 360 if you are interested.

  Are you sure? yes | no

Kosma wrote 12/23/2020 at 09:42 point

Interesting tutorial but not in linux. Im not have a windows

  Are you sure? yes | no

David Tucker wrote 12/23/2020 at 19:20 point

Check out tutorials on LinuxCNC, they will have pointers on how to do cam (convert a 3D model to grbl) in Linux.  And since your already on Linux you can consider using this as your sender and controller as well.

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