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Making pieces that fit together

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Posted · Making pieces that fit together

newbie here. I've got a project made in two parts and now I'm working on how to make the two parts fit together nicely. I don't really want to glue them. I want to insert a top piece into a base. The more I think about it the more complicated it seems. Both to model (FreeCAD) the required shapes and the choice of geometry. Maybe this problem has a name. Anyway, I'd appreciate some tips and/or links.

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Posted · Making pieces that fit together

If you are on Windows, I would suggest you try DesignSpark Mechanical for editing: this is also free (just requires registration) and is much easier and more stable than FreeCAD. There are a lot of good tutorials on Youtube. Have a look at them and see if you like its concept. Or try any of the free online-programs (I have no experience with them).

 

Then for mounting, in my experience:

 

For multiple-part objects, I often mount them with nylon screws and nuts. Provide the holes and hex-indents for the nut and screw in the model, so they sit recessed and are easy to assemble and disassemble.

 

Press-fitting parts requires a lot of testing to get it right, and PLA (and lots of other 3D-printing materials) tend to creep under mechanical loads, so they deform over time. So the fitting may become loose.

 

Snap-lockings work well for PET. For PLA they work when inserting, but after a year the PLA gets harder, and then the snap-locks break apart when you try to unlock them.

 

3D-printing screws and threads is hard, and tapping them is even harder, especially in PLA: this melts almost immediately.

 

Glueing with cyanoacrylate works pretty well for PLA. Depending on the circumstances, this can be a good option.

 

One of many possibilities of a clamping bolt and nut (this one has to slide and be adjustable):

 

ostroncp_v20170104c.thumb.jpg.6dae46fd9b48b292823335415abefbf4.jpg

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Posted · Making pieces that fit together

Hi @geert_2 thanks for the great tips. I'll might have a look at DesignSpark but I'm using FreeCAD because it FOSS software/GNU, has many workbenches, and can be scripted with Python. Yes, it's been quite unstable, but I haven't had v0.18 crash on me yet.

 

I can hear from what you're saying that it's not so easy to click things together. I think I might be better going with a regular screw recessed into the bottom of the base and going into a nearly big enough hole in the bottom of the top section. I'll let you know how it goes (very slow spare time project).

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Posted · Making pieces that fit together

I recommend that you experiment with various concepts on small test models, to see what works for your application.

 

In this spool holder for my UM2 printers, I use several different mechanisms:

- Two snap-fits for the main holder (cyan part), where it fits into the printer, just like the original. This snap-fit has long travel-ways, and the openings in the printer have a lot of dimensional tolerance, so it is not likely to break.

- The pins (red) are clamped and kept in place by other parts.

- The rotating head and bearing are mounted on the main house with a big inox M8 bolt and self-locking nut (mediumblue and darkblue).

- Both halves of the bearing-housing (yellow and spring-green) and the bearing itself (purple) are secured with nylon M4 bolts and nuts.

- The bearing (purple) is a standard 608 bearing (as in skater wheels).

 

image.png.fb5ca13db604e4117fe60bac0007be3a.png

 

image.png.ee70962ea408132816287d7a063c2c38.png

 

My designs often are a compromise between ease of printing, ease of assembly, functionality (it has to work), and strength and stability. For printing, I want a big flat bottom plate sitting on the glass, and an orientation that does not require too much overhangs and bridges (as I have single-nozzle printers). If supports are needed, they should be in an area that does not damage the model (like in the main cyan housing: supports are required and would be on the inside, and it would be printed with the wide opening down).

 

The anti-unwind clamp shown below also uses two M4 nylon nuts and bolts to clamp things, and it can very easily be printed on its back. One bolt clamps the filament, the other clamps the whole thing on the rim of the spool. By itself, the red models snap-fits and freely slides around the rim of this spool.

 

image.thumb.png.78c8f2262d3b2d5beaf962fdad1883c7.png

 

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