New 3D Printer (and some early mods)

Back in 2013 I built a RepRapPro Mendel from a kit.  It took me five consecutive evenings to put it together and get it running.  I learned a lot about the mechanics of cartesian FDM printers (or "glorified glue guns" as a gentleman of my acquaintance put it) from assembling that and then using it for the last 6 years.  Time and market forces move on, though and for 60% of what I paid for that kit I can now get something like the photo on the right, which has a much bigger build volume and a stronger structure.  Those market forces have simply made the old technology cheaper, though - the microcontroller running this, and the firmware running on it are essentially the same as on my old Mendel. Maybe I'll keep the old thing going to try out some more modern controller electronics.

The new printer is a Copymaster 300, supplied by Technology Outlet.  Why did I pick this one?  Well, it was on special offer (<£300, still is at the time of writing), it comes with dual z-axis lead screws and has two y-axis rails where many printers costing up to £300 have only one.

The mechanical structure of this, like most of the current crop of Chinese-built imports is built down to a price point using a simple arrangement of aluminium extrusions butted together at the joints with a couple of T-shaped plates for a bit of extra bending resistance where the z-axis frame joins onto the base.  They work surprisingly well straight out of the box in most cases, as reviews and sales of, for example, Creality's CR-10 and Ender 3 models demonstrates.  The lack of triangles in these structures offends my engineering sensibilites greatly though - for a relatively small amount of extra material the rigidity of the z-axis frame could be improved immensely, and IMO it needs improving.  The motion of the extruder causes left-right oscillations of the frame and the motion of the print bed in the y-axis generates front-back oscillations.  Both of these get amplified as the height of the print increases, and both are made worse by having the filament spool right at the top of the frame.  That reel can hold up to 2kg of plastic filament, having that waving about at the top there could generate some serious vibrations unless you print really slowly.  I decided to do something about this by adding some extra bracing.

The Left-Right Bracing
There's not really much opportunity for improvement here, since the bracing has to leave clear the area swept out by the x-axis beam, the carriages which run up the vertical beams, the extruder assembly and its cabling.  It must also not interfere with the z-axis lead screws.  Fortunately, to allow clearance for the cabling and the filament sensor, the z-axis movement stops about 50mm below the top bar leaving enough room couple of braces could be added.

I used a bit of aluminium right-angle section and some of the extra t-nuts that the manufacturer helpfully supplied with the printer along with the tools for final assembly.  I do wonder if they had something like this in mind, especially considering that the tool kit included allen keys in sizes that aren't needed for the final assembly of the printer, but which would be useful for dismantling it for modification.


The Front-Back Bracing
There's much more space for this, and it's much more important.  The vibration in this direction is more severe for two reasons I can see.  The weight of the print bed is much greater, which will tend to induce vibrations of greater amplitude, plus the vertical beams are thinner in that direction and therefore more susceptible to bending.The only clearance worries here are the tops of the z-axis carriages, the rather large pulley cover, which also serves as a mount point for the x-axis limit switch and the clamping belt tensioner at the right of the x-axis beam which has a tieing point for the extruder cable bundle sticking up from it.

To clear all these, it was necessary to add a triangular bracket to the sides of the vertical beams.  Fixing this to the vertical structure at two points fixes the third point realative to the structure so that a diagonal brace then becomes one side of a second triangle rather than a quadrilateral.  Once again, there is just enough clearance to the z-axis carriages to do this, when they are at the top of their travel.  There is also just enough clearance between the cable tie point and the diagonal brace - it looks in the photo like they are touching, but they're not, quite.  Also, not shown here, I used my old Mendel to print a more compact pulley cover for the x-axis motor which puts the limit switch on top rather than in front and there by allows more clearance for the diagonal brace on the other side.  The hex-headed fastener holding the bracket to the top beam is a self-drilling screw with the drilling part cut off, leaving enough length to hold the bracket on without running into the screws holding the horizontal beam to the vertical beam.

The diagonal braces are made from thin-walled steel tube sections from an old collapsible gazebo, which suffered the inevitable fate of those things in a high wind.  There were some straight ones left.  The ends were formed by crushing them in a vice until they were just thin enough to insert some 3mm thick aluminium strips into the ends.  They were then crushed down onto the aluminium and drilled.  First the holes for the end which attaches to the base was drilled, then it was fixed loosely to the base and the centre of the other hole was marked by inserting a drill through the hole in the bracket and rolling it by hand with its tip against the diagonal beam's end.

This all seems to have made it less susceptible to wobbling, so now I'll just have to go and print something 400mm high and see what the top looks like.  That will probably take about 2 days to print.

Update:  I see that Creality now have a variant of their CR-10 (CR-10 Max) with similar, but IMO slightly inferior diagonal bracing.  Plus an even bigger print volume.  At the time of writing, it's still not available but you can pre-order it.