Seating the new, larger leadscrew in the bicep

Gael quite rightly suspected that the new, stronger lead screw I developed for the bicep would clash when it tried to seat in the recess in bottom of the RotGear.  Raising the hole that the thrust collar axis seats in by 11.5 mm sorted that problem out quite prettily.

Here you can see the revised HighArmSide part that supports the thrust collar.  I've overlaid the revised design over the old.  As you can see the new mounting hole has been moved upward a bit.  I simply plugged the old hole and cut a new one in the STL file.

Here is the forearm at full extension.

Contracting the forearm towards the bicep at the elbow.

Fully contracted, the lead screw slides into the recess in the bottom of the RotGear which allows the bicep to rotate in a perpendicular plane to its axis.

A closer view.

Removing RotGear you can see the lead screw seated where the recess in the bottom of the RotGear is.

A closer look.

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.

Progress on the bicep gear box.

Well, I redesigned the lid on the bicep gear box and got all the pieces printed out.  I discovered that to print big footprint pieces you need to recalibrate the print table when you do a big print.  That doesn't take a long time and appears to be necessary if you intend to print pieces with dimensions over about 90-100 mm.

In any case, I spent a little time upgrading the lid to the gear box so that it take much less after print hand working than it did before.  The involute profile gear and the worm gear are, of course, opposite what one has in the shoulder.  For a 3D printer, that is not a big matter.

I did notice that the axis in the gearbox for the worm gear is about 0.5-1 degree off true.  That doesn't cause any real trouble, but I will remedy that later on.

I slapped on the same servo that I'd modified for the shoulder and, of course, the leads into the potentiometer were backwards to what they needed to be.  :-)

All the same, the HS-805BB servo turned the drive shaft barrel easily without lubrication.  

I got notice from the vendor for the spare gear sets for the HS805BB that they will arrive either this Friday or next Monday or at latest onTuesday.  Of course, they'd promised to deliver them by this Friday when I ordered them.  :-D

This weekend, I hope to have the time to get test the servos on the lead screws that position the forearm and the bicep away from the body.

Printing the gear box

This is just a minor note.  I am using an UP! printer to do the parts for the InMoov robot.  Fortunately, its 120x120x120 mm print volume had been sufficient to the task.  Lately, however, I've encountered a bit of a problem since beefing up the gear box design for the shoulder and bicep.

The UP! has an iron print base with heat applied in the center.  Of course, you get a thermal gradient with such a design and the edges are considerably cooler than the center.  The larger the footprint of a print the more likely you are to get lifting at one of the corners of the print.  The new gearbox has a footprint of 110x120 mm when printed on the large flat side.  It appears that that is about the limit for ABS printed on perf board.  Two prints of the gearbox got noticeable lift on one corner of the print.  These lifts were cosmetically annoying but did not compromise the functionality of the box.

All the same, my aesthetic sense was offended, so I reoriented the print to rest on the new box structure that encloses the servo.

This alteration reduced the footprint to 55x95 mm and does not cause corner curling on the UP!  It does, however, require 40 minutes additional print time and uses about 8 cubic centimeters extra for support material.

Here you can see the printed result.

The UP! leverages the same print preparation software used in Delta Micro Factory's larger, professional printers.  The sophistication of the support structures is readily apparent in the finished print.

Removal of the paper-thin supports is a very simply matter.

Left arm of Inmoov robot complete

Printing an assembly for the left torso, shoulder, bicep, forearm, wrist and hand is complete.

Front view of left torso assembly, shoulder, bicep and base of forearm.

Detail of rear view of upper torso and shoulder.

Left arm raised.

Forearm flexed.

Forearm extended.

Detail of lead screw and collar controlling the left shoulder.

The Hitech HS-805BB servos that drive the torso and arm kinematics are on order.