Threading internal and external

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Threading internal and external

dbvanhorn

My goal is to create a compression fitting by putting a piece of 13mm ID silicone tubing (17mm od) inside two printed parts that will compress the tubing lengthwise, causing it to collapse around the shaft, which is somewhere between 12mm and 11mm.

I have a design that I'd like to pursue, which would require printing two ABS parts that thread into each other.  The nominal thread diameter would be something between 15mm and 20mm and I am thinking of a pitch that's about 4mm per turn.

Can someone with experience in this give me any advice?  I don't have a feel for what's really workable.  The parts don't have to mate with anything else in the universe.  Maybe there's a less complicated way to get there?

I'll be checking mail while I'm hacking away.  Thanks

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Re: Threading internal and external

ednisley
On 02/26/2017 02:52 PM, david vanhorn wrote:
> compress the tubing lengthwise

Looking at the areas, the relaxed silicone tubing occupies 94 mm² and
the 1.5 mm annular gap is 29 mm². Assuming the tubing OD doesn't change,
the compressed tube must occupy 94 + 29 = 123 mm² to barely touch the
shaft, so it will be 76% = 94/123 of its original length.

Compressing a chubby silicone tube by 1/4 of its length (seems like) it
will require entirely too much strength from a 3D printed structure and,
worse, a small length error means a bad joint.

I'd favor an annular clamp, done in simple E-Z printable halves, with
internal ridges to squash the tubing against the shaft. Run a row of
screws along each side, compress the tubing by maybe 1/3 its thickness
(1+ mm) all around, and you're done.

You'll definitely need T-nuts (*) to get enough clamping force; screws
threaded directly into the ABS will strip out.

(*) The non-spiky kind, similar to:
        http://www.t-nuts.cn/en/products/xjlm.html       

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Re: Threading internal and external

MichaelAtOz
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In reply to this post by dbvanhorn
Wouldn't a design like garden hose fittings be more feasible in plastic?
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Re: Threading internal and external

dbvanhorn
Ok, I got a sample of the silicone tubing, and I agree, WAY too stiff.  
Changing to Latex tubing, much softer.

Same question re threads though, but at least it won't need to be printed from titanium.


Garden hose fitting: That looks pretty hard to print.  I thought about doing something like that, and that's what led me to the idea of compressing squishy tubing lengthwise.  Tubing I can cut to length easily and just drop into place.


Joining the parts is the next challenge.  I'm still working on how to apply outer threads to the male part and inner threads to the female part.  I was hoping to generate a "tooth spiral" that I could union onto the male part, and then generate at a slightly larger diameter and difference from the female part.  No luck yet.

Fortunately I don't need to match up to any other thread on the planet, so I don't have that problem as well.
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Re: Threading internal and external

ednisley
On 03/03/2017 10:37 PM, dbvanhorn wrote:
> how to apply outer threads to the male part and inner threads to the female part

I used a crude thread approximation to repair a broom:

https://softsolder.com/2013/04/01/broom-handle-screw-thread-replacement-plug/

A slightly less crude model (with a dedendum!) and better thread shapes
probably won't work for your purposes, because it requires gluing a
solid core:

https://softsolder.com/2013/11/07/broom-handle-screw-with-dedendum-effect-of-printing-orientation/

An internal-thread version to match threaded (!) flexible hose:

https://softsolder.com/2016/07/29/dust-collection-vacuum-fittings/

The basic problem with 3D printed threads is that they have terrible
surface finish with very high friction. If you need a nice, smooth screw
that transfers high loads (which I think you do), then 3D printing isn't
going to work well at all.

In round numbers, printing a thread in the obvious axis-vertical
orientation with a 0.2 mm layer thickness gives you 20 "clicks" of 18°
each around a single turn of 4 mm pitch. It's even worse with the axis
horizontal, because the XY resolution is around 0.5 mm; the threads look
prettier, but have gritty edges in all directions.

That's why I suggested a circumferential clamp printed in halves,
because it applies direct pressure from metal screws. The ribbon cable
clamp I build isn't strong enough for a hose, but should give you the
general idea:

https://softsolder.com/2016/12/27/loop-antenna-splice-reinforcement/

Happy modeling …

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Re: Threading internal and external

dbvanhorn
Hmm..

Well, I need some sort of twist-lock function.  Screwdrivers are not an option.
What other mechanisms might work?
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Re: Threading internal and external

ednisley
On 03/04/2017 10:55 AM, dbvanhorn wrote:
> some sort of twist-lock function

I think consumer-grade 3D printing isn't going to get you to the goal,
because high mechanical forces applied to sliding parts require better
materials and surface finishes than you can get from printed plastic
parts, at least in the sizes you've described.

If you're willing to buy printed parts from, say, Shapeways, you have
access to a wider variety of materials (including metals, with some
caveats) printed with (somewhat) better resolution and, for their
sintered powder processes, built-in support material. That may produce
better results than consumer-grade 3D printers, albeit at a much higher
unit cost.

Based on the surfaces of a metal part I got from Shapeways a while back,
I wouldn't expect smooth-sliding threads:

https://softsolder.com/2014/01/06/browning-hi-power-magazine-capacity-reduction-block-steel-version/

Basically, 3D printing is a process with serious limitations. Used
within those limits, it's /wonderful/ and I print plenty of gadgets, but
… it's definitely not a miracle.

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