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Comment by chasd00 | Hacker News Reader

Next weekend can't come soon enough :)

I'm in the high power rocketry hobby and got a Prusa i3 mk3 for myself for Christmas. It's pretty amazing to sketch a bit while on conference calls, then after work model my sketch in Fusion360, export the mesh, import to Prusa slicr, export gcode to an sdcard, start the printer and goto bed. Next morning, there's my part waiting for me.

Electronics is the next bridge to cross and i really hope this class helps with that. I know a guy in the hobby who went from nothing to selling a custom designed altimeter for rockets on his website in about 6 months which was pretty impressive to me. https://flightsketch.com/store/catalog/flightsketch-mini_1/

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Next thing you should do is get a raspberry pi with octoprint! Then you can drop the gcode in from your web browser :)

Or even easier, push to octoprint directly from your slicer!

What annoyed me was that the slicer does not mantain hole diameters, somehow holes are smaller. In my case I was planning to prototype cnc milled stuff (which is kind of expensive), but this killed to use case.

The slicer does maintain hole diameters, but just like CNC milling HDPE, you should expect dimensions to change a little after printing. If you need to have a tight tolerance fit on something, expect to run a drill or chucking reamer through it after printing. If it's really tight tolerances, it's easy enough to melt or press a drill bushing or threaded insert into an oversized hole to get fits tighter than you're likely able to hit with a mill.

Your expensive parts likely go through many steps before and after manufacturing already that involve things like fixturing the material in place, rotating it and reacquiring datums, running taps through by hand, deburring sharp edges, etc; you should not expect 3D printing to be as easy as sending the STEP file to the machine and getting a complete part out. Sanding, acetone vapor polish, drilling, reaming, tapping, etc. are normal parts of the manufacturing and prototyping process.

Which slicer do you use? It seems to be a known problem of cura.

I use Cura. It’s across FDM printing techniques and is (somewhat) akin to the errors in subtractive manufacturing. Subtractive errors/inaccuracies tend to be of the type that remove too much material. Additive errors/inaccuracies tend to add material.

My models for FDM often include a “hole size compensation” user parameter and I tweak it to introduce an offsetting bias in through-holes. (Non through holes tend to get heat-set brass inserts, which are more forgiving of the 0.1mm or less radial errors.)

You can print a clearance tolerance test to see how accurate your printer is. I use a Prusa mk3s and as accurate as it is, at best my tolerance is 0.1mm. Enough to stop your parts fitting together. I normally build in a 0.2mm tolerance for close fitting parts.

I have had the issue with both Cura and Simplify3D.

For a circular hole, as far as I can tell the slicer commands the hotend path to trace the circle exactly, rather than tracing a slightly bigger circle to account for output thickness.

I imagine if you were to use smaller nozzles the issue would be lesser, but not gone.

Just an fyi drills are not good for tight tolerances. They cut trilobular holes, not circles.

Reaming or boring are the route to tight tolerances.

It is much tighter than 3D printing, though - good enough for most cases if you’re using a drill press

IIRC there's a param in the slicers I've used that can fine tune hole sizes.

i've noticed things shrink a bit when the cool off ( at least with PLA ). What i do is print, test fit, adjust dimensions, then print again. It usually only takes one or two iterations to get it just right. I built a deck once, designed it, spec'd out all the materials i would need, optimized for minimal wastage, the whole 9 yards. I learned the hard way that in the real world dimensions are a matter of degree and what's on the label may or may not be what you get (to a ridiculous degree when it comes to lumber).

I'm use to software where it either compiles or it doesn't, it either works or it doesn't. In the real world with physical objects it's much more a matter of "is it close enough or not".

Lumber is regularly sold up to an inch off the label. But that is standard and expected. Meaning people in the trade know how much lumber to buy given the finished dimensions of what they buy.

I have no idea why this is standard practice and when building out http://MajikBus.co ran into minor issues.

This is, I think, partly due to how the slicer routes the filament paths. I've had this problem moving between nozzle diameters and the only reason I could think of was that it wasn't possible for the slicer to maintain both accurate hole diameters and the correct placement of filament relative to previous extrusions. The STL was identical, but the parts printed with a 0.8 mm nozzle were undersized and only the holes. My experience of this was with Cura and an Ultimaker S5. Generally you should always start printing loose holes and tightening them up as you go to see where the limit is.

The simplest dimension to control is the perimeter. That can, for most parts, be perfectly accurate because there are no constraints on where the nozzle needs to avoid. However if you print a hole close to the edge of a part then the printer somehow has to lay down both an accurate edge to the hole while maintaining suitable separation from previously printed stuff. Essentially your design is quantised by the nozzle diameter within the perimeter of the part (I may be wrong here, but this seems like a simple explanation). You may have better results if you tune your part to the printer/nozzle you're using so that the slicer doesn't have to worry about leaving/filling gaps that are non-integer multiples of the nozzle size. In theory this shouldn't matter, because the hole perimeter can also be printed first, as it's an "outside" edge, but it doesn't always seem to work that way.

Holes with interior dimensions close to the width of the nozzle are hard and need to be calibrated if you absolutely need to print them to size. Basically, the plastic is "spreading" a bit wider than the nozzle tip.

better solutions, rather than making exactly sized holes, is to increase the number of vertical walls, make the hole size slightly under, and use a precision drill and reamer to get to the target size. Also, make sure you're setting your scaling right (for example, I need to set my object size to be 100.6% of its designed size, because PLA shrinks when it cools).

Yeah this is just a thing you have to design around while using FDM printers, as others have mentioned. I really like melting in threaded inserts if you need threads, otherwise use a drill bit or reamer. If you need super tight tolerances (like CNC level tight) then a consumer grade 3D printer likely won't get you there though.

Prusa uses 3D printed parts and I'm a bit surprised just how good the prints are.

I own one and can confirm they are quite nice. Still though, you aren’t getting machining level tolerances (0.001”) on a FDM machine no matter how hard you try.

Might be worth checking out heat-set nut inserts: https://www.mcmaster.com/heat-set-inserts/ I've has good luck pushing these into holes in 3d printed parts with a soldering iron the past.

Chordial deviations add up on smaller holes.

Minimum size line segments and other things come into play on holes less than about 0.2"

What kind of parts do you print? I imagine engine parts are right out, and I'm guessing fins might melt as well. Nose cones? Body or internal stuff?

Not gp, but things you can definitely use 3d prints for related to rocketry: nose cones, fins, motor mounts, electronics mounts, and probably a whole slew of other stuff I'm forgetting at the moment.

my least favorite part of building a rocket is the motor mount. It requires a handful of parts, a decent epoxy system, and is one of the more expensive parts of the airframe. I've designed a modular mount that i can pull out of one airframe and plug into another. So once the motor mount is done i can use it in all my airframes of the same diameter and motor diameter without any bonding. To cover all my current airframe diameters and motor diameter combinations requires 5 prints and adding a new one is pretty easy.

my next thought is modular fin cans. The rule of thumb for trapezoidal fin dimensions are a function of airframe diameter. I would like to be able to make one fin can for each airframe diameter and then re-use those on new airframes. With my current inventory I need 4 fin cans ( 2", 3", 4" and 7.5" diameter airframes )

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