Wednesday, April 27, 2011

Sampo: The return of Darwin...

Actually, Darwin never really left.

Despite all of the hoopla about Mendel, a very robust Reprap machine which is virally spreading like a bad flu, Darwin derivatives have been quietly building up numbers largely through Bits From Bytes' Darwin-derived Rapman printer.  It would be fairly safe to say that there are upwards of two thousand of these Darwin clones of one flavour or another in the field.

I bought a Rapman 3.0 in the Fall of 2009.

Rapman is basically a Darwin re-engineered for laser cut acrylic plastic instead of printed parts.  I've never been able to decide whether Rapman is a crib of the Ponoko laser cut acrylic Darwin or vice versa. The Rapman was a brilliant choice for me.  I had been working on a series of Repstrap systems.  Early on in 2009 I realised that getting Tommelise 2.0 printing was going to take another half year at the rate I was going and what I really wanted to do was design things with a 3D printer in the design loop.  I saw Batist Lehman's video of his Rapman in action and was sold.

Of course, Rapman has its little ways.  It arrived disassembled as a flat pack.  Inside were several sacks of metric nuts and bolts heavy enough, if put in a sock, to serve as a very effective cosh.  Assembly was a daunting task and once finished there was a strong sense that you don't want to fool around with it for fear that it would break or some apart.

I was puzzled at first that lock washers hadn't been included with the nuts and bolts.  It quickly became apparent, however, that if you put enough torque on nuts to make a lock washer compress the high impact acrylic that Bits from Bytes used would shatter.  The net effect of this was that I keep a bowl beside my Rapman to collect the constant drizzle of nuts and bolts which unscrew themselves and fall on my work table.

Since then I've put well over three thousand hours on my Rapman.  It became apparent some months ago that  Rapman was not a heavy duty machine.  I started seeing stress cracks all over the machine and was faced with a x-axis extruder mounting plate that just crumbled away from the heat of the extruder and the mechanical stress of being moved around.  I replaced it with an equivalent aluminum plate.

Early on Rapman users began to design replacement parts for the bits of the Rapman laser cut acrylic that fell apart.  You can see a corner block designed in white ABS by Chylld in the picture above.

You notice that I only put ONE of Chylld's excellently designed, fast printing corner blocks onto my Reprap machine.  Once I got it on it has performed beautifully.  Unfortunately, I had to half way disassemble my Rapman to get the damned thing installed.  That wasn't an experience that I thought worth repeating.

Late in 2010 I decided that I wanted to experiment with dual extruders.  Bits from Bytes had come out with one some months before and I decided that I would get one.  When I called, they were so focussed on their new 3000 printer that they wanted me to switch over to that and did a hard sell.  I never react positively to hard sells, so I put off the purchase.  As well, the dual head Rapman had the same dimensions as the old one which meant that it had a reduced print table area as a result.

After the beginning of the new year Bits from Bytes was acquired by 3D Systems.  I attended the telephone meeting announcing the takeover and was so put off by the 3D Systems CEO, Abe Reichental, that I abandoned any plans to purchase any more Bits from Bytes equipment at all.

That left me with quite a dilemma.  I had never been particularly happy with the Mendel design.  Tommelise had used very similar kinematics.  The problem with it that it requires a larger footprint than the print table and is unstable in the x axis direction.  Mendel addresses this last issue with a electronics mounting board that doubles as a reinforcing plate to cure the x-axis problem.  Because of that, I decided to stay with the Darwin kinematics and run the system through another kaizen exercise.

Basically, I want to look into several possible improvements to my Rapman
  • a return to printed parts.  My experience has shown that laser cut acrylic does not make for a robust machine.
  • a massive reduction in the variety, as opposed to the number, of parts needed to construct the machine.
  • parts design which allows for easy partial demounting of the printer.
  • a design robust enough to be easily shipped fully assembled or quickly broken down and reassembled for exhibitions
  • room for two extruders without compromising the size of the print table.
  • cleaner y and z axis kinematics
  • a more capable controller board
  • shifting to the successful Wade/Adrian pinch wheel extruder 
My experience with trying to install the Chylld corner on my Rapman has caused me to want to rethink the whole issue of parts design for a Reprap printer.  I feel that we have, in a way, gone wrong when we designed the parts for Darwin and Mendel.  Because you can custom tailor individual parts with a 3D printer doesn't, to me, mean that it is good design practice.  Right now, to have a proper set of spares for a Darwin or Mendel you pretty much have to print a full parts set.  A lot of that stems from, in my opinion, from custom tailoring parts.

I don't think too many Reprappers are going to bite the bullet like Adrian Bowyer and Nophead {Chris Palmer} have and print whole sets of parts day after day and week after week.  Most of us want Reprap machines so that we can design and print other things beside new Reprap machines.  To that end, having a limited set of parts types that you can print a few at a time during down time when you are doing design, seems to me to be the way most of us are going to propagate more printers.

With respect to a new y and z axis design I want to revisit the cabling approach that eD did with the Darwin precursor ARNIE back in 2006.

But, enough talk.  Here is what I have so far.

Sampo's frame is built on a 600 mm module to give it that extra space for dual extruder print frame that Rapman's 400 mm does not. So far, the entire frame has been assembled with exactly five different parts.

Here you can see the Sampo frame beyond my Rapman 3.0 for scale.  I am working on the cable housings for the z-axis as this blog entry goes to press.


Marcus Wolschon said...

"The net effect of this was that I keep a bowl beside my Rapman to collect the constant drizzle of nuts and bolts which unscrew themselves and fall on my work table."

I replaced all the nuts that fell off with Nylocs. (locking nuts)

Forrest Higgs said...

Metric Nylocs are hard to come by here. :-(

Nudel said...

Why not use some thread-locking fluid? I don't have a single nylock on my mendel, as I like to be able to take things apart without replacing the nuts.

Forrest Higgs said...

The problem with Rapman is that it is made of laser cut acrylic. Tighten the nuts too much and it cracks or shatters. I'm using printed parts on my new printer. Ordinary lock washers work fine on it. :-)

Erik de Bruijn said...

Hi Forrest,

I've heard similar stories of Acrylic machines falling apart. It's the main reason why the Ultimaker has only birch wood and Delrin. Delrin is much tougher for mechanical parts. Wood isn't as binary (under tightened or cracked) but has a wide range where you have a good mechanical bond without damaging the plywood sheets and bolt-heads will have enough friction without washers or anything. Much faster to assemble.

I've never liked the PCB as structural part of the Mendel either. I've heard PCBs can damage during long periods of flexing. Of course many people solve this by putting a piece of MDF below it and reinforcing with that, but still, yu have very limited stiffness in the X direction compared to what is possible with Darwin with much less material/weight. Tall objects will become less accurate at the top when printing at higher speeds because they'll swing along (inertia of the printed parts), also, the moving head will still cause the X axis to flex.

Forrest, I've made a part that allows you to make sure the Z-stage
isn't over constrained.
I'm sure you'll find a way to do this without adding another unique part.

Putting the bed on 3 rods seems to me to be the better approach to prevent jamming of the Z-axis and and twisting the bed. I've had problems aligning the bed on both of my Darwins and on my second Darwin I converted it to this setup.

It seems like you're going for serial kinematics? Will the X motor be stationary or stacked onto the Y axis? I would suggest making it stationary.

B.t.w: I like how you've kept the part count low. Also allows for more attention can go into optimizing these building blocks.

BodgeIt said...

The solution to avoid cracked parts when using Acrylic is to always have a softer material that the nuts bed into this also provides nut locking.

A low cost thread-locking fluid is a pot of discarded nail polish/varnish they come with neat little brushes. Having 4 daughters it cost nothing to get a discarded pot of nail varnish.

Else any $1 shop will have nail varnish if you have them in the USA.

Or you could just use two half nuts if Nylocks are not easily avalible.

I use Half nuts as they fit flush in a fixit block. Printed parts could also be designed to have a nut counterbore like a fixit block has.

Bogdan Kecman said...

Great work. I do have a question... those linear bearings on the Z slider rods .. how they hell are they suspended on the middle of rod .. they should be sliding to the bottom of the rod due to gravity itself .. maybe you put too much grease in them?

Forrest Higgs said...

LOL! I haven't put any grease on them yet. They came with some light machine oil on them which has no lubricating effect whatsoever. I've greased xy and they work fine. I'll get around to z in the next few days. :-)

Buback said...

I love the Sampo reference! wonderfully obscure.

sanpedro said...

I have a bfb v2 kit, I have been struggling to get the extruder to work properly, I'm giving up with it.
Now I'm looking for a compatible design, wich I can build without having a working reprap.
Any ideas appreciated.

Forrest Higgs said...

When you say that you can't get the extruder to "work properly", could you be a bit more specific?