Thursday, October 28, 2010

Annoyances and changes

UPDATE: I let my anger run away with me in this blog post. I've just spent time with Ulf Lindhe at Netfabb. Ulf has sorted out the problem I was having with the boolean operations and got me upgraded to version 4.6 at no charge.

It's taken a few days, but I can now print at about the same quality with a 0.5 mm extruder as I was able to print with my now ruined 0.3 mm extruder. I've gone back to work on the active telepresence project and the present task is to create linear servos to as the muscles to operate the hand and fingers.  I'm building my own from gearmotors rather than buying after some bad experiences trying to adapt servos designed to operate the flaps and rudders on model airplanes and sailboats to operate hands.  Andreas Maryanto was good at this, but I am definitely not.

Doing this, I am leveraging the herringbone rack and pinion scripts that I wrote.  While Nophead's advice to simply not fiddle with my 0.3 mm hot end settings got me 95% of the way to where I wanted to be, that last 5% took a little tweaking.  I'm quite happy with the final results.

The pinions peeled off of the rafts with a minimum of trouble and only took a touch on the belt sander before they were usable.

That was relatively easy.  When I started working on the rack, however, I quickly ran up against the limits of Art of Illusion.  While it is easy to assemble a set of solids to represent what I want to print in AoI...

Doing the Boolean opeations to make them into one solid is quite beyond AoI's boolean ops capabilities.  Bogdan told me repeatedly that I could do this sort of thing in Netfabb Professional, so I read up on the method and set about to use my now copy and discovered that the Netfabb people had somehow disabled this feature in my copy.  I had a email from them about version 4.6 {I have 4.4} but no clear idea if I was going to get a free upgrade or have to pay them even more to do something that I am already supposed to be able to do.

The more I think about that the more pissed off I get.  If I don't hear from them in the morning, I am going to build boolean ops into Slice and Dice and give it away for free.  The way I feel just now I might just do that anyway.    I am getting sick and tired of getting ripped off.

Wednesday, October 20, 2010

Getting to the bottom of the Rapman hot end failure

In which your narrator gets to the bottom of why it was that his 0.3 mm Rapman hot end failed and perhaps even why it started causing resets prior to the failure.

The hot end failure that I experienced on 14 October is a relatively rare event. The only other person with a Rapman who reports the sort of ABS leakage that I experienced was Klazlo.

Once I had my Rapman repaired and usefully printing again, I turned my attention to doing forensics on the failed hot end. Once I took a hard look at the hot end and the pictures that I took immediately after the failure hints of the means and mode of the failure began to recommend themselves.

Taking a look at one of the forensic pics that I took on the 14th two items jump out at you. I've circled them in red.

If you look closely at the top red ellipse you will notice that one of the three bolts which secures the spacer on the hot end ensemble was either not tightened adequately at the factory or, more likely, worked its way loose via hundreds of heating and cooling cycles while in operation.  This threw the alignment of the hot end out opening a route for hot ABS to escape between the PEEK sleeve and the aluminum extruder barrel sleeve that fits around it.  You can clearly see the billowing flow of ABS in the picture.  ABS was only leaking on the side of the PEEK barrel where the spacer bolt was loose.  That part is straightforward.

The vertical ellipse on the right hand side of the hot end is much more ominous.  If you look closely, you can see that one of the thermistor leads has exposed above the sleeve leading down to the extruder tip where the extruder is housed.  A bit over 4 mm of exposed thermistor lead is open to the air.  You can see that happening a bit more clearly with this picture.

The larger red circle encloses the exposed lead.  The smaller one shows the recession of the purple plastic insulation on the lead that occurred once the ABS began to boil out of the gap between the PEEK and the aluminum and contacted the lead.

At first I thought that the hot ABS had actually caused the gap in the insulation.  Closer inspection, however, revealed that the lead had most likely been stripped too liberally and originally was only just barely covered by the yellow silicone sleeve.  The heat of the billowing ABS almost certainly caused the shrinking of the insulation from just being covered by the silicone sleeve to its present situation.  It's easy to see the shrinkage in the second circle.

I then removed the PEEK cylinder from the aluminum extruder head.  Originally, I had thought that the PEEK sleeve in contact with the aluminum might have shrunk.  It measured 10.09 mm outside of the aluminum sleeve and 9.9 mm inside.  That could have been either because of heat shrinkage of the PEEK over time or some sanding of the PEEK tube at the factor to get a clean fit.  Unless the parts had been measured before the hot end was put into service, it would be impossible to tell for certain.

I then removed the silicone tube from the inside of the PEEK sleeve as you can see here.

This is the most revealing forensic evidence in the whole exercise.  At first, it appears that the silicone tube has shrunk on the hot end.  While that is true to an extent, the reason for the shrinkage is actually because the inside diameter of the PEEK sleeve has shrunk because the PEEK has expanded within the confines of the aluminum sleeve.

It was fortunate that I bought several unassembled hot end kits before BfB began making premade ones because I had some disassembled PEEK and silicone tubes.  The unused PEEK sleeves had an outside diameter of 10.0-10.1 mm and a consistent inside diameter of  6.36-6.40 mm.

When I measured the failed PEEK sleeve the cold end inside diameter was 6.40 mm but the hot end had been reduced to 5.88-5.92 mm.  The silicone tube outside diameter on the hot end was more or less the same as the inside diameter of the PEEK sleeve.

What was revealing, though, was that the hot end of the silicone tube was virtually entirely closed.  It was impossible to slide a 2.9 mm ABS filament through it.  I was able to blow tiny bubbles through the silicone tube into a glass of water.  What that says is that the ABS filament was being melted in the silicone tube and squirted into the aluminum extruder barrel rather than being melted in the extruder barrel.

I don't think that that was how the hot end design was intended to work.

Getting back to that exposed thermistor lead, it is well known that blowing hot air past plastic surfaces will create a static charge.  Not only was that exposed lead within millimeters of billowing hot ABS it was also touching the bottom acrylic plate of the x-axis carriage {part 10034}.  Periodic static discharges off of this heated acrylic surface were most likely at least deranging the temperature measurements off of the thermistor and also causing the MCU board to reset.  During last winter, I walked in socks across the floor and picked up sufficient charge to where when I touched the MCU board the system reset.  


The failure of the hot end had two aspects.  

  • The loosening of the spacer bolt, most likely from repeated heating and cooling cycles of the hot end allowed for a hot end misalignment that created a path for leakage of hot ABS out one side of the hot end between the aluminum extruder sleeve and the PEEK sleeve for the silicone tube.
  • The swelling of the PEEK sleeve over time reduced it's inside diameter almost entirely closing the silicone tube and causing the ABS to have to melt in the silicone tube rather than in the aluminum hot end.  This was certainly not the intent of the hot end designers.

It seems quite obvious that the combination of a PEEK sleeve enclosing a silicone sleeve for the plastic filament need to be rethought.

The placing of an MDF plate and an acrylic plate at the top of the hot end spacers through which the securing bolt has to pass also needs some careful.  Replacing the MDF with a PEEK plate with the bolts secured by lock washers to the PEEK plate and recessed through the acrylic with the acrylic and PEEK secured by a separate trio of short bolts might be more successful.  An aluminum plate as a replacement for the MDF might also be considered.

Tuesday, October 19, 2010

Chylld is a parts design genius

As I mentioned earlier I replaced to destroyed 0.3 mm hot end with one of my spare 0.5 mm hot ends and began to get used to using the larger nozzle size.  After some frustrating tries at adjusting flow rates Nophead suggested that I stop fiddling with things and just use the settings that I'd used before.  He was right, of course.

A quick print of some of the bones of the hand yielded good results, so I decided to have a try at one of Chylld's corner replacements for Rapman cut acrylic.

So far there's been no sign of the wall-to-wall resets I was getting with my Rapman in the hours before the 0.3 mm hot end finally failed.

The corner block printed in 2:15 and  mounted with a minimum of drama.

There was a minimum of post print part processing required.  the x-axis bar was a little tight, but a brushing out of the hole cured that.  All bars had a tight, friction fit and the tightening screws were largely cosmetic except when the printer wanted moving.

Chylld is a parts design genius.  I've never seen anything work quite this easily.

Sunday, October 17, 2010

Operational again at 0.5 mm.

Okay, I got one of the spare 0.5 mm extruder heads mounted. I measured the output and discovered that I was getting swell to about 0.66 mm. That works out to 0.34 mm^2 cross sectional area.

When I was working with the 0.3 mm extruder head I was getting a swell to about 0.42 mm which works out to about 0.14 mm^2 or about 40% as much.

I tried printing a test raft using the old extruder set points and noticed that the base print roads for the raft, which had been well separated before, are overlapping now. In the morning I'll lop 60% off of the extruder's stepper speed and see what happens.

Repairing a catastrophic failure of the Rapman 3.0

Little did I know when I was upgrading Slice and Dice to do prints of laser-cut Rapman parts that I would soon be confronting the problem of replacing such parts without having a 3D printer to print the parts. That is an ENTIRELY different problem.

I had upgraded Slice and Dice to do a better job of printing my sample laser-cut part from last time and was doing trial prints when I noticed that I was getting an enormous number of resets. This was odd because ordinarily I only get one or two resets per month since I uploaded firmware version 4.0.2. I checked the humidity and it was a bit dry, so I fired up the hot mist humidifier and drove the relative humidity up to 52%. No joy.

The next morning I undertook a new print and noticed a mushroom of ABS had formed under the extruder.

When I attempted to remove the extruder, the mushroom of ABS proved to be too big to easily get out of the mounting hole.  I then began to disassemble the x-axis carriage that holds the extruder only to discover that the top plate was being held together by the grace of God and nothing else.  It crumbled into two major pieces and half a hundred small fragments when I began to remove the bolts.

As well, one of the tiny little bars that hold the x-axis belt had also broken in half as is readily visible in the previous picture.

As you might imagine, this all made me very cranky.  A quick calculation revealed that I had historically been spending considerably more on preassembled BfB hot ends that I was on the filament that goes through them.  I run a lot of filament, too.

I spent several hours chatting with both Iain and Andy at BfB.  They were attempting to be as helpful as they could, but somehow I came away with the same feeling that I get when I take my car into the garage and they say recursively, "It MIGHT be X.  We could work on that and see what happens."  I began to steam up a bit when they started talking about my not using "BfB gcode".  They'd latched on to that fact that I'd written my own STL processing software that produces gcode right out of their manual and tagging this as a possible problem.  They then started talking about the fact that I was using very short line segments {0.1-0.1414 mm} and that was possibly touching off heretofore unsuspected bugs in the firmware.  I pointed out that the Rapman is supposed to have 0.1 mm resolution and you can't print objects at that level of resolution unless the firmware can handle that short a line segment.

I finally decided that I had to stop talking before I said things I'd regret later {I do that a lot}, and have a good think.

Here's what it came down to...

  • I needed a new top plate for the x-carriage and a new hot end.
  • I was going to have to pay for these and it was going to take a week to get them
  • It was entirely possible that the hot end had been destroyed by a firmware bug
  • There was nothing to say that the new top plate would last any longer than the last one, viz, ten months.
No matter how I looked at that equation it just didn't seem to balance.  A big bone in my throat was the fact that BfB wanted me to pay to replace a hot end that it was very possible that their firmware had broken.  A bigger bone was that there was no guarantee that if I put the new hot end that I bought into the system that the firmware wouldn't ruin it, too, in short order.

It seemed to me that the most reasonable course was to see if the problem was with the firmware.  I typically print with a 0.3 mm hot end.  I like what that kind of resolution does for my prints.  Before I settled on 0.3 mm, however, I bought two, preassembled 0.5 mm hot ends, so I had those in stock.

I also had Bogdan's experience that replacing the top plate on the x-carriage and grounding the hot end to it would stop the resets.  This from the observation that resets were most often caused by a static charge building up on the plastic of the x-axis carriage and extruder and then discharging, causing a reset.  BfB which is apparently located in a damp environment had never encountered this issue.  I noted that resets tended to get quite common when the relative humidity in the room holding the printer dropped  below about 42%.  I'd sorted out resets, except for the ones I encountered most recently which have led to the hot end failure, I think, by using a hot vapour humidifier.  After seeing how the top plate had crumbled, however, the notion of replacing the acrylic one with an aluminum one began to sound very attractive.

I decided to acquire the means to cut both acrylic and aluminum.  Harbour Freight in Salinas had a very nice scroll saw on sale for $69 which would reputedly do the trick.  

I bought that, a sheet of 0.22 inch (5 mm) acrylic and a billet of 3.35 mm aluminum plate.  I decided to cut an aluminum top plate first.  I began the process by simply tracing the bottom plate, which was identical to the top plate onto the aluminum with a fine tip marker.

I did the rough cut with the scroll saw and dressed it with a grinding wheel and a half round ring file after having set the plate in a small vise.

I then remarked two holes at diagonal corners of the plate, drilled 1/16th inch guide holes and widened them to 13/64th inch {as close as I could get to the 5.2 mm holes as I measured them on the original piece.   I did this with a hand drill after securing the plate in a vise. 

This done I then bolted the acrylic bottom plate to the developing aluminum one and drilled the guide holes for the rest of the holes using my Dremel drill press.

I then removed the acrylic bottom plate, secured the aluminum plate in the vise again and drilled out the rest of the holes.  

Afterwards I cleaned up the finished plate with a wire wheel and checked it for fit on the x-axis.

At this point my dyslexia set in.  The plate is not symmetrical and I'd got it flipped and completely reassembled and tested the carriage this way.  I didn't notice the problem till I tried to fit the extruder into the top plate assembly and discovered the symmetry problem.  The next pic is from the original, incorrect assembly.

The new top plate works smoothly.  Now I've got to swap out my ruined, 0.3 mm extruder with one of my spare 0.5 mm ones.  I will be able to see if I have a serious firmware problem or whether I just have a design fault with the hot end.  It could be either, or both.

I'm entering a big of a crisis with respect to 3D printing.  I bought into BfB's Rapman because I wanted to do some printing instead of screwing around with printer design and problems all the time.  At the time, a year ago, it was a good move.  Rapman was a bit pricy, but it was solid and the components of it were affordable. The 32bit MCU board was a delight after all of the Linux/Arduino/Sanguino/Bullshitino nonsense.  Some months ago there was talk of extending the Rapman MCU to where you could parameterise the firmware setpoints to deal with different machines and extruders, like the Mendel, for example, or even machines you'd designed yourself.  As it stands, it's not clear that BfB can design reliable firmware for their own machine, much less a parametric firmware app that would make it applicable to a wider range of machines.  

On top of that, they've recently jumped the price rather dramatically.

As it stands, BfB's Rapman has two Achille's heels; their firmware and their hot end.  Neither are reliable and the hot end is very difficult to repair.  I'm told that BfB is working on successors to the hot end, but that does me no good at all.  I'd like to shift over to something like Nophead's power resistor driven hot end.  The problem with that, however, is that I'll have to design a MCU to drive it and the printer both.  By the time I've done that,  BfB is out of the picture, since the those two components are what is defensible as corporate worth in the BfB.

I don't know quite what to do.

Sunday, October 10, 2010

Replicating laser-cut parts

In which your narrator confects his own idiosyncratic method of replicating laser cut parts.

The recent acquisition of the UK manufacturer of the reliable Rapman printer, BitsfromBytes {BfB}, by the American firm 3D Systems has exacerbated a long standing problem that the Rapman's users' community has had, vis, getting drawings of the laser cut acrylic parts that make up the system. While BfB uploaded drawing files of one of their early Rapman designs into the Reprap website, they have neglected to do so with version 3.0 and later models to the best of my knowledge.

That wouldn't be such a problem save for the fact that the acrylic parts in higher stress parts of the Rapman tend to develop shear cracks and spalls after hundreds of hours of operation. Rapman owners are then left to either request replacement parts from BfB or print their own spares. Interestingly, there is a healthy spares design effort underway for the Rapman, part of which is hosted on the BfB website itself.

Here the printed white ABS grips for the x-axis linear bearings replace the lower plate for the extruder carriage in a much stronger configuration that the original.  Indeed, at this moment, there is a very exciting redesign of its printable parts underway by an Australian newcomer that greatly reduces the print time required for what were finicky corner blocks.

All this aside, there are many pieces in a laser-cut printer that simply need to be printed as-is in ABS rather than redesigned.  Heretofore, I found myself carefully tracing the parts out on paper and using calipers to get the dimensions.  While that approach works well with many parts some, like the extruder z-depth stop plate are much more problematical.

This particular plate is the antithesis of rectilinear and locating the holes and pockets is tedious to put it mildly.  I'd thought that it ought to be possible to use an ordinary 2D scanner to capture this sort of information.  Today, I gave it a try and discovered that it was not as straightforward an operation as I'd imagined.  

I had an extra copy of this part that I'd bought still in the protective blue film, so I threw it in my Epson Perfection V500 Photo scanner, a very cheap and extremely high resolution machine.  Even with the blue film the image captured didn't have a lot of colour information that would let you think that you could separate the part from its background.

I pulled that one into Slice and Dice and tried to separate out the image with RGB manipulations to no avail.  I then tried putting a intense red background behind the piece figuring that the blue would override the red and tried playing with the colour mixes in both Slice and Dice and Photoshop, again to no avail.

Two things were killing me; the transparency of the part and it's depth, which you can see very clearly in this scan.  When I tried to just capture the edges by going high contrast, the depth of the object spoiled everything.

It occurred to me that I could paint the part on one side with tempra paint, which can be wiped off of slick surfaces.  Rather than drive into town and given that the part was already protected with a blue film I simply spray painted it with red enamel.  I then put it back in the scanner with a piece of dead black HDPE sheet behind it and instantly got the colour separation I needed. 

Popping the scan into Slice and Dice and filling the black with green, I had an image I could work with.  As you can see, it was a little nasty, largely due to the messy laser cutting on some features and a bit of flare here and there.

A few moments in Paint cleared that up.

Now that I had crisp colour separation, defining the part boundary in Slice and Dice was trivial.

While I was in paint I took the pixel counts across the diameter of the outer circular feature boundary and measured the same feature on the part with calipers.  I then adjusted the size of the image in Paint.

With a bit of pushing and shoving in Slice and Dice, I processed the part and created a print file.  Here you can see the print roads for the part.

With a print file, I did a trial print to check the dimensions.  The acrylic part lay precisely over the printed part.

I photographed it again with the original part slightly ajar so that you can see the holes in the print.

If you look closely at the several layers of part print that I ran before aborting it you can see that I need to increase the print flow a touch.  More importantly, with a part of this size and complexity, however, I am going to have to write a routine that makes a better job of reducing transition distance between print roads.  Transitioning was taking far too much of the machine time.  That's on my "to do" list now.

What I've demonstrated here is not a smooth operation on Slice and Dice just yet.  I was mostly trying to prove the concept.  That was a huge success.  What this means is that owners of laser cut reprap machines can readily exchange parts information with nothing more complicated than an ordinary 2D scanner and a set of calipers.  This should give us considerably more flexibility than we have at the moment.

I am certain as I finish this blog entry that someone is going to show me a simpler, faster way to do this in a few hours.  That's certainly happened before.  If not, though, we have this approach.  :-D