Best Z axis coupler?

I'm trying to source parts for a Mendel90, specifically the Z axis coupling which connects my 5mm motor shafts to 8mm threaded rod. Nophead uses Neoprene hose to expand the motor shaft diameter to 8mm, then clamps both shafts in a printed coupling, however finding someone who can sell me a few centimeters of rubber hose for less than silly money proved harder than expected, so I started looking at alternatives.

The professional solution on CNC machines seems to be an aluminium machined coupling, and Ebay has many listings (all of them cheaper than the listings I could find for Neoprene hose/tube!), so that seems to be the way to go, however I've noticed there are 2 slightly different interpretations of the coupler.

1. Couplers like this tighten on the shafts by grub screws directly onto the shaft.

2. The other style, like this, have a split in the coupling wall which is clamped tight by screws.

Seemingly a minor difference, but can anyone think of any advantages/disadvantages between the 2 methods (or does anyone use these professionally, and know what the differences are)?

I suppose what I'm wondering, is either one more, or less likely to lead to an off centre coupling?

If you know someone with a printer, I can send you an stl file of a shaft coupler. Give me shaft diameters, lengths available for grub screws, and overall length of the coupler and I can send the stl. As an alternative, I have an rsdoc file of a design that I can send you if you use DesignSpark Mechanical (it's free!) and you can make the necessary changes your self. You can see a picture of the coupler I am using here: Shaft coupler

Being slightly off center isn't too important- that's why you're using a flexible shaft coupler instead of a non flexible type.

Edited 1 time(s). Last edit at 12/22/2014 08:04PM by the_digital_dentist.

Clamps have less of a tendency to come loose and center the shaft better.

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@Dentist, I'd be interested to see the STL, thanks. Shafts are 5mm motor, 8mm threaded rod, probably (see below), and I'd prefer to use an M3 grub screw (6 or 8mm). I see you specify ABS for your design, any reason that won't work in PLA? I have an i3 box, which I plan to upgrade to the Mendel90 after printing the parts, but I haven't got around to printing in ABS yet (though I've got a reel sitting on the shelf behind me).

@3dkarma, I was wondering if that might be the case, thanks.

Since posting that question I also realised I could use 5mm threaded rod instead of the 8mm (same as my i3) which makes coupling much simpler (I use plastic tube and zip ties). On the face of it 5mm rod gives greater Z resolution (probably more than necessary, though I haven't crunched the numbers) at the expense of slower speed (which on the Z axis, I don't really care about), are there any other good reasons to use 8mm instead of 5mm?

8mm threaded rod is less likely to get/be bent than 5mm.

Motors can get hot when operating for long periods. That includes the shaft. I think ABS would be better, but PLA might be OK depending on how much current you run through the motor.

I edited the file for 5mm and 8mm shafts and M3 grub screws. The shaft holes are just a little oversized so that when they print they'll fit the shafts tightly. You may have a little tweaking to do depending on how well your printer is calibrated. I can send a higher resolution file but this forum limits attachments to 600kb. Slice with Cura and use the support material everywhere option. You'll have to use a scraper or other bladed tool to get the support material out of the spiral slots. Print with about 50% infill.

I can send the original RSDOC file if you can use it. Making changes to it is very easy.

Couplings like this;
ebay plum coupling

Are a couple of quid more but are claimed to be anti backlash. The couplings you linked to can elongate slightly (like a spring) especially if a load is suspended from it.

As stated - the idea of having a coupling is to allow flex for misalignment between your motor and the drive rod. If it's too rigid it will put extra side loads on motor bearings and whatever it is you are driving.

-a

My inclination would be to turn down the end of the rod and use a 5 x 5 coupling.