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Aquarium tubing for Z-coupler - fit over M8 rod?

Posted by cobrageek 
Aquarium tubing for Z-coupler - fit over M8 rod?
March 23, 2013 02:03PM
There's something fishy about the shafts on the Z-motors that came with my kit or with the standard plastic couplers because the couplers can't get tight enough to fit. The shafts are 0.2" or just over 5mm and the couplers look OK. I was considering using tubing anyway as that seems to be a strong recommendation so I bought some aquarium tubing.

The tubing fits perfectly on the motor shaft (nice and tight and figured I'd use zip ties until I'm up and printing then make some clamps). But how do you fit the tubing over the M8 rod? I might be able to fit it on there, but it's not easy. I recalled seeing it on the reprappro mendel build instructions and after checking them again realized that they included M5 rod for this which would work perfectly. But my kit came with M8 rod for the Z-rods that match all the other M8 rods for the frame.

So, what have people done to get the tubing to fit on the M8 shafts? If I cut a little slit to start I might be able to "thread" it in past the slit and then cut off the slit part. Or if I stretch it out some with some needle nose pliers it might loosen up enough to fit. Any thoughts?

Thanks!
Re: Aquarium tubing for Z-coupler - fit over M8 rod?
March 23, 2013 03:38PM
I had success doing this with 1/4" acme rod, which is a little thinner than M8.

Basically, you just force the end of the rod into the tubing and start rotating it to kind of "cut" threads into the tubing. As the threads catch it will pull more rod into the tubing as you rotate it.

If the M8 is too big to fit into the end of the tubing, you could try cutting slits in the tubing to open the end up a bit. This might allow you to get the first thread or two started. Or you could reduce the diameter of the first few mm of rod with a dremel, angle grinder, or lathe, to make getting it started easier.

EDIT

If nothing else works, you can find different tubing that has about a 7mm ID to go over the M8 rod. Put a section of small tubing over the motor shaft and put the big tubing over the small tubing.

Edited 1 time(s). Last edit at 03/23/2013 03:39PM by crispy1.
Re: Aquarium tubing for Z-coupler - fit over M8 rod?
March 23, 2013 04:21PM
Thanks crispy1. I used pliers to get it to fit over the end of the M8 and then "screwed" it in farther. It's definitely in there tight and not going to slip off.

But after playing with the aquarium tubing, it really seems to me like it will be too "spongy". I know it's good to let the Z-rods "wobble" a bit, but this is so "spongy" that it seems like the motor will turn and "twist" the tubing and that all of the rotation will not translate to the rod. It seems like that there will end up being several mm worth of "twist" that is held in the tubing. Doesn't this cause a problem? Or does it not matter because during a print the motors are really only turning in one direction?

Seems like this would be a problem at least at the start of the print. When the Z is turned all the way down for home and then the first rotation of the Z-motors in the "up" direction would be completely absorbed by the "sponginess" of the tubing. Is my tubing just to spongy?
Re: Aquarium tubing for Z-coupler - fit over M8 rod?
March 23, 2013 04:26PM
There are two types of aquarium tubing. The colored stuff made of silicone and the clear stuff made of some other plastic (maybe pvc). I think the people using it as a coupler are using the clear stuff because it's stiff. If you are using it with a plastic clam shell over it like Nopheads coupler then you can use the silicone stuff.


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Re: Aquarium tubing for Z-coupler - fit over M8 rod?
March 23, 2013 05:56PM
Hmm.. Well the stuff I have is clear and is the normal tubing I've seen used for air hoses into aquariums. It looks just like [www.reprap.org] but maybe not quite as thick (it's hard to tell. Seems to have a 4 or 4.5mm ID and 6mm OD. It's pretty stretchy and it had to be in order to fit over the 8mm rod. Maybe it's even worse after being stretched out so far.

I could use the tubing just on the motor shaft and then the nophead couplers to affix it to the rod. I was also thinking about using the tubing on the motor shaft and then putting on a couple layers of heat shrink tubing as the coupler. Do you think that would work? Which do you think wuold work better?

Thanks for the help.
Re: Aquarium tubing for Z-coupler - fit over M8 rod?
March 23, 2013 06:23PM
I like Nopheads coupler myself. There is also an old thread by VDX that shows some other tubing being used for coupler. I think it was hydraulic tubing.


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Re: Aquarium tubing for Z-coupler - fit over M8 rod?
March 23, 2013 07:09PM
I just played around with the nophead coupler using the aquarium tubing over the motor shaft. That combination seems to work perfectly. The rod looks very centered to the eye and the coupler over the tubing allows just a bit of movement in X and Y if needed, but is totally locked rotationally. I think I'll go with that for the initial build and I can improvise something else later if it proves to be an issue.

I found some automotive tubing in the garage that would work for the M8 rod but the ID is too large even for the aquarium tube covered motor shaft. I'm sure I could find something that would work that route or go with heat shrink tubing, but I think this will work well and is easier. Let me know if you have concerns about this plan or a better idea. Thanks.
Re: Aquarium tubing for Z-coupler - fit over M8 rod?
March 24, 2013 03:29AM
tubing is ok as long as you also glue it or add kapton tape around the motor shaft then place the tubing on. the tubing will stretch out when the motors warm up and go weird on you. ideally use polypropylene tubing. silicone tubing seems too slippery for me. any way the idea is to have the surface area with enough grip force to prevent loss of positioning. actually also have this tubing with another piece of larger tubing on the outside if not using zip ties or clamps. the week walls around the tubing will stretch from the wobble motion they absorb.

come to think of it it really was never a good idea for tubing without clamps for a permanent solution.


to fit over 8mm shaft use larger tubing, over smaller tubing that goes to motor shaft. have larger tubing all the way down the shaft. glue in place the 8mm threaded rod. and clamp it with zip ties on the threaded end.


james

Edited 1 time(s). Last edit at 03/24/2013 03:32AM by jamesdanielv.
Re: Aquarium tubing for Z-coupler - fit over M8 rod?
March 24, 2013 10:17AM
Clamping an M8 threaded rod in solidly with a coupler is fine so long as the rod is straight, centred on the motor spindle, and runs in parallel with the Z smooth rod. If any one of those conditions isn't met, you'll tend to get wobbling in prints in the Z axis, especially on a Mendel-style frame, due to over-constraint of the Z-rod. The problem with mating the 5mm motor shaft with the M8 threaded rod is that you need to use some kind of intermediary - nophead's couplers are great, but it still means clamping a bit of aquarium hose to provide a bit of flex, and perhaps still over-constrains Z.

I prefer to use M5 rod for Z (3/16"=4.7625mm is the nearest imperial equivalent), which is the same outside diameter as the motor shaft. A piece of 1/4" OD (has an ID of around 4.5mm) aquarium hose then fits nicely over both the shaft of the motor and the M5 rod, and put a small clamp on each side, just like the RepRapPro Mendel: [reprap.org]



This leaves the Z less constrained, allowing X-Y movement in the Z leadscrew to be absorbed by the flexible coupling rather than being transmitted into the frame. The smaller leadscrew size increases accuracy (0.8mm thread pitch on M5 rather than 1.25mm of M8), and has less mass and strength to effect the X axis.

It does mean you need to have an insert on your x axis ends for M5 nuts rather than M8, or better still for a really unconstrained Z, use a M5-sized z-isolator: [www.thingiverse.com]
EVEN better, get Stainless Steel M5 Z leadscrews, and brass M5 nuts for really smooth movement.

In case you're wondering why we don't use proper motor couplings like in CNC machines, it's because they are coupling them to PROPER leadscrews, which are machined straight, so over-constraining them isn't as much of an issue. Plus the frame of a CNC machine are generally engineered much more rigidly so can resist any leadscrew whip. If there is any, they tend to seize up, or prematurely wear out the leadscrew nut, rather than flex the axis. Put it down to the lightweight frames we use that we have to come up with an alternative solution!
Re: Aquarium tubing for Z-coupler - fit over M8 rod?
June 21, 2013 11:37PM
I just got 1/4" ACME lead screw here. 1/4" Leadscrew I used the poly (HDPE I think) 1/4" tubing that's sold in home improvement stores for hooking up refrigerator ice makers to the home water supply. It's much firmer than PVC aquarium tubing and fits like a glove! I first tried heating the tubing with a rework blower, but it becomes too soft to "thread". Using tools tends to destroy the tubing. Using hand force I was able to thread about 10mm, (though I have strong hands).
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Re: Aquarium tubing for Z-coupler - fit over M8 rod?
June 22, 2013 03:33AM
I use tubing on my z although i havnt finish my printer yet I found just using couplers I got printed of thigiverse made the z wobble too much


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Re: Aquarium tubing for Z-coupler - fit over M8 rod?
June 22, 2013 04:21AM
did you know that placing the tubing in warm water makes it easier to stretch it. soak in warm to boiling water, then fit it over the 8mm rod.

Edited 1 time(s). Last edit at 06/22/2013 04:21AM by jamesdanielv.
Re: Aquarium tubing for Z-coupler - fit over M8 rod?
June 22, 2013 05:27AM
could of needed that knowledge two weeks ago when straining too fit the tubing over my 5mm threaded rod lol


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