Public Lab Research note

Notes on assembling an Aerobee Rig

by mathew | July 08, 2016 20:35 08 Jul 20:35 | #13273 | #13273

I purchased an Aerobee rig from the KAPtery because I'm real excited about the Jerk Pan mechanism. Our unpredictable late spring rains (its still spring in Oregon) have kept me from getting a good flight window to test this rig, but I hope to have it in the air soon.

I got an early version of the instructions and assembly. Overall, the assembly was straightforward and low-stress.


One sticking point was the leg brackets. the clearance for the screws forced me to jam them in at odd angles.


The plastic right under the leg attachment didn't have enough clearance for a nut, and had to be trimmed with a knife:


I'd suggest a change to the 3D printed file to get a little more clearance.

The hitch pins are almost impossible to get into the pendulum kit without stretching them open with pliers, then the kit easily assembles and disassembles:



Pendulum kit:

Some modifications to the pendulum kit are necessary to fit the jerkpan on the bottom. @cfastie may have already changed the instructions on this assembly.

The instructions specify the need for a 5/64", 5/16" drill bits. I used a 1/16", 3/32", 7/64, and 1/8"

they replace the current drill instructions in the following locations:




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Thanks Mathew,

The holes in the aluminum frame for the leg brackets definitely needed modification. I make them bigger now and moved some of them about 1 mm. Some are still a tight fit, but the screws will fit if you bother them enough. Your Aerobee was shipped before I recognized that problem.

The leg brackets have always had that problem that the nuts won't fit back into that crevice far enough. You got a new version of the brackets where I tried to fix that, but there is still not enough room. The instructions for other KAPtery rigs suggest slicing two of the corners off the nylon hex nut, but recently I have been using the Dremel to widen the crevice before I ship them. Before I print another batch of those I will modify the model, although the cruft in that old model makes it hard to work with. Maybe I need Ranon and Fusion 360!

It's really good for me to see what you did with the cotter pins. It's certainly hard to work those in sometimes, but I have never bent one open to make it easier. I just drill the holes larger (and aligned) until the pin slides in. Bending the cotter pin makes it less secure (more likely to slip out -- they are designed to "lock" in that middle bend), so I should make sure people don't do that. The system requires that the holes be just the right size (if they are too big it weakens the suspension shaft). If I could think of a different way of attaching the shaft I would consider it, but it has to be an absolutely secure attachment because the entire rig falls off if it fails.

You're right about the drill bit sizes. I will rework those instructions. The listed bit sizes are probably the ones I used, but a small bit can quickly make a big hole in that fiberglass shaft, so some of the bits I listed are smaller than the hole should be. I should be more explicit about that, but I should also avoid making the instructions more tedious than they already are. It's a challenge to find the proper compromise.

Thanks for the feedback. I wish more people would share their experiences.


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The hitch pins really wouldn't go in until I opened them up. I first focused (as you have above) on opening up the hole and clearing the path. no dice, just couldn't push them in. I gradually opened them to their current state. I think they're quite secure, it still takes a solid tug to get them out.

If anything, I'd suggest pre-opening the pins and shipping them at an easier but still safe opening level to prevent someone from modifying them too much:


Drilling out the fiberglass rod is less than ideal, I agree. I'm not too fond of it as a task in a kit. We've been playing around with under-sized rubberized loop clamps-- 1/8" inner diameter for a 1/4" dowel, and they hold quite firm to the end of a shaft. I wonder if a steel or aluminum one could bite through the vinyl tubing to hold the shaft in place?


@Ranon, want to take a look at @cfasties's leg brackets and see if you can re-model them with some clearer screw mounts?

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I'll play around with those pins. The odd thing is that I have inserted hundreds of those pins in suspension shafts over the past three years, and never once bent one open to make the task easier. So I guess I am not a good test subject for determining how to make the task easier.

I'm pretty sure there should be at least one pin through the shaft at each end to ensure that it cannot slip off. That kind of safety issue cannot be trusted to chance or friction. Most KAP flights are gentle rides, but some unexpectedly become violent wind-whipped journeys that can easily pull things apart (or break wooden dowels with abandon). It's a challenge to find the happy medium between overbuilding rigs and keeping them lightweight enough to be useful, but there are no load bearing wooden dowels or friction mounts on KAPtery rigs.

There are two issues with the KAPtery leg bracket. One is that it needs just a little more room for the nylon nuts to slip in against the angled tube. The other is that the angled tube could be stronger. The bracket is printed on its back, as in the diagram below, so the deposited layers are horizontal. The fused layers are usually the weakest part of a 3D printed part, and a strong dowel inserted into the angled tube can be levered so that the layers could separate. I have made the sides of the tubes thicker, but additional strengthening might be good.


I always print leg brackets in ABS for additional strength, and a several millimeter thick layer of solid ABS is printed halfway up the part where it is most vulnerable (the rest has a dense honeycomb of ABS).

It would be nice to start over and redesign this part, but there are lots of critical dimensions and angles that must be recreated exactly, so I have avoided the task.


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While you're worried about friction mounts, I'm worried about home-drilled holes in rods and the chance of putting cracks or stresses into the rod.

Friction mounts can be pretty good. If I can't pull it apart, we're looking at 60+ lbs break strength. Is that really something to worry about?

Clearly the only answer is a head-to-head shake off.

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Are you going to make me stand under a KAPtery rig with a Canon EOS M on it while you shake test it until it fails? Then you stand under a wooden dowel holding a Mobius ActionCam while I whack you in the head with it until it breaks.

All seriousness aside, I agree that the weak link is the human who puts the rig together. Structure can be compromised by poor drilling or failure to tighten the clamps. Safety might depend on clear instructions more than anything else.

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Agreed-- And some strength tests/inspection points.

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Here is a timely and apropos idea for drilling holes in the pendulum or Picavet shafts. Last week I shipped a Picavet Hardware Kit (everything but the 3D printed Picavet cross) to a customer. Today he post a drilling jig at Thingiverse for drilling the holes though the shaft. This would also work for the pendulum shaft. Maybe I should include one of these with each kit. Thanks bluepop4!.


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wow. nice little jig! more jigs like that would be great. it is especially nice for helping communities do small runs themselves.

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@cfastie I modified the leg bracket in Fusion 360 and have posted it to thingiverse. Leg_Bracket.png

I moved the leg brackets in more towards the center and angled them out more sharply to compensate, does that compromise the support of the legs?

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Nice model. Thanks for doing that. There is plenty of room for nuts now. I don't know how changing the angle will affect function. I would have to print it and try it out.

The angled brackets are shorter now which compromises the friction hold on the dowel legs. There is also much less plastic between the two angled brackets and on either side of them, so it will be more vulnerable to delaminating along printed layers. In fact, the approach should probably be (instead of building it from two tubes) to start with a solid block and then remove the path for the two legs. We need as much plastic as possible to strengthen those angled brackets. The only space that should not be solid is where the four nuts have to fit. Those are very small nuts, so it does not require very much space around the holes.

Thanks, Chris

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Glad to help! I have a design in mind for solving the support issue the way you're describing, I'll get back to you with that model as soon as I get a chance.

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I made a new model designed to address the layer separation issue. It is larger than the previous version, but I think the print time should actually be similar with the infill.


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That looks cool. I will start a print when the current Aerobee Rig camera tray print is done. It looks like the walls on the outside of the angled tubes might be a little thin. I'm not sure why that is but maybe the leg tubes are splayed out more than they were? Although the footprint of the part should stay the same size, the part can get wider near the top. So more material could be added where it will strengthen the end of the tubes which is where cracks will start. Those tubes for the legs really need to be surrounded by a lot of plastic.

Also, the wall in front of the horizontal tube is so thin that a hole appears there after slicing:


In fact, two holes are needed in that horizontal tube. Note the set screws in that tube below. They are to prevent the inserted tubing from rotating (adds to the protective value). So the model should have those 1/16" holes. Note that they should be at an angle so the screw can be reached when the bracket is in the position shown. That will be tough with your current model.


Sorry for all the obsessive details. But while I'm at it, when the brackets are mounted back to back on the Aerobee Rig, there is not quite enough room for the heads of the screws at the arrows below. This could be fixed if each bracket had a small indent for each screw in the base along the horizontal bracket.


I'll stop now.

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Ok I have updated the thingiverse post and I think I've addressed all of the points you brought up, unless I misunderstood them.

Modified_Leg_Bracket.jpg The leg tubes are splayed out at the same angle as before, although it looks like a sharper angle when inside of the structure as they are.

Does the angle for the set screws look like it would be accessible in the configuration you showed?

Also, it looks like the indent for the upper bracket may cut close to the hole in the bracket itself, does the size of that indent need to be reduced?

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Very nice. I printed the last one. You got the spacing so the nuts will fit exactly right.

I think the indents for screw heads on the new model can be a little smaller, but that is exactly what I was thinking of. The wall of the horizontal tube gets reduced to just two printed extrusion layers, so maybe make the indents 50% to 75% as deep as they are now.


I think the set screws will be just barely accessible with a small screwdriver on the Aerobee Rig. It might be nice to have them at an angle just a bit closer to 45°. They are otherwise in a good location.

It looks like the space for the nut might have shrunk in the new version.

It would be good to have more material on the outside of the tube opening. I guess there will have to be a bulge there. Might as well make it a big bulge to get as much strength as possible. Splitting there is a real vulnerability. The bulge can extend back over the screw hole. That will make it hard to use a screwdriver on the screw, but in all of the KAPtery rigs that place is for a nut, not the screw head. So the nut can be accessed from the side.

I will restrain myself and wait to print the current version this time.

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Hey Chris,

I made changes to those areas you pointed out yesterday and have uploaded a new version.


Is this the sort of bulge you were referring to? If so is it large enough to prevent splitting?

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I like the bulge. It might be more forward than it has to be (it blocks screwdriver access to the front screws) and not out to the sides as much as it could. Below is a model with a crude blob I stuck on one side to suggest where the bulge should be to thicken the side wall of the leg tube.


Ignore the shape of my bulge, only the general location is relevant. I haven't learned Fusion 360 well enough to edit your excellent models, so I just glommed a shape onto the side in Sketchup. Below you can see that the bulge extends back over the screw hole. The idea is to beef up the volume on the outer sides of the leg holes.


I guess I need to start watching Fusion 360 videos.

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Thanks Chris!

I designed a new leg support structure and uploaded the updated model.


I designed the support structure using T-spline modeling (sculpt mode in Fusion 360). I'd definitely recommend watching some tutorials on it! It's a great way to quickly model more complex shapes.

Let me know if that's what you had in mind or if there need to be any other modifications.

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Awesome. It is definitely beginning to look like a prop from a Marvel movie. The two leg holes do not seem to be very deep. Can they be extended all the way in? The farther the better. I will print one of these to see how everything else fits.


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Sure thing I'll extend those as far as they'll go. Look forward to hearing how it prints!

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The print looks really good. There is plenty of room for the nuts. There are a few details that could be adjusted:

  • I mentioned the leg holes should be a deep as possible
  • . The set screws are at the proper angle, but are not aimed at the center of the tube. The screws will hold the tubing best if the holes are along the blue line below.


  • If the bulge was even bulgier on the sides like the blue lines below I think the added strength would be appreciable.


  • The indents on the back are not big enough. They probably should be about as big as you had them the first time. Maybe they should be angled like the red line below so the wall of the tube does not get so thin. Maybe the red line should be at an even steeper angle (more vertical).


I watched a bunch of Fusion 360 videos but I'm still useless getting anything done on a model. That might be the steepest learning curve I have encountered in software.

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Hi Chris,

Regarding the Fusion 360 tutorials, I have a former professor who recently published a free course teaching Fusion 360. Here's a link to the course. Hope this helps.

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That UDEMY course is very good. I have been watching videos and making the models as he does. I think there is hope. I still can't make any useful changes to your last model of the leg bracket. Should I be able to modify the stp file of the bracket you put at Thingiverse? I haven't figured out how to do that. I'm just trying to make those last few changes.


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Hi Chris,

Glad you've found the UDEMY course helpful! Actually, there's been a very exciting new update for Fusion that lets you directly edit .stl's without having to first open them in MeshMixer. I haven't gotten a chance to try it yet, but if you're familiar with MeshMixer that may be a good place to start.


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I also had a chance to make some changes to the leg bracket, which I have updated in Thingiverse. The primary change I made was to the bulge in the sculpt mode, which you should be able to find in the timeline as a purple box. If you open the f3d document and right click the purple box then click edit form, you can go in and edit the shape of that bulge.

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The new bulge looks perfect. I will give the new f3d file a spin tonight. Thanks.

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You're welcome, let me know how it turns out!

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Fusion 360 did not allow me to open the f3d file. It said that opening local files was not supported. I was not expecting that.

So I opened the most recent stp file and managed to modify the set screw holes and the indents on the back for the screw heads. I printed one and it looks pretty good. The indents for screw heads should be a little bigger, but I noticed that the angled leg holes are not angled enough. The angle from vertical is good, but the two legs should form an angle of 48 degrees with each other and they are at 30 degrees now. I'm not sure whether I should try to change the angle of the existing holes or delete them and make new holes. I will give it a try.


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The print looks great! For opening the .f3d file, there is a certain way you have to open it which I forgot to mention.


Once you have made a project you'd like to save the file in, click on the "upload" cloud with the arrow on it. You can then select the .f3d file from its save location and open it in Fusion.

To change the angle of those holes, it may be easiest to make new ones, or another option would be to look through the sketches in the .f3d file and select the one that was used to make the hole, then move it and change its angle. (There are a lot of sketches in the .f3d file and I'm not sure if that one is named, I apologize in advance for my organizational skills).

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Thanks, that seems to work to open the f3d file (it's not done uploading it yet). I guess another way is for us to share the files at the Autodesk Central Authoritarian File Server.

Last night I kept working on the version I made from your stp file and deleted the leg holes and made new ones. I couldn't figure out how to control the angles so both legs were perfectly symmetrical, but I got them close.


I printed a draft this morning and it looks good.

Aerobee_20160804-6922.jpg Aerobee_20160804-6916.jpg

I'm printing four of them now with solid plastic in the upper half. This should be strong enough that the slight advantage of ABS over PLA is irrelevant so the brackets can be PLA like everything else.

Ranon, when you started making new versions of the KAPtery rigs in Fusion 360, how did you start? Did you import the old STLs and modify those forms? Or did you import the old STLs and make sketches of the plan and then model from scratch in Fusion 360? Or did you just start from scratch? I might want to tackle another KAPtery part to keep improving my Fusion 360 skills.

Thanks for your help,

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That looks good! To make the leg holes symmetrical, I've found the easiest way is to make a single hole and then mirror the "cut" command from the history bar at the bottom.

To make new versions of the KAPtery rigs, I started by importing the .stls into a blank workspace and using them as a guide to make sketches. This may not be the most efficient way, though, now that there is mesh editing in Fusion 360.


I've tried it out a bit, and it's difficult to edit what's already there, (filleting, extruding, etc.) but it's easy to add features to it using sketches. I guess it really depends on the type of edits you're planning on making.

Glad to help,


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