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Dual motion transmission for a small project

04/08/2022 6:37 AM

Good Morning Smart People! I'm working on a small project, a ring box. The lid is a leaf shutter, like an old camera. The body is a wood cylinder glued to the outer edge of the shutter. The bottom rotating ring of the iris is glued to a pvc tube which is glued to a wooden bottom cap. By rotating the bottom cap, the pvc tube rotates, which rotates the iris open and closed. That's the easy part.

Simultaneously, I want a plunger to raise and lower inside the pvc tube revealing the ring as the iris opens, and moving it lower inside as the iris closes. I've carved spiral grooves in the pvc tube, and put acrylic rod "ears" on the plunger to fit in those grooves. This facilitates the up and down motion. To keep the plunger from rotating along with the pvc tube, thus not moving up and down on the grooves, I've cut large windows in the pvc tube to allow longer acrylic "ears" on the plunger to reach straight vertical grooves in the stationary body of the box. This keeps the plunger from rotating, but allows for the vertical movement. This all happens in one quarter turn of the bottom cap. It works. Most of the time.

The problem is that the four points of contact on the plunger, two in spiral grooves, and two in vertical grooves are a little loose, so they don't bind, and this allows the plunger to wobble, becoming stuck in the pvc tube half way through the motion.

Am I on the right track, and need some simple tweeks, or is there a better way to make the two motions with the one turning bottom cap? Thanks in advance for your suggestions.

Mike

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#1

Re: dual motion transmission for a small project

04/08/2022 6:59 AM

With all that work I hope she says yes.

If you can post pictures or drawings it would be very helpful. You seem to have described your device very well but the devil is in the details. Pictures and drawings are very helpful for details.

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#2
In reply to #1

Re: dual motion transmission for a small project

04/08/2022 7:09 AM

I'll take some pics of what I have so far. I'll post them as soon as I can. My morning just got a little busy, but busy is good. I'll get on it.

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#3

Re: dual motion transmission for a small project

04/08/2022 9:35 AM

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#11
In reply to #3

Re: dual motion transmission for a small project

04/08/2022 1:24 PM

It is not clear why you have a large cut-out in the sides of the pvc pipe for the second through-pin. Is it not going to be following the same spiral path as the first through-pin? If so, then instead of the large cut-out, machine matching spiral slots for it. Then add two more vertical slots in the inside of the stationary outside barrel to mate with the ends of the second through-pin. Maybe this would add stability to the assembly and prevent jamming.

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#12
In reply to #11

Re: dual motion transmission for a small project

04/08/2022 1:46 PM

Wow! Now see, that’s exactly what I was looking for! I made the cut outs assuming I needed to get the pvc pipe out of the way of the longer ears. Thanks SlideWinder for helping me realize that spiral slots would accomplish the same goal. Adding 2 more vertical slots in the stationary outer wall seems to be the right move to sure up the plunger. I’ll machine a new tube like you said and report back.

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#15
In reply to #12

Re: dual motion transmission for a small project

04/08/2022 5:09 PM

Well, I’ve run out of time to play with my project. Won’t be able to pick it up again until Sunday. I’ll keep you all posted on my progress after the weekend. Thanks for all you guys help!

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#4

Re: dual motion transmission for a small project

04/08/2022 9:38 AM

These photos are of a mock up prototype. I'll make the real one with walnut wood.

I think I might have found part of the problem. There seems to be a "bump" in one of the spiral grooves, may be causing the sticking problem. I need to get some small files to smooth it out, and see if that helps. Any other thoughts would be appreciated.

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#5

Re: dual motion transmission for a small project

04/08/2022 10:21 AM

- Could little rollers be fitted to the plunger ears to prevent jamming?

-Also, you write that you "carved" spiral grooves in the pvc tube, suggesting that you cut the grooves by hand. For precision, the spiral grooves should be machined - maybe a router gig to facilitate this?

-Maybe a short piece of aluminium tubing would be a better choice - it could still be router machined and it could be polished smooth - for functionality and aesthetics. Satin finished aluminium looks very nice with dark hardwoods.

-Possibly even the vertical grooves should be aluminium or hard plastic (acetal?) tracks epoxied into slots. Before that, try a bit of hard soap as a lubricant in the wood grooves.

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#6
In reply to #5

Re: dual motion transmission for a small project

04/08/2022 10:34 AM

Sorry, I failed to mention all the parts were milled on my cnc router. No hand carving. I chose the acrylic for it's smoothness as it slides through the spiral grooves. But I notice a little "give" in the acrylic, so perhaps aluminum rods might be better. You won't see any of the internal parts once it's finished, so not worried about the internal aesthetics.

I did think about lining the vertical grooves with a cut up plastic straw. It already has a natural curve, so would fit in the groove, and not pop free from the adhesive trying to hold the curve.

The hard soap is a good tip, to lubricate the parts in motion. Thanks!

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#7

Re: dual motion transmission for a small project

04/08/2022 10:42 AM

Another thing I just thought of, is to weight the bottom of the plunger. It's just a short piece of wooden dowel, so it's very light. The weight of a ring will make it top heavy, so maybe I'll drill a lead weight into the bottom of the plunger, so gravity will help to keep it straight.

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#8

Re: dual motion transmission for a small project

04/08/2022 11:42 AM
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#9
In reply to #8

Re: dual motion transmission for a small project

04/08/2022 12:11 PM

I already have the Iris motion machined and working.

https://www.dropbox.com/s/dr3teasrvrrhzch/IMG_0397.mov?dl=0

Sorry, i don't seem to be able to share the video directly. Copy and Paste the address above to see my diaphragm in motion.

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#10
In reply to #9

Re: dual motion transmission for a small project

04/08/2022 12:39 PM

Sounds like it's binding, and you did say you were having trouble with lack of smooth operation...anyway it's just a suggestion, I believe this is a superior design though...but there are many ways to skin a cat...

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#13
In reply to #8

Re: dual motion transmission for a small project

04/08/2022 2:43 PM

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#14
In reply to #13

Re: dual motion transmission for a small project

04/08/2022 3:31 PM

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#16

Re: Dual motion transmission for a small project

04/20/2022 6:56 PM

Hi Guys,

It's been a while since I reported back on my project. I've finally got all the kinks out, and it's working. After making the prototype work, I made a new one out of Maple. I didn't have enough Walnut lying around.

I used Canadian SlideWinder's idea, using 4 spiral grooves in the pvc pipe, and 4 vertical grooves in the stationary wall. I'm still using acrylic rods as ears for the plunger, and I've lined the vertical grooves that receive the ends of the ears with a cut up plastic straw. The motion is smooth, and the plunger rises and falls nice and straight.

I'd love to show you guys a video of it working, but I won't post it on You Tube. Is there another hosting site available that I can provide a link to? How do you guys post videos here?

I want to take a moment to thank you all for chiming in with suggestions, and helping me to complete my project.

Mike

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#18
In reply to #16

Re: Dual motion transmission for a small project

04/21/2022 9:55 AM

Excellent! A very interesting project. If you are pleased with the result you should consider submitting photos to Fine Woodworking magazine for inclusion in their Reader's Gallery - a feature of every publication. Maybe it would even be a candidate for a short article in the magazine.

It has been my experience that although most people take little interest in the workings of complex, commercially available machinery, those same people are fascinated by simple mechanics in a hand-built craft item.

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#17

Re: Dual motion transmission for a small project

04/20/2022 7:13 PM

Here's a link to the Drop Box file in the meantime.

https://www.dropbox.com/s/6cxsfmu1520iy76/IMG_0416.MOV?dl=0

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#19
In reply to #17

Re: Dual motion transmission for a small project

04/21/2022 10:33 AM

Cool....still sounds like it's binding a bit though....

https://www.dropbox.com/s/6cxsfmu1520iy76/IMG_0416.MOV?dl=0

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#20
In reply to #17

Re: Dual motion transmission for a small project

04/21/2022 1:25 PM

Suggestions:

- Exotic hardwoods, prohibitively expensive for large projects, would add a richness, and would be a good choice for a small object like this.

- The design rule, 'If you can't conceal it, emphasize it', could be applicable here. The edges where the stationary and rotating elements meet could be softened/rounded.

- Study the geometry of various small lidded boxes by skilled turners. The housing of the mechanics of your box needn't be limited to a plain cylinder. The profiles of the bottom, middle, and top could differ, physically identifying them as base, middle, and crown, and thereby adding visual interest.

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