agree with you... sort of....
And gotta say John Coker does a great job of documenting and using engineering on his work (read Johns stuff!)
I do agree the materials and process need to be controlled, with equipment calibrated and all the variables listed, and those that affect the outcome controlled. Having said that, with some basic controls and calibration we could come up with something like a b-basis allowable.
https://ntrs.nasa.gov/api/citations/20040111395/downloads/20040111395.pdf for reference. Having said that, i have other concerns...
1) Do you follow the recipe, exactly......
Material and process control is just that you need to CONTROL the critical processes and Monitor them to make sure they are correct. Very few folks on here have actually calibrated their printer (not just is the bed flat but actually calibrated). Some examples are;
- use the extruder without a nozzle and heat and MEASURE the actual amount of material that is extruded when you feed out a theoretical 1M of filament, how does the actual length compare to theoretical
- weigh a finished part to see if the actual weight correlates with the density and the volume
- Print a cube, measure the corner heights and edges to determine; Overall height tolerance, squareness, shrinkage factors etc.
- Verify the gap from the extruder to the bed
- Measure the bed temperatures over a grid pattern.. how even and how close to the edge before it cools.
2) Do you know what you want....
I would be willing to bet that very few folks on her actually know what mechanical properties they need? Saying, i want it the same as (fill in the material here) doesn't work. Do I need the stiffness of wood, the bearing strength of fiberglass, the toughness of nylon? and how 'stiff or strong' do I need. Is it shear strength, do I need to thread a hole? is a one for one substitution of PLA for fiberglass good enough? most likely not. Most of the 3d parts I have seen fail were due to bad desing not bad material choices - this includes designing in infill or using desinged in ribs or stringers.
- Do you understand what using infill does to the streght of the part?
- And how it is different in the X/Y direction vs the Z
- Have you designed in ribs or just let the slicer do an infill?
- Are strutural filles built in?
- Are you designing for hot wet or cold. Side note from testing, parts sitting in the sun got hotter then motor tubes during a launch, all aprts see heat!
3) are you controlling the process, really....
Following the process means doing EXACTLY the same things each time. For example:
- Do you start with dry filament EVERY time (and yes PET-G does need to be dried)
- Do you replace the nozzle frequently enough that the ACTUAL nozzle diameter is still correct
- DO you have an enclosed machine? If not how are you keeping the temperature and humidity the same
- Same minuimum wall thicknesses? same feed rate? Same supplier for the filament?
- Do you need to and should you aneal the parts ( I use a suis vide machine for this)
- What is the shrinkgage from aneeling?
4) How do the parts interface to the rest of the rocket...
- How do your parts attach to the rocket and have you accounted for the limititions / benifits of the materials?
- Remeber the temperature and fit for heat innserts is also a process, just cranking up the soldering iron isnt process control.
- Have I desinged in bonding aids / bonding features?
Mike (actually works on material and process controls for materials) K
P.S. I did mention John is pretty good at this stuff.