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Double-check your models, kids.

A project log for Hubsan X4 Replacement Frame

Love the product--fun, safe, cheap, easy to fly. The stock frame works and is light enough, but I want to make a few tweaks. For science.

zakqwyzakqwy 03/21/2015 at 18:042 Comments

The SLS Nylon bits arrived last week! I sat on 'em for awhile; I've been digging into another project quite a bit lately (more on that later), so I didn't immediately try putting anything together. When I eventually tried assembling one of the motor mounts, I found that the spare motor shell I had lying around didn't want to fit into its hole, despite the CFRP rod fitting snugly with minimal clean-up.

So what's the deal? Time to get out the busted up calipers.

First up--CFRP rod connection:

The ID should match the CFRP rod, which is 4mm stock. Since the wall thickness is 0.7mm (Shapeways' minimum for this material), the OD should be 5.4mm or so. Everything looks good so far.

Next, the pesky motor connection:

Hmm, the OD matches my expectations (7mm motor diameter + 2x wall thickness). The ID, however... is suspiciously 1mm narrow. Time to check the model...

Yup, that's a problem. If you study the assembly picture I posed a few weeks ago, you can even see that the motor model overlaps the ID of this hole since there isn't a defined edge mark:

Time for some sanding. I broke out the Dremel with some sandpaper and various abrasive bits; eventually, I managed to open the motor ID enough to work (getting the part filthy in the process):

During fitment tests I found that the motor was still a bit too snug, which caused the screw portion to distort (the edges of the adjustment slot would cease to be parallel). To fix this without going completely crazy with the sandpaper, I cut a slot below the screw holes to keep them in line:

The process will take some time for all four legs, but I learned a few valuable lessons. Firstly--double-check your CAD models, and then double-check them again. I believe Cubify (like most assembly-capable modeling programs) has some way to check for interferences or at least highlight overlaps; this would have been a valuable step that I ought to investigate in the future.

Secondly--and this bit is quite positive--I learned that I should place more trust in SLS Nylon. Even knowing that it should have similar (if slightly degraded) properties to molded/extruded/formed Nylon, I wasn't prepared for its resiliency. Despite the granular surface texture, it really is strong and flexible. I guess I'm always prepared for textured surfaces to be brittle; this stuff feels like sandstone to the fingers, but it certainly doesn't act like it. As should be expected with a thermoplastic like this, I had better luck removing material with a sharp blade than I did using abrasive methods. Might be a good candidate for a high speed steel mini end mill for the Dremel if I can keep the temperature low.

Thirdly, I'm starting to get my head around the value of personally-owned FDM printers. I've played around with FDM parts in the past (usually ABS) and I haven't been impressed with their strength, density, flexibility, or surface roughness (pretty much anything, I guess). If I'd used an extrusion-type 3D printer to prototype these bits prior to sending them out for production (it's odd to call rapid prototyping "production"), I would have caught the diameter issue early on. Oh well--maybe it's time to add an FDM setup to the list of projects.

In any case, everything worked out in the end:

I think the open back with the wires tucked will protect them adequately, too. I could cut a little plug out of some rubber, or maybe just put a sticker or a piece of tape on the back. Again--not too worried about wire damage at this point:

For now, I'm going to modify the remaining three legs. Then I'll cut the 0.8mm CFRP sheets to size and start fiddling with the other 3D printed bits.

Discussions

Stryker295 wrote 03/24/2015 at 22:58 point

Looks incredibly fantastic!

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zakqwy wrote 03/24/2015 at 23:31 point

thanks!!

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