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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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  • Collimation of events

    David Tucker3 days ago 0 comments

    I just noticed that NEJE.shop has updated there laser diode module page and there are three new modules now available. One is a cheaper 5w version of my 7.5w module model number N40620, interesting but not exiting.  The exiting one is the new A40630 module.  This is the same 7.5w module I have but it adds a collimating lens to the diode. NEJE calls this a FAC or fast axis correction lens, you can see an example of it on this ebay listing. This has the effect of reducing the speed (angle) at which the fast side of the diode diverges, the end result is the spot size of the laser is more or less square and smaller in area.  

    According to there specs they are reducing the spot size from 0.08 x 0.06 mm to 0.04 x 0.04mm, that takes us from 0.0048 mm^2 to 0.0016 mm^2, a 3x reduction in spot size!  The collimators must reduce the laser power by some percent but even so we should be getting quite a bit more power out of the laser with this.  And this should restore the engraving potential so one laser can be used for both engraving and cutting without compromising quality.

    The newer A40640 15w module also uses this correction lens and claims the same 0.04 mm square spot size.  That combined with double the power output on the laser probably puts this close to 4x more cutting power than my uncorrected module.  A K40 CO2 laser uses a 40 watt laser tube, that is still several times more optical power than the 15w NEJE, but it also has several times larger of a spot size so this may be approaching the cutting power of a $300 CO2 laser.  Keep in mind the CO2 laser uses a very different wavelength (infrared vs blue) and so it can cut some clear materials that the NEJE cant.

    The minimum spot size has everything to do with the wavelength of the light we are using.  Our blue lasers have about a 450 nm wavelength (0.00045 mm), while a CO2 laser has a 0.01 mm wavelength.  If we had a square (round) spot in both cases then the blue laser should be able to focus down to a much smaller spot size.  in practice the CO2 laser is probably producing a 0.3 mm square spot size, while my laser diode is producing around a 0.1 mm square spot, and the new modules above claim a 0.04 mm square spot size.  Assuming this is true, then the new module has a 50x smaller cutting area than the CO2 laser!  That sounds too good to be true, but I suspect a 10x improvement is well within reason.  Also keep in mind this is the ideal cutting area, beam divergence can have a big impact on how thick of a material you can cut.

    Anyway things are getting exiting for laser diode cutters, they are getting close to being powerful enough to compete against low end CO2 lasers, while being much simpler to operate and maintain (no large glass tube, no need for water cooling).  Exiting times indeed.

  • Notes to my future self

    David Tucker4 days ago 0 comments

    I have been thinking it over and I think I messed up my laser depth cut tests earlier.  I need to set the focus height for the optimal first line cut before attempting to test if lowering the laser has any benefit.  I'm fairly certain that I was starting from a lower depth before and there may not have been any benefit to going even lower after that. 

    In addition I want to run a test where I move the laser sideways to cut a wider kerf before attempting to cut deeper.  I suspect running two or three side by side cuts combined with two or three depths may be enough to get through much thicker pieces of wood.

    Finally I need to run some tests to see what the optimal focus height is for engraving.  Is it really true that engraving should be focused at the surface while cutting should be focused deeper?  My tests so far don't seem to show this, but they have been far from scientific up till now.  I need to come up with a way to measure the spot size (or line size) accurately so we can try to find the smallest spot with more precision than the ramp test.  Maybe it is time to dig out the microscope.

  • Blade Runner

    David Tucker6 days ago 0 comments

    I picked up a small drag knife when I first built my machine, but I never worked out how to use it.  Now that I'm messing with the laser I thought it would be good to dust it off and work out the kinks.  The knife I have is similar to a Cricut cutter with a small 4mm diameter blade.  I have it mounted on a 3d printed flexure to help if the tool contacts the work surface. 

    These small blades have an adjustment on the top that extends the blade a fixed amount.  Typically you would extend it just past the depth of your material and then lower things so the blade holder is just in contact with the surface of the material.

    There are larger holders that use a box cutter blade and in this case we usually lower the blade to the surface of the base material and leave excess clearance at the top of the blade.  You can typically raise and lower the blade a bit in the holder to change the blade offset (the amount it trails behind the pivot) to adapt to different material.

    In either case if we try to cut a right angle then the rapid change in direction of the cutter at the corner will cause a small tear in the material to be cut, and if you are really unlucky it can cause the cutter to bind up and break.  We can work around this by either rounding all corners in our cut file or we can add a program step where the cutter is raised up so it just drags on the surface of the material then perform a circular motion with the tool to reorient itself properly.

    GrblGuru has a nice video that illustrates how all of this works. There tool should be able to take any outline and adapt it for use with a drag knife.  It is something I need to look more into.

    I wanted to use Fusion 360 for my CAM work, and I came across this nice looking drag knife plugin by Peter Böker. Not only does it take care of the corners it comes with some great documentation on how to make it all work well.  It does cost around $10 but if it gets the job done then it is a small price to pay.

    Anyway I need to spend some time actually cutting things out to make sure this all works.  But at least we have a start on trying to work out the details.

  • Advice

    David Tucker05/08/2021 at 21:18 0 comments

    It is important to keep in mind who is talking to you when asking for advice online.  I keep seeing people post simple questions like "what is the best laser to get" with little or no context given, and others respond in kind with little or no context as well.  This can lead to bad advice, so be aware of it.

    For example that person that tells you not to waist your time on anything short of a $10,000 100w laser is not wrong, but only if your needs are there needs.  If you are running a small business with the laser running 8 hours a day 5 days a week and you rely on it for your income then for sure that 100w laser is a better buy than a much cheaper $200 5w laser diode.  Your $10,000 machine represents 16 weeks of labor at $15 an hour but if productivity was even just 4x faster for the 20x increase in power then you should be able to make that up within a year.

    On the other extreme if you are not planning on making any money at all and this is strictly a hobby then your time may not be worth nearly as much to you as the money you have to put into the hobby.  In that case going with a $200 machine is a much better investment.

    The point is that you should not ask simple open ended questions online and hope to get an answer that applies to your situation.  And on the other end you should not give simple answers without stating your assumptions or needs.  You need lots of info to make an informed decision.  

    This of course applies to all things in life, not just lasers or CNC's in general.  if your choosing a career from a few lines of text online, or making any other large decision without at least taking context into account then you are probably making a mistake.

  • Laser Post-Mortem

    David Tucker05/07/2021 at 04:59 0 comments

    It is time to take stock of my laser add-on.  Right off the bat I have to say the laser is a lot of fun. It is deceptively simple to use, that can really lead to issues if you are not vigilant.  It takes almost zero setup, once I set the focus height I can add a new sheet of stock any time with little more than a rough alignment under the laser before starting a cut.  And cuts are quiet (thanks to my new air pump) and generally fuss free.  No need to worry about cut order, or to secure the material or really worry a lot about feeds and speeds.  Another plus is the accuracy, this has a 0.1 mm kerf and can easily cut paper thin slices of material, there is no worrying about splintering or dealing with a wide bit.

    On the negative side, this is too easy, it is tempting to not wear the safety glasses or not remain in the room while cutting, It feels too safe.  I have had to impose some rules for using the laser to help keep myself from growing complacent.  The first rule is if the laser is plugged into the wall then I wear my safety glasses, no exceptions.  Next I go through a fixed startup sequence of turning on the air and then my exhaust fan, setting the focus and outlining my cut before I turn on the laser.  The reverse sequence is followed as soon as the cut is complete.  Most importantly I pull the power before messing with the material. 

    It is also very slow, a CO2 laser would be able to cut much faster, and much thicker materials as well.  It took several minutes to cut out the little boxes from my last two posts and those are about as simple a cut as you can make.  However for the price and my applications the speed is not that bad.  I don't have room for a CO2 laser, nor do I have anything major I'm producing so time is not money in my case.

    It does not compare well to a 3D printer, they are really designed to do very different things.  The laser is very much a 2D cutter, basically a drag knife on steroids.  While the 3D printer is mostly used for complex 3D forms.  This is closer in spirit to the CNC, but it is much simpler to setup and operate.  If it was not for the death ray and smoke it would be quite user friendly!  You really need to think hard to turn a 2D cut into a 3D shape, going beyond simple box like structures takes a lot of creativity, but there is still a lot you can do with just a 2D cut. I'm beginning to see the appeal of a cricut cutter, it is lower risk than the laser yet capable of cutting a lot of the same materials.

    The important thing to keep in mind is that this is not a 3D printer, it is very easy to use, but not very safe.  Fire is always a concern with any tool, 3D printers included.  But here we are actually catching an object on fire on purpose in a controlled way.  It needs to be taken seriously for sure.  However if you are vigilant then this is both a fun and rewarding tool.

  • Fusion 360 for laser cutting take 1

    David Tucker05/07/2021 at 04:33 0 comments

    I have been trying to work out how to use Fusion 360 to generate laser cut files.  At first I tried just drawing the cut file as a 2D drawing then exporting as a dfx file.  However LightBurn takes all your reference lines as cut lines, leaving a lot of cleanup and making it difficult to later modify the layout if needed.

    Recently I came across this article from Core Electronics and it has pointed me in a better direction.  They generate a 3D object then flatten it out using the align then move tools.  Finally they create a new 2D sketch and project the face of the boxes onto it to make a simplified drawing.  This is much nicer, it lets you use all the tools available in Fusion when designing.  And the output does not require any cleanup so that is a win as well.

    One trick I'm experimenting with in LightBurn is to use the offset feature to take care of the kerf (width of the cut).  This increases the size of your design by the given amount, so when the laser cuts the excess off you should get a final result that is the exact size you originally designed.  This allows you to do all the design work in Fusion without worrying about the kerf.  Keep in mind that if your laser removes 0.1 mm of material then you need to increase the line size by half that, since half the cut will be on the outside of the line in your waste material.  Also be sure to select delete original object or you will get two lines that will be cut (yikes).  In this case I wanted my pivots to be loose so I did not add an offset to them, but typically you would want to offset all lines that need to be dimensionally accurate.

    If you do the math you can see this adds up to the same 0.2 mm adjustment I made on the Inkscape example. In that case I added the 0.2 mm to one tab, but not the slot, in this case I'm adding 0.05 mm to every line, both on the tab and slot, 4 lines adds up to 0.2 mm of added material.

    It took me a while to work out all the kinks but I think this is a much better way to design complex structures.  You could bring the final dfx file into Inkscape to add any artistic details to the object if that is simpler, but this is much easier to to the technical design work in.

  • Inkscape take 1

    David Tucker05/04/2021 at 01:51 0 comments

    I thought I would make a small note on one way to use Inkscape to design laser cut files.  Today we will make a simple box.

    First off in the document properties we need to setup a template that is the size of our material.  In my case I'm cutting out 300x300 mm 1/8" plywood.

    Next I find it convenient to setup grid lines.  I'm using 0.1 mm grid lines with a major line on 1 mm.

    When drawing your design the grid lines (and snapping in general) can get in the way at times, you can quickly toggle snapping on and off with the button highlighted below.

    For the most part we are only interested in the underlying vector and not the outline or fill on a line.  You can hide all that by enabling outline mode in the view menu.  This shows you exactly what the laser will cut.

    Rather than thinking about drawing shapes, as much as possible we want to think about outlines.  This is easier to do by differencing objects to create complex outlines.  In this case I want to make a box 25 mm square with tabs.  By defining a cut tool I can slice holes in my box to form the tabs while keeping a continuous edge to the outside of my box.

    Notice the X, Y, W, H boxes in the toolbar, you can use these to precisely size and locate an object rather than just relying on the grid.

    Differencing our small box from our large box creates a new complex shape.  This is much easier than trying to manually draw the tabs inside our box outline then remove unwanted lines.

    If you did it all correctly you get something approximating a box in the end.  A few notes,  my laser has a kerf of about 0.1 mm so I need to oversize my tabs by 0.1 mm on each side in order to get a tight fit.  Also my material is 1/8" thick or 3.175 mm so my tabs need to be 3.175 mm wide.  Or possibly a bit less to account for the kerf, in my case I tried 3.14 mm and that seemed to fit well.  

    Remember the kerf may be 0.1 mm but it cuts 0.05 mm from both sides of the lines.  For tabs that error sums back together to 0.1 mm because 0.05 mm of material is removed both on the tab and on the slot.  However in this case we only need to reduce our tab length by 0.05 mm or somewhere around there.

    I think it turned out ok. I still find Inkscape frustrating compared to Fusion360, but I am slowly getting the hang of it.  You can download the final svg file from here.

  • Gray Scale

    David Tucker05/02/2021 at 23:59 0 comments

    I thought I would experiment with etching some wood.  I wrote some code to burn a series of lines from 0-30% power in steps of 0.4% and spaced 0.2mm apart (the long box is 0.4mm spacing)  The lower left box is done at 150 mm/min and using 5 psi of air, the lower right box is running at 400 mm/min and 5 psi of air, and the top left box is 400 mm/min but no air.

    It is hard to see in this photo but the top etch tapers down as the power increases with very little ash, but lots of smoke when etching.  The bottom cut looks like a sea sponge, there are many thin slivers of wood sticking up from the base of the etch that are virtually untouched.  I took a knife and sliced through a layer of the slivers and underneath we have a sloped etch that is similar to the one without air but deeper.

    It may just be the material I am using but I was hoping we would get some sort of a gradient in the burnt wood.  However with air we don't get any gradient, and the scorch marks are very black.  Without air we get a tiny bit of a gradient, but I think this has more to do with the excess smoke and it settling back onto the surface of the wood.

    In short using less air probably improves the look of your engraving a bit, but it does not seem to be essential. Faster is probably better. And you should probably rely on stippling rather than power to convey more than a few shades of color.

    Anyway this was just a first stab at etching, I need to run some more experiments for sure.

  • Particle count

    David Tucker05/02/2021 at 23:37 0 comments

    The air finally dried back out so I thought I would test out my particle counter.  I placed it behind my air filter fan, between the laser and fan, and on the far side of the laser cutter as a double check to make sure the smoke is not just clearing up near the fan.

    You can see that between the laser and fan we have a very high particle count, plenty of smoke is generated.

    On the far side of the fan things are much improved, not nearly as low as the base reading of 0-2 when not cutting, but more than 10x better.

    And on the far side things are even better, the fan is definitely pulling the smoke towards itself rather than just cleaning the air around itself.  

    Still in all cases things could be better.  When I produce enough smoke that you can see a steady trail it is clear that not all is being pulled into the fan.  A lot of the smoke is going down to the ground.  And there is still a smell of smoke in the air, although it does not linger for long. 

    I'm sure a more focused collector near the laser cutter would help, and having an activated charcoal filter would be good as well.  It is something I need to spend more time playing with, but it is good that we are seeing numbers and some benefits of the filter I have on there now.

  • Nozzle prototype

    David Tucker05/01/2021 at 04:47 0 comments

    I tried designing my own nozzle inspired by Darko's prototype.  Here is a cutaway view of the nozzle.  I printed it out and tested it but it has two major flaws.  

    With the nozzle being printed tip down the hole spacing is compromised a bit by the elephants foot on the first layer.  I could overcome that by making the holes even larger to compensate, but that is not very portable.

    More importantly the air flow is fairly unfocused, not nearly as efficient as with a single 2mm hole.  This could be improved a bit by elevating the inner wall 1 mm above the outlet, allowing for the outer wall to be brought in a bit.  The down side is the more we do this the more air will tend to flow back up the inner hole.  Also we would need to print this in a different orientation.  Printing it on its side would probably mess up the nozzle, we need it to be very uniform so the air flows straight down.  So we would need to print it upside down, but that would involve a lot of supports or we would need to drop the top of the barrier.

    This would all be simpler if Cura would print single line walls, that would allow us to make the inner wall half as thick.  However for some reason the default is to not print thin walls.

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Raunchy Butts wrote 04/14/2021 at 22:36 point

Really nice effort on the write-up and parts list, thanks. Have you tried machining aluminum on it yet? Very curious to see how that turns out.

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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?

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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.

https://docs.google.com/document/d/1j7bN3EDspqFdop7lzfCD5yb5QjX1i-MaceniBL3TV7U/edit?usp=sharing

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

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

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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.

http://linuxcnc.org/

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