So long springy shaft couplers!


#1

It’s become apparent that flexible aluminum shaft couplers are not the best way to go for holding the Z axis threaded rods. The deal is, they don’t flex in the directions we could use (side to side) but they do flex up and down with the weight of the extruder (not good). To make matters worse, vibrations cause them to bounce around and in some cases this becomes noticeable as z-wobble on vertical surfaces of prints (granted, it is subtle). So, I’ve started using a printed shaft coupler that I designed, and I wanted to share some info about it today. This is obviously not the first printable shaft coupler design. In fact, my first reprap printer (a Mendel i2) used printed shaft couplers. I remember because they were a pain in the butt to assemble and adjust. This one is a single piece, and uses a double-shell design that gives it a nice amount of flex/compression while still being pretty resistant to cracking (as always, I’m printing them in PLA).

It’s located in the stl folder on the Wilson2 repo (https://github.com/mjrice/Wilson2/blob/master/stl/coupler-5x8.stl).

Assembly is straight-forward: fit two M3x10mm screws through the holes and loosely-attach a hex nut to each one (note that one side has insets for the hex nut and one is smooth for the head of the screw). Thread the 8mm lead screw through the top side until it bottoms out (there are two little “nubs” that are meant to engage the threads, so you can’t just slide the lead screw into place). Next, slide the coupler over the top of your motor shaft (it’s ok to let the threaded rod hit the top of the motor shaft). Then just tighten down the screws nice and snug and you’re all set!


#2

I didn’t know aluminum couplers were so finicky… Definately gonna try these out.

Thanks for sharing.


#3

I just printed a set and to me the 8mm end has almost no clearance between the inner and outer shell once I tighten it up.


#4

Sounds perfect, unless I misunderstood(?)


#5

I thought there would be a bigger gap, with maybe a little more movement but I am probably over thinking this a little.

I will set this up on my Geeetech Prusa I3 X 8mm Acrylic Printer… I have been suffering from what I think you have described here and this might just be the fix I need.

UPDATE:

Here are my results and going a long way to fixing my X axis issue.

Y before (bottom) and after (top)… about the same as I expected.

X before (bottom) and after (top)


#6

I printed a pair of these, and while the external dimensions were spot on, all of the holes were too small. I imagine this is a slicer issue, because if the size issues came from my printer, then the external dimensions would be too small as well. I used the cura settings in the github, with slightly modified temperatures and speed. Any suggestions for getting exact dimensions, beyond just scaling the parts up a bit in the slicer?

Thanks,
Ryan


#7

@schubes, Calibrate e-steps and enter the actual filament diameter in your slicer to ensure the slicer and machine are extruding the correct volume of filament.

More detail on these suggestions here, http://community.reprapwilson3d.com/t/printing-pla-on-bare-glass/108?u=rockey

If it’s mostly your first layer squish interfering then try setting Cura’s “initial layer line width” to 100%. It defaults to 110% for more squish and better first layer adhesion.


#8

@schubes I would just scale them up by 10% or so and try that - I will go back and slice them with Cura to verify, but I do believe that is why they come out too small (I’m slicing with S3D now so didn’t try it). I could also make another version with bigger holes if that doesn’t work for you.


#9

Awesome, I have a print going right now of various scaled versions, so I’ll check that when I get back from work. I’ll also try adjusting my first layer and extruder settings. Thanks guys!

EDIT: I got them working, and they seem to be doing fine. I was wondering if there would be any benefit to adding a flat surface in the motor shaft slot to grip better to the D-shaft? It seems like if the PLA wears out it could start slipping and causing z problems.


#10

Marty,
The fact is, The aluminum couplers do flex very easily from side to side as long as you mount them correctly. In your video, you suggest inserting the threaded rod all the way in and pulling it out slightly. This is just wrong, my friend. The threaded rod is almost the same diameter as the ID of the couplers, and so it restricts the ability to flex! You should mount the rod, AND the motor shaft only in the solid part of the couplers, and leave the spring “cage” EMPTY. You’ll find the couplers are in fact very flexible that way. :slight_smile:


#11

@davef it isn’t really a matter of if you can get side-to-side flex from them, perhaps you can as you say, but the up/down springing is not beneficial and that was the main complaint I had. And even though it is OK like itis most of the time, I hate to spend extra money on something that isn’t actually helping, you know? Moving away from the flexible couplers seems to be becoming the norm in the reprap community, in fact. Joe Prusa recently released a new version of his machine where he went a step further and is just having his motors manufactured with the lead screws permanently (and rigidly) attached for the same reason.


#12

OK, if the up/down thing is a problem, I see your point. But, the side to side flex is plenty when mounted correctly. You can swing the top of the Z rods 6 to 8 inches side to side with the motor sitting flat on the bench and the rod sticking up unrestricted.


#13

I am using a different style of aluminum couplers, and I’ve had great results with them. Unlike the more common spiral cut “springy” types, these have no vertical movement at all. These are a direct bolt-on if you’re using 8mm lead screws.


#14

Those do look nice, and only $16 each :smiley:


#15

Link isn’t working. Can’t seem to find the stl in the github repo- only the sketchup file.


#16

I had some bad luck with the printed couplings failing pretty regularly. I found the above spider couplings cheaper. $10 you can have a pair shipped.


#17

ATM I am testing just straight aluminum couplings and after a few prints I’ll show my results.


#18

I have those rigid couplers on mine they work perfect


#19

Yeah all my test prints so far look good.


#20

I seemed to be have clearance issues with these. The 10mm m3 screws tighten fine and is good. They run into the Z endstops and on left side try to snag the endstop switch wires. Suddenly I realized the Z would be higher since bed would be there. LOL