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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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  • Baltic Birch

    David Tucker17 hours ago 0 comments

    I have some1/8" (3mm) Baltic birch from woodcraft that I thought I would test cut with the laser today.  This is truly 1/8" so it is more like 3.2 mm and is 3 ply.  

    Anyway I wanted to see how well I could cut it with and without air assist.  I did some initial cuts into the side of it at 100-400 mm/min and 75% power with full air, however none of those really made it through.  So I tried again at 80% power and speeds of 25-100 mm/min and full air and this time I made it through nicely.

    Next I wanted to see how important the air assist is so I ran the job again with no air and then with the air set to a light trickle (just barely making any noise).  You can see that no air is a disaster waiting to happen, I don't think you should go near wood with a laser that does not have air assist.  Light air faired a bit better but you really need the speed to keep it from burning.  Neither cut very deep and the cut has a lot more taper to it.

    Next I wanted to compare my two air nozzles, the one that is mounted next to the lenses and blows in from the side vs the one that mounts over the lens and blows straight down into the cut.  I also wanted to verify what speed would successfully cut through my wood in one pass.  It turns out 50 mm/min at 80% power can cut through in one pass.  That is not setting any sort of a speed record, but it is not bad.

    The side air nozzle is a bit of a dud.  It is working way better than no nozzle but it is blowing a lot of smoke across the work in the direction the nozzle is aiming (towards the top of the piece in this case).  The downward air nozzle produced less smoke and appeared to have less of a flair up as well.  Visually it appears the width of the cut is smaller as well.

    I'm still contemplating ways to seal the over the lens nozzle around the lens, I'm not sure if that is worth the effort or not.  Maybe it is time for some experimentation.  But I will be sticking with the over the lens nozzle going forward. Having a loud compressor run is not as bad as a smoked out cut.

    This basically confirms what we already knew.  To cut wood you need a lot of air blowing straight down into the cut to clear away all the burnt material.  And it highlights another point, this is very much like trying to cut wood with a soldering iron, it is a serious fire hazard and all safety precautions should be followed at all times.  Most importantly don't turn your back on the cut, it can flair up really fast.

  • Danger!

    David Tucker2 days ago 0 comments

    It's time to talk safety again.  A member on one of the laser forums I follow posted this picture of there shed recently (I hope it's just there shed).  They left there laser (k40) unattended for a moment and something went terribly wrong.  Everyone is ok but they did spend some time in the hospital.

    Anyway the point is that lasers and cnc machines are dangerous.  Not just in the lose your eye sort of danger but fire and plenty of other risks.  This is not something to goof around with.  You need to treat it like the scariest saw in the shop.  In fact it is far worse than that because it is automated and will continue to cut or burn long after something goes wrong.

    • Always stay close to your machines when they are running. You should be able to smell the smoke, or hear the broken bit as soon as it happened.  Don't rely on a baby monitor or camera to monitor, and never leave things unattended even for a moment.
    • Be wary of anything on the machine that can detach and become embedded under the laser or cutting tool.  This includes air assist nozzles. My design bolts to the laser head, I would not want to risk a friction fit.
    • Don't push the machine too hard, loosing steps can lead to tools being trapped in the work.  In fact anytime the laser or cutter is in continuous contact with any object you are at a high risk for fire.
    • Always wear safety gear.  A laser light can bounce off of anything and even an unfocused light can start a fire or blind you.  Your ears, eyes and mouth all need protection at all times.
    • Don't ever power up the laser when it is just resting on the table.  It is too easy to accidentally bump it and have it spin around.  The laser should be well secured at all times.
    • Make sure your machine is properly maintained with all cables tied up and everything well secured.

    Safety is no joke.  You need to use your brains when building this machine.  It is not my job or anyone elses to keep you safe, that falls squarely on your shoulders.  And it goes without saying but my instructions above are only a guide to how I build my machine, I am not responsible for any harm you may cause to yourself, others, or your property.  Keep safe!

  • Lightburn Take 2

    David Tucker4 days ago 0 comments

    So lightburn is a frustrating app.  It is so close to being great, but out of the box it does not work well with GRBL and I can't see a good reason for it.  All the other GRBL senders just sense how the machine is setup and work with it, but not lightburn.  Anyway after lots of frustration I stumbled onto this document on there website for how to make your grbl machine work.  After spending some time playing with it I came up with the following adjustments and it seems to be working properly now.

    First in the device settings fill in the following options:

    • Our machine dimensions (about 350x450 mm), be sure to measure this accurately from the home position for your machine.
    • Set the origin to the lower left corner (even though our origin is the upper right)
    • Enable z axis support
    • Enable the $J jog mode for smoother jogging.
    • Turn off return to finish position, unless you have a safe position you want to send the cutter to.

    On the console tab we need to add in two macros.  One will put the machine in laser mode and the other will return it to mill mode for regular mill work.  We will send $32=1 to enable laser mode, this has some laser safety like not turning the laser on unless the head is in motion.  And it removes a motion delay when you change the power.  $10=0 sets the machine into relative coordinates.  And G10 L2 P1 X-334 Y-334 offsets the origin relative to our home position so the origin is now in the lower left corner of the workspace.

    Mill mode just reverses the above by turning off laser mode, using absolute coordinates, and removing the origin offset.

    When you first start up lightburn, hit the home button, then hit the laser mode button to set the origin correctly.  Then at shutdown hit the mill button before exiting.

    Finally for your job control set the origin of the job to the lower left corner and the start position to the current position.  That way you can jog to the corner of your work and start a job.  Keep in mind that by default this will start cutting at the current head position.  There may be a better way to handle this, I need to experiment some more.

    If you want when jogging you can also enable continuous jog mode so that the laser moves as long as you hold down the jog buttons.  This works out ok, but the default move is very small so it can be fiddly to try and zero in on a point.

    Anyway these changes seem to take us from broken to just a bit annoying.  Still I wish they had a more intuitive setup out of the box for GRBL.

  • Misc.

    David Tucker5 days ago 0 comments

    I have always wondered why the new 40w module from NEJE cost more than twice that of the 30w module, when the 20w module is only about $10 cheaper than the 30w module.  Well I was poking through the neje.shop website and came across a video where they disassemble the laser.  

    It turns out that they are using two laser diodes with a beam splitter to combine the output back together.  That makes it plausible that they are getting 15w out of the diodes, assuming the diodes were 6w and overdriven by 20% or so.

    I also painted my bed mat black to reduce the flashback and make things a bit more safe.  It works better than I would have though.  We will see how long the paint lasts but I think it was worth the effort.

  • Laser Calorimeter take 2

    David Tucker6 days ago 0 comments

    So my previous laser calorimeter topped out at 50% because it could not handle the heat.  I was eating a bowl from Chipotle and noticed that the tin lid was a nice compromise between too thin and too thick so I thought I would try swapping it in to my rig in place of the tinfoil.  That worked great, and I was able to get readings from across the full power range.

    It is not shown in this picture but the foil is taped onto the points of the holder with tiny pieces of double sided tape.  If I made this again I would have the foil slide into the holder rather than mess with tape...

    I used a 20mm square piece of the lid painted flat black.  I mounted it 100 mm away from the laser with the laser set to a 5mm focal length.  That spread the beam across almost all of the target surface and minimized the chance of hurting the target.  I then placed my IR thermometer about 8 mm behind the target and placed it in continuous mode.  Using my test program I triggered the laser for exactly 3 seconds (+/- 1 ms) and read the temperatures by hand from the display.  Reading the temps is the biggest issue, the display updates probably 10 times a second, so the accuracy is not great.  The thermometer has a max mode, I should experiment with that, but I would need to find a way to hold the button down for that to work.

    Anyway the results look as linear as the previous ones.  This is surprisingly linear across the whole power range.  I was expecting some serious degradation at the top since we are overdriving the laser.  I would still stay away from 100% power to try and extend the life of the laser. 

  • Better air nozzle

    David Tucker04/02/2021 at 04:57 0 comments

    A while ago I made an air nozzle for my laser.  This replaced the red shield on the front of the laser and completely enveloped the lens so the air is flowing in the exact same direction as the beam.  This worked really well except for one issue, more air leaked around the lens than came out of the nozzle.  I could try to seal things up at the lens side of things but I worry that is just going to make a mess and be difficult to reattach when servicing the laser.

    So I decided to redesign this again using a nozzle that runs close to the beam but not quite in the same direction.  This is not as good air wise but hopefully it will help keep my compressor from cycling so often and should allow me to use more pressure. It is not clear in the rendering but the nozzle has a bit of an angle to it internally, that seems to be just enough to push the center of the jet over the center of the cut at a distance of 5mm from the end of the housing.

    At the same time I made a series of test nozzles with various size holes in them to see how the size affects the air flow.  I printed sizes from 0.5mm to 3mm in 0.5mm steps and found that the 2 and 2.5 mm holes produced the best results.  At 3mm the air flow was spread out too much and unfocussed, and at 1 mm it was severely restricted with much lower flow and pressure, 1.5 mm was ok, but 2mm had a detectably stronger flow.

  • Laser Calorimeter

    David Tucker04/02/2021 at 04:45 0 comments

    It would be nice to be able to work out the lasers peak power, and to check out how linear the power response is as we lower or raise the input power.  The tool we use to do that is a laser calorimeter.  This is a device that measures the temperature change on an object from the beam of the laser being shown on it for a fixed amount of time.  It is really nothing much more than a regular oven thermometer that has been calibrated accurately.  The problem is the simplest ones start at $100 and quickly go up to $500 for fancy ones with digital displays.

    The NBS published a nice writeup on how laser calorimeter's work.  It is a bit heavy on the math but the ideas are hiding in there if you are patient.

    We can still try to make our own calorimeter but it will be uncalibrated and only good for relative measurements.  The user Warsky posted an interesting take on a calorimeter on the laser pointer forums.  This is basically a piece of tinfoil of a known size that has been painted black and is held a fixed distance from an IR thermometer.  You hit it with an unfocused laser for a set amount of time and measure the change in temp.  They have some funny math to try and work out the power, but you can ignore that part.

    I decided to take there idea and apply 3D printing to the solution.  I have a small Cen-Tech IR thermometer from Harbor Freight that can be put into continuous mode. I printed a small holder that slips on the front that can hold a square of foil a fixed distance from the thermometer.  The idea is to paint the foil black to increase the absorption of the laser light, shine an unfocused laser at it for a set time and hopefully get a repeatable and linear increase in temperature based on power.

    I wrote a simple program that will turn the laser on for a precise amount of time at a fixed power level.  Then I ran through a series of power levels and recorded the peak temperature. Unfortunately with a 5 second pulse I was only able to go up to 50% before my IR thermometer maxed out  It seems to max out at around 110 C (230F).  I was surprised that an unfocused laser can heat the foil that much in 5 seconds.  I will try again with a much shorter pulse but that will make it less stable and harder to see the temperature climb on the LCD panel. 

    Here are my results, you can see for the lower half of the power range the optical power is very linear with respect to the requested power.  This is great news, it should make engraving more easy to handle.

    Anyway this is working great, even if my laser is a bit too powerful for the test.  I need to find a piece of metal with a bit more mass to it, maybe some steel flashing or something.  It would also be nice to have a lower power laser to compare against so we can see how the power numbers hold up.  Maybe someday...

  • Smoke!

    David Tucker03/28/2021 at 02:02 0 comments

    My initial stab at cutting produced more smoke than it should have and was leaving a lot of smoke marks on my cuts, along with stinking up the place.  So I decided to put some effort into cleaning things up.

    I had been cutting straight on my particle board waste board and that was causing a lot of the smoke from the waste board being damaged by the laser.  I picked up a cheap cookie sheet and cooling rack from Walmart that fit perfectly together to form a raised bed with a metal base.  It allows me to raise the part off the base and provides some fire protection as well.  I used some neodymium magnets to hold paper to the cooling rack so it does not flutter around.  This cost about $9 and seems to work really well.

    Next I picked up a 20"x20"x1" MERV 13 filter and a 20" box fan from Walmart to act as an air filter to suck up any smoke that is produced.  The box fan was a mess, it vibrated like mad and made a horrible noise.  After taking it appart I could see that the blade was just horribly cast, it had flashing all over it and was off balance.  A little time with a file and it is no longer making noise and is much more in balance.  Still it would be worth while spending more than $17 on a fan. 

    Anyway this seems to work well, it has no trouble sucking smoke away from the cut and it seems to suppress flames at the cut as well.  It is not perfect but the smoke smell is much reduced.

    Finally I picked up some 1/4" tubing and a coupler for my air compressor along with a valve to make it easy to turn the air on and off.  Then I printed a nozzle for the laser that goes around the lens and blows straight down on the cut.  This is working great! I'm running about 30 psi through it and it blows quite hard and completely suppresses any flames at the cut.

    This is working well except there is a lot of air leak around the top by the lens.  I'm planning on putting a bit of silicon there to block that end up.  I'm also still trying to work out the proper nozzle size, a small hole makes a fine stream but it really restricts the air flow.  Anyway the core problem is my compressor is crazy loud and all the lost air is making it run nearly all the time.  It is something that needs more attention.

    The results speak for themselves.  The small Tintin was my first cut, and the larger one is after all the modifications.  This is the bottom side of each cut but the top side looks very similar.  The larger Tintin does not even have soot on the edges.  Overall this probably cost me $60 and it has made a big difference in the cut quality and made this more safe as well.  Now to experiment with smaller and more quiet compressors.

  • Progress

    David Tucker03/26/2021 at 01:38 0 comments

    I have my laser wired up and running now with full control over power in GRBL.  So far I have been able to slice up a few sheets of paper and burn some wood.  I have not tried cutting wood yet.

    I went ahead and used the NEJE PWM/Temp module to act as an intermediary between the laser and GRBL.  It is not needed but it has a few nice features. First it can read out the temp of the laser module and display it at the push of a button, even while the laser is running.  It can also be used to manually drive the laser for focusing.  It also displays the requested laser power when under GRBL control, that acts as a nice double check so I can see if GRBL is doing the right thing.  Finally it has an on/off button so you are less likely to accidentally trigger the laser when your not ready for it.

    I printed out a nice housing for the module so it is not quite so sketchy.

    I'm just getting started but here are some observations so far

    • This laser produces smoke, but less than I was picturing.  Still it is enough that it is no fun to breath and it can give you a headache.  I would say it is less smoke than an incense stick.  Still it is not nice smoke, it is enough that I will be getting carbon filters for my respirator and working on a proper air filtration system for the shop.
    • It takes only a tiny amount of air to blow out the flame and smoke produced by the laser. I experimented with different levels of air using a compressed air duster and even the smallest trickle I could produce was good enough.  This explains why fish pumps are so popular.
    • The glasses mess up your vision, they very effectively block all blue and green light and after wearing them for an hour your eyes become overly sensitive to those colors making you feel off.  It takes a half hour for this to go away afterwards. You probably get a similar effect from tinted sun glasses.
    • It is hard to dial down the power on this 30W module, on top of that GRBL is only capable of 256 levels of PWM.  I found that for MDF above about 20% power just burns the wood, and below 5% power has little effect on the wood so you only have around 40 shades of grey that you could produce at best. In practice you are probably limited to something closer to 15 shades if you dial your power in just right.  The spot is definitely rectangular as well and relatively large.  I'm seriously considering picking up the 2W module as well to do engraving.  It is only $30 if you don't get any accessories, and in theory it should fit the same mounting bracket and use the same wires.
    • The controll software side of things is not great: 
      • I really love Universal GCode Sender but it does not officially have a laser mode.  If you could produce gcode from another tool you could still use UGS to control the machine but that sounds complicated.
      • LaserWeb is screwy, I could not make any progress with it.
      • LaserGRBL is nice but it has quirks.  For example you have to tweak the image as you import it, once you bring it in there is no going back and editing the line spacing or speeds.  There is no real layout mode, you import the image, trim it and that is it.  Its jog mode is not perfect either. On the upside it is free, and fairly functional as long as your working with an image that was already processed in another program.
      • LightBurn is reasonably priced at $40, much better than the several hundred dollars needed to get VCarve working on the milling side.  It has very strong page layout tools and some ability to convert images to vectors and edit them.  With that said it is no as intuitive to edit with as Inkscape or Illustrator, but if your doing something fast it works.  On the down side the GRBL support is confusing.  Jogging is horrible, and there is a lot of confusion in my head about where the origin is.  No matter how I set things up it seems to shoot off in a different...
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  • Focus

    David Tucker03/26/2021 at 01:21 0 comments

    I found focusing the laser to be confusing at best, but eventually hit on a bright idea.  Others have done a trick where you tilt a piece of wood at an angle then draw a series of lines at right angles to the slope.  Then by carefully looking at the lines you can work out what one is the thinest and therefore what height is an optimal focus.

    I don't like this trick, it is too complicated and not very accurate.  However I decided to try a different method that seems to work much better.  I used a board at an angle and manually jogged the laser up and down the ramp while adjusting the speed/power till I only burned a line when the laser was roughly in focus.  Then by measuring the line and marking the center point I have a very precise record of the optimal focus point.  Finally I set the laser to a very low power and manually jogged it to that focus point and then measured the height to the base of the laser housing.

    I came up with an optimal laser height of 15 mm to the bottom of the housing.  NEJE says 5mm is a more optimal height, so I need to play with the lenses a bit, but I will save that for another day.

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Discussions

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.

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

  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.

http://linuxcnc.org/

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