moteus servo mk2: Front housing

The front housing is the most complex machined piece in the moteus servo mk2, as it was in the mk1.  It is relatively large and mates with many other components with the associated tight tolerance surfaces.  For mk2, the front housing is even larger in diameter, but otherwise has the same basic features.

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Manufacturing

Building a prototype of this was a real challenge given the tools I have available to me now.  For mk1, I didn’t even try and just had Xometry build my prototypes, and was lucky enough that the first ones worked.  My only CNC currently is the Pocket NC v2-50, which is just barely big enough to deal with this part, and has no convenient workholding that can be used for the stock.  Also, it has a low material removal rate, such that starting from stock here would be prohibitively time consuming.

Manual preparation

My approach was similar to that of the outer housing, in that I prepared the materials on the Artisan’s Asylum manual machines, then did the remainder of the machining on the Pocket NC.

I started with 4″ diameter round stock cut to 1″ +0/0.125:

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Then, on the mill, I faced the parts to the correct 22mm height:

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Then I drilled a 5/8″ hole close to the center:

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At this point, I switched to the lathe and performed some roughing operations, removing some of the OD and ID to reduce the amount of cutting the Pocket NC needed to do.  These were done with some 3D printed spacers to help align the stock in the lathe’s 4 jaw chuck.

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CNC work

Now that the part was roughed out, I first mounted it to the Pocket NC using a machined aluminum plug bolted to a custom 3d printed plate.

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My first attempt used a 3D printed plug, which just delaminated and failed, thus I re-drilled some more holes for this first prototype offset from the original 8.  This operation threaded the holes on the front side, which are used to secure the piece for the second operation.

In the second operation, a second custom 3d printed fixture is bolted to the newly drilled front holes, and then bolted to a plate that is mounted on the B axis.  This enables the Pocket NC to reach the full back side and around the perimeter.  The bracket needed to be slightly offset from the center of the B-plate, otherwise the mill couldn’t reach all the way around.

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Datron 4mm endmill

This was the first part for which I used a Datron 4mm endmill on the V2-50.  It is definitely a good tool, although it has its shortcomings.  The biggest win is that it can remove material at more than double the rate of anything else I’ve got.  Granted, this isn’t exactly fast, but it is fast for a Pocket NC and makes parts like this front housing even remotely feasible.  It also produces a nicer surface finish, and has less deflection so straight walls are straighter.  For simple geometries I used 47k rpm, 0.25mm optimal load and 4mm stepdown and then slowed those down when dealing with the more complex shapes.

There are some downsides though.  One, it uses a 4mm collet, which negates much of the value in having the convenient tool changing handle.  It takes a longish time to switch collets so I have to arrange toolpaths to do all the 4mm collet things together.  Second, it produces more chips than just about any other tool I’ve used on the Pocket NC.  So much so, that I had to schedule breaks every 45-60 minutes in the program just to clear out chips.  Otherwise the chip tray became too full to use effectively.  For some geometries, such as when clearing pockets, the chips are thrown onto the X ways and require even more frequent clearing, otherwise the mill can’t move to the full X extent as it squishes the chips against the front side of the enclosure.

Machining video

Here’s a video of the third one I made, which used finalized fixtures for all operations.

Fit test

And here’s one of the front housings with all the various things mated in place.

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