Yet another 3D printed coupler design

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edma194
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Re: Yet another 3D printed coupler design

Post by edma194 »

That video is interesting. It seems that gluing a PVC shaft to the appropriate ends is viable. I wasn't surprised wood dowels didn't work well, it's like a unidirectional fiber rod.

I don't have a newer coupler like the one in the 'melted/broken power coupler' thread, but I'm surprised to see a failure like that unless misalignment was the cause. I would like to see something to relieve the issues of minor misalignment. I tried a crude experiment using two socket wrench U-joints on either end of a shaft and it seemed to work fine for a shaft displacement of about 1/4". I want to experiment more with this, using real coupler ends and a decent intermediate shaft, but those U-joints would never hold up in real use and I haven't found a practical alternative so far.
Ed from Rhode Island

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RFGuy
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Re: Yet another 3D printed coupler design

Post by RFGuy »

edma194 wrote: Sun Dec 27, 2020 12:46 pm That video is interesting. It seems that gluing a PVC shaft to the appropriate ends is viable. I wasn't surprised wood dowels didn't work well, it's like a unidirectional fiber rod.
Ed,

I think you are missing the point. The wood dowels, I believe are intentional and should be able to "break". He mentions cutting a slit in it to create a fuse and I have read other threads on this forum commenting about how the coupler is intended to be the first item to break if alignment isn't correct between an SPT and the drive hub. When alignment isn't perfect, there is some flex to the coupler that takes up the slack and in the worst case it may break, but in doing so it protects the SPT. Didn't he say in the video that all of the ones he tested had sufficient torque for normal operation? This includes the wooden ones.

One benefit of going with wood dowels like this, is it saves on 3D printing time since only the ends are fabricated. What is interesting from the video is it sounds like he fabricated end pieces and sold them before, but some customers had failures with the PVC connections. Of course, this could be due to mis-alignment or could be they didn't get a good PVC glue joint. He doesn't sell this type any more and only sells short couplers or steel ones that require a wood dowel. Note he does this because making a full steel one would be too rigid and not give a safety should alignment become a problem.
I would still have interest in a full 3D printed one if Paul wants to make a batch for a few of us.

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edma194
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Re: Yet another 3D printed coupler design

Post by edma194 »

RFGuy wrote: Sun Dec 27, 2020 1:28 pm
edma194 wrote: Sun Dec 27, 2020 12:46 pm That video is interesting. It seems that gluing a PVC shaft to the appropriate ends is viable. I wasn't surprised wood dowels didn't work well, it's like a unidirectional fiber rod.
Ed,

I think you are missing the point. The wood dowels, I believe are intentional and should be able to "break". He mentions cutting a slit in it to create a fuse and I have read other threads on this forum commenting about how the coupler is intended to be the first item to break if alignment isn't correct between an SPT and the drive hub. When alignment isn't perfect, there is some flex to the coupler that takes up the slack and in the worst case it may break, but in doing so it protects the SPT. Didn't he say in the video that all of the ones he tested had sufficient torque for normal operation? This includes the wooden ones.
I do get that point. I was only speaking about their relative performance. Sorry not to be clear about that. :)

Short of a clutch, a designed failure point is good idea, but in comparison to the other couplers the wood ones were lower strength. I think wood will break down and weaken over time due to the twisting forces. With the square dowels you can see how the wood eventually delaminates at the point of breaking. But yeah, wood may be better than many materials for a controlled break. I'm recalling how aircraft landing gear may be attached to break-away wood blocks so you don't destroy your wing spar from a hard landing. Couplers cost less than bandsaws and planers.

ETA: And only now did I notice I wrote "did not work well" instead of "did not work as well".
Ed from Rhode Island

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paulrussell
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Re: Yet another 3D printed coupler design

Post by paulrussell »

So, let me lead with a disclaimer:
Like several other people here, I have had issues with the current white coupler. I had one get hot and distort while using my bandsaw, and a second is going bad because the engagement keys have premature wear. So I decided to save a few bucks and make a coupler. And hurray, it works! If it breaks, I can have another printed within two hours.

I posted it here.

Then a few of you voiced similar concerns regarding the "new" white coupler and inquired of mine. But making a coupler that works, and making one that I would feel justified selling are two completely different ballparks. Mind you, I enjoy this kind of endeavor, and playing with various 3D printing techniques and materials is my kind of fun. If this ends up being something of worth for the rest of you, that is icing on the cake.

So that leads me to a thought experiment about what constitutes a good coupler:
1) It needs to join two tools with a firm fit
2) It needs to be able to withstand the radial torque needed to drive the tool
3) It needs to be able to accomodate slight lateral mismatches in alignment
4) It needs to withstand additional lateral forces such as when being "wiggled off"
5) It should withstand the wear and tear of regular use and still firmly mate tools
6) It should be able to withstand the expected temperature extremes of the work environment
7) Per many, it should be strong, but not too strong, and act like a mechanical fuse in case of malfunction

And based completely on personal preference: I want my coupler to be visually "similar" to the 1950's original. Printing a 1.25" tube would be easier than the classic shape. But again: The challenges are my kinda fun.

I *think* I'm heading down a path that can satisfy the above list. No promises other than that I'll keep you posted.
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Re: Yet another 3D printed coupler design

Post by DLB »

Historically, I've had good success with the OEM parts, to the point that I consider them an excellent solution. I think I've had two lifetime failures, one of which was a very old coupler that was probably original to my Greenie, so 60+ at the time of failure. I've had one of each known version fail, the old translucent yellowish one (natural?) and a newer white one that was also at least 20 years old. I think that misalignment of drive shafts contributed to or caused both failures, both of which occurred on bandsaws. IMO neither of my failures had anything to do with the torque limit of the coupler. Heating was a factor in both of my failures, both were hot immediately after fail.

- David
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Re: Yet another 3D printed coupler design

Post by RFGuy »

paulrussell wrote: Sun Dec 27, 2020 4:19 pm Then a few of you voiced similar concerns regarding the "new" white coupler and inquired of mine. But making a coupler that works, and making one that I would feel justified selling are two completely different ballparks. Mind you, I enjoy this kind of endeavor, and playing with various 3D printing techniques and materials is my kind of fun. If this ends up being something of worth for the rest of you, that is icing on the cake.
Paul,

Just wanted to let you know that I would be willing to fund 1 or 2 of these, depending on final cost, purely as an experiment. I have never broken a Shopsmith coupler, but I am still using the same one for 20+ years. I bought a spare from Shopsmith last year and it is the newer white one that may or may not have a problem compared to the old one. I saw the 3D printed couplers posted on Shapeways before, but didn't buy because I really just want a coupler equivalent to the original Shopsmith coupler (old one), i.e. same size and strength. I really don't need the telescoping design even though it is nice to have. I thought maybe you were doing this as a hobby and that is why I offered my support here, but only if you want to print more to share. If the printing doesn't go well or you get busy, don't worry about it. Either way, I appreciate you sharing your work here. I am intrigued as 3D printers advance to being able to fabricate more functional and heavy duty parts like this which hasn't always been the case. I have no immediate plans to purchase one, but just curious what your setup is if you want to share it.
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paulrussell
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Re: Yet another 3D printed coupler design

Post by paulrussell »

Wrestling nylon into 3d printing submission was quite an exercise. But I was able to produce what I think is a decent coupler. I've sent PMs to those who expressed interest. I'd rather get feedback from these folks first before venturing any further.
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tbolish
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Re: Yet another 3D printed coupler design

Post by tbolish »

I’d be interested in check one of these out if you have any more available. Thanks!
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paulrussell
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Re: Yet another 3D printed coupler design

Post by paulrussell »

To those who are getting the first batch of couplers:

You should have gotten a PM from me.

Using a Shopsmith spring on the headstock end will make removal of the coupler much easier. The coupler is intentionally a bit snug when new. After a couple uses the slight "ribbed" texture to the 3d printed part that should smooth out on the mating surfaces.

BTW: I prefer my couplers to have a snugger fit than the white couplers I've purchased over the last few years. The fit is closer to my old-school nylon coupler.
tbolish wrote: Wed Jan 06, 2021 1:56 am I’d be interested in check one of these out if you have any more available. Thanks!
I'll keep you in mind for round 2. I want to get feedback from a few users before I move forward.
Last edited by paulrussell on Mon Jan 11, 2021 10:59 pm, edited 1 time in total.
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dusty
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Re: Yet another 3D printed coupler design

Post by dusty »

I had difficulty removing the springs from broken couplers and ultimately discarded them. I'll never learn. I keep things I never use again and then discard what I need.
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