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JohnInOttawa

PVA support and printing threads

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Good morning everyone!  This might be obvious to the more experienced, but it's new to me, so here goes:

 

I am printing an adapter (reverse ring for a camera lense) with two different sizes of thread, both ISO metric, one M42 x 1, the other M62 x 0.75.  There is a circular flat plate between them that really needs to be flat on both sides. 

 

I first printed each thread  (UM3, PLA) individually on a test cylinder, test fits went perfectly.  So I decided to create the merged STL with the plate between them.  To keep the plate flat, I elected to go with PVA support.  I selected 'everywhere' instead of 'touching build plate' just to see how it would go.

 

Good news first, the plate came out flat as expected.

Bad news, neither the lower, nor upper threads, printed correctly. Both printed fully encased in PVA.   They look fine, but neither thread will start correctly and starts to cross-thread in about 1/8 of a turn.

 

Under magnification, instead of the thread geometry being a smooth, trapezoidal cross section as it was without support, it was more like a histogram, with visible gaps between (very straight and flat) filament layers, almost as if there was interleaving between the PVA and PLA.  Also, the PVA was a challenge to deal with in the fine metric thread, even after prolonged soaking and cleaning with a toothbrush.  I'm still not sure I got it all out.

 

I will try this again with modification to the support to try and move it away from the threads and will ensure it's set to just touching the build plate.  Any other suggestions?

 

Thanks in advance

John

 

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Another option might be to print the thread a little bit too small, so it does not fit. Or print the model without any thread. Then warm it up in an oven (only if electronically controlled), or on the build plate to about 70...80°C, screw it on the equipment immediately, and let it cool to room temp. In this way I have made difficult designs fit around clamps, although without threads. Expect it to take a few trials, or try on obsolete equipment first. Make sure your photo equipment has metal threads, no plastic ones.

 

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Well, of course there may exist better settings; it's just that I don't know them. And I don't have an UM3 with supports anyway, only single nozzle UM2s.

 

Small threads will always be difficult due to the layer lines which distort the shape and cause sort of "derailing". I also found it almost impossible to machine threads in PLA (I tried M3, M4, M5), due to the low melting temp. Even when taking very much time (+10 min) to slowly cut the thread manually, it still melted, and it was not strong enough to fix the screw later on. I think most people will try to avoid threads, and use other methods. That is the background idea for heating and making the whole model softer, and then forcing it on the other part and let cool down. This works well for rough shapes like clamps, but I can imagine that it may not be optimal for finely machined parts like photo equipment. I guess it will require trial and error. If you would find a good solution, please let us know.

 

By the way, any models in PLA will become soft and distort anyway, if you leave them in your car in the sun, even on a mild spring or autumn day.

 

The first image below shows the clamp which had to fit over a big glue clamp, so I heated it and forced it over it. The thin colored plates are supports to get it printed on my single-nozzle UM2. It are separate plates for easier removal.

 

If you don't mind a bit of uglyness, you could also use external screws to hold parts together, as shown in the other models below. These are clamps for laboratory equipment. It takes less time to design and print them, than to search and buy.

 

lijmklem2.thumb.jpg.1fcc38db076f22bf02b539e2962d5c79.jpg

 

darmklem_207_15mm_3b.jpg.55bbee6c20655b2b9f15417fc4826d7b.jpg

 

haak_blaaspist_v3.thumb.jpg.87935c5c5c3fd646592ea7d55eeca330.jpg

 

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I've had a lot of luck printing internal threads, even down to M3 size on my UM2+ (.1mm layer height, PLA, default settings). The caveat is that the holes need to be oriented vertically along the Z axis.

 

The threaded connection is quite strong for my purpose, which is an electronics enclosure. There are holes for M3 bolts in the lid, and pre-printed threaded holes in the box. You can just screw in M3 bolts without worrying about nuts/nut traps so it's very convenient.

 

I can definitely see how it would struggle though if they weren't vertically oriented and had support material to deal with. @geert_2 I agree that tapping PLA does tend to create a mess though.

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