I give up: XYZ part

In building my second MPCNC I am reminded of how much difficulty I had with the outermost nuts on the XYZ part. I have tried the screwdriver trick, printing custom wedges, pre-inserting into the nylon, and everything else I can think of, and I always seem to end up mashing the plastic facets to be round with nothing to grab on to. Depending where I wedge the screwdriver I also managed to crack the part by de-laminating layers.

So I gave up and chopped the model (green shows removed area):

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I’m sorry, Ryan, for defacing your design intent. I feel oddly guilty for mangling the part like this.

From the pictures it should be clear that tightening becomes a non-issue since two opposing sides of the bolt are freely accessible. Now it’s super easy, comparable to most of the other nuts and bolts, and no plastic gets mashed in the process.

I’m not sure how the computation is made for the engineering trade-off between stiffness and cursing, but for me at least this trade seems favorable. :slight_smile:


Honestly i wish Ryan would take those bolts out of the needed hardware list, and say it literally NEEDS to use metric for that part. I purchased the same bolts you have and rather than do the logical thing of going to the store and purchasing some metric bolts, i wasted another hour breaking the part. One thing that does not work is using a heat gun to immobilize the bolthead by softening those 2 damn ears. Just go buy Metric.

I understand the frustration. Most have no issue, some run them together to loosen the nylock first, some just use a regular nut and locktite, new one is a modified part… there are many ways to deal with these 4 bolts that need to be tightened 1 time.

I would absolutely love to have all metric / or imperial hardware. It would be awesome, but that is not the current state of the world.

It does not “literally” need to use metric. I used my imperial bolts just fine, in fact I was confused that the instructions even mentioned it being an issue.

I couldn’t help but think about this a bit more and I am wondering if there might be other alternatives. I took measurements off Ryan’s STL file, and I came up with 15.25 mm diameter (corner-to-corner) of the hexagon. This is twice the distance from the center of the hole to the inside corners of the little half-pocket for the nut.

I put together a test part by adding a rectangular block and carving out a hexagon this same size. I also rotated 30 degrees because the steep overhangs don’t print particularly well for me, and instead I have a horizontal overhang which (for me) prints alright as a bridge. A more universal approach could cut out a little “roof” above the nut so it would have no steep overhangs at all.

I tried tightening a nut and bolt and I was able to snug it down plenty tight without the nut slipping.

This made a bigger difference than I expected – I actually expected the size difference to allow it to still slip, but it doesn’t feel like it’s anywhere close to slipping. I am wondering if this is maybe because being supported on opposite sides, pushing against the plastic is restrained by the opposite side pushing, instead of being supported by a cantilever action pushing the bolt against the bore of the hole. I also get the feeling it fits a bit “too well” making me somewhat doubt whether the dimension is correct for an 8mm nut/bolt. I tried to mimic the existing dimensions from the part because I wanted to maintain the ability to fit metric hardware, but I am not 100% sure I did this correctly.

I’ve got the test part here if anyone else is curious: https://www.thingiverse.com/download:6718742 . It is a bit ugly, looking like it was just tacked on, but it is fine for a functionality experiment.

Food for thought – maybe something like this could one day resolve the nut problem. Maybe the overhang could be eliminated with a a five-sided pocket, or a hexagon-with-roof, and still maintain plenty of strength.

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Why don’t you guys simply use some hex socket screws + a washer instead?

Wouldn’t that solve entirely your problems without the headaches?

Part of what I like about the MPCNC is a certain elegance in design, and a part of that is using a very small number of distinct fasteners. So while I am sure this is a valid solution that would work practically, (or requiring an 8mm bolt,) personally I feel a certain attachment to keeping the hardware list as short as possible.

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I have that dual nut option on the nut traps, it works great until it doesn’t. If I stick with this design I will look into this but I am 90% sure I actually tried and it did have an issue. This part is still the weakest point of the Center gantry, I have added everything I can to keep it rigid. I will always sacrifice convenience for rigidity. If it was the tool mount or something sure, but these literally get done once and there are lots of options. This is one metric preferred part…metric people have to deal with this on every single other nut on the build.

In reality I have very few complaints about this. I am guessing some just get funky tough nylocks or something.

I have a few prototypes of a different style gantry, nothing I am super excited about yet but I always try to keep both systems in mind. I would rather spend time on something much better than a tiny convenience tweak.

Not sure if it makes any difference but I put the bolt heads on the other side to maximize the tool area.

It’s not just four screws that will ruin the entire design, moreover this is a machine for doing stuff, not some kind of art piece you’ll expose in your living room for other people to masturbate on :wink:

If you still can see the screws after a few weeks of use, it only means that you didn’t make enough dust. Just use whatever works, no need to reinvent the wheel :slight_smile:

And if you really put a premium on hardware consistency, then just convert the whole machine to socket hex screws!

I’ve been using button head socket screws lately for a nice low profile, but you still have the nut at the other end. the nut is a problem much more often than the screw head.