Modifying JHack's servobed and cover for easy mounting and removal: part 1

I did CAD work for the mounting block additions to the lower and upper halves of the loser side of the left hand forearm over the weekend.

I began by printing and gluing together the upper and lower halves of the left forearm.

I did a similar thing with Art of Illusion and Netfabb in CAD.  With both a real and virtual forearm bet I added JHack's servo bed.

With that model I was able to locate mounting blocks which snugly held the servo bed in place.

As I began to print test parts around 1100 on Sunday, however, my UPS system died, so I spent five hours getting a new one, which is also faulted but works for now.

I did a partial print of the lower half to see if the blocks holding the lower sides of the servo bed fit properly. Here you can see the blocks in place on the left.

On the right you can see the dummy block for the top of the servo bed in place in the glued lower half of the forearm.

Here I fitted the servo bed into the partial print.  The bed slides into place easily and the blocks are flush with the top of the bed.  

Printing the JHack simple servo bed

Jhack's derivative of the simple servo bed seems to be an improvement over the original simple servo bed that Gael posted recently.  It is certainly simpler to mount servos in it.  It is, however, fragile, in a few places and it isn't obvious how you attach it to the forearm shell save using glue. The same can be said for the cover over the servos that fits on top of the bed.  Once glued down, it would seem to me that you'd have to tear the while forearm apart if a servo failed.  I'm probably going to try to modify that while I'm waiting for the rest of my HK15298B servos to arrive.

The simple servo bed was a touch too big for my UP! Pro 3D printer so I had to tilt 45 degrees on the vertical to fit into my print volume.  Given the height of the resulting print, I had to use the stable support option which took a lot of filament to make the support and a lot of extra print time.  You can see the finished print here.

The extent of the support structure needed is quite clear.

You can get an idea of the nature of the paper-thin support structure in this pic of the support structure being removed.

While the removal of the support structure looks like a formidable task, as a practical matter it took less than ten minutes to clean it from the print.

The cover, not pictured, required a similar support structure.  It was considerably more fragile than the bed and required greater care in removal from the support structure.  I was able to do that successfully, however.

Working on the Inmoov forearm/bicep actuator.

Gael Langevin's Inmoov robot uses a lead screw/thrust collar arrangement to move the forearm with respect to the bicep. He has noted that the lead screws are rather fragile and has, apparently, bought steel replacements.  I've priced lead screws and thrust collars and they are a substantial expense.  Given the goal of keeping the InMoov robot inexpensive it would seem that we should have another look at the notion of printing a lead screw.

While trying to get the servo for the elbow joint going I discovered that the 12.5 mm lead screw printed in ABS tended to suffer a shear failure in extension quite easily if you didn't have the potentiometer and servo settings just right.  After I'd broken three of them, I decided to take a crack at seeing if I could design a more robust lead screw with a larger cross section that would resist shear failures better.

I finally got a working model going this morning.  The cross sectional area of the screw is about 4.75 times larger than the 12.5 mm original. Basically, doubled the diameter of the lead screw and doubled the diameter.