Further 3D Printer Mods

The structural mods I described in the last post do indeed help with rigidity, but there's still a bit of an elephant in the room in the form of the lower frame side rails.  In order for the vertical frame to oscillate left to right, the lower frame rails have to twist where the vertical frame joins to it.  As supplied, those lower frame rails are not very good at resisting torsion - they're just a horizontal 40 x 20mm extrusion hanging in mid air at the point where the vertical frame joins them.  You can see this in the picture, along with my solution.  I found a bit of 45x45 extrusion in a skip which turned out to be long enough to span the full width of the printer with a bit of overhang.  I used the horizontal beam from the top of the vertical frame to mark out four bolt holes that line up with the central tubes of the vertical beams and some longer bolts (M5x60) to to attach it simultaneously to the lower frame and the vertical beams.  This makes a huge difference to the rigidity of the vertical frame.  I might have a go at making a video to show just how big an improvement this gives.

For the next improvement, I think I'll print some feet to support the printer underneath this extra beam, and so unload the horizontal frame rails almost completely.

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.

Mini Integrated Amplifier

For this project, I did something I've never done before - I planned a stripboard layout in advance.  Normally when I build things on stripboard the circuit is so simple I can just make it up as I go along, but this one is a bit too complex for that.  In fact, given the time I spent plannning this layout, I could as easily have done a proper PCB & etched it.  Oh well, it seemed like a good idea at the time.  It works well enough too.  Without further ado, here's the stripboard layout for the preamplifier, as seen from above the component side.



It's intended to be put into its box 'upside down', hence the unconventional layout of the controls - these will be in the reverse order in the finished item.

Also a first for me is the use of electronic analogue switches for input selection - I've had some 4051 8-way multiplexers for a while and, now that I come to use them, I find that TI have stopped making them.  I've used only four inputs (that's plenty for a little amp like this) and, for simplicity have used inputs 0,1,2 and 4 that way I only need to worry about one control line for each input.  Had I chosen to use input 3, I'd need to keep S1 and S2 high without allowing 'false highs' when inputs 1 or 2 are selected.  That would have involved extra diodes.  Incidentally, I've omitted from the drawing the (very much needed) pull-down resistors for S1, S2 & S3 and the link wires to connect those pins to the corresponding ones on the second chip.  The resistors were mounted vertically with the ground connection at the top so that the leads from the 2nd and 3rd resistors could be soldered to the lead from the first.  I wasn't sure how to convey that in the diagram.  The link wires were soldered on underneath to save space.  Likewise, the chips' power supply decoupling capacitors were soldered underneath as I've always found that much easier than trying to connect a capacitor from one corner of a chip to the opposite corner with stripboard.

So what was the big idea with this thing?

My old Creek 4040 amp, which was doing sterling service providing sound in my spare room/workshop, along with my turntable, is now at my mum's so she can play records on something vaguely decent rather than a shabby old plastic thing.  I needed to replace it.  Whilst shopping for sound effect generators for someone else's project, I found this 20W class D power amplifier board:
https://www.adafruit.com/product/1752

It turned out to sound pretty decent, certainly better than I'd expected.  So I thought I'd just bolt it onto a preamp, stuff it in a box and off we go.  This was going to be a quick & simple project, so I had the notion of using up some stripboard for the pre-amp.  Turned out to be a bit less simple than that, especially planning the stripboard layout, then getting all the track breaks in the right place and wiring up the links.

The Preamp circuit

The power amp runs on a 12V single-rail supply, so it seemed obvious that the preamp should also.  This presents the small problem of biasing the inputs so that they never go below the negative rail voltage.  This is essential not just for the preamp circuit itself but also the analogue switches - they're not very good at conducting signals that go outside the scope of their supply rails either.  The solution is shown below in the section of circuit on the left, which uses an op-amp to produce a virtual ground at half the supply voltage.  All the inputs & outputs are referenced to this, rather than to a supply rail.  Any currents which find their way to the virtual ground can be drained away by the op-amp rather than shifting the voltage at the virtual ground. A TL072 isn't really the best choice for this, NE5532 would be better as it can source or sink more current.  It's plenty good enough for this though, especially since all the other op-amps are used in the inverting configuration which doesn't source or sink any currents to the ground unlike the non-inverting configuration.

The tone controls are the Baxandall single-capacitor type, and the circuit shown here is lifted straight from Douglas Self's book 'Small Signal Audio Design'.  It's a bit dramatic for my taste, offering about 16dB of cut or boost at the extremes & I'll probably tame it by adding some more resistors at the ends of both pots and maybe changing the capacitors.

Here's the assembled circuit mounted on the amp front panel.  Input selector switch is on the left, 3.5mm jack on the panel is connected to input 4.

The back panel, with speaker sockets, power & audio inputs plus the power amplifier module

The Box, and the Finished Article

I found this old KVM switch in a skip,  It seemed about the right size for this project.

The front and rear panels needed replacements, with all those big holes in the wrong places and all.

Here's the back panel.  Speaker outputs on the left, power jack in the middle and the first 3 inputs on the right.  Yes, they're DIN plugs.  I like DIN plugs.  They're like professional audio connectors, only smaller.

Here's the front panel.  One day I might paint & label it.  Might also get some more consistently-sized knobs.  From L to R, input selector, volume, balance, treble, bass.  3.5mm jack is the fourth input, not a headphone socket

You know what, though, I reckon there's enough space in there for an RIAA preamp too.  Maybe a headphone amp as well.

So What Does it Sound Like?

Not too bad at all, really.  No nasty noises, no 50Hz humming or buzzing, and not much in the way of clicks or thumps, except when switching off, but even that's pretty subdued.  It does need a good power supply though, the first one I tried produced some horrible high-frequency squealing, not very loud but annoying.  As I mentioned earlier, the tone controls are a bit too dramatic for my taste, but they work as expected.

One very big caveat though - some sources need to have the ground AC coupled as well as the signals, unless it's being run from a battery.  The negative rail of most mains power supplies is connected to the mains earth, and so is the signal ground of many pieces of mains-powered audio equipment.  This will cause problems as the op-amp is trying to maintain the signal ground about 6V above that.  My solution was to modify a signal lead to AC couple both channels and ground to use with such equipment.  I could circumvent this by using the spare poles on my input selector switch to switch the ground to the appropriate input & AC couple that to the preamp's virtual ground, but then I'd also have to add coupling capacitors to all the inputs, otherwise they could stray outside the supply rails.  There might also be thumps & pops when switching inputs.  There's no problem with battery powered sources and 'm only using one mains-powered source so I'll leave it as it is for now.