Dual extruders

I bought a Kraken hot end a while back on a whim, I never really intended to run all four hot ends, but dual extruders have been something I've intended to try for a while now.

My primary use case for dual extruders has always been peel-able support, but I had no real success using ABS as support for PLA or vice-versa, the bond between the materials is just too weak. I did order some T-Glass being a PET derivative it might work with PLA, I'll post an update when I've had a chance to test it.

Having failed to get useful peel-able support I thought I'd try some multicolor printing, and as you can see from the image above it came out rather well.

While going through the process to obtain the prints above one thing that became very apparent was that support for multiple extruders really is still in it's infancy. Really the slicer needs to deal with a few issues, the first is priming the extruders after a layer change, both KissSlicer and Cura provide wipe towers to do this (though IMO Cura's solution is better), the second is providing some mechanism to deal with ooze from the hotend that isn't in use. It doesn't matter how much or how fast you retract the filament, at least a small amount will be left in the melt zone, ooze out and get wiped on the edges of the print. KissSlicer has no real answer for this, Cura provides what IMO is a really good solution to this in the guise of the "ooze guard", basically it's a shell about 2mm out from the edge of the print, as the unused extruder passes over it,  it loses the plastic it's oozing.

You can see above what the print looks like when finished including all the oozed filament wiped on the outside of the shell.

This is the print removed from the bed, you can see the gap between the shell and the print, it's painless to remove the shell since it doesn't actually touch the print.

I also bought a copy of Simplify3D to play with, right now it's dual extruder support is incredibly basic, no support for a wipe tower and no mechanism to deal with the ooze, somewhat disappointing for a $140 product. There is also a new KissSlicer Beta with supposedly improved multi extruder handling that I have yet to try.

If I get a chance after I've used it for more than a couple of prints I'll write up a review of Simplify3D.

Quick filament drive update

It's been a bit hot in my garage to spend any significant time in there, but I did get most of the XY Stage prototype running. There is supposed to be a second plate on the right hand side, but it's stiff enough without it, so I'll wait until it's a bit cooler to machine it.

There is a small geometry error in the carriage design, I set the spacing for the bolts used to secure the fishing line from the outside edge of the bolts instead of the center. This actually results in measurable error, about 0.4mm as the carriage moves from extent to extent across the bar in the H. So I need to print a new carriage.
The XY Stage is attached to the Arm based electronics, and I need to make a couple of changes to the firmware before I can do any real speed testing, since it doesn't really support acceleration. For point of reference in the video the motion is at about 140mm/s with instantaneous change in velocity (i.e. that's the jerk and velocity setting). I also need to add support for the modified kinematics.
In the mean time if I get a chance I'll set up a test to see if there is any measurable error accumulation over a few hours.

E3D V4 Hotend - follow up

In the end I couldn't reliably print PLA with the original E3D V4 Hotend, Sanjay from E3D was nice enough to send me a complete replacement, plus some extras.
I spent 3 or 4 days printing PLA with the new hotend, and while I can get good results from it, I don't think it's an ideal solution for printing PLA.

First I want to sow you how good a print the hot end can produce, this print is in Ultimachine Natural ABS 0.2mm layer height, the model is scaled by 50 %.
I doesn't photograph particularly well, but looks really good, it's hard to find something to criticize about the print. Perhaps the best way to describe prints from the E3D Hotend is "sharp", it does a great job of with detail, as good or better than any Hotend I've used.

 

The following is the small army of buddhas that are my attempts to get a really good PLA print out of the Hotend.

This army does not include the many complete failures I had early on, any attempt with too aggressive a retraction setting (anything over 2mm on my boden setup) resulted in a print failure with the Hotend jamming. Any attempt to print under ~225C resulted in a jam and failure.
The really observant of you will have noticed that although most of the prints are actually very good they all have issues when printing the head. The forwards most print is perhaps the best of them, so lets take a closer look.

It's somewhat hard to see, but if you look closely what happened in the print, is that at the start of a layer the flow of plastic did not immediately resume. Very shortly afterwards when it did resume, the pressure that had built up during the "stall" caused excess plastic to be extruded part way into the layer, producing the distinct ring on the forehead.
What you can't see without watching the print is why this happens, there are two very distinct places when printing the model where the cooling settings cause the print head to greatly slow down, one is at the top of the coat and one at the point you can see the issue. Some of the models have similar issues at the first point.
What I believe happens is that the retracted filament sticks to the inside of the stainless thermal break when it's primed, and the low extrusion rate isn't enough to overcome this additional force. The pressure slowly builds during the layer and when extrusion resumes you get all the excess plastic deposited very quickly.
My reason for believing this is that you can see it happen when the hotend is blocked, it simply will not extrude, the stepper will stall trying to drive more filament in, you wait perhaps 30s or retract 30mm of filament, and suddenly you see a lot of very liquid filament spew out and the Jam is cleared.
You appear to be able to reduce this effect by increasing the temperature of the print, but I don't have enough cooling to print PLA at much more than 235C (where this print was done) without it introducing other issues.

Now this particular model is particularly challenging at this scale, and I have printed several other PLA pieces that look very good, but there are plenty of none all metal Hotends that will also print this model without the issue.

My conclusion is if you don't print a lot of PLA then this Hotend is really very good, and well worth the investment, the fact that E3D have been responsive in dealing with problems is a big plus as well.
If you primarily print PLA I'd wouldn't by any all metal Hotend right now with the intent of using it as your primary Hotend.
I'm beginning to think that there is no perfect single hotend and the right approach is to have at least 2 and make it fast and painless to swap between them, which I suppose gets back to my 3D printer automatic tool changer idea...

A better filament drive?

Ever since I saw Sublime's Tantillus 3D printer using spectra fishing line, I've ben really intrigued by the idea, in fact I built 2 Hbot prototypes using the idea.

The fundamental issue that comes up is that the fishing line "walks" as it wraps around the pulley, this changes the angle of the line, which in turn affects the tension. The problem is worse on a HBot because of the single long belt the number of times that the belt wraps around the pulley to go extent to extent can be large. In fact on my first prototype where I didn't try and address this there is a noticeable change in tension.

There are basically two way to approach the drive Tantillus passes the line through the "pulley" and wraps the filament on and off, this makes it impossible for the line to slip, but it's also difficult to address the walking filament, on Tantillus the combination of short axis and linear line path make this a minor issue that doesn't seem to impact the print quality.

The other option is to have sufficient friction on the drive pulley to prevent slipping, this requires a significant contact area between the line and the pulley and significant tension on the line.

The idea of the figure 8 line drive above is to use an idler to continually reset the filament position, basically the drive pulley in this case has 4 grooves, and the idler 3 that are offset by 1/2 the groove spacing. The filament always rolls onto the top groove on the drive pulley and off the bottom one.

There is no filament walking, and the equivalent of multiple wraps around the drive pulley to provide additional friction.

You can see the prototype running here

As a point of reference I'm running it off the LPC1768 based electronics discussed in other posts.

The idler has 2 9mm bearings in it's center.

I stole the idea for this from somewhere, I think the first time I ever saw it suggested was on the CNC Zone forums regarding wire drives for routers.

I'm pretty happy with the friction from the 3 figure8's, I might reduce the groove spacing on the drive and idler from 1.5mm to 1mm, but 1.5mm seems to work well.

My intent is to use this mechanism on my 4th HBot prototype, though I intend to use the CoreXY mechanics rather than the HBot mechanics, I have most of the X/Y Gantry laid out in CAD, hopefully I can get the CAD work complete this week.

Ultimaker style XY gantry

I've been on and off experimenting with HBot designs, there are some videos of a couple of my prototypes on YouTube here and here if your interested. The issue that arises is that the belt geometry introduces a racking force that if the design is not extremely rigid causes crosstalk in the motion as the carriage moves across the bar in the H.

The Ultimaker design has many of the same advantages, notably static motors resulting in low moving mass,  the above video was a simple X/Y stage I put together so I could better understand the pros and cons.

The obvious con with the Ultimaker design is that the outside travel rods also rotate, so you can'y use LM8UU style bearings there. I'm using 1 inch long bronze bushings on 5/16 drill rod. Again I can't use 8mm rods because an 8mm precision rod won't fit in the 8mm hole of the 608 bearings I'm using to support the rod. The rods for the cross piece are 8mm and on them I'm using LM8UUL bearings since they do not rotate.

There is surprisingly little play in the design, the coupled belts remove almost all of slop.
The design is however extremely sensitive to alignment, towards the end of the video above you can hear one of the bushings vibrating, that's caused by the cross piece being very slightly askew resulting in increased static friction on the bushing. I fixed it just after shooting the video by loosening two of the pulleys and adjusting the alignment.

I have a couple of other issues in this prototype, the printed corner pieces aren't really rigid enough and it needs something to stop the 7/16th's rod moving in the 608 bearings, but it's pretty promising.

I have some parts here for 4th HBot prototype, my intent is to use the CoreXY mechanics this time, it looks like it resolves the issue with unwanted racking forces. So it should make an interesting comparison. I just need to make some time and finish the design.

ARM motion controller Part 2

Ok so we don't want to solve a quadratic equation every time we make a step.

We want all of the moving axis to be move in a synchronized way, and be able to implement acceleration on top of that with a minimal additional cost.

The idea here is to decouple the acceleration from the linear motion, if we pick a fixed step rate and quantize all of the axis motion to that rate, then we could just treat the motion itself as linear and then change that rate independently to implement acceleration.

The nice thing about this is that acceleration calculation  is completely decoupled from the step interrupt, this is nice because the acceleration calculation still involves a divide which isn't cheap on the microcontroller.


The requirement of all the axis being synchronized is now also easy, the problem become analogous to a simple line drawing algorithm, it just happens to be in N dimensions which as we'll see later doesn't actually make it any more complicated.

So what should we select as our initial "fixed step rate"?

It must be high enough so that you are not required to do more than one step on any given axis per interrupt.
We could pick some arbitrarily high number, but this would result in interrupts where no steps are emitted and that would limit our maximum speed. And starve the rest of the system of CPU resources.
Instead it's better to pick the longest axis in our multidimensional line, compute a step rate based on than and it's initial velocity, such that it would output exactly one step per interrupt.
We then just need to determine if for any given step on that axis we need to output a step on the other axis as well.

There are a couple of ways to approach this, we could implement a DDA with fixed or floating point math, though this would accumulate error over the length of the line because of the limited number of bits in the representation, or we could just use the Bresenham line algorithm which uses only integer math, and therefore introduces no error in addition to the necessary quantization.

I'll quickly derive the bits of the Bresenham algorithm we need here, but Wikipedia has a pretty nice description if you're interested, though it only discusses two dimensions extending it to n is trivial.

The basic DDA is something like this for a stepper driver

// Primary axis is trivially the longest one 
select a primary axis P

// Initialize
foreach axis A
    Ad = (Aend -Astart)/(Pend-Pstart)
    Apos = Astart+0.5;   //Center of the "pixel"

// Steps
foreach step in P
    foreach axis A
        Aposnew = Apos + Ad
        if (Aposnew != Apos)
            output step for this axis
        Apos = Aposnew

It's a relatively easy to realize you don't actually need to track positions, just the fractional portions of them and you end up with.


// Primary axis is trivially the longest one 
select a primary axis P

// Initialize
foreach axis A
    Ad = abs((Aend -Astart)/(Pend-Pstart))
    Aerror = 0.5;

// Steps
foreach step in P
    foreach axis A
        Aerror += Ad
        if (Aerror >= 1)
            output step for this axis
            Aerror -= 1;

Looking at the above algoritm the only error we introduce is computing Ad where we have to divide by (Pend-Pstart), so all Bresenham does is multiply everything through by that and you end up with.


// Primary axis is trivially the longest one 
select a primary axis P

// Initialize
Plength = abs(Pend-Pstart)
foreach axis A
    Ad = abs(Aend -Astart)
    Aerror = Plength >> 1;

// Steps
foreach step in P
    foreach axis A
        Aerror += Ad
        if (Aerror >= Plength)
            output step for this axis
            Aerror -= Plength

Now we can do everything with integers, no multiplies, our only divide is a divide by two which can be done as a trivial shift (the bottom bit doesn't really matter). All dropping the bottom bit in the divide by two above does is offset the concept of the pixel center very slightly. It would in fact be entirely "correct" to pick any pixel center, but picking 0.5. 0.5 results im more natural looking lines. If its something that you think is significant you can multiply everything through by two.

What's our accuracy with the new approach?
Since we're quantizing steps in the none primary axis to steps on the primary axis, we are accurate to ~1/2 a step.
In practice I think this ends up being more accurate that the original implementations, because of rounding error introduced in the original when converting velocities to frequencies.

E3D V4 Hotend

I did some printing with Nylon a few months ago, I was running my hotend at ~245C for a lot of it which is getting close to the point where both the PEEK and the PTFE liner would be damaged. In fact when I stripped the hotend down to change the nozzle I did see signs of damage to the PTFE liner.

Several all metal hotends using a stainless steel thermal break have started to appear on the market, and I ordered the E3D one when they went up for sale.
This is what's in the package.

It's pretty much everything you need to build it, including the alan keys for the set screw and the bolts to mount the fan. E3D has assembly instructions on it's website, they are straight forwards and following the yields something like this.

The purple part is a mount designed for a JHead from http://www.thingiverse.com/thing:60539

Since I was replacing the hotend anyway I took the opportunity to add some inline connectors to make removing it easier.

Here it is mounted on the Rostock Max

Some observations, without the fan the heatsink gets very hot, with it, it's cool to the touch even close to the thermal barrier.
I pushed some filament through by and it's somewhat interesting, there is some initial resistance, but one the filament is moving that seems to clear, more on this in a bit.
I plugged everything in, reset the Z Height on Max, I ended up losing about 12mm of Z, though that could be regained with bigger spacers.
I started a print using PLA and the hot end immediately jammed, after some messing about I did get the print started and it worked well until it jammed part way through the print.
After some experimentation I think the issue is that I ended up mounting the fan perhaps 10mm higher than the supplied fan mount and there is insufficient cooling at the base of the heat sink, this in turn is increasing the length of the melt zone and the PLA is doing what PLA does and jamming.

So I tried a different mounting configuration which allowed me to retain the original fan mount.

This seems to work better, but I still had a hell of a time getting the first layer down, it would get half way through and jam. In the end I set the temperature MUCH higher than I usually would for PLA, the print above is running at 250C. It doesn't behave like PLA at 250C, usually it has the consistency of water at that temperature.

So I pulled out my thermocouple and stuck it in the spare hole on the aluminum heater block, it reads 245C, so the thermistor isn't far off.  I've been printing the same plastic at 190C in the old hotend.
Best guess is the cooling fan is causing a significant thermal gradient in the AL block and as a result the temperature at the outside of the block is significantly different than the actual temperature of the brass nozzle. Unfortunately the thermocouple won't fit into the hotend itself, so I can't verify.

Anyway here's the finished print, really pretty good.

No conclusion at this point, I need to run more PLA through it and see if I can get a handle on how best  to make it work, and obviously run some ABS and Nylon through it.