Category Archives: robots

Full rotation quadruped build continued

The next step in (re-)building the quadruped with a full rotation leg was getting all the motors ready.  I had to first install reinforcing rings on 6 of the motors:

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My epoxying station
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4x gearboxes with reinforcing rings installed

Then, I needed to lengthen the power leads on 3 of the motors to serve as the lower leg joint.

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Motors with longer power leads

Then I had to assemble all the new legs:

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Upper leg joints mounted
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Lower leg joints mounted
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All three remaining legs built

I mounted them all to the chassis:

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All the legs!

And then re-installed the battery stud and “resting” feet:

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Next up, will be actually powering them and getting it to walk!

Fusion 360 3D adaptive and thin walls

I have been trying to improve the tool paths for the BE8108 gearbox sun gear holder.  The first time, I ended up slowing things down a lot and actually took some of the initial adaptive passes in several iterations as I fixed problems, so it wasn’t clear that any one iteration would be functional from start to finish.

So,  I tried it on a fresh piece of stock, with settings that I thought would resolve the pullout issues I had seen earlier.  Lo and behold, what did I find but more pullout!  It appeared to happen in exactly the same situations as before.  The adaptive clearance would leave a thin sliver of material, then “round it off” very rapidly, resulting in a large chunk of sliver hitting the mill at once.  Increasing the minimum cutting radius and tolerance helped reduce the problems some, but didn’t get rid of them entirely.

Eventually, I managed to find this thread on the Fusion 360 forum where others were having exactly the same issue.  From there, I discovered there is an undocumented parameter, only available in the “Compare and Edit” screen called “Curve in Radius”.  By default, it is calculated based on the tool diameter, but I found that I could add a random fudge factor of approximately 1mm to whatever the calculated version was and boom, the thin slivers were handled in a much more appropriate way and the tool never rolled over them until the sharp point was gone.

I still have more bugs to work out of this toolpath, but at least that’s one down.

 

Machining a sun gear holder on the Pocket NC v2-50

After doing my first cuts in aluminum, I wanted to actually try and make a real part, to prove that the Pocket NC v2-50 was capable of making things that I can actually use.  My first attempt, was the same part I did my first aluminum cuts in, the sun gear holder from the BE8108 planetary gearbox.

This part attaches to the rotor, the sun gear, the position sense magnet, and has bearing interfaces to the planet input and the back housing.  While not terribly big, the number of features and mating surfaces is relatively large.

First, before we start, here’s a video montage of all the machining, apologies for the bad lighting and focus:

Second, be warned that this is a *long* post!

Programming

As mentioned earlier, I decided to try an approach that would allow the entire part to be machined from a single piece of stock and a single setup.  That is one of the advantages of the 5 axis pocket NC.  Even for parts like this, the 3+2 indexing means that I can access all the faces in one go.  Then to finish it off, at the end I left a thin tab which would be broken by hand and filed away.

Before I get into the details, let me be clear, that this is the first real part I have ever programmed for a CNC and there is a lot I am likely doing in a sub-optimal, if not terrible way.  I won’t document all future parts in this much depth, but writing out my thoughts for this one will, if nothing else, give me a reference to come back to as I find my footing.

I marked all the individual tool paths with an initial bolded number (N:) so that you can correlate them to the video if you like.

Setup

I used 1 x 1.5″ rectangular 6061 stock cut to 2″ in length (so it came approximately 2.125″).  This was placed into the Pocket NC vise longwise, so that the 1″ side just about exactly matches the vise’s set screws.

20190612_sun_gear_stock_setup.png

This part may have *just barely* fit into a 0.75″ x 1.5″ stock, but I didn’t want to risk having so little margin on my first attempt.  I originally chose this orientation because I thought that it would be easier to center by hand, although upon reflection putting the long side against the set screws would allow me to use the narrower 0.75″ stock.

I don’t yet have a reasonable means of cutting stock here, so just had Speedy Metals ship it cut to the correct length.

Back side roughing

1: To start with, I used the 3mm Datron single flute endmill that Pocket NC sells for the bulk roughing of the back side.  The majority of the roughing for that side was accomplished using a single adaptive clear.  The final path looked like:

20190612_sun_gear_holder_back_rough

I started out with the speeds and feeds I had used last time, derived from Ed Kramer’s posts.  Since I was using a 3mm instead of a 2mm end mill, I just upped the stepover to 0.3mm initially and left everything else the same.

This was definitely the tool path that I had the most trouble getting to work — I had a couple of different problems.

First problem – tool pullout

To start with, this tool path worked just fine, if a bit slow.  However, once the path got into more complicated geometry, I had what I eventually diagnosed as a tool pull out event that manifested as the spindle stalling out.  It looked like the tool pulled out about 0.2 – 0.4mm, and then started just hogging through uncut material.  This both caused a defect in the surface and then the spindle to stall out.

My debugging of this problem was complicated by a different issue.  I had not really done any math to select a proper stickout for the tool for this path.  Slightly past the point of pull-out, with my initial 20mm stickout, the machine faulted because on the next pass the Z axis travel limit would have been reached.  However, the fault was so far removed from the program point where the limit would have been reached, I initially assumed it had just pulled out again.  Thus, I made a lot of tweaks, none of which fixed the stickout problem, and some of which may have fixed the pullout problem.

Ultimately, I now believe that the pullout problem was likely caused by the “Tolerance” parameter of the 3D adaptive clearing path in Fusion 360.  My initial settings for this were 0.1mm or 0.15mm of tolerance and 0.3mm optimal load, which I figured would result in a usable amount of error.  However, I hadn’t expected geometries like this:

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When the tolerance was set too large, the tool “cut off” that long piece of material, going from an optimal level of engagement to nearly 100% engagement in a heartbeat.  The v2-50’s NSK spindle doesn’t have a lot of clamping force on the tool, so that drastic change in load must have pulled it out.

Despite that as my ultimate conclusion, I still ran all the rest of my paths a bit slower at 30k rpm 600mm/min and a stepover slightly under 10%, so 0.25mm for the 3mm endmill because that was what I did my final testing for this problem with.  I wanted to get a part in this round, not necessarily optimize speeds and feeds.

Second problem – Z axis travel

As I mentioned above, the second problem I had here was that the Pocket NC Z axis travel was insufficient to reach past approximately the midpoint of my part with a 20mm stickout.  In fact, I had to increase the stickout all the way up to 28mm before I could complete this toolpath.  I didn’t experience a whole lot more chatter after that change, and my surface finish was equally mediocre before and after, so it was probably reasonable at least for cuts that were this gentle.

The rest of the backside

The remaining pieces of the backside were:

  • The central cavity
  • The planet input bearing interface
  • The back most visible surface
  • The “bearing lip” for the planet input bearing
  • The back surface that mates against the rotor

2: First, let’s tackle the central cavity.  I roughed out the cavity with a bore toolpath.  I used a relatively small initial bore diameter, then did multiple stepovers to reach the desired diameter.  I later realized this was moderately inefficient, and I should just do a bore of a single diameter and let adaptive clearing handle the rest, but I didn’t bother updating this path with that strategy.

I was pretty apprehensive running this boring operation, as clearing out a cavity is what broke my first endmill.  I ran this at a very conservative 0.2mm depth per turn, and at 30k rpm with 400mm/min cutter speed.  Also, I discovered that I needed to manually set the top height a bit above the start of the cut to get the cutter to actually start its helix before engaging.  With the default 0mm offset, it ended up plunging straight in and sounded terrible.

Even with the helix moving before engaging, it still sounded pretty bad, but did make it through.  I guess using an actual drill is probably the only way to achieve this depth of cavity on the Pocket NC without having significant chatter.  Fortunately, I didn’t really care about the surface finish, and the chatter didn’t seem bad enough to break anything so I just ran with it.

20190612_sun_gear_back_central_rough.png

3: Then I used a 2D face to finish the back most surface:

20190612_sun_gear_back_finish.png

4: Then I used a 3d adaptive to finish clearing out the bulk material from the central cavity.  This is what I should have just let remove all the material after the initial bore:

20190612_sun_gear_back_roughing_cavity.png

5: Next I used a 3D parallel to finish off the rotor mounting surface.  This was a little bit tricky, as at this point in the program I have the beginnings of a “tab” beneath the part that I don’t want to dig into.  Thus I defined a boundary as a sketch in the model for all the things that I wanted to keep away from that tab.  Here I used used it as the “Machining Boundary” with the “Tool inside boundary” option and added a random additional offset set to make the simulation not get into the tab.

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6/7: Finally, for the back side, I used two 2D circular paths to get the planet input bearing interface, and the “lip” behind it, to their proper diameter.  I used a 3mm stepdown for both, although for the best finish the 3mm endmill is long enough I should have just done it in a single cut.  I also used the “Wear” tool offset compensation, so that I could enter small offsets into the Pocket NC tool diameter to dial in the exact dimensions I wanted.

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The front side

8: For the front side, I initially did as much as possible as I could do with the 3mm end mill.  So, that started with a roughing of the non-cavity pieces.  This used identical parameters to what I settled on for the back side roughing.

20190612_sun_gear_front_roughing.png

9: Then I used a 2D face to finish the front most surface.  This is both ways and has a 0.5mm stepover.

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10: I used a bore path once again to carve out an initial hole in the front cavity up until the narrower part of the hole.  For this one, I also used a few stepovers, here spaced at 0.25mm and also manually set the top height 0.5mm above the hole.

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11: After that, I used another bore to get the narrower hole in the middle.

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12: A 2D parallel finished up the front surface.  Nothing actually mounts to this, but I figured I might as well make it flat.  As with the facing, a 0.5mm stepover was used.  I ended up just selecting everything but the back flat face as a “Avoid/Touch Surface” set to avoid to keep the path from trying to get there.  I also didn’t do anything special to keep this away from the tab, which resulting in it digging a bit into that region.  I was mostly worried about engaging the full width of the cutter, but it didn’t cause too big of a problem, and the 2d parallel path did that a bit anyways as it worked its way around the central post.

20190612_sun_gear_mounting_front_finish.png

13/14: The last two things that I did with the 3mm flute were finishing the interface to the rotor bearing, and the bearing lip using 2D circular paths.

20190612_sun_gear_front_bearing.png

Front side – 2mm end mill

The other features I needed to do on the front side were all too small to complete with the 3mm end mill, so I switched to a 2mm single flute Datron with 5mm long flutes for the subsequent operations.

15: First, bores opened up the four bolt mounting holes that connect the part to the rotor.  For these, I used just a single boring pass, but manually set the height to be a bit above and below the part.  Above so that the helix would start before it engaged, and below so that I’d be sure to actually get all the way through the part.

20190612_sun_gear_radial_mounting_holes.png

16: A 3D adaptive removed most of the remaining material in the front central cavity:

20190612_sun_gear_front_central_hole_rough_2mm

17: And a 2D contour removed the final bit of material:

20190612_sun_gear_front_central_hole_finish.png

18: The last thing I did in this tool setup was to clear away the remaining outside material that was above the tab, as all the “heavy” cutting is done.  This was done with a single 3D adaptive clearing, with the depth of cut set to 3.5mm so it finished in a single stepdown.

20190612_sun_gear_outer_rough.png

Chamfer milling

Now that almost all the material is removed, I switched to a 90 degree chamfer mill.  This I just got from Shars, part 416-3509.  I wanted to break all the sharp edges I could and also needed to cut the countersinks for the mounting bolts.

19: First, the front chamfers.  I used a 2D contour with 0.25mm of “Chamfer Width” and 0.25mm of “Chamfer Tip Offset”:

20190612_sun_gear_front_chamfers

20: Then, while still on the front side, I did the countersinks.  For these I also used the 2D Contour toolpath.  I used as many stepovers at 0.2mm as I could fit in, which worked out to 7 in this case.  Since these chamfers were modeled, I left the “Chamfer Width” and “Chamfer Tip Offset” at 0mm.  In hindsight, I probably wanted some tip offset still, as it left a tiny flat at the start of the chamfer.

20190612_sun_gear_front_countersinks

21: The last thing I did with the chamfer mill was the back chamfers, which were done identically to the front ones, just with a different “Tool Orientation” selected to come from the back side.

20190612_sun_gear_back_chamfers.png

Finishing up and parting off

22: At this point, I switched back to the 2mm end mill from before with the same stickout to get ready for parting.  First, I used a careful selection of boundary to remove all the material laterally, except for the final parting tab with a 3D adaptive.  Because this is reaching in far, and the shank of the tool would otherwise rub, it spends some time carving away at the base of the stock to give room for the shank to reach in.

20190612_sun_gear_leave_tab.png

23: Now, as much of the outer surface is exposed as is going to be exposed, so this is where I stuck my finish pass for that surface.  I used a 3D contour, which let me restrict its operation using the same machining boundary I used before so that it didn’t run itself into the tab.

20190612_sun_gear_outer_profile_finish

24: And finally, I used a 3D adaptive to clear away all but about 0.2mm of the remaining material in the tab.  The selection of tab width was one I was concerned about, never having done this before.  Too thick, and breaking it off would be annoying.  Too thin, and the part might fall off while the mill is still engaged and spinning.  In the end, the 0.2mm was totally reasonable to bend off by hand, but still seemed relatively well fixed as the mill worked under it.

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The final result!

Some pictures of the final part:

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Clearly, there are plenty of areas for improvement.

Surface finish: As seen in the close ups, the surface finish, is at best, “mediocre”.  I didn’t really try to optimize this, but did try to avoid obviously loud chatter.  Perhaps I can run some of the paths with the same tools but at smaller stickouts, or perhaps tweak the RPM to find areas of stability to improve things?  Also, my choice of finishing paths seems to have left a lot of circular swirls on the front side.  I should probably switch to something circular there instead of linear so that the center stud doesn’t cause so much heartache.

Back cavity finish: In this iteration, I didn’t actually do a finish pass on the interior surface of the back cavity.  This is a precision surface that the sun gear fits into, so I definitely need to do that and tune it in.

Accuracy: The bearing surfaces’ accuracy is probably best described as “meh”.  I tweaked the tool diameter offset until I could fit the bearings on, but the difference between “won’t fit on at all because it binds near the base” and “fits pretty loosely around the whole thing” was not really controllable.  There was a moderate taper between the near end of each surface and the far end, despite doing several “spring” passes.  I suspect this is just an inherent limitation of the Pocket NC.  I believe it only has steps for 0.0002″ in position and isn’t that rigid either, so the tool deflection is pretty measurable.  Probably the best I could do would be to run those paths with a smaller stickout tool in the hopes of minimizing deflection based error.

Single run: I did this first part executing each tool path singly with my hand on the estop the whole time.  After I get the back cavity bore path working, I’d like to try one with the only intervention being at tool changes.

Summary

Wow.  This was significantly more involved than I expected, although most of that was due to debugging the tool pull out and the Z axis limit problems.  Even still, I spent a good week getting the tool paths programmed and tested.  I hope that with practice, I’ll get to where I can turn these around more quickly.  Especially since this was the “easy” part I had!

 

Stripping the coaxial quadruped for parts

To switch to the full rotation gear design, I needed to get all my gearbox motors, some bearings, and a lot of other bits and pieces disassembled and ready for re-use.

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The remaining 3 non-broken legs from Maker Faire

Taking everything apart took a surprising amount of time, nearly a full day.  Each leg resulted in quite a collection of fasteners.  Seeing them all in one place made me realize how complex this has become!

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One leg’s worth of parts

I’ve also got the full set of parts printed for the full rotation legs:

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Now I just need to get to assembling and reworking to get them all installed!

First assembled full rotation leg

As I described earlier, the first draft brushless quadruped leg design was insufficiently robust for the gearbox driven motors and I am updating it to a geometry that allows full rotation.  I’ve made at least some progress on that front, so here is an intermediate report.

First, after doing some analysis, it appeared that the 3mm pitch 6mm wide belt was unlikely to be able to carry the full torque from the motors.  So I’ve switched to a 5mm pitch 15mm wide belt, which while still unable to carry the full torque indefinitely is only a factor of 2 or 3 off instead of a factor of 20 off.  Secondly, I added a bearing opposite the upper pulley so that it is supported from both sides.  The recommended belt tension for this belt works out to something like 120lb, which is a fair amount of cantilevering, even over the 16mm wide pulley.  The updated CAD looks like:

20190531-full-rotation-cad.png

And the newly added bearing can be seen in this section view:

20190531-upper-leg-bearing.png

I did a first test print of all these parts and put them together.  While there were a few tweaks necessary for the second revision, it looks like this leg is probably usable.

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The shoulder and upper leg adapter installed, all the leg pieces splayed out
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Lower leg all assembled with belt and upper pulley installed
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All assembled!

Next up is building 3 more of them!

 

Full rotation leg design

Another of the failure modes observed during the 2019 Maker Faire was in my quickly slapped together leg design.  The shoulder joint was required to squeeze two motors together against a strongly tensioned belt, using nothing but a relatively thin section of printed plastic.  This caused it to deform, leading to belt tooth skipping, and then eventually to fail, leading to delamination of the shoulder joint.

My plan to resolve this is to switch to a leg design where the upper and lower leg are in series rather than opposing one another.  This is more like the Mini-Cheetah design from Ben Katz.  This has the benefit of getting the leg out to the side, so the upper leg is free to rotate 360 degrees, only limited by cable harnessing.  As seems to be my pattern, I’ll try making something out of 3d printed PETG first, optimize it some, and if I fail there, switch to metal.  Here’s a render of the current CAD:

full_rotation_leg

 

Eric from CireRobotics helpfully pointed out that I’m way over the design limit for the 6mm Gates belt I was using, so I’ll also be trying to bump up to a beefier belt in this iteration.

 

Working around motor shroud failures

As seen at Mech Warfare 2019, the existing gearbox motor shroud isn’t really up to the task of supporting the weight of a 20lb robot.  While I work on a more comprehensive redesign, I’ve got a short term fix in the form of another 3D print.  This is just a simple reinforcing ring, printed at 3mm thick, with the layer lines oriented so that layer separation will not be the primary failure mode.  It is attached to the outer housing via a thin layer of epoxy.

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The two halves of the reinforcing ring

Due to the unconventional orientation, removing support was a pain, but doable.

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A reinforced motor

Results from Maker Faire 2019

After a concerted push, I managed to get Super Mega Microbot “Junior” walking, for all of 15 minutes, then packed it up and went off to compete in Maker Faire.  Needless to say, with that much testing, I wasn’t expecting stellar results, and I wasn’t disappointed.  However, I did learn a lot.  Here’s some of the things that went wrong:

Gimbal and Turret EMI

For this new revision of SMMB, I updated the gimbal board to use RS485 and support the 5S system voltage.  I tested it some, but apparently not enough.  While I observed no problems during Thursday or Friday’s testing at the site, during the first Saturday match, after firing the gun a few times, the gimbal went into a fault state and stopped applying power.  The control scheme for SMMB relies on the turret being operational, so this not only made it impossible to aim, but also made it nearly impossible to drive.

I did manage to connect to the turret manually after the match to diagnose the problem, and discovered that the IMU had stopped communicating over I2C.  I had some half-baked logic to try and recover from that, but it was broken, and the only effective way to recover was to power cycle the whole unit.

Unfortunately, my matches on Saturday were all close together, so I didn’t have enough time to prepare a fix in between.  Thus, each match I got one or two shots off, and then the machine as a whole became effectively inoperable.

Likely, something in the new board, either in the layout or the decoupling capacitors, results in worse electrical noise than the old one when the AEG is fired.  This shouldn’t be too hard to resolve, either through tweaking the layout, or perhaps moving the AEG control to an entirely separate board.

Walking and Leg Robustness

When I got the gearbox system walking for the first time, I quickly noticed that one or more of the timing belts connecting the lower legs to their motor had a propensity to skip a tooth.  Since there is no position sensing directly on the lower leg, when that occurs the gait logic just has the incorrect position, causing the robot to fall over pretty soon afterwards.  I had never observed any tooth skipping in my previous direct drive leg, even when jumping for over an hour.  The first difference I thought which might be causing the problem was the lower pulley print, which I had initially done at 0.15mm but in the gearbox revision it was at 0.2mm.  So I printed a full set at 0.15mm, and swapped them in.  However, that didn’t fix it and I didn’t have any more time for mechanical solutions, so I tried to work around it by tuning the gait to be as gentle as possible.

Unfortunately, I wasn’t really able to come up with a gait that both could effectively move on the foam mat in the arena, and not occasionally result in belt skips.  Also, as I went along, the skips got worse and worse.  I tried upping the tension on the belt, lowering the tension on the belt, walking with a straighter leg and more bent leg, nothing much made a difference.

Finally, before my third match, I did more examining and realized that the shoulder joint was deforming significantly under the tension of the belt, resulting in the timing belt only contacting maybe half the pulley or less, and the rest dangling off.  Also, the pulley was out of alignment, so the belt was probably only effectively making contact in an even smaller patch.  Unfortunately, there was very little I could do about that aside from hope for the best.  As it turns out, that problem, while limiting the gaits I could use significantly, didn’t result in ending my run.

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Shoulder failure

Gearbox Outer Housing Strength

The entire gearbox effort was undertaken somewhat at the last minute, and with little thought to analysis or design for structural integrity.  At best, I made a gut check of “that’ll probably work”, and at worst, I gave it no thought at all.

It was an instance of the latter that caused the final and fatal failure in SMMBJ at Maker Faire.  In the gearbox chassis design, the lateral servos themselves support the entire weight of the robot.  Those gearbox servos transmit the entire load from the front plate of the servo, through the outer housing, then to the back plate, and finally to the chassis itself.  The problem in this case is that the outer housing is a 1.5mm thick (or rather thin) PETG shroud printed with layer lines perpendicular to the primary load.

On reflection then it was not too surprising that a 20lb robot walking around was enough to cause a motor’s shroud to separate at the layer lines, which is what ended SMMBJs run.  I had a spare motor and could have replaced it, however, it would likely have failed shortly afterwards too, and the shoulder was about to rip itself apart due to the leg tension problems mentioned above.  Thus I turned it into a “static display” and switched to a “show and tell” mode for the rest of the event.

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Crippled SMMBJ

Award

Despite those problems, the kind organizers at RTeam awarded me the “Most Innovative” award for trying to push the limits!

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Fixing the problems

Clearly, all of these issues can be fixed in a variety of ways, both easy and hard.  Keep coming to see my attempts!

Mech Warfare 2019 – First look

Well, Mech Warfare at Maker Faire 2019 has come and gone.  Maker Faire was a really awe inspiring event, and RTeam did an excellent job organizing the Mech Warfare competition.  There were something like 13 teams with moderately functioning mechs who competed across the 3 days.

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Super Mega Microbot “Junior”

My entry, Super Mega Microbot Junior, did manage to walk a bit in 3 matches, but had a previously unseen failure in the turret system that rendered it inoperable a short while into each match.  At the end of the 3rd match, one of the leg joints sheared off, and some other of the 3D printed parts were about to fail as well, so I declared it unrepairable at that point.

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In the arena pre-failure

I’ll write up a more detailed lessons learned and link to the videos of my matches when they get posted.  The videos aren’t all that interesting, given that I only scored perhaps 2 hits across all 3 matches. 😉

Walking and Maker Faire!

Alert!  I’m at Maker Faire Bay Area all weekend in the Mech Warfare area in Zone 2 (May 17-19, 2019 for you time travelers from the future).  Drop by and say hi!

If you were left in suspense last time, yes, the robot can walk!  Getting it to do so in a minimal way was relatively painless.  What I found, which hadn’t happened in earlier iterations, is that many types of dynamic motions would cause the lower leg belts to jump a tooth.  Needless to say, this was nearly universally fatal, as there is no direct position sensing of the lower leg.  This robot is heavy enough that my simulacrum 3d-printed timing belt pulleys just don’t cut it.

Well, there wasn’t enough time to actually get better pulleys now, so I just tuned the walking to be slow and gentle enough that nothing went awry.  Here’s the first bit of a 13 minute video I took of it walking around and shooting targets.

Now, that that was over with, I had a few minor things to finish up before heading out to Maker Faire.  I made some covers for the motors to keep BBs out.

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And I made a bracket so that I could attach the front and rear target panels to shoulder joints:

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And here’s a glamour shot of the whole thing in fighting form!

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Now that it was all ready, time to take it all back apart and pack it for shipping.

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And off to the airport I went!